Nearest-neighbor Coulomb Interaction Research Articles

Tomonaga-Luttinger parameters and spin excitations in the dimerized extended Hubbard model

We study the one-dimensional extended Hubbard model with alternating size of the hopping integrals using the density-matrix renormalization group method. We calculate the spin gap, the Tomonaga-Luttinger parameter, and the charge-density-wave order parameter for various dimerizations, interaction strengths, and band fillings. At half band-filling the spin and charge excitations are gapped but these gaps disappear for infinitesimal hole doping. At quarter filling, the umklapp scattering in the half-filled lower Peierls band generates a gap for the charge excitations but the gapless spin excitations can be described in terms of an effective antiferromagnetic Heisenberg model. Beyond a critical strength for the nearest-neighbor interaction, the dimerized extended Hubbard model at quarter filling develops a charge-density-wave ground state. The dimerization and the nearest-neighbor Coulomb interaction strongly reduce the Tomonaga-Luttinger parameter from its value for the bare Hubbard model. We discuss the relevance of our findings for the Bechgaard salts.

Numerical study of charge ordering in Na xCoO 2

A simple microscopic model of charge ordering in the Na x CoO 2 system is presented. The model takes into account the interplane interactions between the ordered Na ions and d electrons from the CoO 2 layers as well as the nearest-neighbor intraplane Coulomb interactions between d electrons. It is shown that a driving force of charge ordering in the CoO 2 layers is the interplane interaction that alone is able to describe various types of inhomogeneous charge ordering (e.g., the striped phases) as well as to predict correctly the conducting properties of the system.

Possible Charge-Ordered States in Boron–Nitride and Boron–Carbon–Nitride Nanotubes and Nanoribbons

Electronic states in boron–nitride and boron–carbon–nitride nanoribbons with zigzag edges are studied using the extended Hubbard model with nearest-neighbor Coulomb interactions. Charge- and spin-polarized states are considered, and the phase diagram between two states is obtained. Next, electric capacitance is calculated to examine the nanofunctionalities of the system. Owing to the presence of strong site energies, the charge polarized state overcomes the spin polarized state, yielding a large difference in the phase diagram compared with that of a graphite system. Electronic structures are always like that of semiconductors. Capacitance is inversely proportional to ribbon width, owing to the presence of the charge excitation energy gap.

Novel Charge Order and Superconductivity in Two-Dimensional Frustrated Lattice at Quarter Filling

Motivated by the various physical properties observed in $\theta$-(BEDT-TTF)$_2$X, we study the ground state of extended Hubbard model on two-dimensional anisotropic triangular lattice at 1/4-filling with variational Monte Carlo method. It is shown that the nearest-neighbor Coulomb interaction enhances the charge fluctuation and it induces the anomalous state such as charge-ordered metallic state and the triplet next-nearest-neighbor $f$-wave superconductivity. We discuss the relation to the real materials and propose the unified view of the family of $\theta$-(BEDT-TTF)$_2$X.

Spin susceptibility of one-dimensional extended Hubbard model at quarter filling

The ground-state property of one-dimensional extended Hubbard model is investigated at quarter filling. By using spin-charge factorized wave function in the U → ∞ limit, the effective exchange coupling J eff and spin susceptibility χ are calculated as a function of nearest-neighbor Coulomb interaction V. It is found that J eff is suppressed by the effect of V through charge fluctuation. The results indicate these quantities have no singularities at the transition point because of the Berezinskii–Kosterlitz–Thouless-type transition of charge ordering. We also confirm the results by exact diagonalization method in the realistic parameter range. The relevance to the experiments on quasi one-dimensional organic conductor ( DI-DCNQI ) 2 Ag is discussed.

Charge order, spin order and superconductivity in two-dimensional electron systems: Effect of nearest-neighbor Coulomb interaction

Motivated by the charge order (CO) and superconductivity (SC) in θ - ( ET ) 2 X salts, we study the extended Hubbard model in a two-dimensional triangular lattice with variational Monte Carlo method. We show that the nearest-neighbor Coulomb interaction V induces charge fluctuation and the 3-fold CO state is stabilized. We also discuss the possibility of next-nearest-neighbor f-wave SC induced by charge fluctuation.

