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Ruggero Vaia

Publications and source records attributed to Ruggero Vaia.

42 records · Page 3Linked to original sources
arXiv1998

Spectral shapes of solid neon

We present a Path Integral Monte Carlo calculation of the first three moments of the displacement-displacement correlation functions of solid neon at different temperatures for longitudinal and transverse phonon modes. The Lennard-Jones potential is considered. The relevance of the quantum effects on the frequency position of the peak and principally on the line-width of the spectral shape is clearly pointed out. The spectrum is reconstructed via a continued fraction expansion; the approximations introduced using the effective potential quantum molecular dynamics are discussed.

cond-mat↗
arXiv1998

Heisenberg antiferromagnet on the square lattice for S>=1

Theoretical predictions of a semiclassical method - the pure-quantum self-consistent harmonic approximation - for the correlation length and staggered susceptibility of the Heisenberg antiferromagnet on the square lattice (2DQHAF) agree very well with recent quantum Monte Carlo data for S=1, as well as with experimental data for the S=5/2 compounds Rb2MnF4 and KFeF4. The theory is parameter-free and can be used to estimate the exchange coupling: for KFeF4 we find J=2.33 +- 0.33 meV, matching with previous determinations. On this basis, the adequacy of the quantum nonlinear sigma model approach in describing the 2DQHAF when S>=1 is discussed.

cond-mat↗
arXiv1997

Phase-transitions induced by easy-plane anisotropy in the classical Heisenberg antiferromagnet on a triangular lattice: a Monte Carlo simulation

We present the results of Monte Carlo simulations for the antiferromagnetic classical XXZ model with easy-plane exchange anisotropy on the triangular lattice, which causes frustration of the spin alignment. The behaviour of this system is similar to that of the antiferromagnetic XY model on the same lattice, showing the signature of a Berezinskii-Kosterlitz-Thouless transition, associated to vortex-antivortex unbinding, and of an Ising-like one due to the chirality, the latter occurring at a slightly higher temperature. Data for internal energy, specific heat, magnetic susceptibility, correlation length, and some properties associated with the chirality are reported in a broad temperature range, for lattice sizes ranging from 24x24 to 120x120; four values of the easy-plane anisotropy are considered. Moving from the strongest towards the weakest anisotropy (1%) the thermodynamic quantities tend to the isotropic model behaviour, and the two transition temperatures decrease by about 25% and 22%, respectively.

cond-mat.stat-mech↗
arXiv1997

The two-dimensional quantum Heisenberg antiferromagnet: effective Hamiltonian approach to the thermodynamics

In this paper we present an extensive study of the thermodynamic properties of the two-dimensional quantum Heisenberg antiferromagnet on the square lattice; the problem is tackled by the pure-quantum self-consistent harmonic approximation, previously applied to quantum spin systems with easy-plane anisotropies, modeled to fit the peculiar features of an isotropic system. Internal energy, specific heat, correlation functions, staggered susceptibility, and correlation length are shown for different values of the spin, and compared with the available high-temperature expansion and quantum Monte Carlo results, as well as with the available experimental data.

cond-mat.stat-mech↗
arXiv1997

Thermodynamics of dissipative quantum systems by effective potential

Classical-like formulas are given in order to evaluate thermal averages of observables belonging to a quantum nonlinear system with dissipation described by the Caldeira-Leggett model [Phys. Rev. Lett. 46, 211 (1981); Ann. Phys. (N.Y.) 149, 374 (1983)]. The underlying scheme is the pure-quantum self-consistent harmonic approximation, which leads to expressions with a Boltzmann factor involving an effective potential and with a Gaussian average. The latter describes the effect of the fluctuations of purely quantum origin. As an illustration we calculate the coordinate probability distribution for a double-well potential in the presence of various degrees of Ohmic dissipation.

cond-mat.stat-mech↗