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arXiv:2512.09252 · Physical Review A

Thermal liquid-gas phase transition in a quasi-one-dimensional dipolar Fermi gas

Lanxuan Gao, Koki Takayama, Hiroyuki Tajima, Takahiro M. Doi, Haozhao Liang

Published May 1, 2026• 0 citations
arXivHTMLPDF

Abstract

We theoretically investigate the thermodynamic properties of a quasi-one-dimensional single-component dipolar Fermi gas at finite temperatures. A self-bound fermionic droplet can be achieved by exchange correlations with long-range dipole-dipole interactions under quasi-one-dimensional confinement, where the interaction can be tuned by tilting the dipoles along the system coordinate. Using the Hartree-Fock approximation, we show how the liquid-gas phase transition occurs in this system and elucidate the finite-temperature phase structure consisting of a gas phase, liquid phase, gas-liquid coexistence phase, and spinodal phase. We also discuss its similarity to the liquid-gas phase transition in nuclear matter through a comparison with phenomenological models. By examining the experimental conditions for realizing self-bound fermionic droplets, we find that microwave-shielded fermionic polar molecules are promising candidates. Our results will be useful for an interdisciplinary understanding of self-bound fermionic matter as well as an analog quantum simulation of nuclear systems.

DOI
10.1103/z4wr-f6wd

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We theoretically investigate the thermodynamic properties of a quasi-one-dimensional single-component dipolar Fermi gas at finite temperatures. A self-bound fermionic droplet can be achieved by exchange correlations with long-range dipole-dipole interactions under quasi-one-dimensional confinement, where the interaction can be tuned by tilting the dipoles along the system coordinate. Using the Hartree-Fock approximation, we show how the liquid-gas phase transition occurs in this system and elucidate the finite-temperature phase structure consisting of a gas phase, liquid phase, gas-liquid coexistence phase, and spinodal phase. We also discuss its similarity to the liquid-gas phase transition in nuclear matter through a comparison with phenomenological models. By examining the experimental conditions for realizing self-bound fermionic droplets, we find that microwave-shielded fermionic polar molecules are promising candidates. Our results will be useful for an interdisciplinary understanding of self-bound fermionic matter as well as an analog quantum simulation of nuclear systems.
Using a combination of results from exact mappings and from mean-field theory we explore the phase diagram of quasi-one-dimensional systems of identical fermions with attractive dipolar interactions. We demonstrate that at low density these systems provide a realization of a single-component one-dimensional Fermi gas with a generalized contact interaction. Using an exact duality between one-dimensional Fermi and Bose gases, we show that when the dipole moment is strong enough, bound many-body states exist, and we calculate the critical coupling strength for the emergence of these states. At higher densities, the Hartree--Fock approximation is accurate, and by combining the two approaches we determine the structure of the phase diagram. The many-body bound states should be accessible in future experiments with ultracold polar molecules.
Abstract Advances in semiconductor nanostructures have enabled the creation of ultra-cold dipolar fermions, offering a new frontier for exploring many-body fermionic physics. By manipulating electric dipole interactions in these systems, we demonstrate the possibility of cooling and condensing particles. The anisotropic nature of dipole interactions leads to an anisotropic Fermi liquid state, distinct from the isotropic standard, and supports unconventional Cooper pair symmetries involving both singlet and triplet states. Long-range dipolar interactions foster strong correlations within the system. Employing methods such as Gaussian expansion and random-phase-approximation, we analyze how anisotropy influences the spectrum, collective excitations, and correlation functions. In particular, we derive the screened interaction and the polarization function, and we investigate the pairing instability via the Bethe–Salpeter equation in the ladder approximation. The critical temperature for superfluidity is obtained as function of the effective coupling strength. We discuss the the conditions for attractive pairing, the expected order parameter symmetry, and the role of confinement dimensionality. Our results provide a theoretical framework for understanding dipolar Fermi gases in nanostructures and highlight the possibility of realizing unconventional superfluid phases.
The rapid progress in the production and cooling of molecular gases indicates that experimental studies of quantum gases with a strong dipolar interaction is soon within reach. Dipolar gases are predicted to exhibit very rich physics including quantum liquid crystal phases such as density waves as well as superfluid phases, both of which play an important role for our understanding of strongly correlated systems. Here, we investigate the zero temperature properties of the density-wave phase of a two-dimensional (2D) system of fermionic dipoles using a conserving Hartree-Fock theory. We calculate the amplitude of the density waves as a function of the dipole moment and orientation with respect to the 2D plane. The stripes give rise to a 1D Brillouin zone structure, and the corresponding quasiparticle spectrum is shown to have gapped as well as gapless regions around the Fermi surface. As a result, the system remains compressible in the density-wave phase, and it collapses for strong attraction. We show that the density waves have clear signatures in the momentum distribution and in the momentum correlations. Both can be measured in time-of-flight experiments. Finally, we discuss how the striped phase can be realized with experimentally available systems.
We examine collective modes, stability, and BCS pairing in a quasi-two-dimensional gas of dipolar fermions aligned by an external field. By using the (conserving) Hartree-Fock approximation, which treats direct and exchange interactions on an equal footing, we obtain the spectrum of single-particle excitations and long-wavelength collective modes (zero sound) in the normal phase. It appears that exchange interactions result in strong damping of zero sound when the tilting angle between the dipoles and the normal to the plane of confinement is below some critical value. In particular, zero sound cannot propagate if the dipoles are perpendicular to the plane of confinement. At intermediate coupling, we find unstable modes that can lead either to collapse of the system or to the formation of a density wave. The BCS transition to a superfluid phase, on the other hand, occurs at arbitrarily weak strengths of the dipole-dipole interaction, provided the tilting angle exceeds a critical value. We determine the critical temperature of the transition, taking into account many-body effects as well as virtual transitions to higher excited states in the confining potential, and discuss prospects of experimental observations.
We study by quantum Monte Carlo simulations the low-temperature phase diagram of dipolar bosons confined to one dimension, with dipole moments aligned along the direction of particle motion. A hard-core repulsive potential of varying range ($\ensuremath{\sigma}$) is added to the dipolar interaction, in order to ensure stability of the system against collapse. In the $\ensuremath{\sigma}\ensuremath{\rightarrow}0$ limit the physics of the system is dominated by the potential energy and the ground state is quasicrystalline; as $\ensuremath{\sigma}$ is increased the attractive part of the interaction weakens and the equilibrium phase evolves from quasicrystalline to a nonsuperfluid liquid. At a critical value ${\ensuremath{\sigma}}_{c}$, the kinetic energy becomes dominant and the system undergoes a quantum phase transition from a self-bound liquid to a gas. In the gaseous phase with $\ensuremath{\sigma}\ensuremath{\rightarrow}{\ensuremath{\sigma}}_{c}$, at low density attractive interactions bring the system into a ``weak'' superfluid regime. However, gas-liquid coexistence also occurs, as a result of which the topologically protected superfluid regime is not approached.
We present a finite-temperature analysis of a quasi-two-dimensional (Q2D) dipolar gas. To do this, we use the Hartree-Fock-Bogoliubov method within the Popov approximation. This formalism is a set of nonlocal equations containing the dipole-dipole interaction and the condensate and thermal correlation functions, which are solved self-consistently. We detail the numerical method used to implement the scheme. We present density profiles for a finite-temperature dipolar gas in Q2D and compare these results to those for a gas with zero-range interactions. Additionally, we analyze the excitation spectrum and study the impact of the thermal exchange.