Faculty Profiles - SHIGETA Iduru

Papers - SHIGETA Iduru

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Hideki Aoshima, Iduru Shigeta, Akiko Nomura, Kunio Yubuta, Touru Yamauchi, Rie Y. Umetsu, Takeshi Kanomata, Masahiko Hiroi . The critical behavior of magnetization near the Curie temperature in highly spin-polarized Heusler alloy Co<sub>2</sub>TiGa<sub>0.3</sub>Sn<sub>0.7</sub> . IEEE Transactions on Magnetics60 ( 9 ) 4100305-1 - 4100305-5 2024.9Reviewed International journal

Authorship：Corresponding author Language：English Publishing type：Research paper (scientific journal)

We present an analysis of the critical behavior of magnetization near the Curie temperature <italic>T</italic><sub>C</sub> in Heusler alloy Co<sub>2</sub>TiGa<sub>0.3</sub>Sn<sub>0.7</sub>, which is a promising candidate for highly spin-polarized materials that exhibit the characteristic properties of magnetic Weyl states. Weyl semimetals have garnered significant interest for their potential applications in spintronics devices, owing to their anomalous magnetoelectronic and magnetothermal properties. The magnetization of the synthesized Heusler alloy Co<sub>2</sub>TiGa<sub>0.3</sub>Sn<sub>0.7</sub> was investigated in the vicinity of <italic>T</italic><sub>C</sub>. The analysis of magnetic isotherms for Co<sub>2</sub>TiGa<sub>0.3</sub>Sn<sub>0.7</sub> reveals that the critical exponent δ is deduced to be 5.16 in the high field region and 4.96 in the low field region. The obtained value of δ deviates from the value of 3.0 expected by the conventional molecular field theory. However, it is in close agreement with the value of 5.0 predicted by the spin fluctuation theory for weak itinerant electron ferromagnets, exhibiting that the spin fluctuation theory is applicable even to highly spin-polarized Heusler alloys.

DOI： 10.1109/TMAG.2024.3414430

Hideki Aoshima, Iduru Shigeta, Akiko Nomura, Kunio Yubuta, Touru Yamauchi, Rie Y. Umetsu, Takeshi Kanomata, Masahiko Hiroi . The critical behavior of magnetization near the Curie temperature in highly spin-polarized Heusler alloy Co<sub>2</sub>TiGa<sub>0.3</sub>Sn<sub>0.7</sub> . 2024 IEEE International Magnetic Conference Proceedings - INTERMAG 2024 Short Papers 1 - 2 2024.5Reviewed

Masahiko Hiroi, Tsubasa Takamoto, Iduru Shigeta, Masahira Onoue . Mössbauer study of magnetically ordered states in Heusler compounds Fe<sub>3−</sub><i><sub>x</sub></i>Mn<i><sub>x</sub></i>Si . Interactions245 ( 1 ) 58-1 - 58-9 2024.4Reviewed International journal

Language：English Publishing type：Research paper (scientific journal)

It has been known that the Heusler compound Fe3-xMnxSi has a complex magnetic phase diagram. For 0.75 ≤x≤ 1.75, it exhibits a ferromagnetic transition at TC, and at a lower temperature TA, another transition to a phase with both ferromagnetic and antiferromagnetic components. Recently, however, magnetization and specific heat measurements revealed that for x= 1.7, yet another transition occurs at a temperature lower than TA, i.e., there are three transitions at zero magnetic field. In our previous study, the nature of the ordered phases and transitions for x= 1.7 and 1.68 were investigated using Mössbauer spectroscopy, and the existence of a hyperfine field Bhf was confirmed. This demonstrates that the low temperature phases are magnetically ordered phases. In this study, to further investigate the ordered phases and transitions in Fe3-xMnxSi, we measure Mössbauer spectra for x = 1.8 and 1.9, where only a single antiferromagnetic transition appears to exist as judged from magnetization measurements, and the transition temperature is defined as TA2′. The spectra show that Bhf is present for x= 1.8 and 1.9 at low temperatures, and the values of Bhf are comparable with that for x = 1.7. With increasing temperature T, Bhf decreases smoothly, and at T>TA2′ the spectrum becomes a singlet and Bhf is zero. These results confirm the existence of magnetic order for x = 1.8 and 1.9.

DOI： 10.1007/s10751-024-01903-3

Masahiko Hiroi, Kouki Akaishi, Tomohito Nonoyama, Iduru Shigeta, d Masahira, Onoue . Mössbauer Spectroscopy of Magnetic Ordered Phases in Heusler Compounds Fe3−xMnxSi . JPS Conference Proceedings38 011136-1 - 011136-6 2023.5Reviewed International journal

Language：English Publishing type：Research paper (international conference proceedings) Publisher：Physical Society of Japan

For 0.75 ≤ x ≤ 1.75, the Heusler compound Fe3−xMnxSi undergoes two phase transitions: one ferromagnetic at TC and another to a phase with antiferromagnetic properties at a lower temperature, TA. It was recently discovered that for x = 1.7, another antiferromagnetic transition occurs at a temperature even lower than TA, TA2. In addition, the phase below TA2 is defined as the AF2 phase. In this study, Mössbauer spectroscopy measurements were performed for samples with x = 1.5, 1.68, and 1.7 down to ∼10 K to explore the properties of these magnetically ordered phases. At high temperatures, a singlet peak was observed and no hyperfine field was detected. At low temperatures the peak became broader, reflecting the increase in the hyperfine field in the magnetically ordered phases. For x = 1.7 and 1.68, the hyperfine field increased smoothly with decreasing temperature from TC through TA and TA2 to low temperatures. TC estimated via the temperature dependence of the hyperfine field agrees with the results of the magnetization measurements. These findings show that despite the small macroscopic magnetization below TA2, an internal field exists, implying that the AF2 phase is an antiferromagnetic ordered phase.

DOI： 10.7566/JPSCP.38.011136

Hideki Aoshima, Iduru Shigeta, Jun Gouchi, Touru Yamauchi, Takeshi Kanomata, Rie Y. Umetsu, Yoshiya Uwatoko, Shinpei Fujii, Masahiko Hiroi . Magnetic properties of highly spin-polarized Heusler alloy CoFeCrAl . JPS Conference Proceedings38 011133-1 - 011133-6 2023.5Reviewed International journal

Authorship：Corresponding author Language：English Publishing type：Research paper (international conference proceedings) Publisher：Physical Society of Japan

We report the magnetic properties of quaternary Heusler alloy CoFeCrAl as a potential candidate for spin-gapless semiconductor (SGS). The obtained magnetization of CoFeCrAl showed that the spontaneous magnetization Mexps was 2.021 μB/f.u., Mexps was independent of applying pressure, and the high magnetic field susceptibility χexphf was almost independent of magnetic fields. We have further investigated the electronic structure by the first-principles band calculations. The total density of states (DOS) demonstrated typical SGS-like structure, and thus the spin polarization Pcal was deduced to be 95.47% at the Fermi energy EF and the total magnetic moment Mcaltot is 1.999 μB/f.u., respectively. We found that Mexps is good agreement with Mcaltot following the Slater–Pauling rule. Therefore, we conclude that CoFeCrAl is a highly spin-polarized material and it is suitable for applying to counterelectrodes in spintronic devices.

DOI： 10.7566/JPSCP.38.011133

Kouki Akaishi, Iduru Shigeta, Masahiko Hiroi, Jun Gouchi, Yoshiya Uwatoko . Effect of pressure on the magnetic properties of Heusler compound Fe<sub>1.3</sub>Mn<sub>1.7</sub>Si . JPS Conference Proceedings38 011135-1 - 011135-6 2023.5Reviewed International journal

Language：English Publishing type：Research paper (international conference proceedings) Publisher：Physical Society of Japan

The Heusler compound Fe3−xMnxSi has been studied in various compositions and it exhibits complex magnetic properties at low temperatures. In ambient pressure at zero field, when x = 1.7, particularly, it orders ferromagnetically below TC ≈ 85 K, exhibits a transition to a phase with antiferromagnetic components at TA ≈ 67 K; furthermore, it orders antiferromagnetically at an even lower temperature of TA2 ≈ 60 K. For the dependence on the magnetic field of these transition temperatures, TA(B) and TA2(B), with increasing magnetic field, TA(B) changes only slightly, whereas TA2(B) shifts toward lower temperatures. In this study, magnetization measurements under pressures up to 1.2 GPa were performed to investigate pressure effects on transition temperatures and magnetic fields. The B–T magnetic phase diagram is presented, which summarizes the transition temperatures and magnetic fields obtained from these results. With increasing applied pressure, the TA2(B) line shows a shift toward the higher magnetic fields and temperatures, whereas TA(B) changes only slightly, and TC decreases and disappears around ∼1 GPa.

DOI： 10.7566/JPSCP.38.011135

Iduru Shigeta, Shuta Oku, Takahide Kubota, Shojiro Kimura, Takeshi Seki, Bunju Shinozaki, Satoshi Awaji, Koki Takanashi, Masahiko Hiroi . Superconducting fluctuation effect on epitaxially layered films of superconductor NbN and half-metallic Heusler alloy Co<sub>2</sub>MnSi . AIP Advances13 025116-1 - 025116 -5 2023.2Superconducting fluctuation effect on epitaxially layered films of superconductor NbN and half-metallic Heusler alloy Co<sub>2</sub>MnSiReviewed International journal

Authorship：Lead author,　Corresponding author Language：English Publishing type：Research paper (scientific journal) Publisher：AIP Publishing

Superconducting properties were investigated in epitaxially layered films consisting of superconductor NbN and half-metallic Heusler alloy Co2MnSi (CMS). Temperature dependence of the electrical resistivity ρ(T) was measured by applying perpendicular magnetic fields to the surface of NbN/CMS films. With the increase of the CMS thickness dCMS, the upper critical field μ0Hc2 decreased monotonically, but the superconducting transition temperature Tc had the minimum of 10.1 K at dCMS ≈ 5 nm. The Tc behavior was in qualitative agreement with the theory of the π-coupling. The pair-breaking parameter δ determined by the superconducting fluctuation theory took the maximum at dCMS = 3 ∼5 nm, which would be related to the minimum of Tc. The experimental results reveal that the superconductivity of the NbN layer in NbN/CMS films is affected by the interplay between the superconducting NbN layer and the half-metallic CMS layer.

