C. J. Gazza

Superconductivity in the two-dimensional t-J model

Using computational techniques, it is shown that pairing is a robust property of hole-doped antiferromagnetic insulators. In one dimension and for two-leg ladder systems, a BCS-like variational wave function with long-bond spin singlets and a Jastrow factor provides an accurate representation of the ground state of the t-J model, even though strong quantum fluctuations destroy the off-diagonal superconducting long-range order in this case. However, in two dimensions it is argued-and numerically confirmed using several techniques, especially quantum Monte Carlo-that quantum fluctuations are not strong enough to suppress superconductivity.

SCHWINGER-BOSON APPROACH TO QUANTUM SPIN SYSTEMS : GAUSSIAN FLUCTUATIONS IN THE NATURAL GAUGE

We compute the Gaussian-fluctuation corrections to the saddle-point Schwinger-boson results using collective coordinate methods. Concrete application to investigate the frustrated J1-J2 antiferromagnet on the square lattice shows that, unlike the saddle-point predictions, there is a quantum nonmagnetic phase for 0.53 < J2/J1 < 0.64. This result is obtained by considering the corrections to the spin stiffness on large lattices and extrapolating to the thermodynamic limit, which avoids the infinite-lattice infrared divergencies associated to Bose condensation. The very good agreement of our results with exact numerical values on finite clusters lends support to the calculational scheme employed.

Antiferromagnetically coupled alternating spin chains

The effect of antiferromagnetic interchain coupling in alternating spin-(1,1/2) chains is studied by means of spin-wave theory and density-matrix renormalization group (DMRG). In particular, two limiting cases are investigated, the two-leg ladder and its two-dimensional (2D) generalization. Results of the ground-state properties like energy, spin gap, magnetizations, and correlation functions are reported for the whole range of the interchain coupling

Classical antiferromagnetism in kinetically frustrated electronic models.

{J}_{\ensuremath{\perp}}.

Stripes in the Ising limit of models for the cuprates

For the 2D case the spin-wave results predict a smooth dimensional crossover from 1D to 2D keeping the ground state always ordered. For the ladder system, the DMRG results show that any

Spin-Peierls dimerization and frustration in two-dimensional antiferromagnets

{J}_{\ensuremath{\perp}}g0

A modified Holstein-Primakoff approach to frustrated quantum spin systems

drives the system to a gapped ground state. Furthermore the behavior of the correlation functions closely resemble the uniform spin-1/2 ladder. For

Evolution of Nagaoka phase with kinetic energy frustrating hopping

{J}_{\ensuremath{\perp}}

Theoretical study of the magnetism in the incommensurate phase of TiOCl

lower than 0.3, however, the gap behaves quadratically as

The spectral function for Mott insulating surfaces

\ensuremath{\Delta}\ensuremath{\sim}{0.6J}_{\ensuremath{\perp}}^{2}.