Takenori Fujii | The University of Tokyo (original) (raw)

Papers by Takenori Fujii

Large high-quality single crystals of Bi 2 Sr 2 Ca n−1 Cu n O 2n+4+δ (n = 2, 3) were successfully... more Large high-quality single crystals of Bi 2 Sr 2 Ca n−1 Cu n O 2n+4+δ (n = 2, 3) were successfully grown using an improved traveling solvent floating zone (TSFZ) method, which features a slow growth rate and steep temperature gradient along the melting zone. By measuring anisotropic resistivities and susceptibilities of Bi 2 Sr 2 CaCu 2 O 8+δ , the characteristic pseudogap temperature T * was studied as a function of doping. The T * suggest that the pseudogap is not simply a precursor of high-T c superconductivity, but that the pseudogap and the superconducting gap compete with each other. The anisotropic resistivities of Bi 2 Sr 2 Ca 2 Cu 3 O 10+δ were also measured, revealing that the T c remains fixed in the overdoped region while anisotropy decreases continuously. This anomalous behavior will be discussed in terms of the inequivalent hole doping, which occurs between two inequivalent CuO 2 planes in the triple-layer system.

Jun Sugiyamaa,∗ Yutaka Ikedoa Hiroshi Nozakia Peter L. Russob Jess H. Brewerc Eduardo J. Ansaldob... more Jun Sugiyamaa,∗ Yutaka Ikedoa Hiroshi Nozakia Peter L. Russob Jess H. Brewerc Eduardo J. Ansaldob Gerald D. Morrisb Kim H. Chowd Daniel Andreicae Alex Amatof Takenori Fujiig Satoshi Okadah and Ichiro Terasakih aToyota Central Research and Development Laboratories ...

We report a systematic high-resolution ARPES on Bi_2Sr_2Ca_ n-1Cu_nO_2n+4 (n=1-3) to study the un... more We report a systematic high-resolution ARPES on Bi_2Sr_2Ca_ n-1Cu_nO_2n+4 (n=1-3) to study the universality on the electronic structure and interactions. For n=2 and 3, we find that the kink in the energy dispersion becomes pronounced on approaching (pi, 0) in the superconducting state, while a kink appears only around the nodal direction in the normal state. The kink around (pi,

Japanese Journal of Applied Physics, 2014

We have successfully controlled thermoelectric properties of ZnO by changing carrier concentratio... more We have successfully controlled thermoelectric properties of ZnO by changing carrier concentration using an electric double layer transistor (EDLT) which is a field effect transistor gated by electrolyte solution.

We have precisely measured the Nernst effect in Nd-doped La$_{2-x}$Sr$_x$CuO$_4$ single crystals ... more We have precisely measured the Nernst effect in Nd-doped La$_{2-x}$Sr$_x$CuO$_4$ single crystals with controlling the strength (stability) of the stripe order. We found that the onset temperature TonsetT_{onset}Tonset, where the Nernst signal starts increasing, does not change conspicuously in spite of Nd-doping. At low temperatures, on the other hand, the absolute value of the Nernst signal is strongly suppressed in accordance with the strength of the stripe order. These results imply that the fluctuation of (charge) stripe order enhances the Nernst signal below TonsetT_{onset}Tonset at high temperatures, and then the stripe order enhanced by Nd-doping suppresses the superconducting fluctuation to reduce the Nernst signal at low temperatures. We also observed an increase of the Nernst signal below the charge order temperature TchT_{ch}Tch which is observed in diffraction measurement. Comment: 3pages, 2figures

We have performed an angle-resolved photoemission spectroscopy of NaxCoO2 to clarify the origin o... more We have performed an angle-resolved photoemission spectroscopy of NaxCoO2 to clarify the origin of the antiferromagnetic (AF) transition in the highly-doped region (x > 0.75). We determined the electronic band structure along the out-of-plane direction by varying the photon energy, and revealed that a three dimensional electron pocket emerges at the gamma point in the AF sample (x > 0.75), while it is absent in the non-AF counterpart (x < 0.75), showing the transition of the Fermi surface topology with the band filling. The present result suggests that the emergence of the electron pocket is closely related to the origin of the AF transition in highly-doped NaxCoO2.

