arXiv subjects

Yuya Hattori

Publications and source records attributed to Yuya Hattori.

8 recordsLinked to original sources

arXiv2026

Altermagnetic spin-split Fermi surfaces in CrSb revealed by quantum oscillation measurements

We report a comprehensive quantum oscillation study of the prototypical altermagnet CrSb, combining high-field magnetotransport and torque measurements with DFT + $U$ calculations including spin-orbit coupling. Multiple quantum oscillation frequencies were observed and tracked over wide angular ranges. The measured frequency branches are consistently explained by the spin-split Fermi surfaces arising from the altermagnetic electronic structure. Our determined Fermi surface reveals that bands 1 and 2 form closed pockets centered at the A point, rather than the tubular $c$-axis-open sheets or $Γ$-centered closed pockets proposed in previous studies. Our findings establish the Fermi-surface topology of CrSb and provide a firm basis for exploring emergent phenomena in altermagnetic materials.

cond-mat.mtrl-sci↗

arXiv2026

Ideal band structures for high-performance thermoelectric materials with band convergence

We investigate optimal band structures in band-converged systems to achieve high zT using numerical calculations based on a virtual spectral conductivity model. We consider a two parabolic band system, in which multiple band parameters can be independently controlled. Despite its simplicity, this model provides theoretical validation of empirical trends observed in thermoelectric materials. Our results provide a physically transparent set of design principles for band-structure engineering, offering quantitative design guidelines for the development of a wide range of thermoelectric materials. The main conclusions are as follows: (i) When a band does not cross the chemical potential and |μ-E_edge |>5k_B T, the contribution of the band to zT is negligibly small; (ii) To suppress the bipolar effect, a band gap E_g satisfying E_g>5k_B T_op, where T_op is the operating temperature, is required; (iii) In band-converged systems, the energy separation between the band edge ΔE should satisfy ΔE~0 to maximize zT when interband scattering is insignificant; (iv) Achieving high spectral conductivity Σ (high band degeneracy N, density of states effective mass m_DOS^*, and relaxation time τ) near the band edge is essential for achieving high zT.

cond-mat.mtrl-sci↗

arXiv2025

Multipolar Phase Transition in the 4$f^2$ fcc lattice compound PrCdNi$_{4}$

Transport and magnetic properties of a 4$f^{2}$ fcc lattice compound, PrCdNi$_4$, were studied. The magnetic susceptibility, $χ(T)$, follows the Curie--Weiss law from 300 K to 20 K, as expected for a free Pr$^{3+}$ ion. As the temperature decreases below 5 K, $χ(T)$ approaches a constant, indicating van-Vleck paramagnetic behavior. The specific heat, $C(T)$, displays a broad shoulder at around 4 K, which can be reproduced by a doublet triplet two-level model with an energy gap of 12 K. These results suggest a non-magnetic $Γ_3$ doublet ground state of the Pr$^{3+}$ ion in the cubic crystalline electric field. $C(T)$ exhibits a peak at $T_{\rm O}$ = 1.0 K and this peak remains robust against magnetic fields up to 5 T. In powder neutron diffraction measurements, no magnetic reflection was observed at 0.32 K $<$ $T_{\rm O}$. Two anomalies at $B$ = 2.1 and 5.3 T in magnetoresistance $ρ(B)$ at 0.05 K likely originate from switching in the order parameter. These results suggest that the phase transition at $T_{\rm O}$ is ascribed to an antiferro-type order of the electric quadrupole or magnetic octupole of the $Γ_3$ doublet in the 4$f^2$ fcc lattice.

cond-mat.str-el↗

arXiv2025

Topological delocalization against Anderson localization observed in Bi-doped PbSb2Te4 disordered topological insulator

We experimentally investigate the tendency of localization in the bulk and the topological surface states in topological insulators Pb(Bi1-xSbx)2Te4 (x=0.793-0.818) through detailed transport measurements. The bulk electronic states in the range 0.793<=x<0.818 are situated on the insulator side of the Anderson transition, as indicated by k_Fl values (where k_F is the Fermi wavenumber and l is the mean free path) falling below the Ioffe-Regel criterion (i.e., k_F l<1). In contrast, the topological surface states retain high mobility and even exhibit quantum oscillations, demonstrating their resilient nature against strong disorder. These findings highlight the delocalized nature of the topological surface states despite Anderson localization of the bulk electronic states.

