arXiv subjects

Hitoshi Seo

Publications and source records attributed to Hitoshi Seo.

At least 37 records · Page 2Linked to original sources

arXiv2019

Spin current generation in organic antiferromagnets

Spin current--a flow of electron spins without a charge current--is an ideal information carrier free from Joule heating for electronic devices. The celebrated spin Hall effect, which arises from the relativistic spin-orbit coupling, enables us to generate and detect spin currents in inorganic materials and semiconductors, taking advantage of their constituent heavy atoms. In contrast, organic materials consisting of molecules with light elements have been believed to be unsuited for spin current generation. Here we show that a class of organic antiferromagnets with checker-plate type molecular arrangements can serve as a spin current generator by applying a thermal gradient or an electric field, even with vanishing spin-orbit coupling. Our findings provide another route to create a spin current distinct from the conventional spin Hall effect and open a new field of spintronics based on organic magnets having advantages of small spin scattering and long lifetime.

cond-mat.str-el↗

arXiv2019

Photoinduced collective mode, inhomogeneity, and melting in a charge order system

We theoretically investigate photoresponses of a correlated electron system upon stimuli of a pulsed laser light. Real-time dynamics of an interacting spinless fermion model on a one-dimensional chain, as a model of charge order (CO), are numerically simulated using the time-dependent Hartree-Fock method. In particular, we discuss the differences between two situations as the initial state:the homogeneous order and the presence of a domain wall, i.e., a kink structure embedded in the CO bulk. Coherent dynamics are seen in the former case: When the frequency of the pump light $ω_{p}$ is varied, along with single particle excitations across the CO gap ($Δ_\textrm{CO}$), the resonantly-excited collective phase mode near $ω_{p} \simeq Δ_\textrm{CO}/2$ efficiently destabilizes CO. In clear contrast, in the latter case, when $ω_{p}$ is tuned at such in-gap frequencies and the intensity of light is sufficiently large,inhomogeneity spreads out from the kink to the bulk region through kink creations. Moreover, even stronger intensity induces the inhomogeneous melting of CO where the CO gap is destroyed.

cond-mat.str-el↗

arXiv2018

Electronic Structure Calculation and Superconductivity in $λ$-(BETS)$_{2}$GaCl$_{4}$

Quasi-two-dimensional molecular conductor $λ$-(BETS)$_2$GaCl$_4$ shows superconductivity (SC) below 5.5K, neighboring the dimer-type Mott insulating phase. To elucidate the origin of SC and its gap function, we carry out first-principles band calculation and derive a four-band tight-binding model from the maximally localized Wannier orbitals. Considering the spin-fluctuation-mediated mechanism by adding the Hubbard $U$-term to the model, we analyze the SC gap function by applying the random phase approximation. We show that the SC gap changes its sign four times along the Fermi surface (FS) in the unfolded Brillouin zone, suggestive of a $d$-wave-like SC gap, which only has two-fold symmetry because of the low symmetry of the crystal structure. Decomposing the SC gap into the pairing functions along the crystal axes, we compare the result to similar analysis of the well-studied $κ$-type molecular conductors and to the experiments.

cond-mat.supr-con↗

arXiv2017

Phase Competition and Superconductivity in $κ$-(BEDT-TTF)$_2$X: Importance of Intermolecular Coulomb Interactions

We theoretically study the competition among different electronic phases in molecular conductors $κ$-(BEDT-TTF)$_2$X. The ground-state properties of a 3/4-filled extended Hubbard model with the $κ$-type geometry are investigated by a variational Monte Carlo method. We find various competing phases: dimer-Mott insulator, polar charge-ordered insulator, 3-fold charge-ordered metal, and superconductivity, whose pairing symmetry is an "extended-$s$+$d_{x^2-y^2}$"-wave type. Our results show that the superconducting phase is stabilized not on the verge of the Mott metal-insulator transition but near charge order instabilities, clearly indicating the importance of the intradimer charge degree of freedom and the intermolecular Coulomb interactions, beyond the simple description of the half-filled Hubbard model.

cond-mat.supr-con↗

arXiv2016

Phase Competition, Solitons, and Domain Walls in Neutral-Ionic Transition Systems