We present high-pressure magneto-transport data on single crystals of LuPtBi, a member of the ter... more We present high-pressure magneto-transport data on single crystals of LuPtBi, a member of the ternary half-Heusler family. Recent band structure calculations show that LuPtBi is a topological semi-metal at ambient pressure due to strong spin-orbit coupling [1]. By decreasing the lattice parameter, equivalent to increasing pressure, the system should become a trivial insulator We have grown single crystals of LuPtBi and studied both the field dependence and the pressure dependence of their resistivity. The field dependence shows typical semi-metal behaviour, namely a weak temperature dependence and a large magneto-resistance. The pressure dependence shows a significant increase of resistivity and a decrease of magneto-resistance with increasing pressure. We compare our experimental results to the available theoretical work on the transport properties of topological semi-metals [2]. [4pt] [1] Stanislav Chadov, et al. Nature, 9, 541 (2010)[0pt] [2] W. Al-Sawai, et al. PRB, 82, 125208 (...

Physical Review B, 2003

The superconducting gap, the pseudogap, and their doping and temperature dependences have been me... more The superconducting gap, the pseudogap, and their doping and temperature dependences have been measured by the short-pulse interlayer tunneling spectroscopy for the CuO 2 triple-layer high-T c superconducting Bi 2 Sr 2 Ca 2 Cu 3 O 10+ δ system. It is found for a ...

Physical Review B, 2002

The doping dependence of the themopower, in-plane resistivity ρ ab (T ), out-of-plane resistivity... more The doping dependence of the themopower, in-plane resistivity ρ ab (T ), out-of-plane resistivity ρc(T ), and susceptibility has been systematically measured for high-quality single crystal Bi2Sr2Ca2Cu3O 10+δ . We found that the transition temperature Tc and pseudogap formation temperature T * ρc , below which ρc shows a typical upturn, do not change from their optimum values in the "overdoped" region, even though doping actually proceeds. This suggests that, in overdoped region, the bulk Tc is determined by the always underdoped inner plane, which have a large superconducting gap, while the carriers are mostly doped in the outer planes, which have a large phase stiffness.

Journal of Materials Chemistry A, 2013

ABSTRACT Ammonium proton in a solid ionic semiconductor, TTFCOONH4, is shown to be mobile under a... more ABSTRACT Ammonium proton in a solid ionic semiconductor, TTFCOONH4, is shown to be mobile under anhydrous conditions at room temperature by the hydrogen concentration cell method. Isotope substituted TTFCOOND4 exhibits a 2.2 H/D isotope effect in ion carrier mobility with TTFCOONH4. First-principles calculations reveal that an efficient proton-transfer pathway via low-barrier NH+N hydrogen bonds reduces the activation energy to 0.12 eV, which is quite small and comparable to that reported in a bulk water system. The ac conductivity of TTFCOONH4 and TTFCOOND4 is similar at room temperature, reflecting similar hole carrier concentrations. In sharp contrast, the thermopower exhibits a large isotope effect: TTFCOONH4 shows 260 μV K−1, which is twice as large as that predicted by the hole carrier concentration and the value of TTFCOOND4, with 138 μV K−1. The 1.9 H/D isotope effect in thermopower closely relates to the 2.2 H/D isotope effect in ion carrier mobility. Proton carriers in the temperature gradient enhance thermopower without cancelling out the effect of holes in the solid state owing to possession of the same positive charge.