Light absorption spectrum of two-dimensional Mott insulators

We theoretically investigate the light absorption spectrum of two-dimensional (2D) Mott insulators. We employ the numerical diagonalization method to calculate the light absorption spectrum with using the effective Hamiltonian for the extended Hubbard model, which is valid when the on-site Coulomb interaction energy $U$ is much larger than the nearest-neighbor transfer integral $t$ and the nearest-neighbor Coulomb interaction energy $V$. For $V=0$, the absorption spectrum consists of a broad band with a width of about $16t$ and a sharp central peak, and the energy eigenstates contributing to the absorption spectrum do not have the antiferromagnetic (AF) spin order, when $U$ is sufficiently larger than $t$. These features result from the spin-charge interplay inherent in the very strong correlation region where charge transfer term is dominant. With decreasing $U∕t$, the peak structure in absorption spectrum becomes unclear and some low energy eigenstates have the AF spin order, as a result of the spin-spin interaction term. For large $V∕t$, the dominant peak arises in the lower energy region of the spectrum. A large number of holon-doublon bound states, which have nearly the same charge but different spin structures, are responsible for this peak, in contrast to the conventional exciton state. This is also in contrast to the charge bound states in one-dimensional (1D) Mott insulators, where a single energy eigenstate dominates the optical transition moment as a result of the spin-charge separation. The essentially different absorption spectra between the 1D and 2D Mott insulators originate from the difference in the coupling between spin and charge degrees of freedom.

Mean-Field Study of Charge Order with Long Periodicity in θ-(BEDT-TTF)2X

Charge ordering phenomenon in $\theta$-(BEDT-TTF)$_2$X is studied by using 1/4-filled extended Hubbard model on an anisotropic triangular lattice through mean-field approximation. It is found that a metallic charge ordered state with 3-periodicity on lattices (3-fold type charge order) is realized in the realistic parameter region where the nearest neighbor Coulomb interaction V$_{ij}$ is nearly isotropic. This 3-fold state survives up to high temperatures by estimating the free energy. Insulating state with diagonal or vertical-type charge ordering appears as increasing anisotropy of V$_{ij}$. Considering the effect of the structural distortion observed in $\theta$-(BEDT-TTF)$_2$RbZn(SCN)$_4$ at low temperatures, horizontal-type charge ordered insulating phase tends to be stabilized and the region where 3-fold type exists becomes narrower. Our results are consistent with the metallic state with long periodic charge order which can be related to 3-fold type in $\theta$-(BEDT-TTF)$_2$RbZn(SCN)$_4$ at high temperatures and insulating state with horizontal charge order at low temperatures. For $\theta$-(BEDT-TTF)$_2$CsZn(SCN)$_4$, we speculate several kinds of charge-ordered states are energetically competing with each other leading to an inhomogeneous state at low temperatures.

Effects of Charge Ordering on the Spin Degrees of Freedom in One-Dimensional Extended Hubbard Model

The effects of charge ordering on the spin degrees of freedom is studied by using one-dimensional extended Hubbard model. The ground state property is investigated in the U →∞ limit using spin-charge factorized wave function. The effective exchange coupling J eff and spin susceptibility χ are calculated as a function of nearest neighbor Coulomb interaction V for several electron densities. It is found that J eff is suppressed by the effect of V through charge fluctuation and, as a result, χ increases with increasing V . Furthermore, χ at T =0 has no detectable anomaly at the transition point where a charge ordering occurs, because of the Berezinskii–Kosterlitz–Thouless (BKT) type transition of charge ordering. We also calculate χ by exact diagonalization on small clusters in realistic parameter range and obtained qualitatively the same results. These results mean that the charge fluctuation play important roles around the transition point and the charge ordering has little effect on the spin degrees of fr...

Charge Order and Superconductivity in Two-Dimensional Triangular Lattice atn=2/3

To investigate the possibility of charge order and superconductivity in a doped two-dimensional triangular lattice, we study the extended Hubbard model with variational Monte Carlo method. At n=2/3, a commensurate filling for a triangular lattice, it is shown that the nearest-neighbor Coulomb interaction V induces honeycomb-type charge order and antiferromagnetic spin order at U>10t. We also discuss the possibility of superconductivity induced by charge fluctuation and the relation to the superconductivity in Na_{0.35}CoO_{2}1.3H_{2}O and theta-type organic condoctors.

