arXiv subjects

K. Horiba

Publications and source records attributed to K. Horiba.

At least 19 recordsLinked to original sources

arXiv2025

Interface-induced collective phase transition in VO2-based bilayers studied by layer selective spectroscopy

We investigated the origin of collective electronic phase transitions induced at the heterointerface between monoclinic insulating VO2 and rutile metallic electron-doped VO2 layers using in situ soft x-ray photoemission spectroscopy (PES) and x-ray absorption spectroscopy (XAS) on nanoscale VO2/V0.99W0.01O2 (001)R bilayers. Thanks to the surface sensitivity of PES and XAS, we determined the changes in the electronic structure and V-V dimerization in each constituent layer separately. The layer selective observation of the electronic and crystal structures in the upper VO2 layer of the bilayer indicates that the monoclinic insulating phase VO2 layer undergoes a transition to the rutile metallic phase by forming the heterointerface. Detailed temperature-dependent measurements reveal that the rutile metallic phase VO2 undergoes a transition to the monoclinic insulating phase with a decrease in temperature, as in the case of a VO2 single-layer film. Furthermore, during the temperature-induced phase transition in the VO2 layer, the spectra are well described by an in-plane phase separation of the rutile metallic and monoclinic insulating phases. These results suggest that the interface-induced transition from the monoclinic insulating to the rutile metallic phase in the VO2 layer of bilayers occurs as a collective phase transition derived from the static energy balance between the interfacial energy and the bulk free energies of the constituent layers.

cond-mat.str-el↗

arXiv2025

Electronic structure dimensionality of the quantum-critical ferromagnet YbNi$_4$P$_2$

YbNi$_4$P$_2$ is the first known ferromagnetic metal showing a second-order quantum phase transition. Current theoretical understanding rules out second order ferromagnetic quantum criticality in centrosymmetric 2D and 3D metals. Thus, studying the electronic structure of YbNi$_4$P$_2$ is of prime fundamental importance. Using angle-resolved photoemission spectroscopy, we experimentally prove the existence of 1D Fermi surface contours. In addition, our results demonstrate that part of the electronic structure of YbNi$_4$P$_2$ is made of states of higher dimensionality, thereby bringing into question the fact that ferromagnetic quantum criticality in centrosymmetric crystals, is exclusively found in 1D systems. Our experimental data show that the electronic structure of YbNi$_4$P$_2$ is a playground of mixed dimensionality, electron correlations, strong hybridization and spin-orbit coupling, all of them providing new insights in understanding the origin of ferromagnetic quantum criticality.

cond-mat.str-el↗

arXiv2025

Spin-degenerate bulk bands and topological surface states associated with Dirac nodal lines in RuO2

Altermagnets are a novel platform to realize exotic electromagnetic properties distinct from those of conventional ferromagnets and antiferromagnets. We report results of micro-focused angle-resolved photoemission spectroscopy (ARPES) on RuO2, in which its altermagnetic nature has been under fierce debate in connection with crystal-orientation-dependent spintronic functionalities. By elucidating the band structure of the (100), (110) and (101) surfaces of a bulk single crystal using micro-ARPES, we found that, irrespective of the surface orientation, the experimental band structures show a good agreement with the bulk-band calculations for the nonmagnetic phase, but display a severe disagreement with those for the antiferromagnetic phase. Moreover, spin-resolved ARPES signifies a negligible spin polarization in the bulk bands, suggesting the absence of antiferromagnetism and altermagnetic spin splitting. In addition, we identified a nearly flat surface band and a dispersive one near the Fermi level at the (100)/(110) and (101) surfaces, respectively. Our first-principles calculations and analysis of Berry phase attribute these states to the topological surface bands emerging from the bulk Dirac nodal lines around the Fermi level. Our results indicate that such topological surface/interface states must be considered to understand the spintronic functionalities of RuO2, and may provide new insights into its catalytic characteristics.

cond-mat.mtrl-sci↗

arXiv2023

Antiferromagnetic topological insulator with selectively gapped Dirac cones

Antiferromagnetic (AF) topological materials offer a fertile ground to explore a variety of quantum phenomena such as axion magnetoelectric dynamics and chiral Majorana fermions. To realize such intriguing states, it is essential to establish a direct link between electronic states and topology in the AF phase, whereas this has been challenging because of the lack of a suitable materials platform. Here we report the experimental realization of the AF topological-insulator phase in NdBi. By using micro-focused angle-resolved photoemission spectroscopy, we discovered contrasting surface electronic states for two types of AF domains; the surface having the out-of-plane component in the AF-ordering vector displays Dirac-cone states with a gigantic energy gap, whereas the surface parallel to the AF-ordering vector hosts gapless Dirac states despite the time-reversal-symmetry breaking. The present results establish an essential role of combined symmetry to protect massless Dirac fermions under the presence of AF order and widen opportunities to realize exotic phenomena utilizing AF topological materials.

