arXiv subjects

Yasuhiro Tanaka

Publications and source records attributed to Yasuhiro Tanaka.

27 records · Page 2Linked to original sources

arXiv2013

Charge order and possible bias-induced metastable state in the organic conductor β-(meso-DMBEDT-TTF)2PF6: effects of structural distortion

We theoretically investigate charge order and nonlinear conduction in a quasi-two-dimensional organic conductor β-(meso-DMBEDT-TTF)2PF6 [DMBEDT-TTF=dimethylbis(ethylenedithio)tetrathiafulvalene]. Within the Hartree-Fock approximation, we study effects of structural distortion on the experimentally observed checkerboard charge order and its bias-induced melting by using an extended Hubbard model with Peierls- and Holstein-types of electron-lattice interactions. The structural distortion is important in realizing the charge order. The current-voltage characteristics obtained by a nonequilibrium Green's function method indicate that a charge-ordered insulating state changes into a conductive state. Although the charge order and lattice distortions are largely suppressed at a threshold voltage, they remain finite even in the conductive state. We discuss the relevance of the results to experimental observations, especially to a possible bias-induced metastable state.

cond-mat.str-el↗

arXiv2011

Nonlinear Conduction by Melting of Stripe-Type Charge Order in Organic Conductors with Triangular Lattices

We theoretically discuss the mechanism for the peculiar nonlinear conduction in quasi-two-dimensional organic conductors θ-(BEDT-TTF)2X [BEDT-TTF=bis(ethylenedithio)tetrathiafulvalene] through the melting of stripe-type charge order. An extended Peierls-Hubbard model attached to metallic electrodes is investigated by a nonequilibrium Green's function technique. A novel current-voltage characteristic appears in a coexistent state of stripe-type and nonstripe 3-fold charge orders, where the applied bias melts mainly the stripe-type charge order through the reduction of lattice distortion, whereas the 3-fold charge order survives. These contrastive responses of the two different charge orders are consistent with the experimental observations.

cond-mat.str-el↗

arXiv2011

Crossover from bias-induced to field-induced breakdowns in one-dimensional band and Mott insulators attached to electrodes

Nonequilibrium states induced by an applied bias voltage (V) and the corresponding current-voltage characteristics of one-dimensional models describing band and Mott insulators are investigated theoretically by using nonequilibrium Green's functions. We attach the models to metallic electrodes whose effects are incorporated into the self-energy. Modulation of the electron density and the scalar potential coming from the additional long-range interaction are calculated self-consistently within the Hartree approximation. For both models of band and Mott insulators with length L_C, the bias voltage induces a breakdown of the insulating state, whose threshold shows a crossover depending on L_C. It is determined basically by the bias $V_{th}\sim Δ$ for L_C smaller than the correlation length $ξ=W/Δ$ where W denotes the bandwidth and $Δ$ the energy gap. For systems with $L_C\gg ξ$, the threshold is governed by the electric field, $V_{th}/L_C$, which is consistent with a Landau-Zener-type breakdown, $V_{th}/L_C\propto Δ^2/W$. We demonstrate that the spatial dependence of the scalar potential is crucially important for this crossover by showing the case without the scalar potential, where the breakdown occurs at $V_{th}\sim Δ$ regardless of the length L_C.

cond-mat.str-el↗

arXiv2009

Charge, Lattice, and Spin Dynamics in Photoinduced Phase Transitions from Charge-Order-Insulator to Metal in Quasi-Two-Dimensional Organic Conductors

To elucidate different photoinduced melting dynamics of charge orders observed in quasi-two-dimensional organic conductors $ θ$-(BEDT-TTF)$_2$RbZn(SCN)$_4$ and $ α$-(BEDT-TTF)$_2$I$_3$ [BEDT-TTF=bis(ethylenedithio)tetrathiafulvalene], we theoretically study photoinduced time evolution of charge and spin correlation functions on the basis of exact many-electron wave functions coupled with classical phonons in extended Peierls-Hubbard models on anisotropic triangular lattices. In both salts, the so-called horizontal-stripe charge order is stabilized by nearest-neighbor repulsive interactions and by electron-lattice interactions. In $ θ$-(BEDT-TTF)$_2$RbZn(SCN)$_4$ (abbreviated as $ θ$-RbZn), the stabilization energy due to lattice distortion is larger, so that larger quantity of energy needs to be absorbed for the melting of the charge and lattice orders. The photoinduced charge dynamics shows a complex behavior owing to a substantial number of nearly degenerate eigenstates involved. This is related to the high structural symmetry when the lattice is undistorted. In $ α$-(BEDT-TTF)$_2$I$_3$ (abbreviated as $ α$-I$_3$), the lattice stabilization energy is smaller, and smaller quantity of energy is sufficient to melt the charge and lattice orders leading to a metallic phase. The photoinduced charge dynamics shows a sinusoidal oscillation. In $ α$-I$_3$, the low structural symmetry ensures nearly spin-singlet bonds between hole-rich sites, where the spin correlation survives even after photoexcitation.

cond-mat.str-el↗

arXiv2009

Growth Dynamics of Photoinduced Domains in Two-Dimensional Charge-Ordered Conductors Depending on Stabilization Mechanisms

