arXiv subjects

Y. Ohta

Publications and source records attributed to Y. Ohta.

At least 19 recordsLinked to original sources

arXiv2023

Zigzag chain order of LiVSe$_2$ developing away from the vanadium trimer phase transition boundary

The phenomenon of self-assembly of constituent elements to form molecules at low temperatures appears ubiquitously in transition metal compounds with orbital degrees of freedom. Recent progress in local structure studies using synchrotron radiation x-rays is shifting the interest in structural studies in such molecule-forming systems from the low-temperature ordered phase to the short-range order that appears like a precursor at high temperatures. In this study, we discuss both experimentally and theoretically the relationship between the trimer structure that appears in the layered LiV$X_2$ ($X$ = O, S, Se) system with a two-dimensional triangular lattice of vanadium and the zigzag chain-like local structure that appears near the phase transition boundary where molecular formation occurs. The vanadium trimerization that persistently appears in both low-temperature phases of LiVO$_2$ and LiVS$_2$ disappears in LiVSe$_2$, and a regular triangular lattice is thought to be realized in LiVSe$_2$, but this study reveals that the zigzag chain local distortion appears with a finite correlation length. This zigzag chain state local distortions are similar to the motif of local distortions in the high-temperature phase of LiVS$_2$, indicating that the local distortions are persistent away from the trimer phase transition boundary. On the other hand, it is concluded that the zigzag chain order appearing in LiVSe$_2$ is more stable than that in LiVS$_2$ in terms of the temperature variation of atomic displacement and correlation length. The zigzag chain order is considered to be competitive with the trimer order appearing in the LiV$X_2$ system. In this paper, we discuss the similarities and differences between the parameters that stabilize these electronic phases and the local distortions that appear in other molecular formation systems.

cond-mat.str-el↗

arXiv2020

Robust atomic orbital in the cluster magnet LiMoO2

In this study, we present a rutile-related material, LiMoO2, that becomes a cluster magnet and exhibits a spin singlet formation on a preformed molybdenum dimer upon cooling. Unlike ordinary cluster magnets, the atomic dyz orbital robustly survives despite the formation of molecular orbitals, thereby affecting the magnetic properties of the selected material. Such hybrid cluster magnets with the characters of molecular and atomic orbitals realize multiple independent spins on an isolated cluster, leading to an ideal platform to study the isolated spin dimers physics.

cond-mat.str-el↗

arXiv2018

Large entropy change derived from the orbitally assisted three-centered two-electron σ bond formation in a metallic Li0.33VS2

We discuss herein the emergence of a large entropy change in metallic Li0.33VS2 derived from the orbitally assisted loose σ bond formation. Comprehensive structural studies based on synchrotron x-ray and neutron diffraction analyses clarify the fabrication of ribbon chains at 375 K, consisting of multiple three-centered two-electron σ bonds based on the viewpoint of local chemical bonding. Although the metallic conductivity persists down to the lowest temperature measured, exceptionally large entropy change as a metal, as much as ΔS = 6.6 J/mol K, appears at the transition. Emergence of a large entropy change in a metallic state expects us the possible novel functional materials, such as a heat-storage material with rapid thermal response.

cond-mat.str-el↗

arXiv2018

Strong Coupling Nature of the Excitonic Insulator State in Ta$_2$NiSe$_5$

We analyze the measured optical conductivity spectra using the density-functional-theory-based electronic structure calculation and density-matrix renormalization group calculation of an effective model. We show that, in contrast to a conventional description, the Bose-Einstein condensation of preformed excitons occurs in Ta$_2$NiSe$_5$, despite the fact that a noninteracting band structure is a band-overlap semimetal rather than a small band-gap semiconductor. The system above the transition temperature is therefore not a semimetal, but rather a state of preformed excitons with a finite band gap. A novel insulator state caused by the strong electron-hole attraction is thus established in a real material.

cond-mat.str-el↗

arXiv2018

Ferromagnetic Peierls insulator state in $\mathit{A}$Mg$_4$Mn$_6$O$_{15}$ ($\mathit{A}$ = K, Rb, Cs)

Using the density-functional-theory based electronic structure calculations, we study the electronic state of recently discovered mixed-valent manganese oxides $A$Mg$_4$Mn$_6$O$_{15}$ ($A=$ K, Rb, Cs), which are fully spin-polarized ferromagnetic insulators with a cubic crystal structure. We show that the system may be described as a three-dimensional arrangement of the one-dimensional chains of a $2p$ orbital of O and a $3d$ orbital of Mn running along the three axes of the cubic lattice. We thereby argue that in the ground state the chains are fully spin polarized due to the double-exchange mechanism and are distorted by the Peierls mechanism to make the system insulating.

cond-mat.str-el↗

arXiv2017

Reversed Crystal-Field Splitting and Spin-Orbital Ordering in $α$-Sr$_2$CrO$_4$

The origin of successive phase transitions observed in the layered perovskite $α$-Sr$_2$CrO$_4$ is studied by the density-functional-theory-based electronic structure calculation and mean-field analysis of the proposed low-energy effective model. We find that, despite the fact that the CrO$_6$ octahedron is elongated along the $c$-axis of the crystal structure, the crystal-field level of nondegenerate $3d_{xy}$ orbitals of the Cr ion is lower in energy than that of doubly degenerate $3d_{yz}$ and $3d_{xz}$ orbitals, giving rise to the orbital degrees of freedom in the system with a $3d^2$ electron configuration. We show that the higher (lower) temperature phase transition is caused by the ordering of the orbital (spin) degrees of freedom.

