Abstract Atomic Fermi gases provide an ideal platform for studying pairing and superfluid physics, using a Feshbach resonance between closed-channel molecular states and open-channel scattering states. Of particular interest is the strongly interacting regime. We show that the closed-channel fraction ${Z_{{\rm{cc}}}}$ provides an effective probe for important many-body interacting effects, especially through its density dependence, which is absent from two-body theoretical predictions. Here we measure ${Z_{{\rm{cc}}}}$ as a function of interaction strength and the Fermi temperature ${T_{\rm{F}}}$ in a trapped 6Li superfluid throughout the entire Bardeen-Cooper-Schrieffer–Bose-Einstein-condensate crossover, in quantitative agreement with theory when important thermal contributions outside the superfluid core are taken into account. Away from the deep-BEC regime, the fraction ${Z_{\rm cc}}$ is sensitive to ${{{T}}_F}$ . In particular, our data show ${Z_{{\rm{cc}}}} \propto T_{\rm{F}}^\alpha $ with $\alpha = {\rm{1/2}}$ at unitarity, in quantitative agreement with calculations of a two-channel pairing fluctuation theory, and $\alpha $ increases rapidly into the BCS regime, reflecting many-body interaction effects as predicted.Abstract Atomic Fermi gases provide an ideal platform for studying pairing and superfluid physics, using a Feshbach resonance between closed-channel molecular states and open-channel scattering states. Of particular interest is the strongly interacting regime. We show that the closed-channel fraction ${Z_{{\rm{cc}}}}$ provides an effective probe for important many-body interacting effects, especially through its density dependence, which is absent from two-body theoretical predictions. Here we measure ${Z_{{\rm{cc}}}}$ as a function of interaction strength and the Fermi temperature ${T_{\rm{F}}}$ in a trapped 6Li superfluid throughout the entire Bardeen-Cooper-Schrieffer–Bose-Einstein-condensate crossover, in quantitative agreement with theory when important thermal contributions outside the superfluid core are taken into account. Away from the deep-BEC regime, the fraction ${Z_{\rm cc}}$ is sensitive to ${{{T}}_F}$. In particular, our data show ${Z_{{\rm{cc}}}} \propto T_{\rm{F}}^\alpha $ with $\alpha = {\rm{1/2}}$ at unitarity, in quantitative agreement with calculations of a two-channel pairing fluctuation theory, and $\alpha $ increases rapidly into the BCS regime, reflecting many-body interaction effects as predicted.

arXiv:1903.12321National Science ReviewDec 21, 20218 citations

Quantum Field Theory of BECs

José Tito Mendonça, Hugo Terças

Springer series on atomic, optical, and plasma physicsOct 8, 20120 citations

A generalized molecule approach capturing the Feshbach-induced pairing physics in the BEC–BCS crossover

Eloisa Cuestas, A. P. Majtey

Abstract By including the effect of a trap with characteristic energy given by the Fermi temperature T F in a two-body two-channel model for Feshbach resonances, we reproduce the measured binding energy of ultracold molecules in a 40 K atomic Fermi gas. We also reproduce the experimental closed-channel fraction Z across the BEC–BCS crossover and into the BCS regime of a 6 Li atomic Fermi gas. We obtain the expected behavior Z ∝ T F at unitarity, together with the recently measured proportionality constant. Our results are also in agreement with recent measurements of the Z dependency on T F on the BCS side, where a significant quantitative discrepancy between experimental data and theory’s predictions has been repeatedly reported. In order to contrast with future experiments we report the proportionality constant at unitarity between Z and T F predicted by our model for a 40 K atomic Fermi gas.Abstract By including the effect of a trap with characteristic energy given by the Fermi temperature T F in a two-body two-channel model for Feshbach resonances, we reproduce the measured binding energy of ultracold molecules in a 40 K atomic Fermi gas. We also reproduce the experimental closed-channel fraction Z across the BEC–BCS crossover and into the BCS regime of a 6 Li atomic Fermi gas. We obtain the expected behavior Z ∝ T F at unitarity, together with the recently measured proportionality constant. Our results are also in agreement with recent measurements of the Z dependency on T F on the BCS side, where a significant quantitative discrepancy between experimental data and theory’s predictions has been repeatedly reported. In order to contrast with future experiments we report the proportionality constant at unitarity between Z and T F predicted by our model for a 40 K atomic Fermi gas.

Journal of Physics Condensed MatterApr 14, 20211 citation

Shell-model Monte Carlo simulations of the BCS-BEC crossover in few-fermion systems

Nikolaj Thomas Zinner, Klaus Mølmer, C. Özen +2 more

We study a trapped system of fermions with a zero-range two-body interaction using the shell-model Monte Carlo method, providing ab initio results for the low particle number limit where mean-field theory is not applicable. We present results for the $N$ -body energies as function of interaction strength, particle number, and temperature. The subtle question of renormalization in a finite model space is addressed and the convergence of our method and its applicability across the BCS-BEC crossover is discussed. Our findings indicate that very good quantitative results can be obtained on the BCS side, whereas at unitarity and in the BEC regime the convergence is less clear. Comparison to $N=2$ analytics at zero and finite temperature, and to other calculations in the literature for $N>2$ show very good agreement.We study a trapped system of fermions with a zero-range two-body interaction using the shell-model Monte Carlo method, providing ab initio results for the low particle number limit where mean-field theory is not applicable. We present results for the $N$-body energies as function of interaction strength, particle number, and temperature. The subtle question of renormalization in a finite model space is addressed and the convergence of our method and its applicability across the BCS-BEC crossover is discussed. Our findings indicate that very good quantitative results can be obtained on the BCS side, whereas at unitarity and in the BEC regime the convergence is less clear. Comparison to $N=2$ analytics at zero and finite temperature, and to other calculations in the literature for $N>2$ show very good agreement.

arXiv:0903.2263Physical Review AJul 23, 200923 citations

Theory of many-boson systems

Richard David Etters

Jan 1, 19650 citations

{Z_{{\rm{cc}}}}

provides an effective probe for important many-body interacting effects, especially through its density dependence, which is absent from two-body theoretical predictions. Here we measure

{Z_{{\rm{cc}}}}

as a function of interaction strength and the Fermi temperature

{T_{\rm{F}}}

in a trapped 6Li superfluid throughout the entire Bardeen-Cooper-Schrieffer–Bose-Einstein-condensate crossover, in quantitative agreement with theory when important thermal contributions outside the superfluid core are taken into account. Away from the deep-BEC regime, the fraction

{Z_{\rm cc}}

is sensitive to

{{{T}}_F}

. In particular, our data show ${Z_{{\rm{cc}}}} \propto T_{\rm{F}}^\alpha $ with

\alpha = {\rm{1/2}}

at unitarity, in quantitative agreement with calculations of a two-channel pairing fluctuation theory, and $\alpha $ increases rapidly into the BCS regime, reflecting many-body interaction effects as predicted.

N

-body energies as function of interaction strength, particle number, and temperature. The subtle question of renormalization in a finite model space is addressed and the convergence of our method and its applicability across the BCS-BEC crossover is discussed. Our findings indicate that very good quantitative results can be obtained on the BCS side, whereas at unitarity and in the BEC regime the convergence is less clear. Comparison to

N=2

analytics at zero and finite temperature, and to other calculations in the literature for

N>2

show very good agreement.