Physical Review Letters, 2000

We find in the Bi 2 Sr 2 CaCu 2 O 8+δ system that the characteristic temperatures T * χ (below wh... more We find in the Bi 2 Sr 2 CaCu 2 O 8+δ system that the characteristic temperatures T * χ (below which the uniform susceptibilities χ ab (T ) (H⊥c) and χ c (T ) (H c) decrease) and T * ρc (below which the out-of-plane resistivity ρ c (T ) shows typical upturn) coincide for all doping levels. We attribute the T dependence of χ's and ρ c to the anomalous (pseudogapped) density-of-states (DOS) in high-T c cuprates. Furthermore, the anisotropy in the T dependence of χ's is universal, i.e., χ c ∝ 1.6χ ab , showing that there is only a single T -dependent component in the χ's. This implies that the Curie-like behavior (dχ/dT < 0) observed in overdoped samples is also caused by a DOS effect.

Physical Review B, 2008

The magnetism of pure and Pb-doped Bi 2 Sr 2 Co 2 O y ͑BSCO͒ crystals has been investigated with ... more The magnetism of pure and Pb-doped Bi 2 Sr 2 Co 2 O y ͑BSCO͒ crystals has been investigated with positive muon-spin rotation and relaxation ͑ + SR͒ spectroscopy. The entire volume of both materials enters into a magnetic state at low temperatures, occurring below 4.7 K for Pb-doped BSCO and below 1.0 K for pristine BSCO. By combining + SR and susceptibility measurements, it is clarified that Pb-doped BSCO is a ferromagnet with a Curie temperature ͑T C ͒ of 4.7 K and its ordered internal magnetic field ͑H int ͒ is almost parallel to the c axis. Since the relationship between the reduced T and reduced H int for Pb-doped BSCO is very similar to that for BSCO, the origin of the magnetic transition in both crystals is thought to be explained by common physics. Interestingly, we also detect the existence of a magnetic anomaly far above T C . This occurs at Ϸ60 K, coinciding with the metal-to-insulator transition that was observed for both materials by resistivity measurements.

Physical Review B, 2009

ABSTRACT We have performed high-resolution angle-resolved photoemission spectroscopy of Na0.5CoO2... more ABSTRACT We have performed high-resolution angle-resolved photoemission spectroscopy of Na0.5CoO2 to elucidate the origin of the two-step phase transition at Tc1=88 K and Tc2=53 K. At well below Tc2, we find anisotropic energy-gap opening on the remnant Fermi surface (FS). This gap does not disappear at the metal-insulator transition temperature Tc2 and survives up to Tc1. The remnant FS below Tc2 shows a deviation from the normal-state FS above Tc1, suggesting a possible reconstruction of FS caused by the magnetic ordering.

Physical Review B, 2013

We report superconductivity in the ternary half-Heusler compound LuPtBi, with Tc = 1.0 K and Hc2 ... more We report superconductivity in the ternary half-Heusler compound LuPtBi, with Tc = 1.0 K and Hc2 = 1.6 T. The crystal structure of LuPtBi lacks inversion symmetry, hence the material is a noncentrosymmetric superconductor. Magnetotransport data show semimetallic behavior in the normal state, which is evidence for the importance of spin-orbit interaction. Theoretical calculations indicate that the strong spin-orbit interaction in LuPtBi should cause strong band inversion, making this material a promising candidate for 3D topological superconductivity. 74.25.fc, 71.20.E, 71.30.+h I.

Physica C: Superconductivity, 2003

We have measured the superconducting gap, pseudogap and their doping dependence of CuO 2 trilayer... more We have measured the superconducting gap, pseudogap and their doping dependence of CuO 2 trilayer high-T c superconductor Bi 2 Sr 2 Ca 2 Cu 3 O 10þd by short-pulse interlayer tunneling spectroscopy. While both gaps exhibit temperature dependence similar to those for the bilayer system, their doping dependence behaves differently from that of T c , presenting an anomalous relationship between T c and the gap magnitude. In the overdoped region, T c remains almost unchanged from its optimum value irrespective of doping, while the gap decreases with increasing doping. This is suggestive of inequivalent hole doping in the inner and outer planes, which is thought to occur in trilayer systems.

