arXiv subjects

K. Terashima

Publications and source records attributed to K. Terashima.

At least 19 recordsLinked to original sources

arXiv2024

Magnetic properties of AlB$_2$-type holmium silicides and germanides

Discovery of the large magnetocaloric effect in HoB$_2$ has highlighted the practical advantage of heavy rare-earth ions. Other holmium compounds are of interest, and we here report the synthesis and the magnetic properties of HoSi$_{1.67}$ and HoGe$_{1.67}$ which form the same AlB$_2$-type structure but with vacancies. They are found to show the antiferromagnetic order with the Neel temperature 17.6(2)K for HoSi$_{1.67}$ and 9.9(2)K for HoGe$_{1.67}$, and the magnetic entropy changes at the temperature are 0.05(1)J/cm$^3$K for HoSi$_{1.67}$ and 0.08(1)J/cm$^3$K for HoGe$_{1.67}$. Magnetic orders were suppressed by replacing vacancies with nickel, resulting in an increase of magnetic entropy changes. Distance between the in-plane Ho$^{3+}$ ions appears to be an important parameter leading to the transition between the antiferromagnetic (HoSi$_{1.67}$) and the ferromagnetic (HoB$_2$) order. The finding may aid the exploration of other heavy rare-earth compounds for similar applications.

cond-mat.str-el↗

arXiv2022

Estimation of Grüneisen parameter of high-entropy-alloy-type functional materials

In functional materials like thermoelectric materials and superconductors, the interplay between functionality, electronic structure, and phonon characteristics is one of the key factors to improve functionality and to understand the mechanisms. In the first part of this article, we briefly review investigations on lattice anharmonicity in functional materials by Grüneisen parameter (γG). One can find that the γG can be a good scale for large lattice anharmonicity and for detecting a change in anharmonicity amplitude in functional materials. Then, we show original results on estimation of γG for recently-developed high-entropy-alloy-type (HEA-type) functional materials with a layered structure and a NaCl-type structure. As a common trend between those two systems with two- and three-dimensional structures, we find that γG increases by a slight increase in configurational entropy of mixing (ΔSmix), and then γG decreases with increasing ΔSmix in high-entropy region.

cond-mat.mtrl-sci↗

arXiv2019

Enhanced thermoelectricity by controlled local structure in bismuth-chalcogenides

Spectroscopic techniques, including photoelectron spectroscopy, diffuse reflectance, and x-ray absorption, are used to investigate the electronic structure and the local structure of LaOBiS$_{2-x}$Se$_x$ thermoelectric material. It is found that Se substitution effectively suppresses local distortion, that can be responsible for the increased carrier mobility together with a change in the electronic structure. The results suggest a possible way to control thermoelectric properties by tuning of the local crystal structure of these materials.

cond-mat.mtrl-sci↗

arXiv2018

Origins of thermal spin depolarization in half-metallic ferromagnet CrO$_2$

Using high-resolution spin-resolved photoemission spectroscopy, we observed a thermal spin depolarization to which all spin-polarized electrons contribute. Furthermore we observed a distinct minority spin state near the Fermi level and a corresponding depolarization that seldom contributes to demagnetization. The origin of this depolarization has been identified as the many-body effect characteristics of half-metallic ferromagnets. Our investigation opens an experimental field of itinerant ferromagnetic physics focusing on phenomena with sub-meV energy scale.

cond-mat.str-el↗

arXiv2018

Determination of the local structure of Sr$_{2-x}$M$_x$IrO$_4$ (M = K, La) as a function of doping and temperature

The local structure of correlated spin-orbit insulator Sr$_{2-x}$M$_x$IrO$_4$ (M = K, La) has been investigated by Ir L$_3$-edge extended x-ray absorption fine structure measurements. The measurements were performed as a function of temperature for different dopings induced by substitution of Sr with La or K. It is found that Ir-O bonds have strong covalency and they hardly show any change across the Néel temperature. In the studied doping range, neither Ir-O bonds nor their dynamics, measured by their mean square relative displacements, show any appreciable change upon carrier doping, indicating possibility of a nanoscale phase separation in the doped system. On the other hand, there is a large increase of the static disorder in Ir-Sr correlation, larger for K doping than La doping. Similarities and differences with respect to the local lattice displacements in cuprates are briefly discussed.

cond-mat.str-el↗

arXiv2018

Temperature-dependent local structure of superconducting BaPd$_2$As$_2$ and SrPd$_2$As$_2$

The local structures of 122-type paradium arsenides, namely BaPd$_2$As$_2$ and SrPd$_2$As$_2$, are examined by As K-edge extended x-ray absorption fine structure measurements to find a possible correlation between the variation of their superconducting transition temperature and the local structure. The local atomic distances are found to be consistent with average distances measured by diffraction techniques. The temperature dependence of mean square relative displacements reveal that, while BaPd$_2$As$_2$ is characterized by a local As-Pd soft mode, albeit with larger atomic disorder, SrPd$_2$As$_2$ shows anomalous As-Pd correlations with a kink at $\sim$160 K due to hardening by raising temperature. We have discussed implications of these results and possible mechanism of differing superconducting transition temperature in relation with the structural instability.

