arXiv subjects

Takayoshi Yokoya

Publications and source records attributed to Takayoshi Yokoya.

15 recordsLinked to original sources

arXiv2025

Spectroscopic Evidence for Electron-Boson Coupling in Half-metallic CrO2

We report quasiparticle properties of the half-metal ferromagnet CrO2 by means of high-resolution angle-resolved photoemission spectroscopy (ARPES). We clearly observed the Fermi surface (FS) and band dispersion in good agreement with the previous reports. Moreover, the ARPES band dispersion reveals a distinct kink structure around 68 meV, providing the first spectroscopic evidence for the elementary excitations in CrO2. The energy scale of this feature is comparable to the Debye temperature and the $A\subm{1g}$ phonon mode, suggesting the electron-phonon interaction. From the detailed analysis, we have extracted the self-energy and found two characteristic structures in the real part of the self-energy. Assuming the existence of the electron-magnon interaction as well as the electron-phonon interaction, we could reproduce the evaluated real and imaginary parts of the self-energy as well as ARPES intensity. Our findings reveal the renormalized quasiparticle (QP) dynamics in CrO$_2$ and provide valuable insights into the fundamental many-body interactions governing half-metallic ferromagnets.

cond-mat.str-el↗

arXiv2023

Anomalously large spin-dependent electron correlation in nearly half-metallic ferromagnet CoS$_2$

The spin-dependent band structure of CoS$_2$ which is a candidate for a half-metallic ferromagnet was investigated by both spin- and angle-resolved photoemission spectroscopy and theoretical calculations, in order to reappraise the half-metallicity and electronic correlations. We determined the three-dimensional Fermi surface and the spin-dependent band structure. As a result, we found that a part of the minority spin bands is on the occupied side in the vicinity of the Fermi level, providing spectroscopic evidence that CoS$_2$ is not but very close to a half-metal. Band calculations using density functional theory with generalized gradient approximation showed a good agreement with the observed majority spin $e_g$ bands, while it could not explain the observed band width of the minority-spin eg bands. On the other hand, theoretical calculations using dynamical mean field theory could better reproduce the strong mass renormalization in the minority-spin $e_g$ bands. All those results strongly suggest the presence of anomalously enhanced spin-dependent electron correlation effects on the electronic structure in the vicinity of the half-metallic state. We also report the temperature dependence of the electronic structure across the Curie temperature and discuss the mechanism of the thermal demagnetization. Our discovery of the anomalously large spin-dependent electronic correlations not only demonstrates a key factor in understanding the electronic structure of half-metals but also provides a motivation to improve theoretical calculations on spin-polarized strongly correlated systems.

cond-mat.str-el↗

arXiv2022

Weak antilocalization induced by Se substitution in layered BiCh$_2$-based (Ch = S, Se) superconductors LaO$_{1-x}$F$_x$BiS$_{2-y}$Se$_y$

We report transport properties for layered BiCh2-based (Ch = S, Se) superconductors LaO1-xFxBiS2-ySey (x = 0.2, 0.5, y = 0-1.05) and the observation of weak antilocalization (WAL). Electrical resistivity and Hall coefficients for the Se-poor samples increase with decreasing temperature. The increase becomes less pronounced with increasing Se concentration indicating a loss of insulating behavior. Interestingly, the moderately Se-substituted samples exhibit metallic behavior in the high-temperature region and a weak increase in the resistivity in the low-temperature regions, which indicates the existence of carrier localization. The heavily Se-substituted compounds show metallic behavior in the entire-temperature region. Sign changes of the Hall coefficients are observed for the x = 0.2 samples, which possibly is related to a charge-density wave (CDW). Magnetoresistance measurements indicate that WAL is realized in the heavily Se-substituted systems. The WAL behavior is weakened by the changes in F and Se concentrations. A crossover state of the WAL and WL emerges around the moderately F-doped and Se-free LaO0.8F0.2BiS2. The change of the resistivity behavior by the F and Se substitution clearly correlates to the difference of the magnetoconductance. Moreover, the localization regions of the WAL-WL crossover and weak WAL states are possibly associated with the CDW. We propose that the BiCh2-based system is a good platform for studying relationship between WAL, superconductivity, and electronic ordering because those states are tunable by element substitutions with bulk single crystals.

