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Journal of Energy Resources Technology

Thermodynamic Correlation of the Entropy of Bose–Einstein Condensation Transition to the Lambda Points of Superfluids

George-Rafael Domenikos, Emmanouil D. Rogdakis, Irene P. Koronaki

Published May 26, 2022• 5 citations
Publisher

Abstract

Abstract In this work, the authors show an apparent connection in the behaviors of superfluid Helium in its two stable isotopes, where the lambda transition to superfluidity is seen to correspond to a theoretical Bose–Einstein condensation temperature of the ideal parts of the two isotopes. A statistical model for the physical and thermodynamic behaviors is developed based on their known properties, where the equations of the partition function, entropy, and specific heat are constructed in such a way where one term applies to the ideal part of the fluid and the second to its interacting part. The calculated temperatures for the formation of a theoretical Bose–Einstein condensation in the ideal parts of both isotopes almost completely matches their lambda transition temperatures. The models of the two isotopes are very different due to the fermionic nature of Helium-3, for which the model is formed based on the Bardeen–Cooper–Schrieffer (BCS) theory of Cooper pairs of fermions. Using this formulation, the Bose–Einstein condensation temperature for the pairs of Helium-3 atoms is calculated, as opposed to the condensation temperature for Helium-4, which is directly derived from the bosonic atoms, leading to its much lower value than Helium-4.

DOI
10.1115/1.4054652

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•Jan 1, 2006•84 citations
Abstract In this work, the authors show an apparent connection in the behaviors of superfluid Helium in its two stable isotopes, where the lambda transition to superfluidity is seen to correspond to a theoretical Bose–Einstein condensation temperature of the ideal parts of the two isotopes. A statistical model for the physical and thermodynamic behaviors is developed based on their known properties, where the equations of the partition function, entropy, and specific heat are constructed in such a way where one term applies to the ideal part of the fluid and the second to its interacting part. The calculated temperatures for the formation of a theoretical Bose–Einstein condensation in the ideal parts of both isotopes almost completely matches their lambda transition temperatures. The models of the two isotopes are very different due to the fermionic nature of Helium-3, for which the model is formed based on the Bardeen–Cooper–Schrieffer (BCS) theory of Cooper pairs of fermions. Using this formulation, the Bose–Einstein condensation temperature for the pairs of Helium-3 atoms is calculated, as opposed to the condensation temperature for Helium-4, which is directly derived from the bosonic atoms, leading to its much lower value than Helium-4.
The goal of this chapter will be to briefly describe the remarkable properties of helium at low temperatures. After stating some of these properties we will see how they can be understood in terms of the phenomenon of Bose–Einstein condensation described in Chapter 7. We will give the main argument in two different formulations, once using the quasi-particle method of Bogoliubov, and then using a Green function approach. We start with some experimental facts. Helium is a remarkable element. It was predicted to exist from observations of the Sun before it was found on Earth. It is the only element which remains a liquid at zero temperature and atmospheric pressure. Experimentally the phase diagram of 4 He is shown in Figure 10.1. Helium I is a normal fluid and has a normal gas-liquid critical point. Helium II is a mixture of a normal fluid and a superfluid. The superfluid is characterized by the vanishing of its viscosity. Helium I and helium II are separated by a line known as the λ -transition line. At T λ = 2.18 K, P λ = 2.29 Pa, helium I, helium II, and helium gas coexist. The specific heat of liquid helium along the vapor transition line forms a logarithmic discontinuity shown in Figure 10.2. The form of this diagram resembles the Greek letter λ and is the reason for calling the transition a λ -transition. The lack of viscosity of helium II leads to some remarkable experimental consequences, one of which we briefly describe. Let two containers A and B be linked by a thin capillary through which only a fluid with zero (or very low) viscosity can pass freely.
The new generalized Bose–Einstein condensation (GBEC) quantum-statistical theory starts from a noninteracting ternary boson-fermion (BF) gas of two-hole Cooper pairs (2hCPs) along with the usual two-electron Cooper pairs (2eCPs) plus unpaired electrons. Here we obtain the entropy and heat capacity and confirm once again that GBEC contains as a special case the Bardeen–Cooper–Schrieffer (BCS) theory. The energy gap is first calculated and compared with that of BCS theory for different values of a new dimensionless coupling parameter n/n[Formula: see text] where n is the total electron number density and n[Formula: see text] that of unpaired electrons at zero absolute temperature. Then, from the entropy, the heat capacity is calculated. Results compare well with elemental-superconductor data suggesting that 2hCPs are indispensable to describe superconductors (SCs).
This article gives a detailed review of our current theoretical understanding of the crossover from cooperative Cooper pairing to independent bound-state formation and Bose–Einstein condensation in Fermi systems with increasing attractive interactions. Introduction There are two well-known paradigms for understanding the phenomena of superconductivity and superfluidity: (I) BCS theory [1], in which the normal state is a degenerate Fermi liquid that undergoes a pairing instability at a temperature T c ≪ ε f , the degeneracy scale. The formation of Cooper pairs and their condensation (macroscopic occupation of a single quantum state) both occur simultaneously at the transition temperature T c . (II) Bose–Einstein condensation (BEC) of bosons at a T c of the order of their degeneracy temperature. At a fundamental level these bosons (for example, 4 He or excitons) are invariably composite objects made up of an even number of fermions. The composite particles form at some very high temperature scale of the order of their dissociation temperature T dissoc , and these “pre-formed” bosons then condense at the BEC T c ≪ T dissoc . In most cases of experimental interest the system under consideration clearly falls into one category or the other. For instance, 3 He is a Fermi superfluid described by (I) whereas 4 He is a Bose superfluid (II). Essentially all cases of superconductivity in metals that we understand reasonably well are much closer to (I) than to (II), as evidenced by the existence of fermionic quasiparticles above T c .
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.
PACS. 03.75.Hh – Static properties of condensates; thermodynamical, statistical and structural properties. PACS. 03.75.Ss – Degenerate Fermi gases. PACS. 05.30.Fk – Fermion systems and electron gas. Abstract. – We present a theory for a superfluid Fermi gas near the BCS-BEC crossover, including pairing fluctuation contributions to the free energy similar to that considered by Nozières and Schmitt-Rink for the normal phase. In the strong coupling limit, our theory is able to recover the Bogoliubov theory of a weakly interacting Bose gas with a molecular scattering length very close to the known exact result. We compare our results with recent Quantum Monte Carlo simulations both for the ground state and at finite temperature. Excellent agreement is found for all interaction strengths where simulation results are available. The recent experimental realization of strongly interacting Fermi gases of 6 Li and 40 K atoms near a Feshbach resonance has opened up the exciting possibility of investigating the crossover from a Bardeen-Cooper-Schrieffer (BCS) superfluid to a Bose-Einstein condensate (BEC) [1–5]. In these systems, the inter-atomic interaction strength can be varied by tuning