1,354,372 research outputs found
The orbital relaxation: A possible origin of t - J model with large J
No full textWhereas the t - J model has gained wide popularity among physicists as a model for high-TC superconductivity, its physical origin is not at all clear. In this communication we show that the Hubbard model with occupation-dependent hopping (t1 - t3 - U model), recently proposed to account for the relaxation of doubly-occupied orbitals, reduces, in a physically relevant parameter region, to an effective t - J - t′ model with no upper bound on the J/t ratio. Results of exact diagonalization studies on finite size systems demonstrate the equivalence between the t1 - t2 - t3 - U model with intermediate/large U and the t - J - t′ even for very large J/t ratio (up to 16). On the contrary, t - U and t - J models turn out to be equivalent to t - J - t′ model only for very small J values (J much smaller than t). Implications of the results on high-TC superconductivity are briefly discusses
Ab initio estimate of Hubbard modle parameters: a simple procedure applied to BEDT-TTF salts.
No full textBEDT-TTF salts: Microscopic parameters from ab-initio calculations
No full textAbstract: Extensive ab initio calculations are performed on BEDT-TTF dimeric units singled out from the kappa-(BEDT-TTF)(2)Cu[N(CN)(2)]Br (ETBR) structure. From a careful analysis of the results obtained for dimers with total charge ranging from 0 to 4 we are able to get reliable estimates of the Hubbard model parameters
On the ab initio evaluation of Hubbard parameters. II. The k-(BEDT-TTF) Cu[N(CN) ]Br crystal.
No full textINTERACTING ELECTRONS IN THE SOLID-STATE - THE ROLE OF ORBITAL RELAXATION
No full textAbstract: A first principle derivation is presented of a Hubbard-like Hamiltonian describing the motion of electrons on a lattice of soft, non-orthogonal orbitals with a polarizable core. Non-orthogonality of the on-site orbitals is explicitly accounted for by expanding the Hamiltonian at different orders in the nearest neighbor overlap. The resulting Hamiltonian contains terms not included in the Hubbard Hamiltonian, such as non-site-diagonal e-e interactions and occupation-dependent hopping terms. The first principle approach we develop yields reliable a priori estimates of microscopic parameters and leads to the unambiguous definition of the reliability range of the proposed model. The properties of the Hamiltonian are investigated through numerical calculations on finite rings
On the ab initio evaluation of Hubbard parameters. I. The analytical approach in the absence of orbital relaxation.
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Simulations of the Plastic Behavior of Amorphous Glassy Bisphenol-A Polycarbonate
No full textA protocol for studying the plastic deformation of amorphous glassy polymers is presented. The protocol is based on a viable computational procedure which combines constant-stress molecular dynamics simulations and fixed-cell energy minimizations, followed by kinetic, configurational, and energy analyses. It is shown that the computational results can be accounted for within a "potential energy landscape" theoretical framework, in which the plastic transition is interpreted as a crossing between and a collapse onto each other of "ideal (thermodynamic) structures." The procedure is applied to bis-phenol-A-polycarbonate (BPA-PC), but is equally valid for a wide variety of polymeric species. Allowing for the limited size of the simulation cell, the high strain rate, and the fact that the simulation are conducted at low temperature, the values of the density, Young’s modulus, yield strain, yield stress, activation energy, and activation volume are in fair agreement with the experimental data on BPA-PC. The analysis of the results shows that the plastic relaxation for this polymer has both a collective and cooperative character (as in classical percolation theories), involves a significant fraction of the simulation cell, and can be viewed as a "nanoscopic shear band.
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