Phase competition and excitations in the one-dimensional neutral-ionic transition systems are theoretically studied comprehensively. From the semiclassical treatment of the bosonized Hamiltonian, we examine the competition among the neutral (N), ferroelectric-ionic (I$_\mathrm{ferro}$) and paraelectric-ionic (I$_\mathrm{para}$) states. The phase transitions between them can become first-order when the fluctuation-induced higher-order commensurability potential is considered. In particular, the description of the first-order phase boundary between N and I$_\mathrm{ferro}$ enables us to analyze N-I$_\mathrm{ferro}$ domain walls. Soliton excitations in the three phases are described explicitly and their formation energies are evaluated across the phase boundaries. The characters of the soliton and domain-wall excitations are classified in terms of the topological charge and spin. The relevance to the experimental observations in the molecular neutral-ionic transition systems is discussed. We ascribe the pressure-induced crossover in tetrathiafulvalene-$p$-chloranil (TTF-CA) at a high-temperature region to that from the N to the I$_\mathrm{para}$ state, and discuss its consequence.

cond-mat.str-el↗

arXiv2015

Theory of Valence Transition in BiNiO$_3$

Motivated by the colossal negative thermal expansion recently found in BiNiO$_3$, the valence transition accompanied by the charge transfer between the Bi and Ni sites is theoretically studied. We introduce an effective model for Bi-$6s$ and Ni-$3d$ orbitals with taking into account the valence skipping of Bi cations, and investigate the ground-state and finite-temperature phase diagrams within the mean-field approximation. We find that the valence transition is caused by commensurate locking of the electron filling in each orbital associated with charge and magnetic orderings, and the critical temperature and the nature of the transitions are strongly affected by the relative energy between the Bi and Ni levels and the effective electron-electron interaction in the Bi sites. The obtained phase diagram well explains the temperature- and pressure-driven valence transitions in BiNiO$_3$ and the systematic variation of valence states for a series of Bi and Pb perovskite oxides.

cond-mat.str-el↗

arXiv2015

Photo-Induced Phase Transition in Charge Order Systems --Charge Frustration and Interplay with Lattice--

Lattice effects on photo-excited states in interacting charge frustrated system are examined. Real time dynamics in the interacting spinless fermion model on a triangular lattice coupled to lattice vibration are analyzed by applying the exact diagonalization method combined with the classical equation of motion. A photo-induced phase transition from the horizontal stripe-type charge order (CO) to the 3-fold CO occurs through a characteristic intermediate time domain. By analyzing the time evolution in detail, we find that this characteristic dynamics are seen when the electron and lattice sectors are not complementary to each other but show cooperative time evolutions. We also find that there are threshold values in the optical fluorescence and electron-lattice coupling for emergence of the photo-induced 3-fold CO. The dynamics are distinct from those from the vertical stripe-type CO, in which a monotonic CO melting occurs. A scenario of the photo-induced CO phase transition with lattice degree of freedom is presented from a view point of charge frustration.

cond-mat.str-el↗

arXiv2015

First-principles study of hydrogen-bonded molecular conductor $κ$-H$_3$(Cat-EDT-TTF/ST)$_2$

We theoretically study hydrogen-bonded molecular conductors synthesized recently, $κ$-H$_3$(Cat-EDT-TTF)$_2$ and its diselena analog, $κ$-H$_3$(Cat-EDT-ST)$_2$, by first-principles density-functional theory calculations. In these crystals, two H(Cat-EDT-TTF/ST) units share a hydrogen atom with a short O--H--O hydrogen bond. The calculated band structure near the Fermi level shows a quasi-two-dimensional character, with a rather large interlayer dispersion due to the absence of insulating layers in contrast with conventional molecular conductors. We discuss effective low-energy models based on H(Cat-EDT-TTF/ST) units and its dimers, respectively, where the microscopic character of the orbitals composing them are analyzed. Furthermore, we find a stable structure which is different from the experimentally determined structure, where the shared hydrogen atom becomes localized to one of the oxygen atoms, in which charge disproportionation between the two types of H(Cat-EDT-TTF) units is associated. The calculated potential energy surface for the H atom is very shallow near the minimum points, therefore the probability of the H atom can be delocalized between the two O atoms.