cond-mat.supr-con↗

arXiv2020

Soft x-ray irradiation induced metallization of layered TiNCl

We have performed soft x-ray spectroscopy in order to study the photoirradiation time dependence of the valence band structure and chemical states of layered transition metal nitride chloride TiNCl. Under the soft x-ray irradiation, the intensities of the states near the Fermi level (EF) and the Ti3+ component increased, while the Cl 2p intensity decreased. Ti 2p-3d resonance photoemission spectroscopy confirmed a distinctive Fermi edge with Ti 3d character. These results indicate the photo-induced metallization originates from deintercalation due to Cl desorption, and thus provide a new carrier doping method that controls the conducting properties of TiNCl.

cond-mat.mtrl-sci↗

arXiv2019

μ-PES Studies on TiNCl and Quasi-two-dimensional Superconductor Na-intercalated TiNCl

Electron-doped TiNCl is a superconductor, for which exotic mechanisms of the superconductivity have been discussed. However, difficulty in preparing large single-phase samples has prevented the direct observation of its electronic structure and how that changes with electron doping. In this study, micro-photoemission spectroscopy (μ-PES) was used to reveal the electronic structures of TiNCl and Na-intercalated TiNCl (Na-TiNCl). Comparison of Ti 2p core-level spectra shows the enhancement of a spectral feature in Na-TiNCl that suggests an increase in its metallic character. This indicates the introduction of electron carriers to the TiNCl layers. Although the overall valence band electronic structure of parent TiNCl could be reproduced by first-principle calculations, that of Na-TiNCl showed a marked deviation from the rigid band model near the Fermi level (EF). The spectral shape observed near EF of Na-TiNCl was found to be similar to a result in early transition metal oxides that has been attributed to the effect of strong electron correlation. The present study also demonstrated the usefulness of μ-PES to obtain reliable electronic structure data for samples that are difficult to make at large sizes.

cond-mat.supr-con↗

arXiv2017

Observation of Intrinsic Half-metallic Behavior of CrO$_2$ (100) Epitaxial Films by Bulk-sensitive Spin-resolved PES

We have investigated the electronic states and spin polarization of half-metallic ferromagnet CrO$_2$ (100) epitaxial films by bulk-sensitive spin-resolved photoemission spectroscopy with a focus on non-quasiparticle (NQP) states derived from electron-magnon interactions. We found that the averaged values of the spin polarization are approximately 100% and 40% at 40 K and 300 K, respectively. This is consistent with the previously reported result [H. Fujiwara et al., Appl. Phys. Lett. 106, 202404 (2015).]. At 100 K, peculiar spin depolarization was observed at the Fermi level ($E_{F}$), which is supported by theoretical calculations predicting NQP states. This suggests the possible appearance of NQP states in CrO$_2$. We also compare the temperature dependence of our spin polarizations with that of the magnetization.

cond-mat.str-el↗

arXiv2017

Bent Electronic Band Structure Induced by Ferroelectric Polarization

Bent band structures have been empirically described in ferroelectric materials to explain the functioning of recently developed ferroelectric tunneling junction and photovoltaic devices. This report presents experimental evidence for ferroelectric band bending, which was observed in the depth profiles of atomic orbitals of angle-resolved hard x-ray photoemission spectra of ferroelectric BaTiO3 thin films. The ferroelectric bent band structure is separated into three depth regions; the shallowest and deepest regions are slightly modulated by the screening effect at surface and interface, respectively, and the intermediate region exhibits the pure ferroelectric effect. In the pure ferroelectric bent band structure, we found that the binding energy of outer shell electrons shows a larger shift than that of inner shell electrons, and that the difference in energy shift is correlated with the atomic configuration of the soft phonon mode. These findings could lead to a simple understanding of the origin of electric polarization.

cond-mat.mtrl-sci↗

arXiv2016

Observation of a Hidden Hole-Like Band Approaching the Fermi Level in K-Doped Iron Selenide Superconductor