DOI： 10.1063/9.0000584

T. Kanomata, Y. Amako, Y. Ida, Y. Adachi, T. Osaki, T. Eto, H. Nishihara, I. Shigeta, S. Imada, M. Doi . Magnetic properties of ferromagnetic Heusler alloy Co<sub>2</sub>ZrSn . Journal of Physics and Chemistry of Solids164 110635-1 - 110635-7 2022.5Magnetic properties of ferromagnetic Heusler alloy Co<sub>2</sub>ZrSnReviewed International journal

Language：English Publishing type：Research paper (scientific journal)

The results of experimental investigations of the crystal structure, permeability, magnetization and Mössbauer spectroscopy of Co2ZrSn are reported. The sample, which was prepared by arc melting, comprised of a Heusler structure Co2ZrSn that coexisted with a small amount of an impurity phase. The Curie temperature TC of Co2ZrSn was found to be 452.3 K. At 1.8 K the magnetic moment of Co2ZrSn was estimated to be 2.0 μB/f.u., confirming that the magnetic moment of Co2ZrSn obeys the Slater-Pauling rule. The temperature variation of the spontaneous magnetization Ms roughly follows the Brillouin function for spin value S = 1/2. However, in the temperature region of T/TC > 0.6, Ms is distinctly higher than the Brillouin function. The TC of Co2ZrSn was almost independent of pressure. The Mössbauer spectroscopy measurements showed that the isomer shift I.S. (1.5 mm/s at 295 K) was approximately temperature independent. At 95 K the value of the hyperfine magnetic field HhfSn at the Sn nuclei was found to be 101 kOe. The temperature variation of HhfSn of Co2ZrSn roughly followed the Brillouin function for S = 1/2. On the basis of the experimental results, the electronic and magnetic properties of Co2ZrSn are discussed.

DOI： 10.1016/j.jpcs.2022.110635

Soichiro Tsujikawa, Iduru Shigeta, Jun Gouchi, Takeshi Kanomata, Rie Y. Umetsu, Yoshiya Uwatoko, Masahiko Hiroi . Magnetization of Quaternary Heusler Alloy CoFeCrAl . IEEE Transactions on Magnetics58 ( 2 ) 2600505-1 - 2600505-5 2022.2Magnetization of Quaternary Heusler Alloy CoFeCrAlReviewed International journal

Authorship：Corresponding author Language：English Publishing type：Research paper (scientific journal)

We report the magnetovolume effect of quaternary Heusler alloy CoFeCrAl as a potential candidate of spin gapless semiconductor. The crystal structure of CoFeCrAl was confirmed as the single phase of the ordered LiMgPdSn-type structure. From magnetization measurements at ambient pressure, the Curie temperature TC and the spontaneous magnetization Ms were determined to be 575.2 K and 2.01 μ B/f.u., respectively. The TC is higher enough than room temperature and the Ms follows the Slater-Pauling rule. High-pressure magnetization measurements showed that the Ms is almost independent of applying pressures not only at 10 K but also up to 300 K, exhibiting that the electronic state of CoFeCrAl is fully spin polarized and it is preserved even at 300 K. The experimental results reveal that the quaternary Heusler alloy CoFeCrAl is an attractive material for functional electrode ferromagnets in spintronics devices.

DOI： 10.1109/TMAG.2021.3096227

Takaaki Yokoyama, Iduru Shigeta, Akiko Nomura, Kunio Yubuta, Touru Yamauchi, Rie Y. Umetsu, Hironori Nishihara, Takeshi Kanomata, Masahiko Hiroi . Critical Behavior of the Magnetization in Heusler Alloy Co2TiGa0.8Sn0.2 . IEEE Transactions on Magnetics58 ( 2 ) 2600404-1 - 2600404-4 2022.2Critical Behavior of the Magnetization in Heusler Alloy Co2TiGa0.8Sn0.2Reviewed International journal

Language：English Publishing type：Research paper (scientific journal)

We report the critical behavior of the magnetization in Heusler alloy Co2TiGa0.8Sn0.2 as a potential candidate of high spin polarized materials with the characteristics of topological Weyl fermions. The Weyl semimetals are materials of great interest for applying spintronics devices, due to their unusual magnetoelectronic and magnetothermal properties. The magnetic properties of the synthesized Heusler alloy Co2TiGa0.8Sn0.2 are investigated around the Curie temperature TC. The magnetic isotherm measurements of Co2TiGa0.8Sn0.2 around TC exhibit that the critical exponent δ is deduced to be 4.455(4) in high maximum applied field of 50 kOe and 4.823(9) in low maximum applied field of 10 kOe, respectively. Both δ values are different from 3.0 derived by the conventional molecular field theory, but they are close to 5.0 predicted by the spin fluctuation theory for the itinerant electron ferromagnets. Possible origins for the discrepancy between experimental results and theoretical models are also discussed.

DOI： 10.1109/TMAG.2021.3097164

Hiroi Masahiko, Nonoyama Tomohito, Shigeta Iduru, Yamauchi Touru . Multiple Magnetic Phase Transitions in Fe3−xMnxSi Heusler Compounds . Journal of the Physical Society of Japan90 ( 10 ) 103701-1 - 103701-5 2021.10Reviewed International journal

Masahiko Hiroi, Tomohito Nonoyama, Iduru Shigeta, Touru Yamauchi . Multiple magnetic phase transitions in Fe<sub>3−x</sub>Mn<sub>x</sub>Si Heusler compounds . Journal of the Physical Society of Japan90 ( 10 ) 103701-1 - 103701-5 2021.10Multiple magnetic phase transitions in Fe<sub>3−x</sub>Mn<sub>x</sub>Si Heusler compoundsReviewed International journal

Language：English Publishing type：Research paper (scientific journal)

Fe3−xMnxSi (x > 0.75) exhibits a ferromagnetic transition at TC and a transition to an antiferromagnetic phase with a ferromagnetic component at a temperature (TA) lower than TC, defined as the AF phase. Recently, in Fe1.3Mn1.7Si, another magnetic transition at a temperature lower than TA, defined as TA2, was observed. The phase below TA2 is referred to as the AF2 phase and is likely antiferromagnetic. This paper presents evidence for multiple phase transitions in Fe1.3Mn1.7Si obtained from specific heat and magnetization measurements. In addition, the magnetic transitions and magnetic properties of Fe3−xMnxSi (1.65 ≤ x ≤ 1.8) are investigated.

DOI： 10.7566/JPSJ.90.103701

Masahiko Hiroi, Sora Nishiinoue, Iduru Shigeta, Masakazu Ito, Keiichi Koyama, Akihiro Kondo, Koichi Kindo, Isao Watanabe, Muneaki Fujii, Shojiro Kimura, Hirotaka Manaka, Norio Terada . High magnetic field properties in Ru$_{2-x}$Fe$_{x}$CrSi with antiferromagnetic and spin-glass states . Physical Review B103 ( 9 ) 094428-1 - 094428-12 2021.3High magnetic field properties in Ru$_{2-x}$Fe$_{x}$CrSi with antiferromagnetic and spin-glass statesReviewed International journal

Language：English Publishing type：Research paper (scientific journal)

We report experimental studies on Ru2-xFexCrSi focusing on properties related to antiferromagnetism in Ru2CrSi and spin glass (SG) in Ru1.9Fe0.1CrSi. By measuring the temperature dependence of magnetization M(T), specific heat C(T), electrical resistivity ρ(T), and muon spin relaxation (μSR), we observed that Ru2CrSi exhibited an antiferromagnetic (AF) transition at a temperature TN of ∼13 K, and Ru1.9Fe0.1CrSi showed SG properties that could be interpreted as successive SG transitions, where the spin-freezing occurred at temperature Tg, and below that strong irreversibility in M(T) and a gradual peak of M(T) at T∗(>Tg) appeared. When the data for 0≤x≤0.1 were compared, by substituting Fe the AF order was rapidly destroyed and it appeared to change to two anomalies at T∗ and Tg. With increasing x, there was a slight change in Tg from TN for x=0; however, T∗ increased, suggesting that the AF and the SG states are closely related. Furthermore, the results of specific heat, resistivity, and magnetization in high fields were presented and compared. For Ru2CrSi in specific heat under high magnetic fields up to 14 T, the peak shape around TN and the TN value were constant. The resistivity and magnetization in pulsed fields suggested that TN of Ru2CrSi was constant up to over 50 T. These results demonstrated the unusual robustness of the AF transition to magnetic fields. In Ru1.9Fe0.1CrSi, unusual hysteresis in magnetoresistance was observed in static and pulsed magnetic fields, although their appearances differed. These hystereses were considered a manifestation of the curious properties of the SG state with strong irreversibility.

DOI： 10.1103/PhysRevB.103.094428

Tomohito Nonoyama, Ryota Kato, Iduru Shigeta, Masahiko Hiroi, Hirotaka Manaka, Norio Terada . Magnetic transition in the Heusler compounds Fe$_{3- x}$Mn$_{x}$Si . AIP Advances8 ( 11 ) 115018-1 - 115018-5 2018.11Reviewed International journal

Language：English Publishing type：Research paper (scientific journal)

It has been shown that Fe MnSi exhibits a ferromagnetic transition at Tc and an antiferromagnetic transition at a lower temperature, T , to a phase referred to as the AF phase. In a recent study on Fe Mn Si, another antiferromagnetic transition at a temperature lower than T , defined as T , was found, with the phase below T referred to as the AF2 phase. In this study, magnetic properties of Fe Mn Si are investigated for an x range of 1.65 ≤ x ≤ 1.85 in order to study theses transitions with varying x. For x ≥ 1.75, a transition characterized by a rather rapid decrease in the temperature dependence of magnetization is observed at a temperature lower than T at fields higher than ∼2 T. This implies the AF2-AF transition exists for x ≥ 1.75 as for x = 1.7. The magnetic field where the AF2-AF transition occurs increases with x, whereas at lower fields T does not depend strongly on x. Meanwhile, at near zero field, the ferromagnetic transition and spontaneous magnetization disappears for x ≥ 1.75, in contrast to the case for x = 1.7. This implies that the transition from paramagnetism directly to the AF2 phase occurs at low fields. These results are summarized in the B-T magnetic phase diagram. 2 A 1.3 1.7 A A2 A2 3-x x A A2

DOI： 10.1063/1.5080429

Ryota Kato, Tomohito Nonoyama, Ryutaro Ooka, Iduru Shigeta, Masahiko Hiroi . Transport properties of Heusler compounds Fe$_{3- x}$Mn$_{x}$Si ($x$ = 1.6, 1.7 and 1.8) under magnetic fields . AIP Advances8 ( 11 ) 115019-1 - 115019-4 2018.11Reviewed International journal

Language：English Publishing type：Research paper (scientific journal)

Measurements of the electrical resistivity, magnetoresistance (MR), and Hall effects are performed to investigate the transport properties of Fe Mn Si (x = 1.6, 1.7, and 1.8) under magnetic fields. It is known that Fe MnSi exhibits a ferromagnetic transition at T and an antiferromagnetic transition at a lower temperature (T ) to the phase referred to as the AF phase. A recent study on Fe Mn Si found another antiferromagnetic transition at a temperature below T , defined as T , to a phase referred to as the AF2 phase. In this study, in MR measurements for x = 1.7 and 1.8, the change in MR with magnetic field is found to transition from positive to negative at certain magnetic fields. Comparing these results with those of magnetization measurements, these points correspond to the AF2-AF transitions. The results also indicate that these transitions occur at a higher field for x = 1.8 than for x = 1.7. The Hall effect measurements show the anomalous Hall effect for x = 1.6 and 1.7 in the temperature region where spontaneous magnetization is present, but not for x = 1.8, for which the ferromagnetic state appears to be absent. These results are consistent with the magnetization measurements. 3-x x 2 C A 1.3 1.7 A A2

DOI： 10.1063/1.5043075

Iduru Shigeta, Takahide Kubota, Yuya Sakuraba, Shojiro Kimura, Satoshi Awaji, Koki Takanashi, Masahiko Hiroi . Transport properties of epitaxial films for superconductor NbN and half-metallic Heusler alloy Co2MnSi under high magnetic fields . Physica B: Condensed Matter536 310 - 313 2018.5Reviewed International journal

Authorship：Lead author Language：English Publishing type：Research paper (scientific journal) Publisher：Elsevier B.V.