cond-mat.mes-hall↗

arXiv2023

180$^\circ$-twisted bilayer ReSe$_2$ as an artificial noncentrosymmetric semiconductor

We have fabricated a 180$^\circ$-twisted bilayer ReSe$_2$ by stacking two centrosymmetric monolayer ReSe$_2$ flakes in opposite directions, which is expected to lose spatial inversion symmetry. By the second harmonic generation and angle-resolved photoemission spectroscopy, we successfully observed spatial inversion symmetry breaking and emergent band dispersions. The band calculation shows the finite lifting of spin degeneracy (~50 meV) distinct from natural monolayer and bilayer ReSe$_2$. Our results demonstrate that the spin-momentum locked state, which leads to spintronic functions and Berry-curvature-related phenomena, can be realized even with the stacking of centrosymmetric monolayers.

cond-mat.mes-hall↗

arXiv2023

Influence of oxygen-coordination number on the electronic structure of single-layer La-based cuprates

We present an angle-resolved photoemission spectroscopy study of the single-layer T*-type structured cuprate SmLa$_{1-x}$Sr$_x$CuO$_4$ with unique five-fold pyramidal oxygen coordination. Upon varying oxygen content, T*-SmLa$_{1-x}$Sr$_x$CuO$_4$ evolved from a Mott-insulating to a metallic state where the Luttinger sum rule breaks down under the assumption of a large hole-like Fermi surface. This is in contrast with the known doping evolution of the structural isomer La$_{2-x}$Sr$_x$CuO$_4$ with six-fold octahedral coordination. In addition, quantitatively characterized Fermi surface suggests that the empirical $T_\mathrm{c}$ rule for octahedral oxygen-coordination systems does not apply to T*-SmLa$_{1-x}$Sr$_x$CuO$_4$. The present results highlight unique properties of the T*-type cuprates possibly rooted in its oxygen coordination, and necessitate thorough investigation with careful evaluation of disorder effects.

cond-mat.str-el↗

arXiv2023

Oxygen on-site Coulomb energy in Pr$_{1.3-x}$La$_{0.7}$Ce$_x$CuO$_{4}$ and Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$ and its relation with Heisenberg exchange

We study the electronic structure of electron-doped Pr$_{1.3-x}$La$_{0.7}$Ce$_{x}$CuO$_{4}$ (PLCCO ; $T_{c}$ = 27 K, x = 0.1) and hole-doped Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$ (Bi2212 ; $T_{c}$ = 90 K) cuprate superconductors using x-ray absorption spectroscopy (XAS) and resonant photoemission spectroscopy (Res-PES). From Res-PES across the O K-edge and Cu L-edge, we identify the O 2p and Cu 3d partial density of states (PDOS) and their correlation satellites which originate in two-hole Auger final states. Using the Cini-Sawatzky method, analysis of the experimental O 2p PDOS shows an oxygen on-site Coulomb energy for PLCCO to be $U_{p}$ = 3.3$\pm$0.5 eV and for Bi2212, $U_{p}$ = 5.6$\pm$0.5 eV, while the copper on-site Coulomb correlation energy, $U_{d}$ = 6.5$\pm$0.5 eV for Bi2212. The expression for the Heisenberg exchange interaction $J$ in terms of the electronic parameters $U_{d}$, $U_{p}$, charge-transfer energy $Δ$ and Cu-O hopping $t_{pd}$ obtained from a simple Cu$_2$O cluster model is used to carry out an optimization analysis consistent with $J$ known from scattering experiments. The analysis also provides the effective one band on-site Coulomb correlation energy $\tilde{U}$ and the effective hopping $\tilde{t}$. PLCCO and Bi2212 are shown to exhibit very similar values of $\tilde{U}$/$\tilde{t}$ $\sim$9-10, confirming the strongly correlated nature of the singlet ground state in the effective one-band model for both the materials.

cond-mat.str-el↗

arXiv2022

Modulation of magnetic property of double-perovskite La2MnCoO6 films by controlling B-antisite disorder

