Supersolidity Is Born at the Liquid Boundary ⚡ экспресс
It is shown that supersolidity can emerge from the coupling of modes at the interface between superfluid and solid phases for a broad class of superfluid systems, without altering bulk interactions. Using analytical and numerical analysis of the coupled superfluid and phonon fields, an instability criterion for density modulation is obtained, which depends on the system’s dimensionality. In superfluid helium, the instability first sets in at the roton mode, whereas in a Bose-Einstein condensate with contact interaction, it appears at the smallest available wave vector set by the system size. Beyond the instability threshold, the ground state develops a spatial density modulation near the boundary, while the bulk remains superfluid, forming a hybrid superfluid-supersolid phase. This result opens a promising route to supersolidity through interfacial mode coupling, allowing the simultaneous use of the quantum properties of the superfluid bulk and the supersolid boundary.
A supersolid is ice that flows. Think of a pond with a frozen crust, but beneath it, a liquid depth without friction. Usually, creating that crust means changing the liquid's properties throughout its entire volume. A team of researchers studying liquid helium found a simpler way: they made the surface vibrate with special waves, and the liquid grew its own solid 'skin.'
Those vibrations weren't just ordinary ripples—they were quantum whirlpools called rotons, whose nature was uncovered by Feynman. By coupling with the motion, they order the atoms at the boundary into a crystal. In Bose–Einstein condensates, a similar effect occurs when the wavelength matches the trap size. The result is a material whose surface holds its shape while the inside flows freely. It’s possible that similar processes harden the crust of neutron stars.
🎯 The idea of supersolidity is over half a century old, but it wasn’t until 2017 that it was realized by trapping ultracold atoms in a laser cage.