A. A. Aligia

Triplet superconductivity in quasi-one-dimensional systems

We study a Hubbard Hamiltonian, including a quite general nearest-neighbor interaction, parametrized by repulsion V, exchange interactions

Anomalous flux quantization in a hubbard ring with correlated hopping.

{J}_{z}{,J}_{\ensuremath{\perp}},

Phase diagram of the Hubbard chain with two atoms per cell

bond-charge interaction X, and hopping of pairs W. The case of correlated hopping, in which the hopping between nearest neighbors depends upon the occupation of the two sites involved, is also described by the model for sufficiently weak interactions. We study the model in one dimension with usual continuum-limit field theory techniques, and determine the phase diagram. For arbitrary filling, we find a very simple necessary condition for the existence of dominant triplet superconducting correlations at large distance in the spin SU(2) symmetric case:

Phase diagrams from topological transitions: The Hubbard chain with correlated hopping

4V+Jl0.

Interpretation of experimental results on Kondo systems with crystal field

In the correlated-hopping model, the three-body interaction should be negative for positive V. We also compare the predictions of this weak-coupling treatment with numerical exact results for the correlated-hopping model obtained by diagonalizing small chains and using a Berry phase to determine the opening of the spin gap.

Thermal transport in one-dimensional spin heterostructures

We solve exactly a generalized Hubbard ring with twisted boundary conditions. The magnitude of the nearest-neighbor hopping depends on the occupations of the sites involved and the term which modifies the number of doubly occupied sites {ital t}{sub {ital AB}}=0. Although {eta}-pairing state with off-diagonal long-range order are part of the degenerate ground state, the behavior of the energy as a function of the twist rules out superconductivity in this limit. A small {ital t}{sub {ital AB}} breaks the degeneracy and for moderate repulsive {ital U} introduce superconducting correlations which lead to {open_quote}{open_quote}anomalous{close_quote}{close_quote} flux quantization. {copyright} {ital 1996 The American Physical Society.}

Orbital Kondo Spectroscopy in a Double Quantum Dot System

We obtain the quantum phase diagram of the Hubbard chain with alternating on-site energy at half filling. The model is relevant for the ferroelectric perovskites and organic mixed-stack donor-acceptor crystals. For any values of the parameters, the band insulator is separated from the Mott insulator by a dimer phase. The boundaries are determined accurately by crossing of excited levels with particular discrete symmetries. We show that these crossings coincide with jumps of charge and spin Berry phases with a clear geometrical meaning.

Thermopower of an SU(4) Kondo resonance under an SU(2) symmetry-breaking field

The quantum phase diagram of the Hubbard chain with correlated hopping is accurately determined through jumps in

Non-equilibrium conductance through a benzene molecule in the Kondo regime

\pi

Crystal-field effects in the mixed-valence compounds Yb2M3Ga9 (M= Rh, Ir)

in the charge and spin Berry phases. The nature of each thermodynamic phase, and the existence of charge and spin gaps, is confirmed by calculating correlation functions and other fundamental quantities using numerical methods, and symmetry arguments. Remarkably we find striking similarities between the stable phases for moderate on-site Coulomb repulsion: spin Peierls, spin-density-wave and triplet superconductor, and those measured in (TMTSF)