Equilibrium Properties of Quantum Spin Systems with Non-additive Long-Range Interactions

We study equilibrium states of quantum spin systems with non-additive long-range interactions by adopting an appropriate scaling of the interaction strength, i.e., the so called Kac prescription. In classical spin systems, it is known that the equilibrium free energy is obtained by minimizing the free energy functional over the coarse-grained magnetization. Here we show that it is also true for quantum spin systems. From this observation, it is found that when the canonical ensemble and the microcanonical ensemble are not equivalent in some parameter region, it is not necessarily justified to replace the actual long-range interaction by the infinite-range interaction (Curie-Weiss type interaction). On the other hand, in the parameter region where the two ensembles are equivalent, this replacement is always justified. We examine the Heisenberg XXZ model as an illustrative example, and discuss the relation to experiments.Comment: 13 pages, two columns; to appear in Phys. Rev.

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.Comment: 5 pages, 4 figures, published versio

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 \theta-(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.Comment: 5 pages, 4 figures, to appear in J. Phys. Soc. Jp

First-Principles Study of Electronic Structure in α\alpha-(BEDT-TTF)2_2I3_3 at Ambient Pressure and with Uniaxial Strain

Within the framework of the density functional theory, we calculate the electronic structure of $\alpha$-(BEDT-TTF)$_2$I$_3$ at 8K and room temperature at ambient pressure and with uniaxial strain along the $a$- and $b$-axes. We confirm the existence of anisotropic Dirac cone dispersion near the chemical potential. We also extract the orthogonal tight-binding parameters to analyze physical properties. An investigation of the electronic structure near the chemical potential clarifies that effects of uniaxial strain along the a-axis is different from that along the b-axis. The carrier densities show $T^2$ dependence at low temperatures, which may explain the experimental findings not only qualitatively but also quantitatively.Comment: 10 pages, 7 figure

Photon and spin dependence of the resonance lines shape in the strong coupling regime

We study the quantum dynamics of a spin ensemble coupled to cavity photons. Recently, related experimental results have been reported, showing the existence of the strong coupling regime in such systems. We study the eigenenergy distribution of the multi-spin system (following the Tavis-Cummings model) which shows a peculiar structure as a function of the number of cavity photons and of spins. We study how this structure causes changes in the spectrum of the admittance in the linear response theory, and also the frequency dependence of the excited quantities in the stationary state under a probing field. In particular, we investigate how the structure of the higher excited energy levels changes the spectrum from a double-peak structure (the so-called vacuum field Rabi splitting) to a single peak structure. We also point out that the spin dynamics in the region of the double-peak structure corresponds to recent experiments using cavity ringing while in region of the single peak structure, it corresponds to the coherent Rabi oscillation in a driving electromagnetic filed. Using a standard Lindblad type mechanism, we study the effect of dissipations on the line width and separation in the computed spectra. In particular, we study the relaxation of the total spin in the general case of a spin ensemble in which the total spin of the system is not specified. The theoretical results are correlated with experimental evidence of the strong coupling regime, achieved with a spin 1/2 ensemble