Anomalous Conductances in an Ultracold Quantum Wire

Abstract

We analyze the recently measured anomalous transport properties of an ultracold gas through a ballistic constriction [S. Krinner et al., PNAS 201601812 (2016)]. The quantized conductance observed at weak interactions increases several-fold as the gas is made strongly interacting, which cannot be explained by the Landauer theory of single-channel transport. We show that this phenomenon is due to the multichannel Andreev reflections at the edges of the constriction, where the interaction and confinement result in a superconducting state. Andreev processes convert atoms of otherwise reflecting channels into the condensate propagating through the constriction, leading to a significant excess conductance. Furthermore, we find the spin conductance being suppressed by superconductivity; the agreement with experiment provides an additional support for our model. Transport measurements through one-dimensional ballistic channels provide invaluable insight into the complex many-body systems by connecting microscopic quantum dynamics with macroscopic observables, such as the conductance G n , spin conductance G s and heat transport. In the normal state, these quantities exhibit plateaus as a function of the gate potential at integer multiples of the conductance and heat conductance quantum, respectively [1][2][3]. If the channel or leads are made superconducting, a wealth of other phenomena opens up. At a normal-superconducting interface, a fermion incident from the normal metal to the superconductor forms a Cooper pair with another fermion so that they can enter the condensate, while a hole gets reflected from the interface -a process called Andreev reflection (AR) [4][5][6][7]. AR lies at the heart of several interesting transport phenomena, including Andreev bound states [6] [15][16][17][18][19]. Despite the abundance of exotic transport phenomena in electronic condensed matter systems, it has been only very recently that the conductance properties of charge neutral massive particles have been measured, using an ultracold Fermi gas of 6 Li atoms, passed through an optically created one-dimensional constriction, realizing the limiting case of a ballistic wire of a single transmitting transverse channel [20][21][22], see Fig. 1. This system offers tunability of the geometry and interactions, with the opportunity to reach the strongly interacting regime, where the wire becomes superconducting, contacted by normal leads in the experiment of Ref. 22. In condensed matter environments, similar systems of inhomogenous superconductivity have attracted significant attention, providing access to phenomena on the verge between microscopic and mesoscopic physics, such as phase-slips [23], non-local quantum correlations [24] and spatially resolved AR [32]. Superconducting islands immersed in a metallic environment may also comprise a platform for the study of the superconductor-metal transition [25][26][27][28][29][30][31].In the presence of weak interactions, the constriction exhibits conductance plateaus of integer multiples of the ...

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