27,105 research outputs found

### 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 $\alpha$-(BEDT-TTF)$_2$I$_3$ 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

- â€¦