Fluctuation-Induced Supersolidity at the Superfluid-Solid Interface

TL;DR

This paper demonstrates that supersolidity can emerge at the interface between superfluid and solid phases through mode coupling, without altering bulk interactions, leading to hybrid phases with interfacial density modulations.

Contribution

It introduces a novel mechanism for supersolidity arising from interfacial mode coupling, applicable across various superfluids without changing bulk properties.

Findings

Interfacial coupling induces density-modulation instability.

Instability appears at the roton mode in helium.

In BECs, it occurs at the system size limit.

Abstract

Supersolidity, combining superfluid and crystalline orders, has been realized in dipolar Bose-Einstein condensates by tuning interatomic interactions. Here we show that supersolidity can also emerge from mode coupling at a superfluid-solid interface, without modifying bulk interactions and for a broad class of superfluids. Using an analytical and numerical treatment of the coupled superfluid and phonon fields, we derive the criterion for a density-modulation instability driven by interfacial coupling and dependent on dimensionality. In superfluid helium, the instability first appears at the roton mode, while in a Bose-Einstein condensate with contact interactions it occurs at the lowest accessible wave vector set by the system size. Beyond the threshold, the ground state acquires an interfacial density modulation while the bulk remains superfluid, forming a hybrid superfluid-supersolid phase. Our results identify interfacial mode coupling as a promising route to supersolidity, enabling the simultaneous exploitation of interfacial supersolid and bulk superfluid quantum properties.