cond-mat.str-el↗

arXiv2015

Fragment Model Study of Molecular Multi-Orbital System $X$[Pd(dmit)$_2$]$_2$

Electronic properties of quasi-two-dimensional molecular conductors $X$[Pd(dmit)$_2$]$_2$ are studied theoretically. We construct an effective model based on the fragment molecular orbital scheme developed recently, which can describe the multi-orbital degree of freedom in this system. The tight-binding parameters for a series of $β'$-type compounds with different cations $X$ are evaluated by fitting to first-principles band calculations. We find that the transfer integrals within the dimers of Pd(dmit)$_2$ molecules, along the intramolecular and intermolecular bonds including the diagonal ones, are the same order, leading to hybridization between different molecular orbitals. This results in charge disproportionation within each molecule, as seen in our previous ab initio study [T. Tsumuraya et al, J. Phys. Soc. Jpn. 82, 033709 (2013)], and also to a revised picture of an effective dimer model. Furthermore, we discuss broken-symmetry insulating states triggered by interaction effects, which show characteristic features owing to the multi-orbital nature. The on-site Coulomb interaction induces antiferromagnetic states with intramolecular antiparallel spin pattern, while electron-lattice couplings stabilize non-magnetic charge-lattice ordered states where two kinds of dimers with different charge occupation arrange periodically. These states showing different spatial patterns compete with each other as well as with the paramagnetic metallic state.

cond-mat.str-el↗

arXiv2014

Photo-Induced Dynamics in Charge-Frustrated Systems

Photo-excited charge dynamics of interacting charge-frustrated systems are studied using a spinless fermion model on an anisotropic triangular lattice. Real-time evolution of the system after irradiating a pump-photon pulse is analyzed by the exact diagonalization method. We focus on photo-excited states in the two canonical charge-ordered (CO) ground states, i.e., horizontal stripe-type and vertical stripe-type COs, which compete with each other owing to the charge frustration. We find that the photo-induced excited states from the two types of COs are distinct. From the horizontal stripe-type CO, a transition to another CO state called the three-fold CO phase occurs. In sharp contrast, the vertical stripe-type CO phase is only weakened by photo-irradiation. Our observations are attributable to the charge frustration effects occurring in the photo-excited states.

cond-mat.str-el↗

arXiv2014

Magnetic Field Effect in One-Dimensional Charge Ordering Systems

We study effects of an external magnetic field on charge ordering in the one-dimensional extended Hubbard model at quarter filling by means of the quantum Monte Carlo method. We find that the Zeeman coupling enhances the charge order correlation, which is prominent when the system is located in the critical regime near the charge ordering transition at zero magnetic field. This behavior is interpreted in terms of the crossover to the fully spin-polarized limit where the model is exactly solvable. Furthermore, by incorporating the interchain Coulomb repulsion, we show that the charge-ordering transition temperature is raised by the magnetic field. We also discuss the relevance of our results to magnetoresistance effects observed in molecular conductors.

cond-mat.str-el↗

arXiv2014

Long-Period Charge Correlations in Charge-Frustrated Molecular theta-(BEDT-TTF)2X

Charge ordering (CO) with long-periodicities in a geometrically frustrated system, theta-(BEDT-TTF)2X, is numerically studied. We consider interacting fermion models on the anisotropic triangular lattice: the extended Hubbard model and the spinless fermion model, taking account of the inter-site Coulomb interactions on not only the nearest neighbor (NN) but also the next nearest neighbor (NNN) bonds. By applying Lanczos exact diagonalization method, we find that long-period charge correlations become dominant when NNN terms are included, in the charge-frustrated region. The calculated optical excitation spectra show that the charge transfer excitations in the long-period CO extend to a lower energy range compared to those in the short-period CO states including the 3-fold CO stabilized when NNN terms are absent. Our results suggest that the charge correlations experimentally observed in the title compounds near the CO insulating phase can be viewed as a tendency toward `tilted stripe'-type CO with large charge fluctuations.

cond-mat.str-el↗

arXiv2013

Electronic States of Single-Component Molecular Conductors [M(tmdt)2]

The electronic states of isostructural single-component molecular conductors [M(tmdt)2] (M= Ni, Au, and Cu) are theoretically studied. By considering fragments of molecular orbitals as basis functions, we construct a multiorbital model common for the three materials. The tight-binding parameters are estimated from results of first-principles band calculations, leading to a systematic view of their electronic structures. We find that the interplay between a pπ-type orbital (L) on each of the two ligands and a pdσ-type orbital (Mσ) centered on the metal site plays a crucial role: their energy difference controls the electronic states near the Fermi energy. For the magnetic materials (M= Au and Cu), we take into account Coulomb interactions on different orbitals, i.e., we consider the multiorbital Hubbard model. Its ground-state properties are calculated within mean-field approximation where various types of magnetic structures with different orbital natures are found. An explanation for the experimental results in [Cu(tmdt)2] is provided: The quasi-degeneracy of the two types of orbitals leads to a dual state where localized M$σ$ spins appear, and L sites show a nonmagnetic state owing to dimerization. On the other hand, [Au(tmdt)2] locates in the subtle region in terms of the degree of orbital mixing. We propose possible scenarios for its puzzling antiferromagnetic phase transition, involving the Mσorbital in contrast to previous discussions mostly concentrating on the L sector.