One of the ultimate goals of the study of iron-based superconductors is to identify the common feature that produces the high critical temperature (Tc). In the early days, based on a weak-coupling viewpoint, the nesting between hole- and electron-like Fermi surfaces (FSs) leading to the so-called $s\pm$ state was considered to be one such key feature. However, this theory has faced a serious challenge ever since the discovery of alkali-metal-doped FeSe (AFS) superconductors, in which only electron-like FSs with a nodeless superconducting gap are observed. Several theories have been proposed, but a consistent understanding is yet to be achieved. Here we show experimentally that a hole-like band exists in KxFe2-ySe2, which presumably forms a hole-like Fermi surface. The present study suggests that AFS can be categorized in the same group as iron arsenides with both hole- and electron-like FSs present. This result provides a foundation for a comprehensive understanding of the superconductivity in iron-based superconductors.

cond-mat.supr-con↗

arXiv2015

Intrinsic spin polarized electronic structure of CrO2 epitaxial film revealed by bulk-sensitive spin-resolved photoemission spectroscopy

We have performed bulk-sensitive spin-resolved photoemission spectroscopy in order to clarify the intrinsic spin-resolved electronic states of half-metallic ferromagnet CrO2. We used CrO2 epitaxial films on TiO2(100), which shows a peak at 1 eV with a clear Fermi edge, consistent with the bulk-sensitive PES spectrum for CrO2. In spin-resolved spectra at 40 K, while the Fermi edge was observed in the spin up (majority spin) state, no states at the Fermi level (EF) with an energy gap of 0.5 eV below EF was observed in the spin down (minority spin) state. At 300 K, the gap in the spin down state closes. These results are consistent with resistivity measurements and magnetic hysteresis curves of the fabricated CrO2 film, constituting spectroscopic evidence for the half-metallicity of CrO2 at low temperature and reducing the spin polarization at room temperature. We also discuss the electron correlation effects of Cr 3d.

cond-mat.mtrl-sci↗

arXiv2014

Signature of high Tc around 25K in higher quality heavily boron-doped diamond

Diamond has outstanding physical properties: the hardest known material, a wide band gap, the highest thermal conductivity, and a very high Debye temperature. In 2004, Ekimov et al. discovered that heavily boron-doped (B-doped) diamond becomes a superconductor around 4 K. Our group successfully controlled the boron concentration and synthesized homoepitaxially grown superconducting diamond films by a CVD method. By CVD method, we found that superconductivity appears when the boron concentration (nB) exceeds a metal-insulator transition concentration of 3.0x10^20 cm^-3 and its Tczero increases up to 7.4 K with increasing nB. We additionally elucidated that the holes formed at the valence band are responsible for the metallic states leading to superconductivity. The calculations predicted that the hole doping into the valence band induces strong attractive interaction and a rapid increase in Tc with increasing boron concentration. According to the calculations, if substitutional doped boron could be arranged periodically or the degree of disorder is reduced, a Tc of approximately 100 K could be achieved via minimal percent doping. In this work, we have successfully observed zero resistivity above 10 K and an onset of resistivity reduction at 25.2 K in heavily B-doped diamond film. However, the effective carrier concentration is similar to that of superconducting diamond with a lower Tc. We found that the carrier has a longer mean free path and lifetime than previously reported, indicating that this highest Tc diamond has better crystallinity compared to that of other superconducting diamond films. In addition, the susceptibility shows a small transition above 20 K in the high quality diamond, suggesting a signature of superconductivity above 20 K. These results strongly suggest that heavier carrier doped defect-free crystalline diamond could give rise to high Tc diamond.

cond-mat.mtrl-sci↗

arXiv2014

Electronic Structures of CeM2Al10 (M = Fe, Ru, and Os) Studied by Soft X-ray Resonant and High-Resolution Photoemission Spectroscopies

