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arXiv:2510.16150 · The European Physical Journal Plus

Absorbed power in ultracold polarized Fermi mixtures at normal-superfluid separation phase: mass-imbalanced effect

N. Ebrahimian

Published Nov 6, 2025• 0 citations
arXivHTMLPDF
DOI
10.1140/epjp/s13360-025-06980-z

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Thermodynamic Signatures of Phase Separation in Mass Imbalanced Fermi Mixtures: Superfluid Density of States and Quasiparticle Specific Heat in the 163Dy 40K Atomic Mixture

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Mass imbalance changes not only pairing in a polarized Fermi gas but also the thermodynamic condition under which normal and superfluid components can remain phase separated. The low-temperature properties of mass imbalanced two component Fermi gases are investigated within the local density approximation on the BCS side of the BCS BEC crossover, with an unpolarized superfluid spatially separated from a partially polarized normal component. For each interaction strength and mass ratio, the pairing gap, average chemical potential, Hartree Fock potentials, and imbalance chemical potential are obtained self consistently, with the latter determined by the normal superfluid phase separation condition rather than varied independently. The results show that a larger mass ratio requires a larger imbalance chemical potential to maintain phase separation. For the 163Dy 40K mixture, an increase in the absolute value of the interaction strength raises the average and imbalance chemical potentials while reducing the pairing gap. The resulting changes modify the quasiparticle spectrum and the superfluid density of states and are reflected directly in the thermal response, with the quasiparticle specific heat decreasing at a fixed mass ratio. In contrast, increasing the mass ratio enhances the specific heat, which is associated with the larger density of available states near the Fermi surface of the heavier majority component. The results thus connect the normal superfluid phase separation condition with the quasiparticle spectrum and superfluid density of states, and show that the specific heat provides a clear thermal signature of mass imbalance in the phase separated regime.

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We consider ultracold imbalanced Fermi gases with strong attractive interactions, for which Cooper-pair formation plays an important role. The two-component mixtures consist either of identical fermionic atoms in two different hyperfine states, or of two different atomic species. In both cases, the number of atoms for each component is allowed to be different, leading to a spin imbalance or a polarization. In the second case, the two different species also give rise to a mass imbalance for the particles. Imbalanced Fermi mixtures play a fundamental role in condensed-matter, nuclear and astroparticle physics. Experimentally, the atomic mixtures are made strongly interacting with the use of two-channel Feshbach resonances. We start out by treating these in more detail. We review the relevant scattering theory, and apply it to the well-studied case of resonant s-wave collisions with zero angular momentum. Furthermore, we discuss the less-studied case of p-wave collisions with nonzero angular momentum, and the inhomogeneous case due to a trapping potential. Having discussed the two-body physics, we turn to the many-body physics. The pioneering experimental studies of Cooper pairing in lithium-6 mixtures with a spin imbalance were performed by Zwierlein et al. [Science 311, 492 (2006)] and Partridge et al. [Science 311, 503 (2006)]. We study the mean-field theory for the polarized Fermi mixture to obtain a qualitative understanding of these experiments. In the unitarity limit, where the scattering length of the interatomic interaction diverges, the theory gives rise to two exotic superfluid phases that are absent in the balanced mixture. These are the gapless superfluid Sarma phase and the phase-separated phase. By using renormalization group techniques to account for the effects of strong interactions, we obtain also quantitative agreement with Monte-Carlo calculations and experiments. We then generalize our knowledge of the solely spin-imbalanced mixture to the mass-imbalanced case, where in particular the mixture consisting of lithium-6 and potassium-40 is experimentally promising. We study the mean-field phase diagram for this mixture in the unitarity limit, where we not only find phase separation and Sarma superfluidity, but also a Lifshitz point. The latter signals an instability towards a supersolid phase. We also include the effects of fluctuations, which do not alter the topology of the phase diagram. However, fluctuation effects lower the critical temperatures with an experimentally relevant factor of three. Finally, we study superfluidity in a fully polarized gas of potassium-40 atoms near a p-wave Feshbach resonance. In particular, we consider coherent Josephson oscillations between the superfluid components in the two channels of the resonance, whose frequencies contain a clear signature of the quantum phase transition that occurs as a function of applied magnetic field.

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M. Iskin

This thesis focuses on the analysis of Bardeen-Cooper-Schrieffer (BCS) \nto Bose-Einstein condensation (BEC) evolution in ultracold superfluid \nFermi gases when the interaction between atoms is varied. The tuning of \nattractive interactions permits the ground state of the system to evolve from a weak \nfermion attraction BCS limit of loosely bound and largely overlapping Cooper pairs \nto a strong fermion attraction limit of tightly bound small bosonic molecules \nwhich undergo BEC. \nThis evolution is accompanied by anomalous behavior of many superfluid \nproperties, and reveals several quantum phase transitions. \nThis thesis has two parts: \nIn the first part, I analyze zero and nonzero orbital angular momentum \npairing effects, and show that a quantum phase transition occurs for \nnonzero angular momentum pairing, unlike the $s$-wave case where the BCS \nto BEC evolution is just a crossover. \nIn the second part, I analyze two-species fermion mixtures with mass and \npopulation imbalance in continuum, trap and lattice models. In contrast with \nthe crossover physics found in the mass and population balanced mixtures, \nI demonstrate the existence of phase transitions between normal and \nsuperfluid phases, as well as phase separation between superfluid (paired) \nand normal (excess) fermions in imbalanced mixtures as a function of scattering \nparameter and mass and population imbalance.

