The role of reduced dimensionality in high-temperature superconductors is still under debate. Recently, ultracold atoms have emerged as an ideal model system to study such strongly correlated two-dimensional (2D) systems. Here, we report on the realization of a Josephson junction in an ultracold 2D Fermi gas. We measure the frequency of Josephson oscillations as a function of the phase difference across the junction and find excellent agreement with the sinusoidal current phase relation of an ideal Josephson junction. Furthermore, we determine the critical current of our junction in the crossover from tightly bound molecules to weakly bound Cooper pairs. Our measurements clearly demonstrate phase coherence and provide strong evidence for superfluidity in a strongly interacting 2D Fermi gas.The role of reduced dimensionality in high-temperature superconductors is still under debate. Recently, ultracold atoms have emerged as an ideal model system to study such strongly correlated two-dimensional (2D) systems. Here, we report on the realization of a Josephson junction in an ultracold 2D Fermi gas. We measure the frequency of Josephson oscillations as a function of the phase difference across the junction and find excellent agreement with the sinusoidal current phase relation of an ideal Josephson junction. Furthermore, we determine the critical current of our junction in the crossover from tightly bound molecules to weakly bound Cooper pairs. Our measurements clearly demonstrate phase coherence and provide strong evidence for superfluidity in a strongly interacting 2D Fermi gas.

arXiv:1908.09776ScienceJul 2, 202078 citations

Finite-size effects in the ideal Fermi gas

V. Subrahmanyam, Mustansir Barma

The ideal Fermi gas in a finite volume is studied in the low-temperature regime where the thermal correlation length is comparable to the size L of the system. Physical quantities such as the density of fermions and the appropriate ordering susceptibility show oscillatory fluctuations around a smooth background as L is varied. These are investigated numerically and analytically. The amplitude of the fluctuations depends on the observable in question and on the container shape. Finite-size scaling does not hold directly for thermodynamic quantities but is recoverable by averaging the mean-squared fluctuations over an appropriate range in L.The ideal Fermi gas in a finite volume is studied in the low-temperature regime where the thermal correlation length is comparable to the size L of the system. Physical quantities such as the density of fermions and the appropriate ordering susceptibility show oscillatory fluctuations around a smooth background as L is varied. These are investigated numerically and analytically. The amplitude of the fluctuations depends on the observable in question and on the container shape. Finite-size scaling does not hold directly for thermodynamic quantities but is recoverable by averaging the mean-squared fluctuations over an appropriate range in L.

Journal of Physics A Mathematical and GeneralJun 7, 198913 citations

Spin correlation function of a one-dimensional Fermi gas with strong interaction

Alexander A. Ovchinnikov, Anton Vladimirovich Zabrodin

Physics Letters AJun 1, 19895 citations

Pair correlation function of a one-dimensional Fermi gas with strong interactions

Anton Vladimirovich Zabrodin, А. А. Овчинников

Journal of Experimental and Theoretical PhysicsApr 1, 19850 citations

Virial expansion for a strongly correlated Fermi gas with imbalanced spin populations

Xia-Ji Liu, Hui Hu

Quantum virial expansion provides an ideal tool to investigate the high-temperature properties of a strongly correlated Fermi gas. Here, we construct the virial expansion in the presence of spin-population imbalance. Up to the third order, we calculate the high-temperature free energy of a unitary Fermi gas as a function of spin imbalance, with infinitely large attractive or repulsive interactions. In the latter repulsive case, we show that there is no itinerant ferromagnetism when quantum virial expansion is applicable. We therefore estimate an upper bound for the ferromagnetic transition temperature ${T}_{c}$ . For a harmonically trapped Fermi gas at unitarity, we find that $({T}_{c}){}_{\mathrm{upper}}<{T}_{F}$ , where ${T}_{F}$ is the Fermi temperature at the center of the trap. Our result for the high-temperature equations of state may confront future high-precision thermodynamic measurements.Quantum virial expansion provides an ideal tool to investigate the high-temperature properties of a strongly correlated Fermi gas. Here, we construct the virial expansion in the presence of spin-population imbalance. Up to the third order, we calculate the high-temperature free energy of a unitary Fermi gas as a function of spin imbalance, with infinitely large attractive or repulsive interactions. In the latter repulsive case, we show that there is no itinerant ferromagnetism when quantum virial expansion is applicable. We therefore estimate an upper bound for the ferromagnetic transition temperature ${T}_{c}$. For a harmonically trapped Fermi gas at unitarity, we find that $({T}_{c}){}_{\mathrm{upper}}<{T}_{F}$, where ${T}_{F}$ is the Fermi temperature at the center of the trap. Our result for the high-temperature equations of state may confront future high-precision thermodynamic measurements.

arXiv:1008.1457Physical Review AOct 26, 201028 citations

{T}_{c}

. For a harmonically trapped Fermi gas at unitarity, we find that

({T}_{c}){}_{\mathrm{upper}}<{T}_{F}

, where

{T}_{F}

is the Fermi temperature at the center of the trap. Our result for the high-temperature equations of state may confront future high-precision thermodynamic measurements.