Jose Riera

Susceptibility and excitation spectrum of (VO)2P2O7 in ladder and dimer-chain models.

We present numerical results for the magnetic susceptibility of a Heisenberg antiferromagnetic spin ladder as a function of temperature and the spin-spin interaction strengths [ital J][sub [perpendicular]] and [ital J][sub [parallel]]. These are contrasted with bulk-limit results for the dimer chain. A fit to the experimental susceptibility of the candidate spin-ladder compound vanadyl pyrophosphate, (VO)[sub 2]P[sub 2]O[sub 7], gives the parameters [ital J][sub [perpendicular]]=7.82 meV and [ital J][sub [parallel]]=7.76 meV. With these values we predict a singlet-triplet energy gap of [ital E][sub gap]=3.9 meV, and give a numerical estimate of the ladder triplet dispersion relation [omega]([ital k]). In contrast, a fit to the dimer-chain model leads to [ital J][sub 1]=11.11 meV and [ital J][sub 2]=8.02 meV, which predicts a gap of [ital E][sub gap]=4.9 meV.

Magnetic susceptibility in the spin-Peierls system CuGeO3.

We study numerically, using a one-dimensional Heisenberg model coupled to the lattice in the adiabatic approximation, the spin-Peierls transition in the linear

Model for the magnetic order and pairing channels in Fe pnictide superconductors.

{\mathrm{Cu}}^{2+}

Indications of d_{x^2 - y^2} Superconductivity in the Two Dimensional t-J Model

spin-1/2 chains in the inorganic compound

Influence of the anion potential on the charge ordering in quasi-one-dimensional charge-transfer salts

{\mathrm{CuGeO}}_{3}

Coexistence of charge-density waves, bond-order waves, and spin-density waves in quasi-one-dimensional charge-transfer salts

, which has been recently observed experimentally. We suggest that the magnetic susceptibility, the temperature dependence of the spin gap, and the spin-Peierls transition temperature of this material can be reasonably described by including nearest and next-nearest-neighbor antiferromagnetic interactions along the chain. We estimate that the nearest-neighbor exchange parameter J is approximately 160 K, and that the next-nearest-neighbor exchange parameter is approximately 0.36J.

LOCAL ENHANCEMENT OF ANTIFERROMAGNETIC CORRELATIONS BY NONMAGNETIC IMPURITIES

A two-orbital model for Fe-pnictide superconductors is investigated using computational techniques on two-dimensional square clusters. The hopping amplitudes are derived from orbital overlap integrals, or by band structure fits, and the spin frustrating effect of the plaquette-diagonal Fe-Fe hopping is remarked. A spin striped state is stable in a broad range of couplings in the undoped regime, in agreement with neutron scattering. Adding two electrons to the undoped ground state of a small cluster, the dominant pairing operators are found. Depending on the parameters, two pairing operators were identified: they involve inter-xz-yz orbital combinations forming spin singlets or triplets, transforming according to the B2g and A2g representations of the D4h group, respectively.

ENHANCEMENT OF ANTIFERROMAGNETIC CORRELATIONS INDUCED BY NONMAGNETIC IMPURITIES : ORIGIN AND PREDICTIONS FOR NMR EXPERIMENTS

Superconducting correlations in the two dimensional t-J model at zero temperature are evaluated using numerical techniques. At the fermionic density (n)∼1/2, strong signals of d x2-y2x superconductivity were observed in the ground state. These conclusions are based on a study of static pairing correlations, the Meissner effect, and flux quantization as indicators of superconductivity. A tentative phase diagram of the two dimensional t-J model is presented

COEXISTENCE OF CHARGE-DENSITY WAVE AND SPIN-PEIERLS ORDERS IN QUARTER-FILLED QUASI-ONE-DIMENSIONAL CORRELATED ELECTRON SYSTEMS

We examine the various instabilities of quarter-filled strongly correlated electronic chains in the presence of a coupling to the underlying lattice. To mimic the physics of the tetramethyltetrathiafulvene

PHASE DIAGRAM OF ELECTRONIC MODELS FOR TRANSITION METAL OXIDES IN ONE DIMENSION

(\mathrm{TMTTF}{)}_{2}X