cond-mat.supr-con↗

arXiv2017

Direct observation of double valence-band extrema and anisotropic effective masses of the thermoelectric material SnSe

Synchrotron-based angle-resolved photoemission spectroscopy is used to determine the electronic structure of layered SnSe, which was recently turned out to be a potential thermoelectric material. We observe that the top of the valence band consists of two nearly independent hole bands, whose tops differ by ~20 meV in energy, indicating the necessity of a multivalley model to describe the thermoelectric properties. The estimated effective masses are anisotropic, with in-plane values of 0.16-0.39 m$_0$ and an out-of-plane value of 0.71 m$_0$, where m$_0$ is the rest electron mass. Information of the electronic structure is essential to further enhance the thermoelectric performance of hole-doped SnSe.

cond-mat.mtrl-sci↗

arXiv2017

Evolution of remnant Fermi surface state in lightly-doped correlated spin-orbit insulator Sr$_{2-x}$La$_x$IrO$_4$

Electronic structure has been studied in lightly electron doped correlated spin-orbit insulator Sr$_2$IrO$_4$ by angle-resolved photoelectron spectroscopy. We have observed coexistence of the lower Hubbard band and the in-gap band, the momentum dependence of the latter traces that of the band calculations without on-site Coulomb repulsion. The in-gap state remained anisotropically gapped in all observed momentum area, forming a remnant Fermi surface state, evolving towards the Fermi energy by carrier doping. These experimental results show a striking similarity with those observed in deeply underdoped cuprates, suggesting the common nature of the nodal liquid states observed in both compounds.

cond-mat.str-el↗

arXiv2014

Proximity to Fermi-surface topological change in superconducting LaO0.54F0.46BiS2

The electronic structure of nearly optimally-doped novel superconductor LaO$_{1-x}$F$_x$BiS$_2$ (${\it x}$ = 0.46) was investigated using angle-resolved photoemission spectroscopy (ARPES). We clearly observed band dispersions from 2 to 6 eV binding energy and near the Fermi level (${\it E}_{\rm F}$), which are well reproduced by first principles calculations when the spin-orbit coupling is taken into account. The ARPES intensity map near ${\it E}_{\rm F}$ shows a square-like distribution around the $Γ$(Z) point in addition to electronlike Fermi surface (FS) sheets around the X(R) point, indicating that FS of LaO$_{0.54}$F$_{0.46}$BiS$_2$ is in close proximity to the theoretically-predicted topological change.

cond-mat.supr-con↗

arXiv2010

Electron correlation in FeSe superconductor studied by bulk-sensitive photoemission spectroscopy

We have investigated the electronic structures of recently discovered superconductor FeSe by soft-x-ray and hard-x-ray photoemission spectroscopy with high bulk sensitivity. The large Fe 3d spectral weight is located in the vicinity of the Fermi level (EF), which is demonstrated to be a coherent quasi-particle peak. Compared with the results of the band structure calculation with local-density approximation, Fe 3d band narrowing and the energy shift of the band toward EF are found, suggesting an importance of the electron correlation effect in FeSe. The self energy correction provides the larger mass enhancement value (Z^-1=3.6) than in Fe-As superconductors and enables us to separate a incoherent part from the spectrum. These features are quite consistent with the results of recent dynamical mean-field calculations, in which the incoherent part is attributed to the lower Hubbard band.

cond-mat.supr-con↗

arXiv2009

Fermi surface nesting induced strong pairing in iron-based superconductors

The discovery of high-temperature superconductivity in iron pnictides raised the possibility of an unconventional superconducting mechanism in multiband materials. The observation of Fermi-surface(FS)-dependent nodeless superconducting gaps suggested that inter-FS interactions may play a crucial role in superconducting pairing. In the optimally hole-doped Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$, the pairing strength is enhanced simultaneously (2$Δ$/Tc$\sim$7) on the nearly nested FS pockets, i.e. the inner holelike ($α$) FS and the two hybridized electronlike FSs, while the pairing remains weak (2$Δ$/Tc$\sim$3.6) in the poorly-nested outer hole-like ($β$) FS. Here we report that in the electron-doped BaFe$_{1.85}$Co$_{0.15}$As$_2$ the FS nesting condition switches from the $α$ to the $β$ FS due to the opposite size changes for hole- and electron-like FSs upon electron doping. The strong pairing strength (2$Δ$/Tc$\sim$6) is also found to switch to the nested $β$ FS, indicating an intimate connection between FS nesting and superconducting pairing, and strongly supporting the inter-FS pairing mechanism in the iron-based superconductors.

cond-mat.supr-con↗

arXiv2009

Evolution of a pairing-induced pseudogap from the superconducting gap of (Bi,Pb)2Sr2CuO6