Physica C: Superconductivity, 2003

Gap inhomogeneity in Bi 2 Sr 2 CaCu 2 O 8 has been studied by low-temperature STM. We found a sys... more Gap inhomogeneity in Bi 2 Sr 2 CaCu 2 O 8 has been studied by low-temperature STM. We found a systematic relation between the local gap energy and local background density of states. A model analysis of the dip-hump structure gave evidence of in-plane charge inhomogeneity. The temperature/doping dependences of the pattern shape of inhomogeneity as well as the gap distribution function were measured. We could identify the signature of phase separation into the superconducting and pseudogapped phases as the appearance of a double-peak structure in the distribution function.

Physica C: Superconductivity, 2010

Local density of states (LDOS) and the lattice structure of highly underdoped Bi 2 Sr 2 CaCu 2 O ... more Local density of states (LDOS) and the lattice structure of highly underdoped Bi 2 Sr 2 CaCu 2 O 8+d with T c = 22 K and 30 K were investigated by a low temperature scanning tunneling microscope. The modulation structure of the Bi-O surface was strongly depressed in the highly underdoped samples. The depression was observed only in the samples subject to the strong reduction annealing process, suggesting that the strong reduction in excess oxygen could destroy the modulation structure. At a time, patch-like inhomogeneity in the gap map sometimes disappeared, indicating that the existence of excess oxygen has an important role in the patch formation. Analysis on the LDOS with various doping levels showed that there was no crossover energy, which separates a pseudogap and a superconducting gap and is proportional to T c .

Physica B: Condensed Matter, 2009

The magnetism of a Pb-doped Bi 2 Sr 2 Co 2 O y (BSCO) crystal has been investigated by positive m... more The magnetism of a Pb-doped Bi 2 Sr 2 Co 2 O y (BSCO) crystal has been investigated by positive muon-spin rotation and relaxation (m þ SR) spectroscopy. Weak transverse field (wTF)-m þ SR measurements show that the whole sample enters into a magnetic state below 4:5K.Combiningtheresultsofzero−field(ZF)−mþSRexperimentwithsusceptibilitymeasurements,itisclarifiedthatthesampleisinaferromagneticorderedphasewithaCurietemperature(TC)of4.7Kandwiththeorderedinternalmagneticfieldparalleltothec−axiswithin4:5 K. Combining the results of zero-field (ZF)-m þ SR experiment with susceptibility measurements, it is clarified that the sample is in a ferromagnetic ordered phase with a Curie temperature (T C ) of 4.7 K and with the ordered internal magnetic field parallel to the c-axis within 4:5K.Combiningtheresultsofzero−field(ZF)−mþSRexperimentwithsusceptibilitymeasurements,itisclarifiedthatthesampleisinaferromagneticorderedphasewithaCurietemperature(TC)of4.7Kandwiththeorderedinternalmagneticfieldparalleltothec−axiswithin AE 15 . On the other hand, a pure BSCO crystal is also found to exhibit a bulk magnetic transition at 1.0 K by m þ SR. Since the relationship between the reduced T and reduced internal magnetic field for BSCO is almost equivalent to that for Pb-doped BSCO, the origin of the magnetic transition for both crystals is thought to be explained by common physics.

New Journal of Physics, 2011

We have performed angle-resolved photoemission spectroscopy of layered cobalt oxide Na x CoO 2 fo... more We have performed angle-resolved photoemission spectroscopy of layered cobalt oxide Na x CoO 2 for a wide doping range (0.35 x 0.85) with synchrotron radiation, and determined the doping dependence of the Fermi surface and band structure. The two-dimensional (2D) cylindrical a 1g Fermi surface at x = 0.35 gradually increases the dimensionality upon Na doping and eventually transforms into a 3D-like Fermi surface at x = 0.77, indicating the strong inter-layer coupling between adjacent CoO 2 layers in the highly doped region. We found that the characteristic shape of the top of the a 1g band is closely related to the various anomalies of physical properties in Na x CoO 2 .

