H. M. Cataldo

Structure of vortices in two-component Bose-Einstein condensates

We develop a three-dimensional analysis of the phase separation of two-species Bose-Einstein condensates in the presence of vorticity within the Thomas-Fermi approximation. We find different segregation features according to whether the more repulsive component is in a vortex or in a vortex-free state. An application of this study is aimed at describing systems formed by two almost immiscible species of rubidium-87 that are commonly used in Bose-Einstein condensation experiments. In particular, in this work we calculate the density profiles of condensates for the same conditions as the states prepared in the experiments performed at JILA [Matthews et al., Phys. Rev. Lett. 83, 2498 (1999)]

Two-mode effective interaction in a double-well condensate

Fil: Jezek, Dora Marta. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Oficina de Coordinacion Administrativa Ciudad Universitaria. Instituto de Fisica de Buenos Aires; Argentina;

Dynamical aspects of phonon-phonon coupling in collective mode damping

We present an extension of the Quantal Brownian Motion (QBM) model of vibration damping that incorporates phonon-phonon or phonon-(two-particle-two-hole) interactions as sources of dissipative evolution of the excited mode. Starting from the Schrodinger-on Neumann equation of motion, a reduction procedure combined with the proper approximations leads to coupled, nonlinear master equations for the density vectors of the separate oscillators. The fermionic heat bath remains equilibrated at temperature T. The evolution of the phonon system is numerically analyzed under different initial conditions that simulate excitation of one or more collective vibrations, for several strengths of mode-mode coupling. It is found that in the majority of cases the system reaches statistical equilibrium with relaxation times that can be extracted from the numerical treatment.

Stability of the hard-sphere icosahedral quasilattice

The stability of the hard-sphere icosahedral quasilattice is analyzed using the differential formulation of the generalized effective liquid approximation. We find that the icosahedral quasilattice is metastable with respect to the hardsphere crystal structures. Our results agree with recent findings by McCarley and Ashcroft [Phys. Rev. B 49, 15600 (1994)] carried out using the modified weighted density approximation.

Holomorphy of the Fourier transformed memory kernel in the quantum Langevin equation

Abstract A basic hypothesis of holomorphy of the Fourier transformed memory kernel is used to introduced non-Markovian corrections to the usual quantum Langevin equation with approximate memory kernel of vanishing lifetime. An application is presented for an electron in the blackbody radiation field showing that the lowest order corrections lead to a quantum stochastic version of the Abraham-Lorentz equation of motion.

Non-Markovian relaxation of a quantum system

A non-Markovian treatment of the relaxation of a spin j is developed. The corresponding reduced collision operator admits a discrete spectral representation whose eginevalues are extracted in a high temperature limit. Then, assuming a phonon reservoir with a Debye-like density of phonon states, the time evolution of the collision operator is explicitly given, showing that the corresponding time decay arises from the branch points of its Laplace transform. The extraction of the analytic continuation of the transform allows the memory effects on the relaxation frequencies (resolvent poles) to be analyzed and the validity of the rotating-wave approximation to be tested.

Dynamics in asymmetric double-well condensates

Fil: Cataldo, Horacio Maximo. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Oficina de Coordinacion Administrativa Ciudad Universitaria. Instituto de Fisica de Buenos Aires. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Fisica de Buenos Aires; Argentina

Stability of a hard-sphere binary quasicrystal

The stability of a quasicrystalline structure, recently obtained in a molecular dynamics simulation of rapid cooling of a binary melt, is analysed for binary hard-sphere mixtures within a density-functional approach. It is found that this quasicrystal is metastable relative to a crystalline and fluid phases for diameter ratios above 0.83. Such a trend is partially reversed for lower diameter ratios, since the quasicrystal becomes stable with respect to the crystal but does not reach a coexistence with the fluid.

Dissipative dynamics of vortex lines in superfluid {sup 4}He

We propose a Hamiltonian model that describes the interaction between a vortex line in superfluid {sup 4}He and the gas of elementary excitations. An equation of irreversible motion for the density operator of the vortex, regarded as a macroscopic quantum particle with a finite mass, is derived in the frame of generalized master equations. This enables us to cast the effect of the coupling as a drag force with one reactive and one dissipative component, in agreement with the assumption of the phenomenological theories of vortex mutual friction in the two fluid model. {copyright} {ital 1996} {ital The American Physical Society}

Singularity spectrum of the resolvent in a nonmarkovian master equation

Abstract The well-known standard nonmarkovian master equation is used to describe the damping of a harmonic macroscopic coordinate coupled to a thermally equilibrated Fermi gas. The time evolution of the phonon distribution is expressed in terms of a resolvent whose spectrum of singularities is computed and analyzed. It is seen that an apparent limitation of the model related to the appearance of undesired oscillations does not inhibit the expected behavior of the average phonon number and phonon number dispersion.