We have performed a photoemission spectroscopy (PES) study of CeM2Al10 (M = Fe, Ru, and Os) to directly observe the electronic structure involved in the unusual magnetic ordering. Soft X-ray resonant (SXR) PES provides spectroscopic evidence of the hybridization between conduction and Ce 4f electrons (c-f hybridization) and the order of the hybridization strength (Ru < Os < Fe). High-resolution (HR) PES of CeRu2Al10 and CeOs2Al10, as compared with that of CeFe2Al10, identifies two structures that can be ascribed to structures induced by the c-f hybridization and the antiferromagnetic ordering, respectively. Although the c-f hybridization-induced structure is a depletion of the spectral intensity (pseudogap) around the Fermi level (EF) with an energy scale of 20-30 meV, the structure related to the antiferromagnetic ordering is observed as a shoulder at approximately 10-11 meV within the pseudogap. The energies of the shoulder structures of CeRu2Al10 and CeOs2Al10 are approximately half of the optical gap (20 meV), indicating that EF is located at the midpoint of the gap.

cond-mat.str-el↗

arXiv2014

Soft x-ray photoemission study of new BiS$_{2}$-layered superconductor LaO$_{1-x}$F$_{x}$BiS$_{2}$

We use core level and valence band soft x-ray photoemission spectroscopy (SXPES) to investigate electronic structure of new BiS$_{2}$ layered superconductor LaO$_{1-x}$F$_{x}$BiS$_{2}$. Core level spectra of doped samples show a new spectral feature at the lower binding energy side of the Bi 4${f}$ main peak, which may be explained by core-hole screening with metallic states near the Fermi level ($E_{\rm F}$). Experimental electronic structure and its ${x}$ dependence (higher binding energy shift of the valence band as well as appearance of new states near $E_{\rm F}$ having dominant Bi 6${p}$ character) were found to be consistent with the predictions of band structure calculations in general. Noticeable deviation of the spectral shape of the states near $E_{\rm F}$ from that of calculations might give insight into the interesting physical properties. These results provide first experimental electronic structure of the new BiS$_{2}$ layered superconductors.

cond-mat.supr-con↗

arXiv2013

Magnetoelectric Effect driven by Magnetic Domain Modification in LuFe2O4

Magneto-capacitance effect was investigated using the impedance spectroscopy on single crystals of LuFe2O4. The intrinsic impedance response could be separated from the interfacial response and showed a clear hysteresis loop below TFerri ~ 240 K under the magnetic field. The neutron diffraction experiment under the magnetic field proves the origin of dielectric property related to the motion of nano-sized ferromagnetic domain boundary. These results imply that the modification of the microscopic domain structure is responsible for the magnetoelectric effect in LuFe2O4.

cond-mat.str-el↗

arXiv2008

Electronic Structure of Superconducting FeSe Studied by High-Resolution Photoemission Spectroscopy

We have performed soft x-ray and ultrahigh-resolution laser-excited photoemission measurements on tetragonal FeSe, which was recently identified as a superconductor. Energy dependent study of valence band is compared to band structure calculations and yields a reasonable assignment of partial densities of states. However, the sharp peak near the Fermi level slightly deviates from the calculated energy position, giving rise to the necessity of self-energy correction. We have also performed ultrahigh-resolution laser photoemission experiment on FeSe and observed the suppression of intensity around the Fermi level upon cooling.

cond-mat.supr-con↗

arXiv2008

Electronic Structure of Noncentrosymmetric Superconductor Li$_2$(Pd$_x$Pt$_{1-x}$3)B Studied by Photoemission Spectroscopy

We have performed x-ray photoemission spectroscopy on the system of noncentrosymmetric superconductor, Li$_2$(Pd$_x$Pt$_{1-x}$3)B. For Li$_2$Pt$_3$B, we found 2 major peaks with 2 other weak components, and the band calculations were in agreement with the observation. The assignment of valence band features using the calculated partial density of states determined that Pt 5d and B 2p contribute to the density of states at the Fermi level. The effect of antisymmetric spin-orbit coupling on the band structure might have been probed, and the analysis on the effect of Pt incorporation into the system indicates the smooth evolution of electronic structures. We presented the measurements of core levels (Pd 3d, Pt 4f, and B 1s) and discussed the chemical bonding states and electronic structures from them.

cond-mat.supr-con↗