Journal of the Physical Society of Japan, 2007

ABSTRACT

Large high-quality single crystals of Bi 2 Sr 2 Ca n−1 Cu n O 2n+4+δ (n = 2, 3) were successfully... more Large high-quality single crystals of Bi 2 Sr 2 Ca n−1 Cu n O 2n+4+δ (n = 2, 3) were successfully grown using an improved traveling solvent floating zone (TSFZ) method, which features a slow growth rate and steep temperature gradient along the melting zone. By measuring anisotropic resistivities and susceptibilities of Bi 2 Sr 2 CaCu 2 O 8+δ , the characteristic pseudogap temperature T * was studied as a function of doping. The T * suggest that the pseudogap is not simply a precursor of high-T c superconductivity, but that the pseudogap and the superconducting gap compete with each other. The anisotropic resistivities of Bi 2 Sr 2 Ca 2 Cu 3 O 10+δ were also measured, revealing that the T c remains fixed in the overdoped region while anisotropy decreases continuously. This anomalous behavior will be discussed in terms of the inequivalent hole doping, which occurs between two inequivalent CuO 2 planes in the triple-layer system.

Jun Sugiyamaa,∗ Yutaka Ikedoa Hiroshi Nozakia Peter L. Russob Jess H. Brewerc Eduardo J. Ansaldob... more Jun Sugiyamaa,∗ Yutaka Ikedoa Hiroshi Nozakia Peter L. Russob Jess H. Brewerc Eduardo J. Ansaldob Gerald D. Morrisb Kim H. Chowd Daniel Andreicae Alex Amatof Takenori Fujiig Satoshi Okadah and Ichiro Terasakih aToyota Central Research and Development Laboratories ...

We report a systematic high-resolution ARPES on Bi_2Sr_2Ca_ n-1Cu_nO_2n+4 (n=1-3) to study the un... more We report a systematic high-resolution ARPES on Bi_2Sr_2Ca_ n-1Cu_nO_2n+4 (n=1-3) to study the universality on the electronic structure and interactions. For n=2 and 3, we find that the kink in the energy dispersion becomes pronounced on approaching (pi, 0) in the superconducting state, while a kink appears only around the nodal direction in the normal state. The kink around (pi,

Japanese Journal of Applied Physics, 2014

We have successfully controlled thermoelectric properties of ZnO by changing carrier concentratio... more We have successfully controlled thermoelectric properties of ZnO by changing carrier concentration using an electric double layer transistor (EDLT) which is a field effect transistor gated by electrolyte solution.

We have precisely measured the Nernst effect in Nd-doped La$_{2-x}$Sr$_x$CuO$_4$ single crystals ... more We have precisely measured the Nernst effect in Nd-doped La$_{2-x}$Sr$_x$CuO$_4$ single crystals with controlling the strength (stability) of the stripe order. We found that the onset temperature TonsetT_{onset}Tonset, where the Nernst signal starts increasing, does not change conspicuously in spite of Nd-doping. At low temperatures, on the other hand, the absolute value of the Nernst signal is strongly suppressed in accordance with the strength of the stripe order. These results imply that the fluctuation of (charge) stripe order enhances the Nernst signal below TonsetT_{onset}Tonset at high temperatures, and then the stripe order enhanced by Nd-doping suppresses the superconducting fluctuation to reduce the Nernst signal at low temperatures. We also observed an increase of the Nernst signal below the charge order temperature TchT_{ch}Tch which is observed in diffraction measurement. Comment: 3pages, 2figures

We have performed an angle-resolved photoemission spectroscopy of NaxCoO2 to clarify the origin o... more We have performed an angle-resolved photoemission spectroscopy of NaxCoO2 to clarify the origin of the antiferromagnetic (AF) transition in the highly-doped region (x > 0.75). We determined the electronic band structure along the out-of-plane direction by varying the photon energy, and revealed that a three dimensional electron pocket emerges at the gamma point in the AF sample (x > 0.75), while it is absent in the non-AF counterpart (x < 0.75), showing the transition of the Fermi surface topology with the band filling. The present result suggests that the emergence of the electron pocket is closely related to the origin of the AF transition in highly-doped NaxCoO2.

