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arXiv:cond-mat/0012011 · Physical Review A

Ground state and quasiparticle spectrum of a two-component Bose-Einstein condensate

Christopher P. Search, Alberto G. Rojo, P. R. Berman

Published Jun 7, 2001• 33 citations
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Abstract

We consider a dilute homogenous atomic Bose-Einstein condensate with two nondegenerate internal energy levels that achieve a state of chemical equilibrium in the presence of an external radiation field that couples the two states. The presence of the radiation field can result in new ground states for the condensate as a consequence of the lowering of the condensate energy due to the interaction energy with the field. We analyze the ground-state energy as a function of the coupling constants for the two-body interactions, the Rabi frequency of the radiation field, and the detuning of the field, and we give explicit expressions for the quasiparticle excitation spectrum.

DOI
10.1103/physreva.64.013615

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We consider a dilute homogenous atomic Bose-Einstein condensate with two nondegenerate internal energy levels that achieve a state of chemical equilibrium in the presence of an external radiation field that couples the two states. The presence of the radiation field can result in new ground states for the condensate as a consequence of the lowering of the condensate energy due to the interaction energy with the field. We analyze the ground-state energy as a function of the coupling constants for the two-body interactions, the Rabi frequency of the radiation field, and the detuning of the field, and we give explicit expressions for the quasiparticle excitation spectrum.
We consider a two-component weakly interacting Bose-Einstein condensate in the presence of an external field which couples the two components. We express the Hamiltonian in terms of the energy eigenstates of the single-body part of the Hamiltonian. These eigenstates are the atomic dressed states of quantum optics. When the energy difference between the two dressed states is much larger than the mean-field interactions, two-body interactions in the dressed state basis that do not conserve the number of atoms in each of the two dressed states are highly suppressed. The two-body interactions then take on a simplified form in the dressed basis with effective coupling constants that depend on the intensity and frequency of the external field. This implies that the chemical potential as well as the quasiparticle spectrum may be controlled experimentally in a simple manner. We demonstrate this by showing that one may achieve significant variations in the speed of sound in the condensate, a quantity which has been measured experimentally.
We study the dynamics of weakly interacting Bose-Einstein condensates consisting of alkali atoms with two or more internal eigenstates. Three separate problems are examined in this thesis, which look at the behavior of the condensates in the presence of electromagnetic fields, which couple the internal states of the atoms. First, we examine the ground state and quasiparticle excitations for a condensate with a spatially uniform and time independent field, which couples two of the internal states. In particular, we address the nature of the instabilities that have been predicted in these systems. Second, we look at the time evolution of small condensates with a time dependent field, which couples two states. It is shown that the atomic population in the two components, as a function of time, is very different for a condensate with a precisely defined number of atoms and one that exhibits spontaneous symmetry breaking. Finally, we propose two methods for mapping information about the quantum state of the condensate to a mode of an electromagnetic cavity. The first method, which is irreversible, is only able to transfer the population statistics of the condensate to the cavity field. The second method involves the adiabatic transfer of the quantum state of the condensate to the cavity field.
Abstract We study a many-body system of interacting fermionic atoms of two species that are in thermodynamic equilibrium with their condensed heteronuclear bound states (molecules). In order to describe such an equilibrium state, we use a microscopic approach that involves the Bogoliubov model for a weakly interacting Bose gas and approximate formulation of the second quantization method in the presence of bound states of particles elaborated earlier by the authors. This microscopic approach is valid at low temperatures, when the average kinetic energy of all the components in the system is small in comparison with the bound state energy. The coupled equations, which relate the chemical potentials of fermionic components and molecular condensate density, are obtained within the proposed theory. At zero temperature, these equations are analyzed both analytically and numerically, attracting the relevant experimental data. We find the conditions at which a condensate of heteronuclear molecules coexists in equilibrium with degenerate components of a Fermi gas. The ground state energy and single-particle excitation spectrum are found. The boundaries of the applicability of the developed microscopic approach are analyzed.
The features of Bose condensation in an equilibrium ideal gas consisting of two types of charged fermions and their bound states—hydrogenlike atoms—in the presence of equilibrium between the photons and matter are investigated. It is shown that under such conditions the main influence on the Bose–Einstein condensation comes from the existence of levels concerned with the hyperfine splitting of the ground state of the hydrogenlike atom. The critical temperature and condensate density as functions of magnetic field are determined by considering effects due to the additional splitting of the levels of the hyperfine structure of the ground state in an external uniform static magnetic field (the Zeeman and Paschen–Back effects). It is found that under conditions of total statistical equilibrium in the system, a condensate is formed only by atoms found in the lowest energy state. It is shown that in the absence of equilibrium between radiation and matter, in the region of ultralow temperatures and low densities, the system can be treated as a multicomponent ideal gas of hydrogenlike atoms. The existence of a hierarchy of individual transition temperatures of each of the samples to the state with Bose–Einstein condensation is established. Expressions are found for the critical temperatures and number densities of particles in the condensate for each of the system components.
We investigate the properties of a dilute boson-fermion mixture at zero temperature. The ground-state energy of the system is calculated to second order in the Bose-Fermi coupling constant. The Green's function formalism is applied to obtain results for the effect of Bose-Fermi interactions on the spectrum of phonon excitations, the momentum distribution, the condensate fraction, and the superfluid density of the bosonic component. A quantitative discussion of these effects is presented in the case of mixtures with attractive interspecies interaction.