The predicted physical properties of double perovskites have usually been compromised by their intrinsic B-antisite (AS) disorder effect, but the relationship between them is still unclear for the La2MnCoO6 (LMCO) system. This study focuses on controlling the AS disorder and quantitatively reveals correlations with magnetic and electronic states in epitaxial LMCO films grown on (111) SrTiO3 substrates. The AS fraction was precisely controlled by tuning the growth conditions as evaluated from saturation magnetization and x-ray reflection profiles. The saturation magnetization at 5 K decreased linearly from 6 uB per formula unit by 50% as the AS fraction increased from 0 (perfectly ordered) to 0.5 (fully disordered). The x-ray absorption spectroscopy revealed that valence states of Mn and Co were 4+ (3d^3) and 2+ (3d^7), respectively, regardless of AS fraction. A local-spin-moment model is proposed to explain the net magnetizations with and without AS disorder and Mn/Co valence states, suggesting that only the rearrangement of local spin moment of Co at the antisite plays a key role in tuning the magnetism in this LMCO system.

cond-mat.mtrl-sci↗

arXiv2022

Rhombic Fermi surfaces in a ferromagnetic MnGa thin film with perpendicular magnetic anisotropy

Mn$_{1-x}$Ga$_x$ (MnGa) with the $L1_0$ structure is a ferromagnetic material with strong perpendicular magneto-crystalline anisotropy. Although MnGa thin films have been successfully grown epitaxially and studied for various spintronics devices, fundamental understandings of its electronic structure are still lacking. To address this issue, we have investigated $L1_0$-MnGa thin films using angle-resolved photoemission spectroscopy (ARPES). We have observed a large Fermi surface with a rhombic shape in the $k_x$-$k_y$ plane overlapping neighboring Fermi surfaces. The $k_z$ dependence of the band structure suggests that the band dispersion observed by ARPES comes from the three-dimensional band structure of MnGa folded by a $\sqrt{2} \times \sqrt{2}$ reconstruction. The band dispersion across the corner of the rhombic Fermi surface forms an electron pocket with a weak $k_z$ dependence. The effective mass and the mobility of the bands crossing the Fermi level near the corner are estimated from the ARPES images. Based on the experimental findings, the relationship between the observed band structure and the spin-dependent properties in MnGa-based heterostructures is discussed.

cond-mat.mtrl-sci↗

arXiv2021

Superconducting gap and pseudogap in the surface states of the iron-based superconductor PrFeAsO$_{1-y}$ studied by angle-resolved photoemission spectroscopy

In order to study the possible superconductivity at the polar surfaces of 1111-type iron-based superconductors, which is doped with a large amount of holes in spite of the electron doping in bulk materials, we have performed angle-resolved photoemission spectroscopy (ARPES) studies on superconducting PrFeAsO$_{1-y}$~crystals. We have indeed observed the opening of a superconducting gap on surface-derived hole pockets as well as on a bulk-derived hole pocket. The superconducting gap is found to open on the surface-derived hole pockets below the bulk $T_c$, which suggests that the surface superconductivity is possibly induced by proximity effect from the bulk. We have also observed the opening of a large pseudogap on the surface-derived hole pockets, which is similar to the pseudogap in 122-type bulk superconductors doped with a smaller amount of holes. This suggests that the opening of a large pseudogap is a characteristic property of hole-doped iron-based superconductors.

cond-mat.supr-con↗

arXiv2021

Extended superconducting dome of electron-doped cuprates after protect annealing revealed by ARPES

The electron-doped cuprates are usually characterized by a more robust antiferromagnetic phase and a much narrower superconducting (SC) dome than those of the hole-doped counterparts. Recently, bulk single crystals of Pr1.3-xLa0.7CexCuO4-δ (PLCCO) prepared by the protect annealing method have been studied extensively and revealed many intriguing properties that were different from those obtained from samples annealed by the conventional methods. Here, we report on a systematic angle-resolved photoemission spectroscopy study of PLCCO single crystals after protect annealing. The results indicate that the actual electron concentration (nFS ) estimated from the Fermi-surface area is significantly larger than the Ce concentration x and the new nFS-based SC dome of PLCCO is more extended towards the overdoped side than the x-based SC dome derived for samples prepared using the conventional annealing method.

cond-mat.supr-con↗

arXiv2020

Electronic structure of a 3x3-ordered silicon layer on Al(111)

Electronic structure of the 3x3 ordered-phase of a silicon (Si) layer on Al(111) has been studied by angle resolved photoemission spectroscopy (ARPES) technique using synchrotron radiation and modeled by a trial atomic model. A closed Fermi surface originating from linearly dispersing band is identified. A band structure calculation of a trial atomic model of the honeycomb silicene on Al(111) implies that the metallic band originates from the Al-Si hybrid state that has the Dirac cone-like dispersion curves. The Si layer on Al(111) can be a model system of Xene to realize the massless electronic system through the overlayer-substrate interaction.

cond-mat.mtrl-sci↗

arXiv2020

Thickness dependence of electronic and crystal structures in VO$_2$ ultrathin films: suppression of the collaborative Mott-Peierls transition