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Understanding the Mass-Imbalanced Highly-Polarized Fermi Gases

Michael Kolodrubetz, Bryan K. Clark

Bulletin of the American Physical Society•Mar 22, 2011•0 citations
Mass imbalance changes not only pairing in a polarized Fermi gas but also the thermodynamic condition under which normal and superfluid components can remain phase separated. The low-temperature properties of mass imbalanced two component Fermi gases are investigated within the local density approximation on the BCS side of the BCS BEC crossover, with an unpolarized superfluid spatially separated from a partially polarized normal component. For each interaction strength and mass ratio, the pairing gap, average chemical potential, Hartree Fock potentials, and imbalance chemical potential are obtained self consistently, with the latter determined by the normal superfluid phase separation condition rather than varied independently. The results show that a larger mass ratio requires a larger imbalance chemical potential to maintain phase separation. For the 163Dy 40K mixture, an increase in the absolute value of the interaction strength raises the average and imbalance chemical potentials while reducing the pairing gap. The resulting changes modify the quasiparticle spectrum and the superfluid density of states and are reflected directly in the thermal response, with the quasiparticle specific heat decreasing at a fixed mass ratio. In contrast, increasing the mass ratio enhances the specific heat, which is associated with the larger density of available states near the Fermi surface of the heavier majority component. The results thus connect the normal superfluid phase separation condition with the quasiparticle spectrum and superfluid density of states, and show that the specific heat provides a clear thermal signature of mass imbalance in the phase separated regime.
We consider ultracold imbalanced Fermi gases with strong attractive interactions, for which Cooper-pair formation plays an important role. The two-component mixtures consist either of identical fermionic atoms in two different hyperfine states, or of two different atomic species. In both cases, the number of atoms for each component is allowed to be different, leading to a spin imbalance or a polarization. In the second case, the two different species also give rise to a mass imbalance for the particles. Imbalanced Fermi mixtures play a fundamental role in condensed-matter, nuclear and astroparticle physics. Experimentally, the atomic mixtures are made strongly interacting with the use of two-channel Feshbach resonances. We start out by treating these in more detail. We review the relevant scattering theory, and apply it to the well-studied case of resonant s-wave collisions with zero angular momentum. Furthermore, we discuss the less-studied case of p-wave collisions with nonzero angular momentum, and the inhomogeneous case due to a trapping potential. Having discussed the two-body physics, we turn to the many-body physics. The pioneering experimental studies of Cooper pairing in lithium-6 mixtures with a spin imbalance were performed by Zwierlein et al. [Science 311, 492 (2006)] and Partridge et al. [Science 311, 503 (2006)]. We study the mean-field theory for the polarized Fermi mixture to obtain a qualitative understanding of these experiments. In the unitarity limit, where the scattering length of the interatomic interaction diverges, the theory gives rise to two exotic superfluid phases that are absent in the balanced mixture. These are the gapless superfluid Sarma phase and the phase-separated phase. By using renormalization group techniques to account for the effects of strong interactions, we obtain also quantitative agreement with Monte-Carlo calculations and experiments. We then generalize our knowledge of the solely spin-imbalanced mixture to the mass-imbalanced case, where in particular the mixture consisting of lithium-6 and potassium-40 is experimentally promising. We study the mean-field phase diagram for this mixture in the unitarity limit, where we not only find phase separation and Sarma superfluidity, but also a Lifshitz point. The latter signals an instability towards a supersolid phase. We also include the effects of fluctuations, which do not alter the topology of the phase diagram. However, fluctuation effects lower the critical temperatures with an experimentally relevant factor of three. Finally, we study superfluidity in a fully polarized gas of potassium-40 atoms near a p-wave Feshbach resonance. In particular, we consider coherent Josephson oscillations between the superfluid components in the two channels of the resonance, whose frequencies contain a clear signature of the quantum phase transition that occurs as a function of applied magnetic field.
This thesis focuses on the analysis of Bardeen-Cooper-Schrieffer (BCS) \nto Bose-Einstein condensation (BEC) evolution in ultracold superfluid \nFermi gases when the interaction between atoms is varied. The tuning of \nattractive interactions permits the ground state of the system to evolve from a weak \nfermion attraction BCS limit of loosely bound and largely overlapping Cooper pairs \nto a strong fermion attraction limit of tightly bound small bosonic molecules \nwhich undergo BEC. \nThis evolution is accompanied by anomalous behavior of many superfluid \nproperties, and reveals several quantum phase transitions. \nThis thesis has two parts: \nIn the first part, I analyze zero and nonzero orbital angular momentum \npairing effects, and show that a quantum phase transition occurs for \nnonzero angular momentum pairing, unlike the sss-wave case where the BCS \nto BEC evolution is just a crossover. \nIn the second part, I analyze two-species fermion mixtures with mass and \npopulation imbalance in continuum, trap and lattice models. In contrast with \nthe crossover physics found in the mass and population balanced mixtures, \nI demonstrate the existence of phase transitions between normal and \nsuperfluid phases, as well as phase separation between superfluid (paired) \nand normal (excess) fermions in imbalanced mixtures as a function of scattering \nparameter and mass and population imbalance.