Mapping out spin and particle conductances in a quantum point contact

TL;DR

This study investigates how particle and spin conductances behave in a quantum point contact with a tunable, strongly interacting Fermi gas, revealing deviations from universal quantization and signs of superfluidity.

Contribution

It provides the first detailed measurement of spin and particle conductance across different interaction regimes in a quantum point contact with a Fermi gas.

Findings

Quantized conductance observed at weak attraction.

Broad maximum in spin conductance indicating Cooper pairing.

Enhanced particle conductance beyond expected superfluid transition.

Abstract

We study particle and spin transport in a single mode quantum point contact using a charge neutral, quantum degenerate Fermi gas with tunable, attractive interactions. This yields the spin and particle conductance of the point contact as a function of chemical potential or confinement. The measurements cover a regime from weak attraction, where quantized conductance is observed, to the resonantly interacting superfluid. Spin conductance exhibits a broad maximum when varying the chemical potential at moderate interactions, which signals the emergence of Cooper pairing. In contrast, the particle conductance is unexpectedly enhanced even before the gas is expected to turn into a superfluid, continuously rising from the plateau at 1/h for weak interactions to plateaux-like features at non-universal values as high as 4/h for intermediate interactions. For strong interactions, the particle conductance plateaux disappear and the spin conductance gets suppressed, confirming the spin-insulating character of a superfluid. Our observations document the breakdown of universal conductance quantization as many-body correlations appear. The observed anomalous quantization challenges a Fermi liquid description of the normal phase, shedding new light on the nature of the strongly attractive Fermi gases.