cond-mat.str-el↗

arXiv2015

Novel Electronic Structures of Ru-Pnictides Ru$Pn$ ($Pn$ = P, As, Sb)

Density-functional-theory-based electronic structure calculations are made to consider the novel electronic states of Ru-pnictides RuP and RuAs where the intriguing phase transitions and superconductivity under doping of Rh have been reported. We find that there appear nearly degenerate flat bands just at the Fermi level in the high-temperature metallic phase of RuP and RuAs; the flat-band states come mainly from the $4d_{xy}$ orbitals of Ru ions and the Rh doping shifts the Fermi level just above the flat bands. The splitting of the flat bands caused by their electronic instability may then be responsible for the observed phase transition to the nonmagnetic insulating phase at low temperatures. We also find that the band structure calculated for RuSb resembles that of the doped RuP and RuAs, which is consistent with experiment where superconductivity occurs in RuSb without Rh doping.

cond-mat.str-el↗

arXiv2014

Excitonic Bose-Einstein condensation in Ta2NiSe5 above room temperature

We show that finite temperature variational cluster approximation (VCA) calculations on an extended Falicov-Kimball model can reproduce angle-resolved photoemission spectroscopy (ARPES) results on Ta2NiSe5 across a semiconductor-to-semiconductor structural phase transition at 325 K. We demonstrate that the characteristic temperature dependence of the flat-top valence band observed by ARPES is reproduced by the VCA calculation on the realistic model for an excitonic insulator only when the strong excitonic fluctuation is taken into account. The present calculations indicate that Ta2NiSe5 falls in the Bose-Einstein condensation regime of the excitonic insulator state.

cond-mat.str-el↗

arXiv2014

Important Roles of Te 5p and Ir 5d Spin-orbit Interactions on the Multi-band Electronic Structure of Triangular Lattice Superconductor Ir1-xPtxTe2

We report an angle-resolved photoemission spectroscopy (ARPES) study on a triangular lattice superconductor Ir$_{1-x}$Pt$_{x}$Te$_2$ in which the Ir-Ir or Te-Te bond formation, the band Jahn-Teller effect, and the spin-orbit interaction are cooperating and competing with one another. The Fermi surfaces of the substituted system are qualitatively similar to the band structure calculations for the undistorted IrTe$_2$ with an upward chemical potential shift due to electron doping. A combination of the ARPES and the band structure calculations indicates that the Te $5p$ spin-orbit interaction removes the $p_x/p_y$ orbital degeneracy and induces $p_x \pm ip_y$ type spin-orbit coupling near the A point. The inner and outer Fermi surfaces are entangled by the Te $5p$ and Ir $5d$ spin-orbit interactions which may provide exotic superconductivity with singlet-triplet mixing.

cond-mat.supr-con↗

arXiv2014

Order, criticality and excitations in the extended Falicov-Kimball model

Using exact numerical techniques we investigate the nature of excitonic (electron-hole) bound states and the development of exciton coherence in the one-dimensional half-filled extended Falicov-Kimball model. The ground-state phase diagram of the model exhibits, besides band insulator and staggered orbital ordered phases, an excitonic insulator (EI) with power-law correlations. The criticality of the EI state shows up in the von Neumann entropy. The anomalous spectral function and condensation amplitude provide the binding energy and coherence length of the electron-hole pairs which, on their part, point towards a Coulomb interaction driven crossover from BCS-like electron-hole pairing fluctuations to tightly bound excitons. We show that while a mass imbalance between electrons and holes does not affect the location of the BCS-BEC crossover regime it favors staggered orbital ordering to the disadvantage of the EI. Within the BEC regime the quasiparticle dispersion develops a flat valence-band top in accord with the experimental finding for Ta$_2$NiSe$_5$.

cond-mat.str-el↗

arXiv2013

Te 5p orbitals bring three-dimensional electronic structure to two-dimensional Ir0.95Pt0.05Te2

We have studied the nature of the three-dimensional multi-band electronic structure in the twodimensional triangular lattice Ir1-xPtxTe2 (x=0.05) superconductor using angle-resolved photoemission spectroscopy (ARPES), x-ray photoemission spectroscopy (XPS) and band structure calculation. ARPES results clearly show a cylindrical (almost two-dimensional) Fermi surface around the zone center. Near the zone boundary, the cylindrical Fermi surface is truncated into several pieces in a complicated manner with strong three-dimensionality. The XPS result and the band structure calculation indicate that the strong Te 5p-Te 5p hybridization between the IrTe2 triangular lattice layers is responsible for the three-dimensionality of the Fermi surfaces and the intervening of the Fermi surfaces observed by ARPES.

