Quantum Fluctuations and Vortex-Antivortex Unbinding in the 2D Bcs-Bec Crossover

Abstract

Very recently, quasi two-dimensional (2D) systems made of attractive fermionic alkali-metal atoms with a widely tunable interaction due to Fano-Feshbach resonances have been realized. In this way, it has been achieved the 2D crossover from the Bardeen-Cooper-Schrieffer regime of weakly-interacting Cooper pairs to the Bose-Einstein condensate regime strongly bound dimers. These experiments pave the way to the investigation of 2D strongly-interacting attractive fermions during the Berezinskii-Kosterlitz-Thouless (BKT) transition from a low-temperature superfluid phase characterized by quasi-condensation to a high-temperature normal phase, where vortex proliferation driven by quantum and thermal fluctuations completely destroys superfluidity. In this paper, we discuss our preliminar theoretical results on the behavior of the BKT critical temperature across the crossover. Our microscopic calculations are based on functional integration taking into account renormalized Gaussian fluctuations and the crucial 2D effect of vortex-antivortex unbinding

Substrate mediated nitridation of niobium into superconducting Nb2N thin films for phase slip study

“…Hence, the dissipation related to current-induced unbound vortex-antivorex movement can also be disregarded. In reduced dimension, phase fluctuation is one of the dominant mechanisms behind the resistive tailing & residual resistance 44 and above T BKT , the phase fluctuation leads to vortex proliferation which eventually brings the system to the normal state 45 . In quasi 0D granular superconductors, depending on the dimension of the nano-grains, phase fluctuations can lead to total suppression of superconductivity and may lead to even an insulating state 46,47 .…”

Pseudogap Regime of a Two-dimensional Uniform Fermi Gas

“…Evaluating Eq. (17) in TMA, we have ∆ PG /ε F = 1.75. Although this is consistent with E PG /ε F = 1.686 evaluated from the TMA result in Fig.…”

“…Using T * , we identify the pseudogap regime (where the pseudogap appears in ρ(ω)) in the phase diagram of a two-dimensional Fermi gas with respect to the temperature and the interaction strength. We briefly note that a two-dimensional Fermi gas has recently attracted much attention both theoretically [9][10][11][12][13][14][15][16][17][18] and experimentally, [19][20][21][22][23][24][25][26] especially since the observation of the Berezinskii-Kosterlitz-Thouless(BKT) transition [27][28][29][30] in a 6 Li Fermi gas. 25,26) The importance of studying two-dimensional pseudogap phenomena is related to the current debate for this phenomenon in a three-dimensional Fermi gas.…”

Fermi-to-Bose crossover in a trapped quasi-2D gas of fermionic atoms

“…and plot the dimensionless critical temperature T c /ε F as a function of g, then T BEC c /ε F ∼ 1/[4 ln(6.4g)], T BCS c /ε F ∼ 2 exp(g) and we reproduce the same qualitative picture of the BCS-BEC crossover in figure 9 as the one presented in figure 8. Note that at low temperatures T ≪ ε F ≤ |E b |/2 the oneparticle chemical potential in BEC domain at first (in the intermediate situation |E b | ≥ 2ε F and for shallow bound states) has the same form as in (30) and contains the factor ε F . However for deep levels in 2D (when ε F ≪ 2 /(mr 2 0 ) ≪ |E b | ∼ |U |) the chemical potential (neglecting corrections of the order of ε F /W , W/|E b |) is just µ ≃ −|E b |/2.…”