Effects of the electron-phonon interaction of the charge ordering in quasi-one-dimensional charge-transfer salts

The Bechgaard salts and their sulfur analogs are considered to be in quasi-one-dimensional correlated electron systems with a quarter-filled π band and exhibit various types of charge and/or spin ordering. A variety of mechanisms have been proposed in an attempt to interpret the variety of instabilities, featuring dimensionality, which can be turned by pressure as well as chemically, intrachain dimerization due to the anion columns, and competing on-site and even long-range Coulomb repulsions. Here we investigate quantum and thermal phase transitions in the Bechgaard salts and their sulfur analogs with particular emphasis on lattice degree of freedom. We consider the one-dimensional extended Peierls-Hubbard model with long-range Coulomb and electron-lattice interactions. Due to the intrinsic dimerization accompanied by the anion columns, we introduce alternation to transfer integrals, nearest-neighbor Coulomb interactions and spring constants. We fully demonstrate the interplay between charge ordering and lattice instability within the Hartree approximation. Besides the purely electronic charge and spin density waves which are well established both experimentally and theoretically, we reveal a lattice-tetramerized phase stabilized essentially in Peierls-Hubbard model. Our results inspected and compared with the results of experiments.

Magnetic vs. charge ordered states, and electric capacitance in zigzag nanographite ribbons

Nanographite materials show novel electronic properties: spin glass like behaviors [1], the on/off switching of magnetism with molecule adsorptions [2], and so on. Here, we study electronic states in nanographite ribbons with zigzag edges. Effects of the nearest neighbor Coulomb interactions are investigated using the extended Hubbard model. The nearest Coulomb interactions stabilize a novel electronic state with the opposite electric charges separated and localized along both edges, resulting in a finite electric dipole moment pointing from one edge to the other. Next, electric capacitance is calculated to examine nano functionalities. We find that the behavior of the capacitance is widely different depending on whether the system is in the magnetic or charge polarized phases. In the magnetic phase, the capacitance is dominated by the presence of the edge states while the ribbon width is small. As the ribbon becomes wider, the capacitance remains with large magnitudes as the system develops into metallic zigzag nanotubes. It is proportional to the inverse of the width, when the system corresponds to the semiconducting nanotubes and the system is in the charge polarized phase also. The latter behavior could be understood by the presence of an energy gap for charge excitations.

Calculations of electric capacitance in carbon and BN nanotubes, and zigzag nanographite (BN, BCN) ribbons

Electronic states in nanographite ribbons with zigzag edges are studied using the extended Hubbard model with nearest neighbor Coulomb interactions. The electronic states with the opposite electric charges separated along both edges are analogous as nanocondensers. Therefore, electric capacitance, defined using a relation of polarizability, is calculated to examine nano-functionalities. We find that the behavior of the capacitance is widely different depending on whether the system is in the magnetic or charge polarized phases. In the magnetic phase, the capacitance is dominated by the presence of the edge states while the ribbon width is small. As the ribbon becomes wider, the capacitance remains with large magnitudes as the system develops into metallic zigzag nanotubes. It is proportional to the inverse of the width, when the system corresponds to the semiconducting nanotubes and the system is in the charge polarized phase also. The latter behavior could be understood by the presence of an energy gap for charge excitations. In the BN (BCN) nanotubes and ribbons, the electronic structure is always like that of semiconductors. The calculated capacitance is inversely proportional to the distance between the positive and negative electrodes.

Electric Capacitance as Nanocondensers in Zigzag Nanographite Ribbons and Zigzag Carbon Nanotubes

Electronic states in nanographite ribbons with zigzag edges and zigzag carbon nanotubes are studied using the extended Hubbard model with nearest-neighbor Coulomb interactions. The nearest-neighbor Coulomb interactions stabilize electronic states with the opposite electric charges separated and localized along both edges. Such states are analogous to nanocondensers. Therefore, electric capacitance, defined using a relation of polarizability, is calculated to examine the nano-functionalities of the system. We find that the behavior of the capacitance varies widely depending on whether the system is in the magnetic phase or charge-polarized phase. In the magnetic phase, capacitance is dominated by the presence of the edge states when the ribbon width is small. Even when the ribbon becomes wider, capacitance remains having a large magnitude as the system develops into metallic zigzag nanotubes. It is proportional to the inverse of ribbon width, when the system corresponds to the semiconducting nanotubes and the system is in the charge-polarized phase also. The latter behavior could be understood by the presence of an energy gap for charge excitations.