We present high-pressure magneto-transport data on single crystals of LuPtBi, a member of the ter... more We present high-pressure magneto-transport data on single crystals of LuPtBi, a member of the ternary half-Heusler family. Recent band structure calculations show that LuPtBi is a topological semi-metal at ambient pressure due to strong spin-orbit coupling [1]. By decreasing the lattice parameter, equivalent to increasing pressure, the system should become a trivial insulator We have grown single crystals of LuPtBi and studied both the field dependence and the pressure dependence of their resistivity. The field dependence shows typical semi-metal behaviour, namely a weak temperature dependence and a large magneto-resistance. The pressure dependence shows a significant increase of resistivity and a decrease of magneto-resistance with increasing pressure. We compare our experimental results to the available theoretical work on the transport properties of topological semi-metals [2]. [4pt] [1] Stanislav Chadov, et al. Nature, 9, 541 (2010)[0pt] [2] W. Al-Sawai, et al. PRB, 82, 125208 (...

Physical Review B, 2003

The superconducting gap, the pseudogap, and their doping and temperature dependences have been me... more The superconducting gap, the pseudogap, and their doping and temperature dependences have been measured by the short-pulse interlayer tunneling spectroscopy for the CuO 2 triple-layer high-T c superconducting Bi 2 Sr 2 Ca 2 Cu 3 O 10+ δ system. It is found for a ...

Physical Review B, 2002

The doping dependence of the themopower, in-plane resistivity ρ ab (T ), out-of-plane resistivity... more The doping dependence of the themopower, in-plane resistivity ρ ab (T ), out-of-plane resistivity ρc(T ), and susceptibility has been systematically measured for high-quality single crystal Bi2Sr2Ca2Cu3O 10+δ . We found that the transition temperature Tc and pseudogap formation temperature T * ρc , below which ρc shows a typical upturn, do not change from their optimum values in the "overdoped" region, even though doping actually proceeds. This suggests that, in overdoped region, the bulk Tc is determined by the always underdoped inner plane, which have a large superconducting gap, while the carriers are mostly doped in the outer planes, which have a large phase stiffness.

Journal of Materials Chemistry A, 2013

ABSTRACT Ammonium proton in a solid ionic semiconductor, TTFCOONH4, is shown to be mobile under a... more ABSTRACT Ammonium proton in a solid ionic semiconductor, TTFCOONH4, is shown to be mobile under anhydrous conditions at room temperature by the hydrogen concentration cell method. Isotope substituted TTFCOOND4 exhibits a 2.2 H/D isotope effect in ion carrier mobility with TTFCOONH4. First-principles calculations reveal that an efficient proton-transfer pathway via low-barrier NH+N hydrogen bonds reduces the activation energy to 0.12 eV, which is quite small and comparable to that reported in a bulk water system. The ac conductivity of TTFCOONH4 and TTFCOOND4 is similar at room temperature, reflecting similar hole carrier concentrations. In sharp contrast, the thermopower exhibits a large isotope effect: TTFCOONH4 shows 260 μV K−1, which is twice as large as that predicted by the hole carrier concentration and the value of TTFCOOND4, with 138 μV K−1. The 1.9 H/D isotope effect in thermopower closely relates to the 2.2 H/D isotope effect in ion carrier mobility. Proton carriers in the temperature gradient enhance thermopower without cancelling out the effect of holes in the solid state owing to possession of the same positive charge.

