A strictly truncated (weak-coupling) perturbation theory is applied to the attractive Holstein and Hubbard models in infinite dimensions. These results are qualified by comparison with essentially exact Monte Carlo results. The second order iterated perturbation theory is shown to be quite accurate in calculating transition temperatures for retarded interactions, but is not as accurate for the ...

We present analytic results for ground-state properties of Hubbard-type models in terms of the Gutzwiller variational wave function with nonzero values of the magnetization m. In dimension D51 approximation-free evaluations are made possible by appropriate canonical transformations and an analysis of umklapp processes. We calculate the double occupation and the momentum distribution, as well as...

A theory of magnetic short–range order for high–Tc cuprates is presented on the basis of the one–band t–t–Hubbard model combining the four–field slave–boson functional integral technique with the Bethe cluster method. The ground–state phase diagram evaluated self–consistently at the saddle–point and pair–approximation levels shows the experimentally observed suppression of magnetic long–range o...

We study the conductivity, density of states, and magnetic correlations of a two-dimensional, two-band fermion Hubbard model using determinant quantum Monte Carlo (DQMC) simulations. We show that an orbitally selective Mott transition (OSMT) occurs in which the more weakly interacting band can be metallic despite complete localization of the strongly interacting band. The DQMC method allows us ...

We report on an introductory study used to gauge the significance of random weakedge disorder on the coherent transport properties of ultra-thin zig-zag nanoribbons (ZGNRs) beyond the simple (i.e., first nearest-neighbour) tight-binding approximation. Such extensions include up to third nearest-neighbour hopping in an extended tight-binding model, as well as a mean-field Hubbard-U . The effect ...

We study the basic features of the two-dimensional quantum Hubbard Model at half-filling by means of the Lüscher algorithm and the algorithm based on direct update of the determinant of the fermionic matrix. We implement the Lüscher idea employing the transfer matrix formalism which allows to formulate the problem on the lattice in (2+1) dimensions. We discuss the numerical complexity of the Lü...

Bosonic Josephson junctions can be realized by confining ultracold gases of bosons in multiwell traps and studied theoretically with the M-site Bose-Hubbard model. We show that canonical equilibrium states of the M-site Bose-Hubbard model may be approximated by mixtures of coherent states, provided the number of atoms is large and the total energy is comparable to kBT. Using this approximation,...

We discuss the spin-triplet pairing mechanism induced by the Hund rule ferromagnetic exchange. We include explicitly the e ect of interband hybridization and treat the problem by starting from an extended Hubbard model for a doubly degenerate band, making the simplest Hartree Fock approximation for the part involving the pairing and the Hubbard interaction. The conditions of stability of variou...

We propose a way of obtaining effective low energy Hubbard-like model Hamiltonians from ab initio quantum Monte Carlo calculations for molecular and extended systems. The Hamiltonian parameters are fit to best match the ab initio two-body density matrices and energies of the ground and excited states, and thus we refer to the method as ab initio density matrix based downfolding. For benzene (a ...

The ground state of Sn/Si(111) and Sn/Ge(111) surface alpha phases is reexamined theoretically, based on ab initio calculations where correlations are approximately included through the orbital dependence of the Coulomb interaction (in the local density+Hubbard U approximation). The effect of correlations is to destabilize the vertical buckling in Sn/Ge(111) and to make the surface magnetic, wi...