Photoinduced melting of horizontal-stripe charge orders in quasi-two-dimensional organic conductors θ-(BEDT-TTF)2RbZn(SCN)4[BEDT-TTF=bis(ethylenedithio)tetrathiafulvalene] and α-(BEDT-TTF)2I3 is investigated theoretically. By numerically solving the time-dependent Schrödinger equation, we study the photoinduced dynamics in extended Peierls-Hubbard models on anisotropic triangular lattices within the Hartree-Fock approximation. The melting of the charge order needs more energy for θ-(BEDT-TTF)2RbZn(SCN)4 than for α-(BEDT-TTF)2I3, which is a consequence of the larger stabilization energy in θ-(BEDT-TTF)2RbZn(SCN)4. After local photoexcitation in the charge ordered states, the growth of a photoinduced domain shows anisotropy. In θ-(BEDT-TTF)2RbZn(SCN)4, the domain hardly expands to the direction perpendicular to the horizontal-stripes. This is because all the molecules on the hole-rich stripe are rotated in one direction and those on the hole-poor stripe in the other direction. They modulate horizontally connected transfer integrals homogeneously, stabilizing the charge order stripe by stripe. In α-(BEDT-TTF)2I3, lattice distortions locally stabilize the charge order so that it is easily weakened by local photoexcitation. The photoinduced domain indeed expands in the plane. These results are consistent with recent observation by femtosecond reflection spectroscopy.

cond-mat.str-el↗

arXiv2008

Charge Order with Structural Distortion in Organic Conductors: Comparison between θ-(ET)2RbZn(SCN)4 and α-(ET)2I3

Charge ordering with structural distortion in quasi-two-dimensional organic conductors θ-(ET)2RbZn(SCN)4 (ET=BEDT-TTF) and α-(ET)2I3 is investigated theoretically. By using the Hartree-Fock approximation for an extended Hubbard model which includes both on-site and intersite Coulomb interactions together with Peierls-type electron-lattice couplings, we examine the role of lattice degrees of freedom on charge order. It is found that the experimentally observed, horizontal charge order is stabilized by lattice distortion in both compounds. In particular, the lattice effect is crucial to the realization of the charge order in θ-(ET)2RbZn(SCN)4, while the peculiar band structure whose symmetry is lower than that of θ-(ET)2RbZn(SCN)4 in the metallic phase is also an important factor in α-(ET)2I3 together with the lattice distortion. For α-(ET)2I3, we obtain a phase transition from a charge-disproportionated metallic phase to the horizontal charge order with lattice modulations, which is consistent with the latest X-ray experimental result.

cond-mat.str-el↗

arXiv2007

Effects of Electron-Lattice Coupling on Charge Order in θ-(ET)2X

Charge ordering accompanied by lattice distortion in quasi-two dimensional organic conductors θ-(ET)2X (ET=BEDT-TTF) is studied by using an extended Hubbard model with Peierls-type electron-lattice couplings within the Hartree-Fock approximation. It is found that the horizontal-stripe charge-ordered state, which is experimentally observed in θ-(ET)2RbZn(SCN)4, is stabilized by the self-consistently determined lattice distortion. Furthermore, in the presence of the anisotropy in nearest-neighbor Coulomb interactions Vij, the horizontal charge order becomes more stable than any other charge patterns such as diagonal, vertical and 3-fold-type states. At finite temperatures, we compare the free energies of various charge-ordered states and find a first-order transition from a metallic state with 3-fold charge order to the insulating state with the horizontal charge order. The role of lattice degrees of freedom in the realization of the horizontal charge order and the relevance to experiments on θ-(ET)2X are discussed.

cond-mat.str-el↗

arXiv2003

Superconductivity in Na_xCoO_2yH_2O by charge fluctuation

A new mechanism for superconductivity in the newly discovered Co-based oxide is proposed by using charge fluctuation. A single-band extended Hubbard model on the triangular lattice is studied within random phase approximation. $f$-wave triplet superconductivity is stabilized in the vicinity of charge-density-wave instability, which is in sharp contrast with the square-lattice case. The physical origin of the realization of the $f$-wave triplet state as well as the relevance to experiments are discussed.

cond-mat.supr-con↗

arXiv2003

Charge fluctuation induced superconducting state in two-dimensional quarter-filled electron systems

We investigate the superconducting (SC) state in two-dimensional extended Hubbard model at a quarter-filling with the on-site repulsive interaction ($U$) and the nearest-neighbor one ($V$), which lead to the spin fluctuation and the charge fluctuation, respectively. The effect of the charge fluctuation on the onset of the SC state is calculated by estimating the pairing interaction within the random phase approximation. Solving the linearized Eliashberg equation with the anomalous self-energy as the function of both momentum and frequency, we examined the singlet SC state on the plane of $U$ and $V$ with the fixed temperature. We found a novel result that the SC state with the $d_{xy}$ symmetry retains for most parameters of $U$ and $V$, although the pairing interaction changes the sign from repulsive one into attractive one with increasing $V$. The reentrant transition as the function of temperature occurs close to the onset of the SC state.

cond-mat.supr-con↗