Kenji Yonemitsu

Coexistence of SDW and purely-electronic CDW in quarter-filled organic conductors

We have performed a mean field calculation in order to determine the ground state of a modified Hubbard model of a quarter-filled one-dimensional chain. It is found that the coexistence of 2 k F spin density wave (SDW) and purely-electronic 2 k F charge density wave (CDW) is realized due to the effect of next-nearest-neighbor Coulomb repulsion. Phase diagrams are obtained as the values of nearest- and next-nearest-neighbor Coulomb interaction energies are varied. Competition between the two coexistent states with 2 k F CDW and 4 k F CDW is revealed, which gives a first order transition. Our result supports the X-ray result confirming the presence of a coexisting phase of purely-electronic 2 k F CDW and 2 k F SDW in (TMTSF) 2 PF 6 , a quarter-filled one-dimensional chain in terms of holes.

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) 2 RbZn(SCN) 4 (ET = BEDT-TTF) and α-(ET) 2 I 3 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) 2 RbZn(SCN) 4 , while the peculiar band structure whose symmetry is lower than that of θ-(ET) 2 RbZn(SCN) 4 in the metallic phase is also an important factor in α-(ET) 2 I 3 together with the lattice distortion. For α-(ET) 2 I 3 , we obtain a phase transition from a charge-disproportionated metallic phase to the horizontal charge order with lattice modulations, which is c...

Photoinduced Metallic Properties of One-Dimensional Strongly Correlated Electron Systems

We study photoinduced optical responses of one-dimensional strongly correlated electron systems. Optical conductivity spectra are calculated for the ground and photoexcited states in a one-dimensional Hubbard model at half filling by the exact diagonalization method. It is found that, in the Mott insulator phase, the photoexcited state has large spectral weights including the Drude weight below the optical gap. As a consequence, the spectral weight above the optical gap is markedly reduced. These results imply that a metallic state is induced by photoexcitation. A comparison between the photoexcited and hole-doped states shows that photoexcitation is similar to chemical doping.

Charge ordering and lattice modulation in MMX chains

Abstract Ground state phase diagrams for the MMX chains are studied in a one-dimensional two-band model by exactly diagonalizing 18-site clusters. Experimentally observed phases are reproduced by changing relative strengths of an electron–lattice coupling, the interdimer transfer integral, and an elastic constant, whose variation is roughly estimated from the interatomic spacing, the species of the halogen ion, and the presence/absence of counter ions.

Structures and electronic phases of the bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF) clusters and κ-(BEDT-TTF) salts: A theoretical study based on ab initio molecular orbital methods

Electronic and geometrical structures of bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF) molecules are studied using ab initio molecular orbital methods. The optimized structure of a BEDT-TTF monomer is close to the experimental one within errors of 0.02 A and 0.5 deg in bond length and angle, respectively, except the ethylene group. Ab initio parameters such as transfer integrals and Coulomb interactions are determined from the BEDT-TTF dimer and tetramer calculations. Using model Hamiltonians with the ab initio parameters, we investigate the electronic states based on the exact diagonalization method. The results show that the ground state has antiferromagnetic correlation which is consistent with experimental results. We study the effects of long-range Coulomb interactions employing the 2-D extended Hubbard model with the Hartree–Fock approximation. It is found that the ground state shows various phases; antiferromagnetic, charge ordering, and paramagnetic ones, controlled by the long-range interactions.

Photoinduced melting of charge order in a quarter-filled electron system coupled with different types of phonons

Photoinduced melting of charge order is calculated by using the exact many-electron wave function coupled with classically treated phonons in the one-dimensional quarter-filled Hubbard model with Peierls and Holstein types of electron-phonon couplings. The model parameters are taken from recent experiments on

Effects of electron-lattice coupling on charge order in θ-(ET)2X

{(\mathrm{EDO}\text{\ensuremath{-}}\mathrm{TTF})}_{2}{\mathrm{PF}}_{6}

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

(\mathrm{EDO}\text{\ensuremath{-}}\mathrm{TTF}=\text{ethylenedioxy}\text{\char21{}}\text{tetrathiafulvalene})

Electronic and Lattice Dynamics in the Photoinduced Ionic-to-Neutral Phase Transition in a One-Dimensional Extended Peierls-Hubbard Model

with a (0110) charge order, where transfer integrals are modulated by molecular displacements (bond-coupled phonons) and site energies by molecular deformations (charge-coupled phonons). The charge-transfer photoexcitation from (0110) to (0200) configurations and that from (0110) to (1010) configurations have different energies. The corresponding excited states have different shapes of adiabatic potentials as a function of these two phonon amplitudes. The adiabatic potentials are shown to be useful in understanding differences in the photoinduced charge dynamics and the efficiency of melting, which depend not only on the excitation energy but also on the relative phonon frequency of the bond- and charge-coupled phonons.