Through ${in~situ}$ photoemission spectroscopy, we investigated the change in the electronic and crystal structures of dimensionality-controlled VO$_2$ films coherently grown on TiO$_2$(001) substrates. In the nanostructured films, the balance between the instabilities of a bandlike Peierls transition and a Mott transition is controlled as a function of thickness. The characteristic spectral change associated with temperature-driven metal-insulator transition in VO$_2$ thick films holds down to 1.5 nm (roughly corresponding to five V atoms along the [001] direction), whereas VO$_2$ films of less than 1.0 nm exhibit insulating nature without V-V dimerization. These results suggest that the delicate balance between a Mott instability and a bandlike Peierls instability is modulated at a scale of a few nanometers by the dimensional crossover effects and confinement effects, which consequently induce the complicated electronic phase diagram of ultrathin VO$_2$ films.

cond-mat.str-el↗

arXiv2020

Switching of band inversion and topological surface states by charge density wave

Topologically nontrivial materials host protected edge states associated with the bulk band inversion through the bulk-edge correspondence. Manipulating such edge states is highly desired for developing new functions and devices practically using their dissipation-less nature and spin-momentum locking. Here we introduce a transition-metal dichalcogenide VTe$_2$, that hosts a charge density wave (CDW) coupled with the band inversion involving V3$d$ and Te5$p$ orbitals. Spin- and angle-resolved photoemission spectroscopy with first-principles calculations reveal the huge anisotropic modification of the bulk electronic structure by the CDW formation, accompanying the selective disappearance of Dirac-type spin-polarized topological surface states that exist in the normal state. Thorough three dimensional investigation of bulk states indicates that the corresponding band inversion at the Brillouin zone boundary dissolves upon CDW formation, by transforming into anomalous flat bands. Our finding provides a new insight to the topological manipulation of matters by utilizing CDWs' flexible characters to external stimuli.

cond-mat.mtrl-sci↗

arXiv2020

Tunable two-dimensional electron system at the (110) surface of SnO$_2$

We report the observation of a two-dimensional electron system (2DES) at the $(110)$ surface of the transparent bulk insulator SnO$_2$, and the tunability of its carrier density by means of temperature or Eu deposition. The 2DES is insensitive to surface reconstructions and, surprisingly, it survives even after exposure to ambient conditions --an extraordinary fact recalling the well known catalytic properties SnO$_2$. Our data show that surface oxygen vacancies are at the origin of such 2DES, providing key information about the long-debated origin of $n$-type conductivity in SnO$_2$, at the basis of a wide range of applications. Furthermore, our study shows that the emergence of a 2DES in a given oxide depends on a delicate interplay between its crystal structure and the orbital character of its conduction band.

cond-mat.mtrl-sci↗

arXiv2019

Non-trivial surface states of samarium hexaboride at the (111) surface

The peculiar metallic electronic states observed in the Kondo insulator, samarium hexaboride (SmB$_6$), has stimulated considerable attention among those studying non-trivial electronic phenomena. However, experimental studies of these states have led to controversial conclusions mainly to the difficulty and inhomogeneity of the SmB$_6$ crystal surface. Here, we show the detailed electronic structure of SmB$_6$ with angle-resolved photoelectron spectroscopy measurements of the three-fold (111) surface where only two inequivalent time-reversal-invariant momenta (TRIM) exist. We observe the metallic two-dimensional state was dispersed across the bulk Kondo gap. Its helical in-plane spin polarisation around the surface TRIM suggests that SmB$_6$ is topologically non-trivial, according to the topological classification theory for weakly correlated systems. Based on these results, we propose a simple picture of the controversial topological classification of SmB$_6$.

cond-mat.str-el↗

arXiv2019

Evolution of electronic states and emergence of superconductivity in the polar semiconductor GeTe by doping valence-skipping In

GeTe is a chemically simple IV-VI semiconductor which bears a rich plethora of different physical properties induced by doping and external stimuli. These include, among others, ferromagnetism, ferroelectricity, phase-change memory functionality, and comparably large thermoelectric figure of merits. Here we report a superconductor - semiconductor - superconductor transition controlled by finely-tuned In doping. Our results moreover show the existence of a critical doping concentration around $x = 0.12$ in Ge$_{1-x}$In$_{x}$Te, where various properties take either an extremum or change their characters: The structure changes from polarly-rhombohedral to cubic, the resistivity sharply increases by orders of magnitude, the type of charge carriers changes from holes to electrons, and the density of states diminishes at the dawn of an emerging superconducting phase. By core-level photoemission spectroscopy we find indications of a change in the In-valence state from In$^{3+}$ to In$^{1+}$ with increasing $x$, suggesting that this system is a new promising playground to probe valence fluctuations and their possible impact on superconductivity.

cond-mat.supr-con↗

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↗