Observing the Influence of Reduced Dimensionality on Fermionic Superfluids

BCS–BEC crossover in a quasi-two-dimensional Fermi superfluid

“…We have found that the order parameter and pair size for 2D and Q2D systems with different trapping frequencies show universal relations with chemical potential in the BCS limit, but behave distinctively with increasing interaction. Within GPF theory, our results of superfluid gap, chemical potential and pair size for Q2D systems show quantitatively agreement with the recent experiment of 6 Li atomic gases [25]. These results reveal the notable effect of reduced dimensionality and pair fluctuations, which can also be concluded from the speed of first sound and convexity of Goldstone modes.…”

“…Here, a bg is background scattering length, µ co is the difference in magnetic moments between the open and close channels, W is the resonance width, and B 0 is the resonance position. To compare with the experiment of Q2D 6 Li atomic gases [25], we consider an effective 2D density per spin state of n = 0.8 atoms/µm 2 and an axial trapping frequency ω 0 = 2π × 9.2 kHz, and use the parameters a bg = −1405a 0 , W = 300 G, and µ co = 2µ B for the wide Feshbach resonance near 834 G, where a 0 and µ B are respectively the Bohr radius and Bohr magneton.…”

“…In condensed matter systems, precise control of carrier density, geometric strain and lattice superstructures significantly bring forward the study of multiband layered superconductors such as FeSe [12], FeSe 1−x S x [13,14], Li x ZrNCl [15], and magic-angle twisted trilayer graphene [16]. In atomic systems, highly anisotropic traps or one-dimensional optical lattices have been implemented to realize quasi-two-dimensional (Q2D) Fermi gases with tunable inter-particle interaction via Feshbach resonances [17][18][19][20][21][22][23][24][25][26]. A very recent experiment in ultracold gases of 6 Li atoms [25] has now provided benchmark results on Q2D superfluidity by evolving the system from a weakly interacting Bardeen-Cooper-Schrieffer (BCS) superfluid to a tightly-bound Bose-Einstein condensate (BEC).…”

“…The gray dots represent the experimental raw data obtained from the onset momentum ko of the pair breaking continuum for Q2D gases of6 Li atoms[25]. The red diamonds are modified Q2D pair sizes calculated via equation (22) using the superfluid gap and the chemical potential obtained from reference[25]. The black dotted line in panel (b) presents the coherence length ξp = kF/π m∆.…”

Following the Higgs mode across the BCS-BEC crossover in two dimensions

“…In this respect, the pole in χ ↓ H (z, q) exists on an unphysical Riemann sheet, different from the physical Riemann sheet where Im χ ↓ H (ω + iδ, q) is measured in spectroscopic probes [33,34]. However, due to the analyticity of χ ↓ H (z, q) across the real axis for ω > 2∆, poles on this unphysical Riemann sheet lead to observable peaks in the spectral function Im χ ↓ H (ω + iδ, q).…”