cond-mat.mtrl-sci↗

arXiv2024

Decoupled charge and heat transport for high-performance Fe$_2$VAl composite thermoelectrics

Decoupling charge and heat transport is essential for optimizing thermoelectric materials. Strategies to inhibit lattice-driven heat transport, however, also compromise carrier mobility, limiting the performance of most thermoelectrics, including Fe$_2$VAl Heusler compounds. Here, we demonstrate an innovative approach, which bypasses this tradeoff: via liquid-phase sintering, we incorporate the archetypal topological insulator Bi$_{1-x}$Sb$_{x}$ between Fe$_2$V$_{0.95}$Ta$_{0.1}$Al$_{0.95}$ grains. Structural investigations alongside extensive thermoelectric and magneto-transport measurements reveal distinct modifications in the microstructure, and a reduced lattice thermal conductivity and enhanced carrier mobility are simultaneously found. This yields a huge performance boost $-$ far beyond the effective-medium limit $-$ and results in one of the highest figure of merits among both half- and full-Heusler compounds, $z\approx 1.6\times 10^{-3}\,$K$^{-1}$ ($zT\approx 0.5$) at 295 K. Our findings highlight the potential of secondary phases to decouple charge and heat transport and call for more advanced theoretical studies of multiphase composites.

cond-mat.mtrl-sci↗

arXiv2023

Topological surface states hybridized with bulk states of Bi-doped PbSb2Te4 revealed in quasiparticle interference

Topological surface states of Bi-doped PbSb2Te4 [Pb(Bi0.20Sb0.80)2Te4] are investigated through analyses of quasiparticle interference (QPI) patterns observed by scanning tunneling microscopy. Interpretation of the experimental QPI patterns in the reciprocal space is achieved by numerical QPI simulations using two types of surface density of states produced by density functional theory calculations or a kp surface state model. We found that the Dirac point (DP) of the surface state appears in the bulk band gap of this material and, with the energy being away from the DP, the isoenergy contour of the surface state is substantially deformed or separated into segments due to hybridization with bulk electronic states. These findings provide a more accurate picture of topological surface states, especially at energies away from the DP, providing valuable insight into the electronic properties of topological insulators.

cond-mat.mtrl-sci↗

arXiv2023

Experimental verification of band convergence in Sr and Na codoped PbTe

Scanning tunneling microscopy and transport measurements have been performed to investigate the electronic structure and its temperature dependence in heavily Sr and Na codoped PbTe, which is recognized as one of the most promising thermoelectric materials. Our main findings are as follows: (i) Below T=4.5 K, all carriers are distributed in the first valence band at the L point (L band), which forms tube-shaped Fermi surfaces with concave curvature. With Sr and Na doping, the dispersion of the L band changes, and the band gap increases from 200 meV to 300 meV. (ii) At T=4.5 K, the Fermi energy is located ~100 meV below the edge of the L band for the Sr/Na codoped PbTe. The second valence band at the Sigma point (Sigma band) is lower than the L band by 150 meV, which is significantly smaller than that of pristine PbTe (200 meV). The decrease in the band offset, leading to band convergence, provides a desirable condition for thermoelectric materials.(iii) With increasing temperature, the carrier distribution to the Sigma band starts at T=100 K and we estimate that about 50 percent of the total carriers are redistributed in the Sigma band at T=300 K.Our work demonstrates that scanning tunneling microscopy and angular dependent magnetoresistance measurements are particularly powerful tools to determine the electronic structure and carrier distribution. We believe that they will provide a bird's eye view of the doping strategy towards realizing high-efficiency thermoelectric materials.

cond-mat.mtrl-sci↗

arXiv2023

Fermi surface and light quasi particles in hourglass nodal chain metal \b{eta}-ReO2

Quantum oscillations in magnetic torque and electrical resistivity were measured to investigate the electronic structure of \b{eta}-ReO2, a candidate hourglass nodal chain metal (Dirac loop chain metal). All the de Haas-van Alphen oscillation branches measured at 30 mK in magnetic fields of up to 17.5 T were consistent with first-principles calculations predicting four Fermi surfaces (FSs). The small-electron FS of the four FSs exhibited a very small cyclotron mass, 0.059 times that of the free electrons, which is likely to be related to the linear dispersion of the energy band. The consistency between the quantum oscillation results and band calculations indicates the presence of the hourglass nodal chain predicted for \b{eta}-ReO2 in the vicinity of the Fermi energy.

cond-mat.str-el↗