We have performed an ultrahigh-resolution angle-resolved photoemission spectroscopy study of slightly-overdoped (Bi,Pb)2Sr2CuO6 to elucidate the origin of pseudogap. By using a newly developed xenon-plasma light source, we determined the comprehensive momentum and temperature dependences of the superconducting gap and the pseudogap. We found that the antinodal pseudogap persists far above the superconducting transition temperature and is smoothly connected to the nodal gap. The characteristic temperature of the pseudogap scales well with the superconducting-gap size irrespective of the momentum location. The present experimental results point to the pairing origin of the pseudogap.

cond-mat.supr-con↗

arXiv2009

Electronic structure of heavily electron-doped BaFe$_{1.7}$Co$_{0.3}$As$_2$ studied by angle-resolved photoemission

We have performed high-resolution angle-resolved photoemission spectroscopy on heavily electron-doped non-superconducting (SC) BaFe$_{1.7}$Co$_{0.3}$As$_2$. We find that the two hole Fermi surface pockets at the zone center observed in the hole-doped superconducting Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$ are absent or very small in this compound, while the two electron pockets at the M point significantly expand due to electron doping by the Co substitution. Comparison of the Fermi surface between non-SC and SC samples indicates that the coexistence of hole and electron pockets connected via the antiferromagnetic wave vector is essential in realizing the mechanism of superconductivity in the iron-based superconductors.

cond-mat.supr-con↗

arXiv2008

Superconducting Gap and Pseudogap in Iron-Based Layered Superconductor La(O$_{1-x}$F$_x$)FeAs

We report high-resolution photoemission spectroscopy of newly-discovered iron-based layered superconductor La(O$_{0.93}$F$_{0.07}$)FeAs (Tc = 24 K). We found that the superconducting gap shows a marked deviation from the isotropic s-wave symmetry. The estimated gap size at 5 K is 3.6 meV in the s- or axial p-wave case, while it is 4.1 meV in the polar p- or d-wave case. We also found a pseudogap of 15-20 meV above Tc, which is gradually filled-in with increasing temperature and closes at temperature far above Tc similarly to copper-oxide high-temperature superconductors.

cond-mat.supr-con↗

arXiv2007

Anomalous dispersion in the autocorrelation of ARPES data of Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$

We find that peaks in the autocorrelation of angle resolved photoemission spectroscopy data of Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$ in the superconducting state show dispersive behavior for binding energies smaller than the maximum superconducting energy gap. For higher energies, though, a striking anomalous dispersion is observed that is a consequence of the interaction of the electrons with collective excitations. In contrast, in the pseudogap phase, we only observe dispersionless behavior for the autocorrelation peaks. The implications of our findings in regards to Fourier transformed scanning tunneling spectroscopy data are discussed.

cond-mat.supr-con↗

arXiv2006

Bulk and surface low-energy excitations in YBa2Cu3O7-d studied by high-resolution angle-resolved photoemission spectroscopy

We have performed high-resolution angle-resolved photoemission spectroscopy on YBa2Cu3O7-delta (Y123; delta = 0.06; Tc = 92 K). By accurately determining the Fermi surface and energy band dispersion, we solve long-standing controversial issues as to the anomalous electronic states of Y-based high-Tc cuprates. We unambiguously identified surface-bilayer-derived bonding and antibonding bands, together with their bulk counterparts. The surface bands are highly overdoped (hole concentration x = 0.29), showing no evidence for the gap opening or the dispersion anomaly in the antinodal region, while the bulk bands show a clear dx2-y2-wave superconducting gap and the Bogoliubov quasiparticle-like behavior with a characteristic energy scale of 50-60 meV indicative of a strong electron-boson coupling in the superconducting state. All these results suggest that the metallic and superconducting states coexist at the adjacent bilayer of Y123 surface.

cond-mat.supr-con↗

arXiv2006

Shadow bands in single-layered Bi_2Sr_2CuO_6 studied by angle-resolved photoemission spectroscopy

We have performed systematic angle-resolved photoemission spectroscopy (ARPES) on single-layered cuprate superconductor Bi2Sr2CuO6 to elucidate the origin of shadow band. We found that the shadow band is exactly the c(2x2) replica of the main band irrespective of the carrier concentration and its intensity is invariable with respect to temperature, doping, and substitution constituents of block layers. This result rules out the possibility of antiferromagnetic correlation and supports the structural origin of shadow band. ARPES experiments on optimally doped La1.85Sr0.15CuO4 also clarified the existence of the c(2x2) shadow band, demonstrating that the shadow band is not a unique feature of Bi-based cuprates. We conclude that the shadow band is related to the orthorhombic distortion at the crystal surface.

cond-mat.supr-con↗

arXiv2005

Non-dispersive Fermi arcs and absence of charge ordering in the pseudogap phase of Bi2Sr2CaCu2O8+d

The autocorrelation of angle resolved photoemission data from the high temperature superconductor Bi2Sr2CaCu2O8+d shows distinct peaks in momentum space which disperse with binding energy in the superconducting state, but not in the pseudogap phase. Although it is tempting to attribute a non-dispersive behavior in momentum space to some ordering phenomenon, a de-construction of the autocorrelation reveals that the non-dispersive peaks arise not from ordering, but rather from the tips of the Fermi arcs, which themselves do not change with binding energy.

cond-mat.supr-con↗