Takenori Fujii

Distinct Fermi-momentum dependent energy gaps in deeply underdoped Bi2212.

We used angle-resolved photoemission spectroscopy applied to deeply underdoped cuprate superconductors Bi2Sr2Ca(1–x)YxCu2O8 (Bi2212) to reveal the presence of two distinct energy gaps exhibiting different doping dependence. One gap, associated with the antinodal region where no coherent peak is observed, increased with underdoping, a behavior known for more than a decade and considered as the general gap behavior in the underdoped regime. The other gap, associated with the near-nodal regime where a coherent peak in the spectrum can be observed, did not increase with less doping, a behavior not previously observed in the single particle spectra. We propose a two-gap scenario in momentum space that is consistent with other experiments and may contain important information on the mechanism of high–transition temperature superconductivity.

Crossover behavior of the anomalous hall effect and anomalous nernst effect in itinerant ferromagnets

The anomalous Hall effect (AHE) and anomalous Nernst effect (ANE) are experimentally investigated in a variety of ferromagnetic metals including pure transition metals, oxides, and chalcogenides, whose resistivities range over 5 orders of magnitude. For these ferromagnets, the transverse conductivity sigma{xy} versus the longitudinal conductivity sigma{xx} shows a crossover behavior with three distinct regimes in accordance qualitatively with a recent unified theory of the intrinsic and extrinsic AHE. We also found that the transverse Peltier coefficient alpha{xy} for the ANE obeys the Mott rule. These results offer a coherent and semiquantitative understanding of the AHE and ANE to an issue of controversy for many decades.

Thermal conductivity of the thermoelectric layered cobalt oxides measured by the Harman method

In-plane thermal conductivity of the thermoelectric layered cobalt oxides has been measured using the Harman method, in which thermal conductivity is obtained from temperature gradient induced by applied current. We have found that the charge reservoir block (the block other than the CoO2 block) dominates the thermal conduction, where a nano-block integration concept is effective for material design. We have further found that the thermal conductivity shows a small but finite in-plane anisotropy between a and b axes, which can be ascribed to the misfit structure.

Superconductivity in the noncentrosymmetric half-Heusler compound LuPtBi: A candidate for topological superconductivity

We report superconductivity in the ternary half-Heusler compound LuPtBi, with Tc = 1. 0Ka ndHc2 = 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. The combination of strong spin-orbit coupling and noncentrosymmetric crystal structure make LuPtBi a strong candidate for 3D topological superconductivity.

Large In-Plane Anisotropy on Resistivity and Thermopower in the Misfit Layered Oxide Bi2-xPbxSr2Co2Oy

We investigated the in-plane anisotropy on the resistivity and thermopower of Bi2-xPbxSr2Co2Oy single crystals, which have a misfit structure between the hexagonal CoO2 layer and the rock salt Bi2Sr2O4 layer. The resistivity and thermopower show significant anisotropy, which exceeds two at maximum. This anisotropy is thought to arise from the anisotropic pseudogap formation enhanced by the misfit structure. The thermopower changes with Pb doping to take a maximum at x=0.4. The misfit structure improves the thermoelectric properties through chemical pressure. The power factor is as large as 9 ?W/cm?K2 at 100 K for x=0.6, which is the highest value obtained for thermoelectric oxides at 100 K.

A momentum-dependent perspective on quasiparticle interference in Bi2Sr2CaCu2O8+|[delta]|

Angle Resolved Photoemission Spectroscopy (ARPES) probes the momentum-space electronic structure of materials, and provides invaluable information about the high-temperature superconducting cuprates. Likewise, cuprates real-space, inhomogeneous electronic structure is elucidated by Scanning Tunneling Spectroscopy (STS). Recently, STS has exploited quasiparticle interference (QPI) - wave-like electrons scattering off impurities to produce periodic interference patterns - to infer properties of the QP in momentum-space. Surprisingly, some interference peaks in Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8+{delta}} (Bi-2212) are absent beyond the antiferromagnetic (AF) zone boundary, implying the dominance of particular scattering process. Here, we show that ARPES sees no evidence of quasiparticle (QP) extinction: QP-like peaks are measured everywhere on the Fermi surface, evolving smoothly across the AF zone boundary. This apparent contradiction stems from different natures of single-particle (ARPES) and two-particle (STS) processes underlying these probes. Using a simple model, we demonstrate extinction of QPI without implying the loss of QP beyond the AF zone boundary.

Gap inhomogeneity, phase separation and a pseudogap in Bi2Sr2CaCu2O8+δ

Abstract 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.

Comparative study of transport properties of Bi2Sr2Ca2Cu3O10+δ and Bi2Sr2CaCu2O8+δ single crystals

Abstract The doping dependence of the in-plane resistivity ρ a ( T ) and the out-of-plane resistivity ρ c ( T ) have been systematically measured for the triple-layered system, Bi 2 Sr 2 Ca 2 Cu 3 O 10+ δ (Bi-2223). In comparison with the bilayered system, Bi 2 Sr 2 CaCu 2 O 8+ δ (Bi-2212), we found in Bi-2223 that the superconducting transition temperature T c and pseudogap formation temperature T * , below which ρ c shows a typical upturn, do not change from their optimum values in the overdoped region, even though doping actually proceeds. This result suggests that inequivalent hole doping occurs between the outer and inner planes. In the overdoped region, the carriers are mostly doped in the outer plane and the inner plane remains at an optimum doping level.

Static magnetic order in metallic K0.49CoO2.

By means of muon-spin spectroscopy, we have found that K0.49CoO2 crystals undergo successive magnetic transitions from a high-T paramagnetic state to a magnetic ordered state below 60 K and then to a second ordered state below 16 K, even though K0.49CoO2 is metallic at least down to 4 K. An isotropic magnetic behavior and wide internal-field distributions suggest the formation of a commensurate helical spin density wave (SDW) state below 16 K, while a linear SDW state is likely to exist above 16 K. It was also found that exhibits a further transition at 150 K presumably due to a change in the spin state of the Co ions. Since the dependence of the internal-field below 60 K was similar to that for Na0.5CoO2, this suggests that magnetic order is more strongly affected by the Co valence than by the interlayer distance or interaction and/or the charge ordering.

Control of thermoelectric properties of ZnO using electric double-layer transistor structure

We have successfully controlled the thermoelectric properties of ZnO by changing the carrier concentration using an electric double-layer transistor (EDLT), which is a field-effect transistor gated by electrolyte solution. The resistivity and thermopower decreased abruptly by applying a gate voltage VG larger than a threshold voltage (~2 V), indicating an increase in carrier concentration on the ZnO surface. The temperature dependence of the resistivity became metallic, which is characterized by the weak temperature dependence of the resistivity, when the gate voltage exceeded 2 V. Corresponding to the resistivity, the temperature dependence of the thermopower changed markedly. The thickness of the induced metallic layer was estimated to be about 10 nm and the power factor at VG = 4 V was calculated to ~8 × 10−5 W m−1 K−2, which is six or seven orders of magnitude larger than that of the bulk ZnO (VG = 0 V). Thus, the EDLT is considered to be a useful way to optimize thermoelectric properties by tuning the carrier concentration.