Transport properties were investigated for epitaxial films of superconductor NbN and half-metallic Heusler alloy Co2MnSi under high magnetic fields up to 17 T. The superconducting transition temperature Tc of NbN/Co2MnSi/Au trilayer films was determined to be 16.1K in the absence of magnetic field. Temperature dependence of the resistivity ρ(T) was measured in both magnetic fields parallel and perpendicular to the surface of NbN/Co2MnSi/Au trilayer films. The activation energy U0(H) for vortex motion of the trilayer films in both magnetic fields was well fitted above 5 T by the similar model with the exponents in the field dependence of the pinning force density. From the resistivity ρ(T) measurements under high magnetic fields, the upper critical field Hc2(0) at 0 K was also deduced to be μ0Hc2 ∥(0)=23.2T for the parallel magnetic filed and μ0Hc2 ⊥(0)=15.8T for the perpendicular magnetic field, respectively. The experimental results under magnetic fields revealed the superconductivity of the NbN layer was affected by the interplay between the superconducting NbN layer and the half-metallic Co2MnSi layer.

DOI： 10.1016/j.physb.2017.09.074

Ryutaro Ooka, Iduru Shigeta, Rie Y. Umetsu, Akiko Nomura, Kunio Yubuta, Touru Yamauchi, Takeshi Kanomata, Masahiko Hiroi . Temperature dependence of differential conductance in Co-based Heusler alloy Co2TiSn and superconductor Pb junctions . Physica B: Condensed Matter536 289 - 292 2018.5Reviewed International journal

Authorship：Corresponding author Language：English Publishing type：Research paper (scientific journal) Publisher：Elsevier B.V.

We investigated temperature dependence of differential conductance G(V) in planar junctions consisting of Co-based Heusler alloy Co2TiSn and superconductor Pb. Ferromagnetic Co2TiSn was predicted to be half-metal by first-principles band calculations. The spin polarization P of Co2TiSn was deduced to be 60.0% at 1.4K by the Andreev reflection spectroscopy. The G(V) spectral shape was smeared gradually with increasing temperature and its structure was disappeared above the superconducting transition temperature Tc. Theoretical model analysis revealed that the superconducting energy gap Δ was 1.06meV at 1.4K and the Tc was 6.8K, indicating that both values were suppressed from bulk values. However, the temperature dependent Δ(T) behavior was in good agreement with that of the Bardeen-Cooper-Schrieffer (BCS) theory. The experimental results exhibit that the superconductivity of Pb attached to half-metallic Co2TiSn was kept the conventional BCS mechanism characterized strong-coupling superconductors while its superconductivity was slightly suppressed by the superconducting proximity effect at the Co2TiSn/Pb interface.

DOI： 10.1016/j.physb.2017.10.055

Masahiko Hiroi, Tomohito Nonoyama, Genta Adachi, Iduru Shigeta, Hirotaka Manaka, Norio Terada . Magnetic properties of Heusler compound Fe1.3Mn1.7Si . Journal of Physics: Conference Series969 ( 1 ) 012098-1 - 012098-6 2018.4Reviewed International journal

Language：English Publishing type：Research paper (international conference proceedings) Publisher：Institute of Physics Publishing

The magnetic properties of two Heusler compounds Fe1.3Mn1.7Si and Fe1.4Mn1.6Si are compared. Fe1.4Mn1.6Si exhibits a ferromagnetic transition at TC ∼ 117 K and another transition to a phase with antiferromagnetic components at TA ∼ 66 K, which is the same as the behavior of Fe2MnSi. Further, it is known that for Fe3-xMn x Si, with increasing x, TC decreases but TA remains almost constant, and they appear to meet around x ∼1.7. The careful measurements of magnetization at various magnetic fields reveal that Fe1.3Mn1.7Si has three transitions at low fields
for instance, at 0.01 T, the transition temperatures are found to be 85 K, 67 K, and 60 K. The lowest transition at 60 K is characterized by a rapid decrease in magnetization with decreasing temperature and significant magnetic field dependence, in contrast to the transition at TA , which is relatively independent of the magnetic field, for x = 1 and 1.6. The critical field at 0 K is ∼3 T. The transition at the lowest temperature was not found for x ≤ 1.6, and it is considered a different transition from that at TA . While the transition at the highest temperature 85 K is a ferromagnetic transition, the transition at 67 K has the characteristics of the transition at TA , and it is found to be insensitive to magnetic fields. They are revealed to be the same transitions as those at TC and TA for x ≤ 1.6.

DOI： 10.1088/1742-6596/969/1/012098

Masahiko Hiroi, Syunya Ishikuma, Iduru Shigeta, Keiichi Koyama, Akihiro Kondo, Koichi Kindo, Hirotaka Manaka, Norio Terada . Magnetization and magnetic phase diagram of Heusler compounds Fe3-y(Mn1-xV x ) y Si (y = 1 and 1.5) . Journal of Physics: Conference Series969 ( 1 ) 012099-1 - 012099-6 2018.4Reviewed International journal

Language：English Publishing type：Research paper (international conference proceedings) Publisher：Institute of Physics Publishing

Fe2MnSi exhibits a ferromagnetic transition at TC and another transition to a phase with antiferromagnetic components at a low temperature of TA . By substituting V for Mn, so as to obtain Fe2Mn1-xV x Si, TA decreases with x and then vanishes around x ∼ 0.2. In this study, the magnetic transitions are investigated by magnetization measurements for Fe2Mn1-xV x Si (y = 1, 0 ≤ x ≤ 0.2) in high magnetic fields up to ∼70 T and for Fe1.5(Mn1-xV x )1.5Si (y = 1.5, 0 ≤ x ≤ 0.1) up to 5 T. For y = 1.5, with increasing x, TC increases and TA decreases as for y = 1, and the rate of the decrease in TA with x is slightly smaller than for y=1. In the low magnetic field region, TA for y = 1 does not significantly change due to the magnetic field but a distinct decrease is observed in high fields. The critical field at 0 K of this transition in Fe2MnSi is found to be larger than 70 T, and with increasing x, the critical field decreases corresponding to the decrease of TA at zero field.

DOI： 10.1088/1742-6596/969/1/012099

Iduru Shigeta, Yutaro Fujimoto, Ryutaro Ooka, Yuya Nishisako, Masahito Tsujikawa, Rie Y. Umetsu, Akiko Nomura, Kunio Yubuta, Yoshio Miura, Takeshi Kanomata, Masafumi Shirai, Jun Gouchi, Yoshiya Uwatoko, Masahiko Hiroi . Pressure effect on the magnetic properties of the half-metallic Heusler alloy Co2TiSn . Physical Review B97 ( 10 ) 104414-1 - 104414-8 2018.3Reviewed International journal

Authorship：Lead author Language：English Publishing type：Research paper (scientific journal) Publisher：American Physical Society

Precise magnetization measurements of a half-metallic ferromagnet Co2TiSn with the Heusler type structure have been carried out under pressure up to 1.27 GPa. At ambient pressure, the spontaneous magnetic moment Msexp and the Curie temperature TCexp are experimentally obtained to be 1.993(3)μB/f.u. and 376.76(7) K, respectively. The Msexp is independent of applying pressure. The TCexp decreases with the increase of pressure and the pressure derivative of the Curie temperature dTCexp/dp is estimated to be -3.15(9) K/GPa. Furthermore, the pressure effect on the electronic and magnetic properties of Co2TiSn has been theoretically investigated on the basis of first-principles density functional calculations by using the projector augmented wave method in order to evaluate the half metallicity of Co2TiSn. Both the experimental and the theoretical results under pressure reveal that Co2TiSn is half metal. The experimental results of pressure-dependent magnetic properties are finally discussed on the basis of the spin fluctuation theory for itinerant electron magnetism.

DOI： 10.1103/PhysRevB.97.104414

Iduru Shigeta, Takahide Kubota, Yuya Sakuraba, Cor G. Molenaar, Joost N. Beukers, Shojiro Kimura, Alexander A. Golubov, Alexander Brinkman, Satoshi Awaji, Koki Takanashi, Masahiko Hiroi . Epitaxial contact Andreev reflection spectroscopy of NbN/Co2FeSi layered devices . Applied Physics Letters112 ( 7 ) 072402-1 - 072402-5 2018.2Reviewed International coauthorship International journal

Authorship：Lead author Language：English Publishing type：Research paper (scientific journal) Publisher：American Institute of Physics Inc.

We investigated the spin polarization P of Co-based Heusler alloy Co2FeSi by epitaxial contact Andreev reflection (ECAR) spectroscopy using epitaxially grown superconductor NbN and Heusler alloy Co2FeSi layered devices. Ferromagnetic Co2FeSi possesses the highest Curie temperature (TC ? 1100 K) and the largest spontaneous magnetic moment (ps ? 6 μB) in the class of Heusler alloys. The ECAR measurements revealed that the P value of Co2FeSi was 54 ± 2% with a finite barrier parameter Z, indicating that an intrinsic P value in ECAR spectroscopy would exceed reported values in point-contact Andreev reflection spectroscopy. We therefore established not only the epitaxial integration of ferromagnetic Co2FeSi with superconductor NbN on an MgO substrate but also the fabrication and evaluation techniques of their ECAR devices. This highly versatile superconducting spintronic system enables fundamental superconducting spintronic studies, and it is also a candidate for practical superconducting spintronic devices.

DOI： 10.1063/1.5007287

Masakazu Ito, Tatsuya Furuta, Keita Kai, Atsushi Taira, Keijiro Onda, Iduru Shigeta, Masahiko Hiroi . Thermodynamic Properties of Heusler Fe2-xCox MnSi . JOURNAL OF MAGNETISM AND MAGNETIC MATERIALS428 390 - 393 2017.4Reviewed International journal

Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER SCIENCE BV

We investigated the thermodynamic properties of Heusler compounds Fe2-xCox mnSi (0.00 <= x <= 2.00). The specific heats CP(T) for compounds with x <= 0.1 exhibit a.-type anomaly arising from spin rearrangements at TR. With increasing x, TR decreases linearly and vanishes at x similar to 0.169. The magnetic entropy, S-TR, derived from the magnetic specific heat, Cm(T), released at TR decreases by increasing x. This means the canting angle of spins from the [111] direction decreases by the substitution of Fe atoms with Co atoms, based on the magnetic structure model of Fe2MnSi proposed by Miles et al. For compounds with 0.5 <= x, CP(T) in the low-T range can be reproduced by Debye T-3 law. The electronic specific heat coefficient decreases monotonically with x.

