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Journal of Experimental and Theoretical Physics

BKT phase in systems of spinless strongly interacting one-dimensional fermions

V. V. Afonin, Victor Yu. Petrov

Published Oct 1, 2008• 8 citations
Publisher

Abstract

We present the ground-state wavefunctions for a system of spinless one-dimensional fermions in the limit of an infinitely strong interaction, and we demonstrate explicitly that the system symmetry is lower than the original symmetry of the Hamiltonian. As a result, the system in this limit undergoes a second-order phase transition into a phase with finite density of chiral pairs. The phase transforms continuously into a Berezinskii-Kosterlitz-Thouless (BKT) phase if the interaction in the model decreases. Therefore, just the BKT phase is realized in nature. The temperature of the smearing phase transition is calculated.

DOI
10.1134/s1063776108100026

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We present the ground-state wavefunctions for a system of spinless one-dimensional fermions in the limit of an infinitely strong interaction, and we demonstrate explicitly that the system symmetry is lower than the original symmetry of the Hamiltonian. As a result, the system in this limit undergoes a second-order phase transition into a phase with finite density of chiral pairs. The phase transforms continuously into a Berezinskii-Kosterlitz-Thouless (BKT) phase if the interaction in the model decreases. Therefore, just the BKT phase is realized in nature. The temperature of the smearing phase transition is calculated.
PT\mathcal{PT}PT-Symmetry Enhanced Berezinskii-Kosterlitz-Thouless Superfluidity
Berezinskii-Kosterlitz-Thouless (BKT) transition, the topological phase transition to a quasi-long range order in a two-dimensional (2D) system, is a hallmark of low-dimensional topological physics. The recent emergence of non-Hermitian physics, particularly parity-time (PT\mathcal{PT}PT) symmetry, raises a natural question about the fate of low-dimensional orders (e.g., BKT transition) in the presence of complex energy spectrum. Here we investigate the BKT phase transition in a 2D degenerate Fermi gas with an imaginary Zeeman field obeying PT\mathcal{PT}PT-symmetry. Despite complex energy spectrum, PT\mathcal{PT}PT-symmetry guarantees that the superfluid density and many other quantities are real. Surprisingly, the imaginary Zeeman field enhances the superfluid density, yielding higher BKT transition temperature than that in Hermitian systems. In the weak interaction region, the transition temperature can be much larger than that in the strong interaction limit. Our work showcases a surprising interplay between low-dimensional topological defects and non-Hermitian effects, paving the way for studying non-Hermitian low-dimensional phase transitions.
A paradigmatic example of a phase transition taking place in the absence of symmetry-breaking is provided by the Berezinkii-Kosterlitz-Thouless (BKT) transition in the two-dimensional XY model. In the framework of canonical ensemble, this phase transition is defined as an infinite-order one. To the contrary, by tackling the transitional behavior of the two dimensional XY model in the microcanonical ensemble, we show that the BKT phase transition is of second order. This provides a new example of statistical ensemble inequivalence that could apply to a broad class of systems undergoing BKT phase transitions.
The Berezinskii-Kosterlitz-Thouless (BKT) transition is the paradigmatic example of a topological phase transition without symmetry breaking, where a quasiordered phase, characterized by a power-law scaling of the correlation functions at low temperature, is disrupted by the proliferation of topological excitations above the critical temperature T_{BKT}. In this Letter, we consider the effect of long-range decaying couplings ∼r^{-2-σ} on the BKT transition. After pointing out the relevance of this nontrivial problem, we discuss the phase diagram, which is far richer than the corresponding short-range one. It features-for 7/4<σ<2-a quasiordered phase in a finite temperature range T_{c}<T<T_{BKT}, which occurs between a symmetry broken phase for T<T_{c} and a disordered phase for T>T_{BKT}. The transition temperature T_{c} displays unique universal features quite different from those of the traditional, short-range XY model. Given the universal nature of our findings, they may be observed in current experimental realizations in 2D atomic, molecular, and optical quantum systems.
The Berezinskii-Kosterlitz-Thouless (BKT) transition is the prototype of a phase transition driven by the formation and interaction of topological defects in two-dimensional (2D) systems. In typical models these are vortices: above a transition temperature TBKTT_{\rm BKT}TBKT​ vortices are free, below this transition temperature they get confined. In this work we extend the concept of BKT transition to quantum systems in two dimensions. In particular, we demonstrate that a zero-temperature quantum BKT phase transition, driven by a coupling constant can occur in 2D models governed by an effective gauge field theory with a diverging dielectric constant. One particular example is that of a compact U(1) gauge theory with a diverging dielectric constant, where the quantum BKT transition is induced by non-relativistic, purely 2D magnetic monopoles, which can be viewed also as electric vortices. These quantum BKT transitions have the same diverging exponent zzz as the quantum Griffiths transition but have nothing to do with disorder.
We perform large-scale, numerically exact calculations on the two-dimensional interacting Fermi gas with a contact attraction. Reaching much larger lattice sizes and lower temperatures than previously possible, we determine systematically the finite-temperature phase diagram of the Berezinskii-Kosterlitz-Thouless (BKT) transitions for interaction strengths ranging from BCS to crossover to BEC regimes. The evolutions of the pairing wave functions and the fermion and Cooper pair momentum distributions with temperature are accurately characterized. In the crossover regime, we find that the contact has a nonmonotonic temperature dependence, first increasing as temperature is lowered, and then showing a slight decline below the BKT transition temperature to approach the ground-state value from above.