Physical Review Letters, 2000

We find in the Bi 2 Sr 2 CaCu 2 O 8+δ system that the characteristic temperatures T * χ (below wh... more We find in the Bi 2 Sr 2 CaCu 2 O 8+δ system that the characteristic temperatures T * χ (below which the uniform susceptibilities χ ab (T ) (H⊥c) and χ c (T ) (H c) decrease) and T * ρc (below which the out-of-plane resistivity ρ c (T ) shows typical upturn) coincide for all doping levels. We attribute the T dependence of χ's and ρ c to the anomalous (pseudogapped) density-of-states (DOS) in high-T c cuprates. Furthermore, the anisotropy in the T dependence of χ's is universal, i.e., χ c ∝ 1.6χ ab , showing that there is only a single T -dependent component in the χ's. This implies that the Curie-like behavior (dχ/dT < 0) observed in overdoped samples is also caused by a DOS effect.

Physical Review B, 2008

The magnetism of pure and Pb-doped Bi 2 Sr 2 Co 2 O y ͑BSCO͒ crystals has been investigated with ... more The magnetism of pure and Pb-doped Bi 2 Sr 2 Co 2 O y ͑BSCO͒ crystals has been investigated with positive muon-spin rotation and relaxation ͑ + SR͒ spectroscopy. The entire volume of both materials enters into a magnetic state at low temperatures, occurring below 4.7 K for Pb-doped BSCO and below 1.0 K for pristine BSCO. By combining + SR and susceptibility measurements, it is clarified that Pb-doped BSCO is a ferromagnet with a Curie temperature ͑T C ͒ of 4.7 K and its ordered internal magnetic field ͑H int ͒ is almost parallel to the c axis. Since the relationship between the reduced T and reduced H int for Pb-doped BSCO is very similar to that for BSCO, the origin of the magnetic transition in both crystals is thought to be explained by common physics. Interestingly, we also detect the existence of a magnetic anomaly far above T C . This occurs at Ϸ60 K, coinciding with the metal-to-insulator transition that was observed for both materials by resistivity measurements.

Physical Review B, 2009

ABSTRACT We have performed high-resolution angle-resolved photoemission spectroscopy of Na0.5CoO2... more ABSTRACT We have performed high-resolution angle-resolved photoemission spectroscopy of Na0.5CoO2 to elucidate the origin of the two-step phase transition at Tc1=88 K and Tc2=53 K. At well below Tc2, we find anisotropic energy-gap opening on the remnant Fermi surface (FS). This gap does not disappear at the metal-insulator transition temperature Tc2 and survives up to Tc1. The remnant FS below Tc2 shows a deviation from the normal-state FS above Tc1, suggesting a possible reconstruction of FS caused by the magnetic ordering.

Physical Review B, 2013

We report superconductivity in the ternary half-Heusler compound LuPtBi, with Tc = 1.0 K and Hc2 ... more We report superconductivity in the ternary half-Heusler compound LuPtBi, with Tc = 1.0 K and Hc2 = 1.6 T. The crystal structure of LuPtBi lacks inversion symmetry, hence the material is a noncentrosymmetric superconductor. Magnetotransport data show semimetallic behavior in the normal state, which is evidence for the importance of spin-orbit interaction. Theoretical calculations indicate that the strong spin-orbit interaction in LuPtBi should cause strong band inversion, making this material a promising candidate for 3D topological superconductivity. 74.25.fc, 71.20.E, 71.30.+h I.

Physica C: Superconductivity, 2003

We have measured the superconducting gap, pseudogap and their doping dependence of CuO 2 trilayer... more We have measured the superconducting gap, pseudogap and their doping dependence of CuO 2 trilayer high-T c superconductor Bi 2 Sr 2 Ca 2 Cu 3 O 10þd by short-pulse interlayer tunneling spectroscopy. While both gaps exhibit temperature dependence similar to those for the bilayer system, their doping dependence behaves differently from that of T c , presenting an anomalous relationship between T c and the gap magnitude. In the overdoped region, T c remains almost unchanged from its optimum value irrespective of doping, while the gap decreases with increasing doping. This is suggestive of inequivalent hole doping in the inner and outer planes, which is thought to occur in trilayer systems.