DOI： 10.1016/j.jmmm.2016.12.070

Masahiko Hiroi, Hiroaki Sano, Tomoya Tazoko, Iduru Shigeta, Masakazu Ito, Keiichi Koyama, Hirotaka Manaka, Norio Terada, Muneaki Fujii, Akihiro Kondo, Koichi Kindo . Magnetic and electrical properties of Heusler compounds Ru2Cr1-xXxSi (X = V, Ti) . JOURNAL OF ALLOYS AND COMPOUNDS694 1376 - 1382 2017.2Reviewed International journal

Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER SCIENCE SA

Two series of Heusler compounds Ru2Cr1-xXxSi (X = V, Ti) are synthesized, and their magnetic and electrical properties are investigated. Overall, these two series of compounds are identical in numerous properties. The measurements of magnetic properties reveal that an antiferromagnetic transition observed for x = 0 becomes smeared and it changes into a spin-glass state with increasing x. Around x = 0.5 this behavior disappears and it becomes paramagnetic. High-field magnetization for x = 03 and 0.4 up to 62 T exhibits no sign of magnetic transition or saturation of the magnetization. In the temperature dependence of resistivity, the kink that indicates the antiferromagnetic transition observed for x = 0 becomes invisible even for x similar to 0.1. This also exemplifies that the antiferromagnetic order is destroyed by the substitution and is replaced by the spin-glass state. Concerning the resistivity, these samples exhibit semiconducting behavior for small x, whereas with increasing x >= 0.5 they turn to show metallic behavior and Ru2VSi becomes a paramagnetic metal. Nevertheless, Ru2TiSi shows a tendency for semiconductivity again. (C) 2016 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.jallcom.2016.09.276

Masahiko Hiroi, Tomoya Tazoko, Hiroaki Sano, Iduru Shigeta, Keiichi Koyama, Akihiro Kondo, Koich Kindo, Hirotaka Manaka, Norio Terada . High-field magnetization of Heusler compound Fe2Mn1-xVxSi . PHYSICAL REVIEW B95 ( 1 ) 014410-1 - 014410-5 2017.1Reviewed International journal

Language：English Publishing type：Research paper (scientific journal) Publisher：AMER PHYSICAL SOC

Fe2MnSi exhibits a ferromagnetic transition at T-C similar to 230 K and another transition to a phase with antiferromagnetic components at T-A similar to 60 K. By substituting V for Mn, so as to obtain Fe2Mn1-xVxSi, T-A is revealed to decrease with x and then vanish around x similar to 0.2. In this study, the phase boundary of the transition at T-A in the high-field range is found for 0 <= x <= 0.15 with pulsed fields up to similar to 70 T. The magnetization of Fe2Mn1-xVx Si slowly increases even at the highest field of similar to 70 T, though it occurs more gradually as x increases. We compare the magnetization for 0 <= x <= 0.20 at 62 T with the Slater-Pauling rule, which holds when a Heusler compound is a half-metal, and find fairly good agreement. This suggests an intimate relation between the high-field phase and the half-metallic electronic structure, and that at the high-field limit the phase approaches the half-metallic state, which has been predicted by band-structure calculations.

DOI： 10.1103/PhysRevB.95.014410

Ryutaro Ooka, Iduru Shigeta, Yoshimi Sukino, Yutaro Fujimoto, Rie Y. Umetsu, Yoshio Miura, Akiko Nomura, Kunio Yubuta, Touru Yamauchi, Takeshi Kanomata, Masahiko Hiroi . Magnetization and Spin Polarization of Heusler Alloys Co2TiSn and Co2TiGa0.5Sn0.5 . IEEE MAGNETICS LETTERS8 3101604-1 - 3101604-4 2017Reviewed International journal

Authorship：Corresponding author Language：English Publishing type：Research paper (scientific journal) Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC

The substitution effect on half-metallicity for Co-based Heusler alloys Co2TiGa1-xSnx was investigated by experimental measurements and first-principles band calculations. The synthesized alloys crystalized in the ordered L2(1) crystal structure. The spontaneous magnetization obeyed the Slater-Pauling rule, indicating the possibility that Co2TiSn and Co2TiGa0.5Sn0.5 are half-metals. The first-principles band calculations revealed that Co2TiSn is half-metal and Co2TiGa0.5Sn0.5 is nearly half-metal. The spin polarization P was determined by the Andreev reflection technique. The intrinsic P values were 58.9% for Co2TiSn and 58.2% for Co2TiGa0.5Sn0.5, respectively. The P values of Co2TiGa1-xSnx were relatively higher than other Co-based Heusler alloys. Thus, Co-based Heusler alloys Co2TiGa1-xSnx are possible candidates for half-metallic electrode materials in spintronic devices.

DOI： 10.1109/LMAG.2016.2623257

Iduru Shigeta, Takahide Kubota, Kazumasa Makise, Shojiro Kimura, Satoshi Awaji, Bunju Shinozaki, Keiichi Koyama, Koki Takanashi, Masahiko Hiroi . Fabrication and Characterization of Epitaxial Films of Superconductor NbN and Highly Spin- Polarized Heusler Alloy CO2Fe0.4Mn0.6Si . IEEE MAGNETICS LETTERS8 3305605-1 - 3305605-5 2017Reviewed International journal

Authorship：Lead author Language：English Publishing type：Research paper (scientific journal) Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC

We report structural, magnetic, and transport properties of epitaxial films of superconductor NbN and highly spin- polarized Heusler alloy CO2Fe0.4Mn0.6Si (CFMS). The saturation magnetization Ms of the film was 870 emu/cm(3), which is slightly smaller than values reported earlier. With an increase of CFMS thickness d(CFMS), the sheet resistance R-sq decreased and the upper critical field mu H-0(c2)(0) decreased monotonically, while the superconducting transition temperature Tc exhibited a minimum around d(CFMS) = 3 nm. A theoretical analysis of superconducting fluctuations revealed that the high spin polarization of the CFMS layer affected the pair breaking parameter d, the superconducting coherence length. (0), and the diffusion coefficient D.

DOI： 10.1109/LMAG.2017.2687878

Ryutaro Ooka, Iduru Shigeta, Yoshimi Sukino, Yutaro Fujimoto, Rie Y. Umetsu, Akiko Nomura, Touru Yamauchi, Takeshi Kanomata, Kunio Yubuta, Masahiko Hiroi . Spin polarization and magnetization of Heusler alloys Co₂TiGa_1−xSn_x (x = 1.0, 0.5) . IEEE Magnetics Letters 2016.10Spin polarization and magnetization of Heusler alloys Co₂TiGa_1−xSn_x (x = 1.0, 0.5)Reviewed

Masahiko Hiroi, Hiroaki Sano, Tomoya Tazoko, Iduru Shigeta, Masakazu Ito, Keiichi Koyama, Hirotaka Manaka, Norio Terada, Muneaki Fujii, Akihiro Kondo, Koichi Kindo . Magnetic and electrical properties of Heusler compounds Ru₂Cr_1−xX_xSi (X = V, Ti) . Journal of Alloys and Compounds 2016.9Magnetic and electrical properties of Heusler compounds Ru₂Cr_1−xX_xSi (X = V, Ti)Reviewed

Qing Tao Meng, Shingo Komaki, Takuya Tsuneoka, Hirotoshi Hanada, Shohei Maeda, Taiki Murano, Fusao Ichikawa, Kikuo Itoh, Yuya Nishisako, Iduru Shigeta, Masahiko Hiroi . Tunneling spectroscopy of the superconductor-insulator transition on Bi₂Sr₂Ca_1-xY_xCu₂O_8+δ and (Bi_1-yPb_y)₂Sr₂Ca_1-xY_xCu₂O_8+δ . IEEE Transactions on Applied Superconductivity24 ( 5 ) 7000504(1) - (4) 2014.10Tunneling spectroscopy of the superconductor-insulator transition on Bi₂Sr₂Ca_1-xY_xCu₂O_8+δ and (Bi_1-yPb_y)₂Sr₂Ca_1-xY_xCu₂O_8+δReviewed

Qing Tao Meng, Shingo Komaki, Takuya Tsuneoka, Hirotoshi Hanada, Shohei Maeda, Taiki Murano, Fusao Ichikawa, Kikuo Itoh, Yuya Nishisako, Iduru Shigeta, Masahiko Hiroi . Tunneling Spectroscopy of the Superconductor-Insulator Transition on Bi2Sr2Ca1-xYxCu2O8+delta and (Bi1-yPby)(2)Sr2Ca1-xYxCu2O8+delta . IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY24 ( 5 ) 7000504-1 - 7000504-4 2014.10Reviewed International journal

Language：English Publishing type：Research paper (scientific journal) Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC

We report tunnel experiments on Bi2Sr2Ca1-xYxCu 2O8+delta (Y-Bi2212) crystals with x= 0, 0.01, 0.05, 0.1, 0.11, and 0.12. The results are discussed about Cooper pairs existing. Tunneling spectra were measured for Y-Bi2212 crystal-SiO-Ag planar junctions. For superconducting samples the V-shaped gap structure and zero bias conductance peaks (ZBCP) were observed. While neither superconducting gap nor ZBCP was observed for insulator samples. These results suggest that Cooper pairs break up on the insulating side of superconductor- insulator transition (SIT). We also measured tunneling spectra on (Bi1-Pb-y(y))(2)Sr2Ca1-xYxCu2O8+delta samples. In their spectra there were also several characteristic behaviors.