Metal-insulator transition in the quarter-filled frustrated checkerboard lattice

We study the electronic structure and correlations in the geometrically frustrated two dimensional checkerboard lattice. In the large U limit considered here we start from an extended Hubbard model of spinless fermions at half-filling. We investigate the model within two distinct Green's function approaches: In the first approach a single-site representation decoupling scheme is used that includes the effect of nearest neighbor charge fluctuations. In the second approach a cluster representation leading to a 'multiorbital' model is investigated which includes intra-cluster correlations exactly and those between clusters on a mean field basis. It is demonstrated that with increasing nearest-neighbor Coulomb interaction V both approaches lead to a metal-insulator transition with an associated 'Mott-Hubbard' like gap caused by V. Within the single site approach we also explore the possibility of charge order. Furthermore we investigate the evolution of the quasiparticle bands as funtion of V.

Quantum melting of charge order due to frustration in two-dimensional quarter-filled systems

The effect of geometrical frustration in a two-dimensional 1/4-filled strongly correlated electron system is studied theoretically, motivated by layered organic molecular crystals. An extended Hubbard model on the square lattice is considered, with competing nearest neighbor Coulomb interaction, V, and that of next-nearest neighbor along one of the diagonals, V', which favor different charge ordered states. Based on exact diagonalization calculations, we find a metallic phase stabilized over a broad window at V' ~ V even for large Coulomb repulsion strengths as a result of frustrating the charge ordered states. Slightly modifying the lattice geometry relevant to the actual organic compounds does not alter the results, suggesting that this `quantum melting' of charge order is a robust feature of frustrated strongly correlated 1/4-filled systems.

STABILITY OF PHOTOINDUCED SUPERCONDUCTING STATES IN STRONGLY CORRELATED TWO-DIMENSIONAL ELECTRON SYSTEMS

We theoretically investigate the properties of multiphoton excited states in strongly correlated two-dimensional electron systems at half-filling using the extended Hubbard model in the strong correlation case. We find that the photoinduced d-wave superconducting states are stable against the nearest neighbor Coulomb interaction within the realistic Coulomb parameters.

Superconductivity due to Charge Fluctuation in θ-Type Organic Conductors

Superconductivity close to charge ordering in quasi-two-dimensional 3/4-filled organic compounds is investigated for the θ-type structure. The effect of spin and charge fluctuations is calculated within random phase approximation for a relevant extended Hubbard model with both on-site ( U ) and intersite ( V ) Coulomb interactions on an anisotropic triangular lattice. It is found that both spin and charge fluctuations induce superconductivity with extended s -wave singlet pairing, which is similar to the d x y pairing state in the square lattice. In the vicinity of charge-density-wave instability, f -wave triplet pairing is also stable due to the effect of nearest-neighbor Coulomb interaction. The temperature dependences of superconductivity and charge-density-wave phase boundaries are also examined.

Dynamical density matrix renormalization group study of photoexcited states in one-dimensional Mott insulators

One-dimensional Mott insulators exhibit giant nonlinear optical response. Based on the dynamical density matrix renormalization group method, photoexcited states and optical response in the insulators are studied as functions of the on-site and the nearest neighbor Coulomb interactions, $U$ and $V$, respectively. We find that the lowest optically-allowed and forbidden excited states, which have odd and even parities, respectively, are degenerate for $V/t\ltsim 2$ with $t$ being the hopping integral of an electron between nearest neighbor sites. For $V/t\gtsim 2$, the bound states with even and odd parities occur and are not degenerate. The nature of the degeneracy and its effect on the optical response are examined.

Pseudogap, competing order, and the coexistence of staggered flux andd-wave pairing in high-temperature superconductors

We study the t-J-V model of a doped Mott insulator in connection to high-T_c superconductors. The nearest neighbor Coulomb interaction (V) is treated quantum mechanically on equal footing as the antiferromagnetic exchange interaction (J). Motivated by the SU(2) symmetry at half-filling, we construct a large-N theory which allows a systematic study of the interplay between staggered flux order and superconductivity upon doping. We solve the model in the large-N limit and obtain the ground state properties and the phase diagram as a function of doping. We discuss the competition and the coexistence of the staggered flux and the d-wave superconductivity in the underdoped regime and the disappearance of superconductivity in the overdoped regime