Sub-unity superfluid fraction of a supersolid from self-induced Josephson effect

TL;DR

This paper demonstrates a method to measure the superfluid fraction in a supersolid using a self-induced Josephson effect, revealing a significant sub-unity superfluid fraction in a cold-atom dipolar supersolid.

Contribution

It introduces a novel approach to measure superfluid fraction via spontaneous Josephson effect in supersolids, highlighting the existence of a sub-unity superfluid fraction.

Findings

Superfluid fraction in a supersolid is significantly less than one.

Self-induced Josephson junctions enable local superfluid fraction measurement.

Results suggest new phenomena like partially quantized vortices are possible.

Abstract

Recently, a new category of superfluids and superconductors has been discovered in various systems. These could be linked to the idea of a supersolid phase, featuring a macroscopic wavefunction with spatial modulation resulting from simultaneous, spontaneous breaking of gauge and translational symmetries. However, this relation has only been recognized in some cases and there is the need for universal properties quantifying the differences between supersolids and ordinary superfluids/superconductors or crystals. A key property is the superfluid fraction, which measures the reduction in superfluid stiffness due to spatial modulation, leading to the non-standard superfluid dynamics of supersolids. Here we employ the Josephson effect, common in superfluids and superconductors, to measure the superfluid fraction in a supersolid. Even without a physical barrier, the Josephson effect arises spontaneously in a supersolid due to spatial modulation. Individual lattice cells act as self-induced Josephson junctions, allowing the direct determination of the local superfluid fraction. We studied a cold-atom dipolar supersolid, revealing a significant sub-unity superfluid fraction. Our results open new research directions, enabling the exploration of novel phenomena like partially quantized vortices and supercurrents, potentially unifying the understanding of supersolid-like systems, and introducing a new type of Josephson junction.