DOI： 10.1109/TASC.2014.2351263

Sora Nishiinoue, Hidekazu Suwa, Iduru Shigeta, Masahiko Hiroi, Keiichi Koyama, Shojiro Kimura, Kazuo Watanabe, Muneaki Fujii . Magneto-transport properties in the Heusler compounds Ru_2-xFe_xCrSi (0 ≦ x ≦ 0.1) . JPS Conference Proceedings3 017018(1) - (6) 2014.6Magneto-transport properties in the Heusler compounds Ru_2-xFe_xCrSi (0 ≦ x ≦ 0.1)Reviewed

Hidekazu Suwa, Sora Nishiinoue, Kensuke Koseki, Iduru Shigeta, Masakazu Ito, Masahiko Hiroi, Hirotaka Manaka, Norio Terada . Study on the half-Heusler compounds Co_1+δTiSb with excess cobalt . JPS Conference Proceedings3 017017(1) - (6) 2014.6Study on the half-Heusler compounds Co_1+δTiSb with excess cobaltReviewed

Masahiko Hiroi, Tomonari Higashiuchi, Takuro Ogawa, Iduru Shigeta, Masakazu Ito, Hirotaka Manaka, Norio Terada . Substitution effects in the antiferromagnetic Heusler compound Ru₂CrSi . JPS Conference Proceedings3 014024(1) - (6) 2014.6Substitution effects in the antiferromagnetic Heusler compound Ru₂CrSiReviewed

Masahiko Hiroi, Tomonari Higashiuchi, Takuro Ogawa, Iduru Shigeta, Masakazu Ito, Hirotaka Manaka, Norio Terada . Substitution effects in the antiferromagnetic Heusler compound Ru<sub>2</sub>CrSi . JPS Conference Proceedings3 014024-1 - 014024-6 2014.6Substitution effects in the antiferromagnetic Heusler compound Ru<sub>2</sub>CrSiReviewed International journal

Hidekazu Suwa, Sora Nishiinoue, Kensuke Koseki, Iduru Shigeta, Masakazu Ito, Masahiko Hiroi, Hirotaka Manaka, Norio Terada . Study on the half-Heusler compounds Co<sub>1+δ</sub>TiSb with excess cobalt . JPS Conference Proceedings3 017017-1 - 017017-6 2014.6Study on the half-Heusler compounds Co<sub>1+δ</sub>TiSb with excess cobaltReviewed International journal

Sora Nishiinoue, Hidekazu Suwa, Iduru Shigeta, Masahiko Hiroi, Keiichi Koyama, Shojiro Kimura, Kazuo Watanabe, Muneaki Fujii . Magneto-transport properties in the Heusler compounds Ru<sub>2−x</sub>Fe<sub>x</sub>CrSi (0 ≦ x ≦ 0.1) . 3 017018-1 - 017018-6 2014.6Magneto-transport properties in the Heusler compounds Ru<sub>2−x</sub>Fe<sub>x</sub>CrSi (0 ≦ x ≦ 0.1)Reviewed International journal

Iduru Shigeta, Yuya Nishisako, Shinpei Urakawa, Osamu Murayama, Masakazu Ito, and Masahiko Hiroi . Fabrication and measurement of high-temperature superconductor YBa₂Cu₃O_7-δ: Activity report of science club . JPS Conference Proceedings1 017013(1) - (4) 2014.3Fabrication and measurement of high-temperature superconductor YBa₂Cu₃O_7-δ: Activity report of science clubReviewed

Iduru Shigeta, Yuya Nishisako, Shinpei Urakawa, Osamu Murayama, Masakazu Ito, Masahiko Hiroi . Fabrication and measurement of high-temperature superconductor YBa<sub>2</sub>Cu<sub>3</sub>O<sub>7−δ</sub>: Activity report of science club . JPS Conference Proceedings1 017013-1 - 017013-4 2014.3Fabrication and measurement of high-temperature superconductor YBa<sub>2</sub>Cu<sub>3</sub>O<sub>7−δ</sub>: Activity report of science clubReviewed International journal

Iduru Shigeta, Yuki Maeda, Kohei Harumori, Yuya Nishisako, Masakazu Ito, Touru Yamauchi, and Masahiko Hiroi . Spin polarization measurements of Heusler alloy Ru_0.2Fe_1.8CrSi by Andreev reflection technique . IEEE Transactions on Magnetics50 ( 1 ) 1401304(1) - (4) 2014.1Spin polarization measurements of Heusler alloy Ru_0.2Fe_1.8CrSi by Andreev reflection techniqueReviewed

Iduru Shigeta, Yuki Maeda, Kohei Harumori, Yuya Nishisako, Masakazu Ito, Touru Yamauchi, Masahiko Hiroi . Spin Polarization Measurements of Heusler Alloy Ru0.2Fe1.8CrSi by Andreev Reflection Technique . IEEE TRANSACTIONS ON MAGNETICS50 ( 1 ) 1401304-1 - 1401304-4 2014.1Reviewed International journal

Authorship：Lead author Language：English Publishing type：Research paper (scientific journal) Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC

We report spin polarization P of Fe-rich Heusler alloy Ru0.2Fe1.8CrSi, which was determined by the Andreev reflection technique. Heusler alloy Ru2-xFexCrSi was predicted to be half- metal by the first- principle band calculations. Fe- rich Ru0.2Fe1.8CrSi compounds are ferromagnetic and the saturation magnetization M-0 almost coincideswith that expected fromthe Slater- Pauling rule. Two theoretical approaches in the determination of spin polarization by Andreev reflection are discussed and compared from an experimental perspective. The spin polarization more than P = 0.60 is obtained for Ru0.2Fe1.8CrSi specimens and it is highest value in all Ru2-xFexCrSi compounds. The difference in values of spin polarization as determined by two models is less than 0.02.

DOI： 10.1109/TMAG.2013.2278034

西迫裕也，大久保亮成，春森浩平，重田出，梅津理恵，伊藤昌和，小山佳一，貝沼亮介，廣井政彦 . アンドレーエフ反射法によるハーフメタル型ホイスラー合金Co₂MnSiのスピン分極率測定 . 鹿児島大学理学部紀要46 1 - 8 2013.12アンドレーエフ反射法によるハーフメタル型ホイスラー合金Co₂MnSiのスピン分極率測定

Masahiko Hiroi, Kaori Uchida, Iduru Shigeta, Masakazu Ito, Keiichi Koyama, Shojiro Kimura, Kazuo Watanabe . Antiferromagnetic transition in Ru2CrSi in magnetic fields . Journal of the Korean Physical Society62 ( 12 ) 2068 - 2072 2013.7Reviewed International journal

Language：English Publishing type：Research paper (scientific journal)

In the Heusler compound Ru2CrSi, an antiferromagnetic transition at T N =14 K was revealed by specific heat and magnetization measurements. In this study the electrical resistivity is measured in magnetic fields up to 14.5 T, and the antiferromagnetic transition in magnetic fields is investigated. In the temperature dependence of the resistivity at zero field, a clear dip at ∼15 K and a hump with a maximum at ∼ 9 K are observed. This dip is considered to be due to the antiferromagnetic transition. With increasing magnetic field, the magnitude of the resistivity slightly increases around T N
i.e., a positive magnetoresistance is observed. The temperature dependence of the resistivity is hardly affected by increasing the magnetic field. The transition temperature decreases only by ∼ 0.3 K even with applying 14.5 T. These results demonstrate that the antiferromagnetic state in Ru2CrSi is unusually unaffected by a strong magnetic field. © 2013 The Korean Physical Society.

DOI： 10.3938/jkps.62.2068

Masakazu Ito, Toru Hisamatsu, Tsugumi Rokkaku, Osamu Murayama, Iduru Shigeta, Masahiko Hiroi . Thermodynamic and transport properties of Ru2-x Fe (x) CrSi (1.3 a parts per thousand currency sign x a parts per thousand currency sign 1.8) . JOURNAL OF THE KOREAN PHYSICAL SOCIETY62 ( 12 ) 2063 - 2067 2013.7Reviewed International journal

Language：English Publishing type：Research paper (scientific journal) Publisher：KOREAN PHYSICAL SOC

We have measured the temperature (T) dependences of the electric resistivity, rho(T), and the specific heat, C (p) (T), of the ferromagnetic full-Heusler compound Ru2-x Fe (x) CrSi (1.3 a parts per thousand currency sign x a parts per thousand currency sign 1.8). rho(T) shows a metallic behavior (a,rho/a,T > 0) for 20 a parts per thousand(2) T a parts per thousand currency sign 300 K and nominal upturn at around 20 K for all samples. In the low-temperature range, C (p) (T) can be described by Debye's T (3) law. Although the coefficient of the lattice part of C (p) (T), beta, is insensitive for the Fe concentration x, the electronic specific heat part, gamma, decreases with increasing x. The values of the total densities of states (D (total) (E (F) )) estimated from gamma were 12.8, 10.1, and 9.6 states/f.u. eV for x = 1.3, 1.6, and 1.8, respectively. The values of majority spin band (D (a dagger)(E (F) )) and the minority spin band (D (a dagger")(E (F) )) were also estimated using gamma and the spin polarization P defined by Andreev-reflection experiments.

DOI： 10.3938/jkps.62.2063

Masahiko Hiroi, Kaori Uchida, Iduru Shigeta, Masakazu Ito, Keiichi Koyama, Shojiro Kimura, Kazuo Watanabe . Antiferromagnetic Transition in Ru₂CrSi in Magnetic Fields . Journal of the Korean Physical Society62 2068 - 2072 2013.6Antiferromagnetic Transition in Ru₂CrSi in Magnetic FieldsReviewed

Masakazu Ito, Toru Hisamatsu, Tsugumi Rokkaku, Osamu Murayama, Iduru Shigeta, Masahiko Hiroi . Thermodynamic and transport properties of Ru_2-xFe_xCrSi (1.3 ≦ x ≦ 1.8) . Journal of the Korean Physical Society62 2063 - 2067 2013.6Thermodynamic and transport properties of Ru_2-xFe_xCrSi (1.3 ≦ x ≦ 1.8)Reviewed

Iduru Shigeta, Yuya Sakuraba, Shojiro Kimura, Keiichi Koyama, Kazuo Watanabe, Koki Takanashi, Masahiko Hiroi . Transport and magnetic properties of fully epitaxial superconducting NbN/half-metallic Heulser alloy Co₂MnSi bilayer flms . Journal of the Magnetics Society of Japan37 222 - 226 2013.5Transport and magnetic properties of fully epitaxial superconducting NbN/half-metallic Heulser alloy Co₂MnSi bilayer flmsReviewed

Transport and magnetic properties of fully epitaxial superconducting NbN/half-metallic Heulser alloy Co<sub>2</sub>MnSi bilayer flms . 37 222 - 226 2013.3Transport and magnetic properties of fully epitaxial superconducting NbN/half-metallic Heulser alloy Co<sub>2</sub>MnSi bilayer flmsReviewed International journal

Qing Tao Meng, Shingo Komaki, Takuya Tsuneoka, Hirotoshi Hanada, Shohei Maeda, Taiki Murano, Fusao Ichikawa, Kikuo Itoh, Yuya Nishisako, Iduru Shigeta, Masahiko Hiroi . Tunneling Spectroscopy of the Superconductor-Insulator Transition on Bi2Sr2Ca1-xYxCu2O8+delta and (Bi1-yPby)(2)Sr2Ca1-xYxCu2O8+delta . 2013 IEEE International Conference on Applied Superconductivity and Electromagnetic Devices (ASEMD) 57 - 58 2013Reviewed International journal

Language：English Publishing type：Research paper (international conference proceedings) Publisher：IEEE

The purpose of our present study is to understand the Superconductor-Insulator Transition. In this paper we report tunnel experiments on Bi2Sr2Ca1-xYxCu2O8+delta (Y-Bi2212) crystals with x = 0, 0.01, 0.05, 0.1, 0.11 and 0.12. The results are discussed about Copper pairs existing. Tunneling spectra were measured for Y -Bi2212 crystal-SiO-Ag planar junctions. For superconducting samples the V-shape gap structure and zero bias conductance peaks (ZBCP) were observed. While neither superconducting gap nor ZBCP was observed for insulating samples. These results suggest that Cooper pairs break up on the insulating side of SIT. In our spectra there were also several characteristic behaviors.