Physica C: Superconductivity, 2003

Gap inhomogeneity in Bi 2 Sr 2 CaCu 2 O 8 has been studied by low-temperature STM. We found a sys... more Gap inhomogeneity in Bi 2 Sr 2 CaCu 2 O 8 has been studied by low-temperature STM. We found a systematic relation between the local gap energy and local background density of states. A model analysis of the dip-hump structure gave evidence of in-plane charge inhomogeneity. The temperature/doping dependences of the pattern shape of inhomogeneity as well as the gap distribution function were measured. We could identify the signature of phase separation into the superconducting and pseudogapped phases as the appearance of a double-peak structure in the distribution function.

Physica C: Superconductivity, 2010

Local density of states (LDOS) and the lattice structure of highly underdoped Bi 2 Sr 2 CaCu 2 O ... more Local density of states (LDOS) and the lattice structure of highly underdoped Bi 2 Sr 2 CaCu 2 O 8+d with T c = 22 K and 30 K were investigated by a low temperature scanning tunneling microscope. The modulation structure of the Bi-O surface was strongly depressed in the highly underdoped samples. The depression was observed only in the samples subject to the strong reduction annealing process, suggesting that the strong reduction in excess oxygen could destroy the modulation structure. At a time, patch-like inhomogeneity in the gap map sometimes disappeared, indicating that the existence of excess oxygen has an important role in the patch formation. Analysis on the LDOS with various doping levels showed that there was no crossover energy, which separates a pseudogap and a superconducting gap and is proportional to T c .

Physica B: Condensed Matter, 2009

The magnetism of a Pb-doped Bi 2 Sr 2 Co 2 O y (BSCO) crystal has been investigated by positive m... more The magnetism of a Pb-doped Bi 2 Sr 2 Co 2 O y (BSCO) crystal has been investigated by positive muon-spin rotation and relaxation (m þ SR) spectroscopy. Weak transverse field (wTF)-m þ SR measurements show that the whole sample enters into a magnetic state below 4:5K.Combiningtheresultsofzero−field(ZF)−mþSRexperimentwithsusceptibilitymeasurements,itisclarifiedthatthesampleisinaferromagneticorderedphasewithaCurietemperature(TC)of4.7Kandwiththeorderedinternalmagneticfieldparalleltothec−axiswithin4:5 K. Combining the results of zero-field (ZF)-m þ SR experiment with susceptibility measurements, it is clarified that the sample is in a ferromagnetic ordered phase with a Curie temperature (T C ) of 4.7 K and with the ordered internal magnetic field parallel to the c-axis within 4:5K.Combiningtheresultsofzero−field(ZF)−mþSRexperimentwithsusceptibilitymeasurements,itisclarifiedthatthesampleisinaferromagneticorderedphasewithaCurietemperature(TC)of4.7Kandwiththeorderedinternalmagneticfieldparalleltothec−axiswithin AE 15 . On the other hand, a pure BSCO crystal is also found to exhibit a bulk magnetic transition at 1.0 K by m þ SR. Since the relationship between the reduced T and reduced internal magnetic field for BSCO is almost equivalent to that for Pb-doped BSCO, the origin of the magnetic transition for both crystals is thought to be explained by common physics.

New Journal of Physics, 2011

We have performed angle-resolved photoemission spectroscopy of layered cobalt oxide Na x CoO 2 fo... more We have performed angle-resolved photoemission spectroscopy of layered cobalt oxide Na x CoO 2 for a wide doping range (0.35 x 0.85) with synchrotron radiation, and determined the doping dependence of the Fermi surface and band structure. The two-dimensional (2D) cylindrical a 1g Fermi surface at x = 0.35 gradually increases the dimensionality upon Na doping and eventually transforms into a 3D-like Fermi surface at x = 0.77, indicating the strong inter-layer coupling between adjacent CoO 2 layers in the highly doped region. We found that the characteristic shape of the top of the a 1g band is closely related to the various anomalies of physical properties in Na x CoO 2 .

Journal of the Physical Society of Japan, 2007

ABSTRACT