cond-mat.supr-con↗

arXiv2013

Exact-diagonalization study of exciton condensation in electron bilayers

We report on small-cluster exact-diagonalization calculations which prove the formation of electron-hole pairs (excitons) as prerequisite for spontaneous interlayer phase coherence in bilayer systems described by the extended Falicov-Kimball model. Evaluating the anomalous Green's function and momentum distribution function of the pairs, and thereby analyzing the dependence of the exciton binding energy, condensation amplitude, and coherence length on the Coulomb interaction strength, we demonstrate a crossover between a BCS-like electron-hole pairing transition and a Bose-Einstein condensation of tightly bound preformed excitons. We furthermore show that a mass imbalance between electrons and holes tends to suppress the condensation of excitons.

cond-mat.str-el↗

arXiv2013

Antiferromagnetic topological insulator state in the correlated Bernevig-Hughes-Zhang model

We study the effects of electron correlations on the topological phase transition in the Bernevig-Hughes-Zhang model using the variational cluster approach where the short-range spatial correlations are taken into account exactly. We calculate the spin Chern number and local magnetic moment to show that the topologically nontrivial antiferromagnetic order exists and that the magnetic transition is of the second order. We furthermore demonstrate that under the spin-quantized condition the topological phase transition is caused by the closing of the bulk band gap.

cond-mat.str-el↗

arXiv2013

Magnetic properties and Mott transition in the square-lattice Hubbard model with frustration

The magnetic properties and Mott transition of the Hubbard model on the square lattice with frustration are studied at half-filling and zero temperature by the variational cluster approximation. When the on-site repulsion $U$ is large, magnetically disordered state is realized in highly frustrated region between the Néel and collinear phases, and no imcommensurate magnetic states are found there. As for the Mott transition, in addition to the Mott gap and double occupancy, which clarify the nature of the transition, the structure of the self-energy in the spectral representation is studied in detail below and above the Mott transition point. The spectral structure of the self-energy is almost featureless in the metallic phase, but clear single dispersion, leading to the Mott gap, appears in the Mott insulator phase.

cond-mat.str-el↗

arXiv2013

Mott transition and ferrimagnetism in the Hubbard model on the anisotropic kagomé lattice

Mott transition and ferrimagnetism are studied in the Hubbard model on the anisotropic kagomé lattice using the variational cluster approximation and the phase diagram at zero temperature and half-filling is analyzed. The ferrimagnetic phase rapidly grows as the geometric frustration is relaxed, and the Mott insulator phase disappears in moderately frustrated region, showing that the ferrimagnetic fluctuations stemming from the relaxation of the geometric frustration is enhanced by the electron correlations. In metallic phase, heavy fermion behavior is observed and mass enhancement factor is computed. Enhancement of effective spatial anisotropy by the electron correlations is also confirmed in moderately frustrated region, and its effect on heavy fermion behavior is examined.

cond-mat.str-el↗

arXiv2012

Orthorhombic-to-Monoclinic Phase Transition of Ta2NiSe5 Induced by the Bose-Einstein Condensation of Excitons

Using the band structure calculation and mean-field analysis of the derived three-chain Hubbard model with phonon degrees of freedom, we discuss the origin of the orthorhombic-to-monoclinic phase transition of the layered chalcogenide Ta$_2$NiSe$_5$. We show that the Bose-Einstein condensation of excitonic electron-hole pairs cooperatively induces the instability of the phonon mode at momentum $q\rightarrow 0$ in the quasi-one-dimensional Ta-NiSe-Ta chain, resulting in the structural phase transition of the system. The calculated single-particle spectra reproduce the deformation of the band structure observed in the angle-resolved photoemission spectroscopy experiment.

cond-mat.str-el↗

arXiv2012

Quantum phase transitions in the honeycomb-lattice Hubbard model

Quantum phase transitions in the Hubbard model on the honeycomb lattice are investigated in the variational cluster approximation. The critical interaction for the paramagnetic to antiferromagnetic phase transition is found to be in remarkable agreement with a recent large-scale quantum Monte Carlo simulation. Calculated staggered magnetization increases continuously with $U$ and thus we find the phase transition is of a second order. We also find that the semimetal-insulator transition occurs at infinitesimally small interaction and thus a paramagnetic insulating state appears in a wide interaction range. A crossover behavior of electrons from itinerant to localized character found in the calculated single-particle excitation spectra and short-range spin correlation functions indicates that an effective spin model for the paramagnetic insulating phase is far from a simple Heisenberg model with a nearest-neighbor exchange interaction.

cond-mat.str-el↗

arXiv2012

Double Exchange Ferromagnetism in the Peierls Insulator State

We study the effects of opening of the band gap on the double exchange ferromagnetism. Applying the density-matrix renormalization group method and an analytical expansion from the dimer limit to the one-dimensional double exchange model, we demonstrate for a relevant region of the exchange coupling that, in the weak dimerization regime, the Peierls gap opens in the fully spin-polarized conduction band without affecting its ferromagnetism, whereas in the strong dimerization regime, the ferromagnetism is destroyed and the Mott gap opens instead, leading the system to the antiferromagnetic quasi-long-range order. An insulator version of the double exchange ferromagnetism is thus established.

cond-mat.str-el↗