DOI： 10.1109/ASEMD.2013.6780707

Iduru Shigeta, Yukio Tanaka, Alexander A. Golubov, Masahiko Hiroi . Spin polarized conductance in ferromagnet / insulator / conventional superconductor junctions . Journal of Physics: Conference Series400 032083(1) - -4 2012.12Spin polarized conductance in ferromagnet / insulator / conventional superconductor junctionsReviewed

Iduru Shigeta, Shinpei Urakawa, Masakazu Ito, Masahiko Hiroi . Magnetization and spin polarization of Co_2-xFe_xMnSi Heusler alloys . Journal of Physics: Conference Series400 032082(1) - -5 2012.12Magnetization and spin polarization of Co_2-xFe_xMnSi Heusler alloysReviewed

Masahiko Hiroi, Iori Yano, Kenta Sezaki, Iduru Shigeta, Masakazu Ito, Hirotaka Manaka, Norio Terada . Substitution effect on the magnetic transitions of Fe₂MnSi . Journal of Physics: Conference Series400 032021(1) - -4 2012.12Substitution effect on the magnetic transitions of Fe₂MnSiReviewed

Masahiko Hiroi, Haku Ko, Shingo Nakashima, Iduru Shigeta, Masakazu Ito, Hirotaka Manaka, Norio Terada . Spin-glass and antiferromagnetic transition in Ru_2-xFe_xCrSi . Journal of Physics: Conference Series400 032020(1) - -4 2012.12Spin-glass and antiferromagnetic transition in Ru_2-xFe_xCrSiReviewed

廣井政彦，小山佳一，伊藤昌和，重田出 . ヘリウム液化装置 . 鹿児島大学理学部紀要44 27 - 32 2011.12ヘリウム液化装置

Iduru Shigeta, Osamu Murayama, Toru Hisamatsu, Alexander Brinkman, Alexander A. Golubov, Yukio Tanaka, Masakazu Ito, Hans Hilgenkamp, Masahiko Hiroi . Spin polarization of Fe-rich ferromagnetic compounds in Ru_2-xFe_xCrSi Heusler alloys . Journal of Physics and Chemistry of Solids72 604 - 607 2011.5Spin polarization of Fe-rich ferromagnetic compounds in Ru_2-xFe_xCrSi Heusler alloysReviewed

Iduru Shigeta, Osamu Murayama, Toru Hisamatsu, Alexander Brinkman, Alexander A. Golubov, Yukio Tanaka, Masakazu Ito, Hans Hilgenkamp, Masahiko Hiroi . Spin polarization of Fe-rich ferromagnetic compounds in Ru2-xFexCrSi Heusler alloys . JOURNAL OF PHYSICS AND CHEMISTRY OF SOLIDS72 ( 5 ) 604 - 607 2011.5Reviewed International coauthorship International journal

Authorship：Lead author Language：English Publishing type：Research paper (scientific journal) Publisher：PERGAMON-ELSEVIER SCIENCE LTD

We report spin polarization P of Ru2-xFexCrSi Heusler alloys by the Andreev reflection technique. Ru2-xFexCrSi with L2(1)-type structure and saturation magnetic moment of 2 mu(B) per formula unit is theoretically predicted to be half-metals in the wide range of the composition x. We had clarified that the experimental results of saturation magnetic moment in Fe-rich compounds had coincided with the theoretical prediction. Therefore, we have measured the differential conductance of Ru2-xFexCrSi/Pb planar-type junctions. The P value of Ru2-xFexCrSi was determined by fitting the differential conductance with the modified Blonder-Tinkham-Klapwijk theory. We have found that the behavior of P for Ru2-xFexCrSi was independent of the composition x in the Fe-rich region: P=0.53 for both of x=1.5 and 1.7. The spin polarization is the similar value to Co-based Heusler alloys. (C) 2010 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.jpcs.2010.10.062

Masakazu Ito, Toru Hisamatsu, Shingo Nakashima, Iduru Shigeta, Kazuyuki Matsubayashi, Yoshiya Uwatoko, Masahiko Hiroi . Transport properties of Heusler compound Ru_2-xFe_xCrSi under pressure . Journal of Physics: Conference Series266 012011(1) - -4 2011.1Transport properties of Heusler compound Ru_2-xFe_xCrSi under pressureReviewed

Masahiko Hiroi, Shingo Nakashima, Kouhei Nakao, Tsugumi Rokkaku, Masakazu Ito, Iduru Shigeta, Hirotaka Manaka, Norio Terada . Magnetic properties of Ru-rich Ru_2-xFe_xCrZ (Z = Si, Ge) . Journal of Superconductivity and Novel Magnetism24 753 - 756 2011.1Magnetic properties of Ru-rich Ru_2-xFe_xCrZ (Z = Si, Ge)Reviewed

Masahiko Hiroi, Shingo Nakashima, Kohei Nakao, Tsugumi Rokkaku, Masakazu Ito, Iduru Shigeta, Hirotaka Manaka, Norio Terada . Magnetic Properties of Ru-rich Ru(2-x)Fe(x)CrZ (Z=Si, Ge) . JOURNAL OF SUPERCONDUCTIVITY AND NOVEL MAGNETISM24 ( 1-2 ) 753 - 756 2011.1Reviewed International journal

Language：English Publishing type：Research paper (scientific journal) Publisher：SPRINGER

The properties of Heusler compounds Ru2-x Fe (x) CrGe are investigated and compared with those of Ru1.9Fe0.1CrSi. Ru2CrGe is confirmed to exhibit an antiferromagnetic transition with Ne,l temperature 13 K by magnetic susceptibility and specific heat measurements. When Fe is substituted for Ru, a peak in the magnetic susceptibility is observed, and in the lower temperature region irreversibility between temperature dependences of magnetization for zero-field-cooling and field-cooling conditions is found. Nevertheless, in specific heat of Ru1.9Fe0.1CrGe, there is no anomaly to indicate phase transition. The specific heat is almost identical to that for Ru1.9Fe0.1CrSi. The above results demonstrate that the magnetic states in the low temperature region of Fe-substituted samples are spin-glass-like states. Slight substitution of Fe for Ru destroys the long-range-order and lead to peculiar spin-glass-like states.

DOI： 10.1007/s10948-010-0967-6

Masakazu Ito, Toru Hisamatsu, Shingo Nakashima, Iduru Shigeta, Kazuyosi Matsubayashi, Yoshiya Uwatoko, Masahiko Hiroi . Transport Properties of Heusler Compound Ru2-xFexCrSi under Pressure . 2ND INTERNATIONAL SYMPOSIUM ON ADVANCED MAGNETIC MATERIALS AND APPLICATIONS (ISAMMA 2010)266 012011-1 - 012011-4 2011Reviewed International journal

Language：English Publishing type：Research paper (scientific journal) Publisher：IOP PUBLISHING LTD

We investigated transport properties of the full-Heusler compound Ru2-xFexCrSi (x = 0.1), which is an insulator, under pressure P up to 8 GPa. The temperature dependence of electric resistance R(T) for Ru1.9Fe0.1CrSi is insensitive to pressure. R(T) is not described by the simple activation-type but -logT in the wide T and all P range we measured. This might be due to atomic disorder in the crystalline.

DOI： 10.1088/1742-6596/266/1/012011

Iduru Shigeta, Osamu Murayama, Toru Hisamatsu, Masakazu Ito, Masahiko Hiroi . Measurements of spin polarization of Ru_2-xFe_xCrSi Heusler alloys by Andreev reflection . Physica C: Superconductivity470 S806 - S807 2010.12Measurements of spin polarization of Ru_2-xFe_xCrSi Heusler alloys by Andreev reflectionReviewed

Iduru Shigeta, Takehito Yokoyama, Yasuhiro Asano, Masahiko Hiroi, Yukio Tanaka . Effect of spin fluctuations on charge transport in diffusive normal metal / d-wave superconductor junctions . Physica C: Superconductivity470 S854 - S856 2010.12Effect of spin fluctuations on charge transport in diffusive normal metal / d-wave superconductor junctionsReviewed

Iduru Shigeta, Takehito Yokoyama, Yasuhiro Asano, Masahiko Hiroi, Yukio Tanaka . Effect of spin fluctuations on charge transport in diffusive normal metal/d-wave superconductor junctions . PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS470 S854 - S856 2010.12Reviewed International journal

Authorship：Lead author Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER SCIENCE BV

The effect of spin fluctuations on the transport properties in diffusive normal metal/d-wave superconductor (DN/DS) junctions is studied under various situations by solving the Usadel equation with Nazarov's generalized boundary condition. Tunneling conductance of the DN/DS junctions is calculated by changing the magnitude of the resistance in ON, Thouless energy in DN, the transparency of the insulating barrier, and the angle between the lobe direction of the d-wave pair potential and the normal to the interface, together with the magnitude of spin fluctuations. A zero-bias conductance dip (ZBCD) and a zero-bias conductance peak (ZBCP) occur in line shapes by way of a coherent Andreev reflection (CAR) around zero energy in the system of DN/DS junctions. We have found that both of the ZBCD and the ZBCP become narrower with the increasing magnitude of spin fluctuations. (C) 2010 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physc.2009.12.015

Iduru Shigeta, Osamu Murayama, Toru Hisamatsu, Masakazu Ito, Masahiko Hiroi . Measurements of spin polarization of Ru2-xFexCrSi Heusler alloys by Andreev reflection . PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS470 S806 - S807 2010.12Reviewed International journal

Authorship：Lead author Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER SCIENCE BV

We report spin polarization P of Ru2-xFexCrSi Heusler alloys by Andreev reflection technique. Ru2-xFexCrSi with the L2(1)-type structure is theoretically predicted to be half-metals in the wide range of the composition x. The experimental results of saturation moment in Fe-rich compounds measurements also support the theoretical prediction. We have measured the differential conductance of Ru2-xFexCrSi/Pb planar-type junctions. The P value of Ru2-xFexCrSi was determined by fitting the differential conductance with the modified Blonder-Tinkham-Klapwijk theory. The behavior of P was independent of the composition x in the Fe-rich region; P = 0.53 for x = 1.5 and P = 0.52 for x = 1.7, respectively. We have found that the spin polarization of Ru2-xFexCrSi in Fe-rich compounds was the similar values to Co-based Heusler alloys. (C) 2010 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physc.2010.01.044

Iduru Shigeta, Masakazu Ito . Advanced experiments of high-temperature superconductors for undergraduate students . Proceedings of the 4th International Symposium on Advances in Technology Education (ISATE2010) 1C6(1) - -7 2010.9Advanced experiments of high-temperature superconductors for undergraduate students

Masakazu Ito, Toru Hisamatsu, Tsugumi Rokkaku, Iduru Shigeta, Hirotaka Manaka, Norio Terada, Masahiko Hiroi . Low-temperature magnetic properties of Heusler compounds Ru_2-xFe_xCrSi (x = 0.1, 0.3, and 0.5) . Physical Review B82 024406(1) - -7 2010.7Low-temperature magnetic properties of Heusler compounds Ru_2-xFe_xCrSi (x = 0.1, 0.3, and 0.5)Reviewed

Masakazu Ito, Toru Hisamatsu, Tsugumi Rokkaku, Iduru Shigeta, Hirotaka Manaka, Norio Terada, Masahiko Hiroi . Low-temperature magnetic properties of Heusler compounds Ru2-xFexCrSi (x=0.1, 0.3, and 0.5) . PHYSICAL REVIEW B82 ( 2 ) 024406-1 - 024406-7 2010.7Reviewed International journal

Language：English Publishing type：Research paper (scientific journal) Publisher：AMER PHYSICAL SOC

We carried out magnetization M(T) and specific-heat C-P(T) measurements of the new Heusler compounds Ru2-xFexCrSi (x=0.1, 0.3, and 0.5), which exhibit spin-glass freezing. M(T) has peak- and irreversibility-type anomalies. The temperatures at which these anomalies appear have magnetic field dependence described by the Gabay-Toulouse and de Almeida-Thouless lines in the low-field range. For the magnetic specific heat C-m(T), we did not observe a discontinuity indicating long-range magnetic phase transition but a broad hump characteristic of spin-glass freezing. C-m(T) in the low-temperature range is described as a combination of linear- and quadratic-T terms. The quadratic-T dependence of C-m(T) is probably associated with excitation of the Ising component of the freezing spins.

