The energy gap of the 1D AF-Heisenberg model in the presence of both uniform (H) and staggered (h) magnetic fields is investigated using the exact diagonalization technique. The opening of the gap in the presence of a staggered field is found to scale with h<Superscript>ν</Superscript>, where ν=ν(H) is the critical...</superscript>

Motived by the necessity of explicit and reliable calculations, as a valid contribution to clarify the effectiveness and, possibly, the limits of the Tsallis thermostatistics, we formulate the Two-Time Green Functions Method in nonextensive quantum statistical mechanics within the optimal...

We study the one-dimensional isotropic spin-1 Heisenberg magnet with antiferromagnetic nearest-neighbor (nn) and next-nearest-neighbor (nnn) interactions by using the modified spin wave theory (MSWT). The ground state energy and the singlet-triplet energy gap are obtained for several values of...

We investigate the ground-state magnetic long-range order of quasi-one-dimensional quantum Heisenberg antiferromagnets for spin quantum numbers s=1/2 and s=1. We use the coupled cluster method to calculate the sublattice magnetization and its dependence on the inter-chain coupling J<Subscript>⊥</Subscript>. We find...</subscript>

We have studied the behavior of the energy gap of the 1D AF spin-<InlineEquation ID="Equ1"> <EquationSource Format="TEX">$\frac{1}{2}$</EquationSource> </InlineEquation> XXZ model in a transverse magnetic field (h) using the exact diagonalization technique. The ground state phase diagram consists of two spin-flop and paramagnetic phases. Using a modified finite-size scaling...</equationsource></inlineequation>

We study the low-temperature thermodynamic properties of a number of frustrated quantum antiferromagnets which support localized magnon states in the vicinity of the saturation field. For this purpose we use (1) a mapping of the low-energy degrees of freedom of spin systems onto the hard-core...

The Jordan-Wigner transformation is applied to study the ground state properties and dimerization transition in the J<Subscript>1</Subscript>-J<Subscript>2</Subscript> XXZ chain. We consider different solutions of the mean-field approximation for the transformed Hamiltonian. Ground state energy and the static structure factor are compared...</subscript></subscript>