cond-mat.str-el↗

arXiv2013

Cation Dependence of the Electronic States in Molecular Triangular Lattice System β'-X[Pd(dmit)_2]_2: A First-principles study

The electronic structure of an isostructural series of molecular conductors, β'-X[Pd(dmit)_2]_2 is systematically studied by a first-principles method based on the density-functional theory. The calculated band structures are fitted to the tight-binding model based on Pd(dmit)_2 dimers on the triangular lattice. We find systematic variation in the anisotropy of the transfer integrals along the three directions of the triangular lattice taking different values. The transfer integral along the face-to-face stacking direction of Pd(dmit)_2 dimers is always the largest. Around the quantum spin liquid, X = EtMe_3Sb, the other two transfer integrals become comparable. We also report sensible differences in the distribution of wavefunctions near the Fermi level between the two dmit ligands of the Pd(dmit)_2 molecule.

cond-mat.str-el↗

arXiv2012

Tuning the magnetic dimensionality by charge ordering in the molecular TMTTF salts

We theoretically investigate the interplay between charge ordering and magnetic states in quasi-one-dimensional molecular conductors TMTTF$_2X$, motivated by the observation of a complex variation of competing and/or coexisting phases. We show that the ferroelectric-type charge order increases two-dimensional antiferromagnetic spin correlation, whereas in the one-dimensional regime two different spin-Peierls states are stabilized. By using first-principles band calculations for the estimation for the transfer integrals and comparing our results with the experiments, we identify the controlling parameters in the experimental phase diagram to be not only the interchain transfer integrals but also the amplitude of the charge order.

cond-mat.str-el↗

arXiv2012

Spin frustration, charge ordering, and enhanced antiferromagnetism in TMTTF$_2$SbF$_6$

We theoretically investigate the effects of charge order and spin frustration on the spin ordering in TMTTF salts. Using first-principles band calculations, we find that a diagonal inter-chain transfer integral $t_{q1}$, which causes spin frustration between the inter-chain dimers in the dimer-Mott insulating state, strongly depends on the choice of anion. Within the numerical Lanczos exact diagonalization method, we show that the ferroelectric charge order changes the role of $t_{q1}$ from the spin frustration to the enhancement of the two-dimensionality in spin sector. The results indicate that $t_{q1}$ assists the cooperative behavior between charge order and antiferromagnetic state observed in TMTTF$_2$SbF$_6$.

cond-mat.str-el↗

arXiv2011

Incommensurate Antiferromagnetic Insulating State in (MDT-TS)(AuI$_2$)$_{x}$

We theoretically study the metal-insulator transition in a molecular conductor (MDT-TS)(AuI$_2$)$_{x}$ composed with an incommensurate ratio ($x = 0.441$), where the conduction band originated from the HOMO of donor MDT-TS molecules is incommensurately filled. We consider a two-dimensional Hubbard model taking account of anisotropic transfer integrals in the donor layer, under a periodic potential due to the anions (AuI$_2$)$^-$ which mismatches the donor lattice period, and investigate the ground state within mean-field approximation. An antiferromagnetic insulating state with induced charge disproportionation is obtained in the large $U$ region; this corresponds to the incommensurate Mott insulating state predicted previously [H. Yoshioka {\it et al.}: \jo{\JPSJ}{74}{2005}{1922}] based on a simplified one-dimensional model.

cond-mat.str-el↗

arXiv2010

Multi-Orbital Molecular Compound (TTM-TTP)I_3: Effective Model and Fragment Decomposition

The electronic structure of the molecular compound (TTM-TTP)I_3, which exhibits a peculiar intra-molecular charge ordering, has been studied using multi-configuration ab initio calculations. First we derive an effective Hubbard-type model based on the molecular orbitals (MOs) of TTM-TTP; we set up a two-orbital Hamiltonian for the two MOs near the Fermi energy and determine its full parameters: the transfer integrals, the Coulomb and exchange interactions. The tight-binding band structure obtained from these transfer integrals is consistent with the result of the direct band calculation based on density functional theory. Then, by decomposing the frontier MOs into two parts, i.e., fragments, we find that the stacked TTM-TTP molecules can be described by a two-leg ladder model, while the inter-fragment Coulomb energies are scaled to the inverse of their distances. This result indicates that the fragment picture that we proposed earlier [M.-L. Bonnet et al.: J. Chem. Phys. 132 (2010) 214705] successfully describes the low-energy properties of this compound.

cond-mat.mtrl-sci↗