DOI： 10.1103/PhysRevB.82.024406

Masahiko Hiroi, Tsugumi Rokkaku, Kazuhisa Matsuda, Toru Hisamatsu, Iduru Shigeta, Masakazu Ito, Takuo Sakon, Keiichi Koyama, Kazuo Watanabe, Shintaro Nakamura, Tsutomu Nojima, Tomohito Nakano, Lin Chen, Tetsuya Fujiwara, Yoshiya Uwatoko, Hirotaka Manaka, Norio Terada . Ferromagnetism and spin-glass transitions in the Heusler compounds Ru_2-xFe_xCrSi . Physical Review B79 224423(1) - -10 2009.6Ferromagnetism and spin-glass transitions in the Heusler compounds Ru_2-xFe_xCrSiReviewed

Hiroi Masahiko, Rokkaku Tsugumi, Matsuda Kazuhisa, Hisamatsu Toru, Shigeta Iduru, Ito Masakazu, Sakon Takuo, Koyama Keiichi, Watanabe Kazuo, Nakamura Shintaro, Nojima Tsutomu, Nakano Tomohito, Chen Lin, Fujiwara Tetsuya, Uwatoko Yoshiya, Manaka Hirotaka, Terada Norio . Ferromagnetism and spin-glass transitions in the Heusler compounds Ru2-xFexCrSi . PHYSICAL REVIEW B79 ( 22 ) 224423-1-10 2009.6Ferromagnetism and spin-glass transitions in the Heusler compounds Ru2-xFexCrSiReviewed International journal

Iduru Shigeta, Yasunari Tanuma, Yasuhiro Asano, Masahiko Hiroi, Yukio Tanaka . Midgap Andreev resonant state affected by superconducting proximity effect at junction interface of high-T_c cuprate attached to diffusive normal metal . Journal of Physics: Conference Series150 052235(1) - -4 2009.3Midgap Andreev resonant state affected by superconducting proximity effect at junction interface of high-T_c cuprate attached to diffusive normal metalReviewed

Yusuke Kondo, Iori Yano, Iduru Shigeta, Masakazu Ito, Masahiko Hiroi, Hirotaka Manaka, Norio Terada . Magnetic properties of the Heusler alloys Fe_2-xCo_xMnSi . Journal of Physics: Conference Series150 042099(1) - -4 2009.3Magnetic properties of the Heusler alloys Fe_2-xCo_xMnSiReviewed

Iduru Shigeta, Takehito Yokoyama, Yasuhiro Asano, Masahiko Hiroi, Yukio Tanaka . Effect of spin fluctuations on tunneling conductance in diffusive normal metal / conventional superconductor junctions . Journal of Physics: Conference Series150 052236(1) - -4 2009.3Effect of spin fluctuations on tunneling conductance in diffusive normal metal / conventional superconductor junctionsReviewed

Iduru Shigeta, Yasunari Tanuma, Yasuhiro Asano, Masahiko Hiroi, Yukio Tanaka . Tunneling study through analysis of spatial variations of the pair potential in diffusive normal metal/insulator/high-T-c superconductor junctions . JOURNAL OF PHYSICS AND CHEMISTRY OF SOLIDS69 ( 12 ) 3042 - 3045 2008.12Reviewed International journal

Language：English Publishing type：Research paper (scientific journal) Publisher：PERGAMON-ELSEVIER SCIENCE LTD

We report how spatial variations of the pair potential with an anisotropic symmetry influence tunneling conductance spectra in diffusive normal metal/insulator/d(x2-y2)-wave superconductor (DN/I/DS) junctions. A zero-bias conductance peak (ZBCP) was observed for the (110)-oriented interface in Ag/SiO/Bi2Sr2CaCu2O8+delta (Bi-2212) planar junctions. The experimental ZBCP was analyzed by the circuit theory, which was improved by the introduction of spatial variations of the pair potential near the junction interface between a DN conductor and a DS electrode. Our experimental results are well explained by the extended circuit theory for DN/I/DS junctions. (C) 2007 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.jpcs.2008.06.014

Iduru Shigeta, Takehito Yokoyama, Yasuhiro Asano, Fusao Ichikawa, Yukio Tanaka . Threshold temperatures of zero-bias conductance peak and zero-bias conductance dip in diffusive normal metal / superconductor junctions . Proceedings of the International Symposium on Mesoscopic Superconductivity and Spintronics 2006 (MS+S2006), World Scientific Publishers, Singapore 111 - 117 2008.6Threshold temperatures of zero-bias conductance peak and zero-bias conductance dip in diffusive normal metal / superconductor junctionsReviewed

Iduru Shigeta, Yasunari Tanuma, Yasuhiro Asano, Masahiko Hiroi, Yukio Tanaka . Tunneling study through analysis of spatial variations of the pair potential in diffusive normal metal / insulator / high-T_c superconductor junctions . Journal of Physics and Chemistry of Solids69 3042 - 3045 2008.1Tunneling study through analysis of spatial variations of the pair potential in diffusive normal metal / insulator / high-T_c superconductor junctionsReviewed

Iduru Shigeta, Takehito Yokoyama, Yasuhiro Asano, Fusao Ichikawa, Yukio Tanaka . Temperature dependence of tunneling conductance in diffusive normal metal / triplet superconductor junctions . Physica C: Superconductivity460-462 1319 - 1320 2007.9Temperature dependence of tunneling conductance in diffusive normal metal / triplet superconductor junctionsReviewed

Iduru Shigeta, Takehito Yokoyama, Yasuhiro Asano, Fusao Ichikawa, Yukio Tanaka . Temperature dependence of tunneling conductance in diffusive normal metal/triplet superconductor junctions . PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS460 1319 - 1320 2007.9Reviewed International journal

Authorship：Lead author Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER SCIENCE BV

The thermal broadening effect on the tunneling conductance in diffusive normal metal/triplet superconductor (DN/TS) junctions is studied based on the Usadel equation under the generalized boundary condition. The zero-bias conductance peak (ZBCP) enhances in the antinodal direction, due to the coexistence of the proximity effect and the midgap Andreev resonant states at the DN/TS junction interface. It is found that the ZBCP is affected by the thermal broadening effect especially for tunnel junctions with the low transparency and the small Thouless energy. (c) 2007 Elsevier B.V. All tights reserved.

DOI： 10.1016/j.physc.2007.04.007

Iduru Shigeta, Yukio Tanaka, Fusao Ichikawa, Yasuhiro Asano . Smearing origin of zero-bias conductance peak in Ag-SiO-Bi₂Sr₂CaCu₂O_8+δ planar tunnel junctions: influence of diffusive normal metal verified with the circuit theory . The European Physical Journal B - Condensed Matter and Complex Systems54 ( 2 ) 141 - 149 2006.11Smearing origin of zero-bias conductance peak in Ag-SiO-Bi₂Sr₂CaCu₂O_8+δ planar tunnel junctions: influence of diffusive normal metal verified with the circuit theoryReviewed

I. Shigeta, Y. Tanaka, F. Ichikawa, Y. Asano . Smearing origin of zero-bias conductance peak in Ag-SiO-Bi2Sr2CaCu2O8+delta planar tunnel junctions: influence of diffusive normal metal verified with the circuit theory . EUROPEAN PHYSICAL JOURNAL B54 ( 2 ) 141 - 149 2006.11Reviewed International journal

Authorship：Lead author Language：English Publishing type：Research paper (scientific journal) Publisher：SPRINGER

We propose a new approach of smearing origins of a zero-bias conductance peak (ZBCP) in high-T-c superconductor tunnel junctions through the analysis based on the circuit theory for a d-wave pairing symmetry. The circuit theory has been recently developed from conventional superconductors to unconventional superconductors. The ZBCP frequently appears in line shapes for this theory, in which the total resistance was constructed by taking account of the effects between a d-wave superconductor and a diffusive normal metal (DN) at a junction interface, including the midgap Andreev resonant states (MARS), the coherent Andreev reflection (CAR) and the proximity effect. Therefore, we have analyzed experimental spectra with the ZBCP of Ag-SiO-Bi2Sr2CaCu2O8+delta (Bi-2212) planar tunnel junctions for the {110}-oriented direction by using a simplified formula of the circuit theory for d-wave superconductors. The fitting results reveal that the spectral features of the ZBCP are well explained by the circuit theory not only excluding the Dynes's broadening factor but also considering only the MARS and the DN resistance. Thus, the ZBCP behaviors are understood to be consistent with those of recent studies on the circuit theory extended to the systems containing d-wave superconductor tunnel junctions.

