A. E. Trumper

Long-Range Néel Order in the Triangular Heisenberg Model

We have studied the Heisenberg model on the triangular lattice using several Quantum Monte Carlo (QMC) techniques (up to 144 sites), and exact diagonalization (ED) (up to 36 sites). By studying the spin gap as a function of the system size we have obtained a robust evidence for a gapless spectrum, confirming the existence of long range Neel order. Our best estimate is that in the thermodynamic limit the order parameter m= 0.41 +/- 0.02 is reduced by about 59% from its classical value and the ground state energy per site is e0=-0.5458 +/- 0.0001 in unit of the exchange coupling. We have identified the important ground state correlations at short distance.

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

Finite-size spin-wave theory of the triangular Heisenberg model

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

ROTATIONAL INVARIANCE AND ORDER-PARAMETER STIFFNESS IN FRUSTRATED QUANTUM SPIN SYSTEMS

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

Classical antiferromagnetism in kinetically frustrated electronic models.

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

Spin-Peierls dimerization and frustration in two-dimensional antiferromagnets

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

A modified Holstein-Primakoff approach to frustrated quantum spin systems

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

Evolution of Nagaoka phase with kinetic energy frustrating hopping

lower than 0.3, however, the gap behaves quadratically as

1/T1 nuclear relaxation time of κ-(BEDT–TTF)2Cu[N(CN)2]Cl : effects of magnetic frustration

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