Band and Correlated Insulators of Cold Fermions in a Mesoscopic Lattice

Abstract

We investigate the transport properties of neutral, fermionic atoms passing through a one-dimensional quantum wire containing a mesoscopic lattice. The lattice is realized by projecting individually controlled, thin optical barriers on top of a ballistic conductor. Building an increasingly longer lattice, one site after another, we observe and characterize the emergence of a band insulating phase, demonstrating control over quantum-coherent transport. We explore the influence of atom-atom interactions and show that the insulating state persists as contact interactions are tuned from moderately to strongly attractive. Using bosonization and classical Monte Carlo simulations, we analyze such a model of interacting fermions and find good qualitative agreement with the data. The robustness of the insulating state supports the existence of a Luther-Emery liquid in the one-dimensional wire. Our work realizes a tunable, site-controlled lattice Fermi gas strongly coupled to reservoirs, which is an ideal test bed for nonequilibrium many-body physics.

Alternate abstract:

Plain Language Summary

Electrical properties of a material—such as whether it is a conductor or an insulator—rely on microscopic details such as the strength of interactions between electrons, the number of directions in which they are free to propagate, and the presence of impurities. However, these properties are challenging to predict. In particular, for particles that are attracted to one another in a periodic potential, a competition is expected to occur between superfluidity, which is associated with a large conductance, and interferences, which turn the material into an insulator. Our experimental work, supported by simulations, reveals the remarkable outcome of this competition in one dimension, using ultracold fermions with widely tunable interactions in a quantum wire.

We create a short one-dimensional lattice structure connected to two reservoirs of ultracold lithium-6 atoms, which allows us to measure the wire’s conductance. We first witness the emergence of a band-insulating phase with weak interactions by observing a conductance gap. By changing the length and height of the lattice, as well as the temperature, we can investigate the coherent character of particle transport. As interactions are tuned from weakly to strongly attractive, we discover that this insulating state persists, hinting at the presence of a Luther-Emery liquid, an original phase distinctive of the one-dimensional character of the structure.

These results demonstrate the simultaneous control of interactions and quantum interferences in cold-atom devices, allowing the engineering of complex structures with new functionalities not available in electronic systems.