DOI： 10.1140/epjb/e2006-00440-2

Iduru Shigeta, Takehito Yokoyama, Yasuhiro Asano, Yukio Tanaka . Thermal broadening effect on the tunneling conductance in diffusive normal metal / superconductor junctions . AIP Conference Proceedings850 999 - 1000 2006.9Thermal broadening effect on the tunneling conductance in diffusive normal metal / superconductor junctionsReviewed

Iduru Shigeta, Yasuhiro Asano, Yukio Tanaka, Fusao Ichikawa, Satoshi Kashiwaya . Tunneling study based on the circuit theory of d-wave superconductors . Journal of Physics and Chemistry of Solids67 175 - 177 2006.1Tunneling study based on the circuit theory of d-wave superconductorsReviewed

Shigeta, I, Y Asano, Y Tanaka, F Ichikawa, S Kashiwaya . Tunneling study based on the circuit theory in d-wave superconductors . JOURNAL OF PHYSICS AND CHEMISTRY OF SOLIDS67 ( 1-3 ) 175 - 177 2006.1Reviewed International journal

Authorship：Lead author Language：English Publishing type：Research paper (scientific journal) Publisher：PERGAMON-ELSEVIER SCIENCE LTD

We propose a new interpretation about zero-bias conductance peak (ZBCP) behaviours in high-T-c superconductors. The ZBCP has been frequently observed in many tunneling spectroscopic experiments for unconventional superconductors. The ZBCP mechanism was well understood by the Andreev bound states theory for unconventional superconductors. On the other hand. the circuit theory has been recently developed from conventional superconductors to unconventional superconductors. The ZBCP often appears in this theory as well. Therefore, we have analyzed experimental spectra with the ZBCP of Bi2Sr2CaCu2O8+delta planar tunnel junctions by using both theories for d-wave paring symmetry. The experimental spectra are consistent with those of recent studies on the circuit theory for d-wave superconductors. (c) 2005 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.jpcs.2005.10.096

Iduru Shigeta, Yukio Tanaka, Fusao Ichikawa, Yasuhiro Asano, Satoshi Kashiwaya . Zero-bias conductance peak reduction by diffusive normal metal in high-T_c superconductor tunnel junctions . Journal of Physics and Chemistry of Solids66 1401 - 1404 2005.8Zero-bias conductance peak reduction by diffusive normal metal in high-T_c superconductor tunnel junctionsReviewed

Shigeta, I, Y Tanaka, F Ichikawa, Y Asano, S Kashiwaya . Zero-bias conductance peak reduction by diffusive normal metal in high-T-c superconductor tunnel junctions . JOURNAL OF PHYSICS AND CHEMISTRY OF SOLIDS66 ( 8-9 ) 1401 - 1404 2005.8Reviewed International journal

Language：English Publishing type：Research paper (scientific journal) Publisher：PERGAMON-ELSEVIER SCIENCE LTD

We report investigations of the smearing origin about a zero-bias conductance peak (ZBCP) observed in high-T-c superconductor (HTSC) tunnel junctions from the point of view of the circuit theory analysis for d-wave superconductors. The ZBCP enhancement was measured in Bi2Sr2CaCU2O8+delta-SiO-Ag planar tunnel junctions only for ab-plane directions. On the other hand, the circuit theory has been recently generalized from conventional superconductors to unconventional superconductors. The analysis in terms of the circuit theory reveals that the tunneling spectral behaviors of the ZBCP were well explained even when the Dynes's broadening factor was excluded from a tunneling spectral formula. Thus, the smearing effect in the ZBCP is understood by the influence of a diffusive normal metal resistance in the systems of HTSC tunnel junctions. (c) 2005 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.jpcs.2005.05.019

Iduru Shigeta, Yukio Tanaka, Fusao Ichikawa, Yasuhiro Asano, Satoshi Kashiwaya . Circuit theory analysis of ab-plane tunnel junctions of unconventional superconductor Bi₂Sr₂CaCu₂O_8+δ . Proceedings of the International Symposium on Mesoscopic Superconductivity and Spintronics 2004 (MS+S2004), World Scientific Publishers, Singapore 161 - 166 2005.7Circuit theory analysis of ab-plane tunnel junctions of unconventional superconductor Bi₂Sr₂CaCu₂O_8+δReviewed

Shigeta, I, T Abiru, K Abe, A Nishida, Y Matsumoto . Temperature and field dependence of magnetization of MgB2 polycrystals . PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS392 359 - 363 2003.10Reviewed International journal

Authorship：Lead author Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER SCIENCE BV

We present magnetization properties of magnesium diboride (MgB2). MgB2 polycrystals were fabricated by using the vapor transport method inside a sealed stainless tube. The critical current density J(c) was estimated from the magnetization hysteresis DeltaM by using the Bean model. The temperature dependence of J(c) (T) showed a linear behavior below 15 K and deviated from such behavior in the higher temperature region. This behavior indicates that a flux creep effect influences the wide temperature range for the MgB2 polycrystals. Furthermore, small values of DeltaM show that the pinning force density F-p is very weak. The temperature dependence of the upper critical field H-c2 (T) was also obtained and revealed a linear behavior far from T-c. From our experimental results, we conclude that the MgB2 polycrystals are of good quality with less pinning centers, compared to the commercially available MgB2 powder. (C) 2003 Elsevier B.V. All rights reserved.

DOI： 10.1016/S0921-4534(03)01049-9

A Nishida, C Taka, Shigeta, I . Effects of Y substitution on the temperature and field dependence of magnetic hystereses in YxGd1-xBa2Cu3O7-delta . PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS392 349 - 352 2003.10Reviewed International journal

Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER SCIENCE BV

Effects of Y substitution on the magnetic hystereses are investigated in Y-substituted YxGd1-xBa2Cu3O7-delta ceramics. Magnetizations are first measured at respective constant temperatures, where the hysteresis DeltaM is estimated by the magnetization difference between field increase and decrease. When the variations of DeltaM for the identical field against temperatures are examined, DeltaM is found to obey DeltaM = A exp[-T/T-0] with the single To for a given field. The characteristic temperature T-0 is in the range of 10-30 K, and the field dependence is compared with the function: H-1/n. The field dependence of T-0 for the x = 0.2 (nominal) sample is larger than that for the pure Gd123 ceramics of x = 0, reflecting larger T-0 at lower magnetic fields. This is owing to larger DeltaM at higher temperatures and lower fields in the Y-substituted system. Improvement of the intergrain coupling by the Y substitution is discussed in comparison with magnetic behaviors in the pure sample. (C) 2003 Elsevier B.V. All rights reserved.

DOI： 10.1016/S0921-4534(03)00848-7

Y Matsumoto, Shigeta, I, T Abiru, Y Terasaki, T Akune, N Sakamoto . Critical current density and flux pinning characteristics of powdered MgB2 specimens . PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS388 163 - 164 2003.5Reviewed International journal

Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER SCIENCE BV

Field and temperature dependence of magnetization and also time dependent magnetization (flux creep) of powdered MgB2 specimens have been measured by using a SQUID magnetometer in the temperature range of 4.5 less than or equal to T less than or equal to 37 K and under the field up to 1 T. Scaling plots of normalized pinning force density F-p/F-p max as a function of normalized flux density B/B-max in the temperature range of 25 less than or equal to T less than or equal to 37 K showed that a two dimensional pinning is dominant for the flux pinning of the specimens. Another distinct feature is that a linear reduction of J(c) with increasing temperature at temperatures lower than 30 K and a quadratic decrease at higher temperatures than 35 K have been observed. (C) 2003 Elsevier Science B.V. All rights reserved.

DOI： 10.1016/S0921-4534(02)02707-7

Shigeta, I, F Ichikawa, T Fukami, T Aomine . Observation of a large superconducting gap in Bi-based cuprates by tunneling spectroscopy . PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS388 291 - 292 2003.5Reviewed International journal

Authorship：Lead author Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER SCIENCE BV

We report that a large superconducting gap has been observed in some tunneling spectra of Bi2Sr2CaCu2O8+delta (Bi2212). The observed gap magnitude is roughly twice as large as those of the recent studies of Bi2212. Magnetic susceptibility measurements reveal that the single crystals retain a single phase of Bi2212. Therefore, the experimental results suggest that the large gap arises by way of the superconductor-insulator-superconductor (SIS) tunnel junction, due to the junction interface of the crystal boundary. (C) 2003 Elsevier Science B.V. All rights reserved.

DOI： 10.1016/S0921-4534(03)00627-0

A Nishida, S Teshima, C Taka, Shigeta, I . Field- and temperature-dependent magnetic hysteresis in GdBa2Cu3O7-delta ceramics . PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS388 419 - 420 2003.5Reviewed International journal

Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER SCIENCE BV

Magnetic hysteresis measurements are performed in GdBa2Cu3O7-delta ceramics, and effects of magnetic field, temperature and substitution are studied. The hysteresis DeltaM is found to decay exponentially with temperature: DeltaM proportional to exp[-T/T-0]. The activation temperature T-0 varies mildly around 20 K, which seems to depend on the magnetic field as H-1/n. Substitution tends to increase DeltaM at lower fields. The results are examined in relation to ceramic nature of the sample. (C) 2003 Elsevier Science B.V. All rights reserved.

DOI： 10.1016/S0921-4534(02)02549-2

Shigeta, I, F Ichikawa, T Aomine . Temperature dependence of zero-bias conductance peak in Bi2Sr2CaCu2O8+delta-SiO-Ag planar tunnel junctions . PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS378 316 - 319 2002.10Reviewed International journal

Authorship：Lead author Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER SCIENCE BV

We present the temperature dependence of tunneling spectra of as-grown Bi2Sr2CaCu2O8+delta single crystals. An enhancement of the zero-bias conductance peak (ZBCP) was observed for the ab-plane tunnel junctions. The ZBCP has appeared below the bulk superconducting critical temperature T-c. This experimental result suggests that the ZBCP obviously arises by way of the superconducting gap and not the pseudogap. This result is consistent with recent studies of the Andreev bound states model for d-wave superconductors. (C) 2002 Elsevier Science B.V. All rights reserved.

DOI： 10.1016/S0921-4534(02)01434-X

I. Shigeta, T. Uchida, Y. Tominari, T. Arai, F. Ichikawa, T. Fukami, T. Aomine, V. M. Svistunov . In-plane tunneling spectroscopy in Bi2Sr2CaCu2O8+δ-SiO-Ag planar junctions . Physica C: Superconductivity and its Applications357-360 205 - 207 2001Reviewed International coauthorship International journal

Authorship：Lead author Language：English Publishing type：Research paper (scientific journal)

We report the experimental results of quasiparticle tunneling using Bi2Sr2CaCu2O8+δ-SiO-Ag planar junctions. We have measured the (0 0 1)-, (1 0 0)- and (1 1 0)-oriented junctions. For the in-plane ab oriented junctions, a reproducible zero bias conductance peak has been observed. We have analyzed the data by using the extended Andreev bound states model for d-wave superconductors and have considered the roughness effect by averaging over in-plane directions. The experimental results are consistent with the theoretical calculations. © 2001 Elsevier Science B.V. All rights reserved.

DOI： 10.1016/S0921-4534(01)00207-6

Shigeta, I, T Uchida, Y Tominari, T Arai, F Ichikawa, T Fukami, T Aomine, VM Svistunov . Zero bias conductance peak enhancement in Bi2Sr2CaCu2O8+delta-SiO-Ag planar tunnel junctions . JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN69 ( 9 ) 2743 - 2746 2000.9Reviewed International coauthorship International journal

Authorship：Lead author Language：English Publishing type：Research paper (scientific journal) Publisher：PHYSICAL SOC JAPAN

We report the experimental results of quasiparticle tunneling using Bi2Sr2CaCu2O8+delta (BSCCO)-SiO-Ag planar junctions. For the {001}-oriented junctions, we have measured a well-known V-shaped gap structure expected for d-wave superconductors. For the {100}- and {110}-oriented junctions, a reproducible zero bias conductance peak (ZBCP) enhancement has been observed. The temperature dependence of the ZBCP is also presented. The ZBCP was observed below T-c. Our results are consistent with those of recent studies on the Andreev reflection between a d-wave superconductor and a normal metal.

DOI： 10.1143/JPSJ.69.2743

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