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Progress of Theoretical Physics

Bose-Einstein Condensation and Quantum Hydrodynamics

Masahiko HIROOKA, Shuichiro Yamasaki

Published Nov 1, 1980• 2 citations
Publisher

Abstract

The velocity operator vk studied in a previous paper admits further reduction in a Bose-Einstein condensed system. The reduced expression manifests the longitudinal nature of the operator vk(k≠0), and is proportional to the operator φk canonically conjugate to the density fluctuation, ρk. The value of v0 is the momentum to mass ratio of a condensed particle. As a dynamical variable describing the Hamiltonian, v0 plays peculiar role reminiscent of the super-velocity introduced in phenomenological theories. The collective formalism of condensed boson systems is found to provide the microscopic foundation of the quantum hydrodynamics incorporating the super-velocity.

DOI
10.1143/ptp.64.1521

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The velocity operator vk studied in a previous paper admits further reduction in a Bose-Einstein condensed system. The reduced expression manifests the longitudinal nature of the operator vk(k≠0), and is proportional to the operator φk canonically conjugate to the density fluctuation, ρk. The value of v0 is the momentum to mass ratio of a condensed particle. As a dynamical variable describing the Hamiltonian, v0 plays peculiar role reminiscent of the super-velocity introduced in phenomenological theories. The collective formalism of condensed boson systems is found to provide the microscopic foundation of the quantum hydrodynamics incorporating the super-velocity.
Concrete expressions are given to the operators φk forming a set of canonically conjugate variables to the density fluctuations ρl in a Bose-Einstein condensed many-boson system. The Fourier transform of φk to the coordinate space admits in classical terms an interpretation as the phase of the boson field ψ(x). Various objections against the existence of canonically conjugate variables to the density are critically examined. They do not apply to the proposed operator φk which is non local and canonically conjugate only to the density fluctuation.
We have developed a microscopic theory of superfluidity in a two-dimensional, slightly nonideal Bose gas. The theory is rooted in the fact that a number of bosons are concentrated in the small momentum region at low temperatures when the interaction is weak, in spite of the fact that there is no Bose condensate in the two-dimensional case when T≠0. This fact lets us reduce the description of the system from one employing creation ak+ and annihilation ak operators for bosons to one based on combinations of these operators ρk and φk(ρk = S−1Σqaq+aq+k, φk· = −(i/ns) Σ(kq/k2 + l)aq+aq+k, where n is the boson density, and S is the area of the system). The description in terms of ρk and φk is simple because the hamiltonian is quadratic with respect to ρk and φk. The operators ρk and φk can also be used to express the occupation number Nk = ak+ak and current operators. We find the elementary excitation spectrum (which is identical to the Bogolyubov spectrum) and the momentum distribution of the bosons. We calculate the normal density due to thermal oscillations in the Bose gas and to interactions with impurity centers in the substrate.
The Bose-Einstein condensation mechanism is shown to be capable of accounting for the existence of separate hydrodynamic velocity fields for the normal and the superfluid, provided suitable assumptions are made with regard to the single-particle energy spectrum. We consider the effect on the microcanonical gas distribution of imposing a nonzero value of the total momentum P. For an ordinary gas the effect is trivial, the whole distribution being merely shifted in momentum space. However, in the case of a degenerate Bose gas whose single-particle energy spectrum has a sharp minimum (gap or cusp), only the excited part of the gas (normal fluid) participates in the imposed motion, the condensate (superfluid) remaining "frozen" in momentum space. This rigidity of the condensate in momentum space plays the same role as the rigidity of the superelectrons on imposition of a magnetic field in the London theory of superconductivity. P acts as an additional thermodynamic variable, states with P \ensuremath{\ne} 0 being macroscopically metastable and corresponding to the existence of a relative velocity between normal and superfluid. The basic hydrodynamic assumption of the two-fluid model is thus reduced to an assumption concerning the form of an effective single-particle energy spectrum, and the parallelism between the theories of superconductivity and superfluidity is clearly exhibited. The present theory permits, in particular, the introduction of the mathematical form of Landau's phonon and "roton" spectra within the framework of the Bose-Einstein condensation picture. The statistical-thermodynamic formulas are derived and are shown to lead to characteristic two-fluid equations derived previously from a variational principle.
Dynamics of Bose-Einstein Condensate (BEC) is treated on the basis of the Mori theory of non-equilibrium thermodynamics in the hydrodynamic region. The local values of conserved quantities, i.e., particle number, momentum and energy, and Ψ(x), the wave function of BEC, are adopted as a complete set of macrovariables. The set of equations of motion thus obtained is useful to describe the dynamical behavior of superfluid 4He near the λ-point. The density of condensate is not conserved, but its equation of motion has the term which is relevant to the renormalization of condensate to superfluid. The expression for the relaxation time τc of condensate density to its local-equilibrium is given and it is shown that the anomaly of τc near the λ-point coincides with the behavior experimentally determined if nc(∂µc/∂nc) is proportional to (Tλ-T) as in the Landau theory of phase transition. In case ωτc ≪1 the ordinary two-fluid hydrodynamics is derived by eliminating the motion of condensate density.
The existence of a Bose-Einstein condensation in an interacting many-boson system at T = 0°K is proved under certain conditions on the particle density and the interparticle potential. Starting with the tentative assumption that the condensation exists, we study the fluctuation in the occupation number of the condensate with due regard to its interactions (1) with particles outside the condensate as well as (2) with the fluctuation itself. If the condensate fluctuation has a normalizable ground state, then the assumed existence of the condensation is tenable. For the case of the pair-Hamiltonian model satisfying the conditions for condensation, the interactions of the second category are of no importance. In the limit of infinite volume, this Hamiltonian can be diagonalized in an irreducible representation of a Bose-field operatorφ(x), whereφ(x) has nonvanishing ground state expectation value, in accordance with the usual c-number replacement of creation and destruction operators for the condensate particles. The full Hamiltonian for a system of pairwise interacting bosons is studied only in a low-density limit. Bose-Einstein condensation exists when the over-all space integral of the interparticle potential is positive. In this case the interactions of the second category play an important role in ensuring a normalizable ground state for the condensate fluctuation. There is an indication that in the limit of infinite volume the Hamiltonian cannot be diagonalized in any irreducible representation of the field operatorφ. Yet the c-number replacement of the condensate operators is legitimate as far as states of particles outside the condensate are concerned. Some speculations are made as to what may happen for systems of moderate density.