Superfluidity of vacancies in solid 4He

Under the adiabatic approximation, we present a superfluid state of vacancy motion in solid He-4. The vacancies in solid He-4 can sense an attractive effective interaction by exchange of virtual longitudinal acoustic phonons. Such an interaction leads to a Bardeen-Cooper-Schrieffer (BCS) condensation state, in which the vacancies with opposite wave vectors are bound into pairs and single, unpaired bare vacancies are transformed into a new kind of quasiparticle, the dressed vacancy. A dressed vacancy is a vacancy clothed with a cloud of virtual longitudinal-acoustic phonons. The vacancy system undergoes a first-order phase transition from the normal state to the BCS condensation state. The critical temperature may be the transition temperature of the supersolid state. We derive the low-temperature expressions of vacancy heat capacity and vacancy number density in the BCS condensation state. The BCS condensation state of vacancies is the superfluid state of vacancies, which possesses potentially important and novel effects in condensed matter physics. (C) 2021 Elsevier B.V. All rights reserved.

Automated summary

The study presents a superfluid state of vacancy motion in solid He-4, where vacancies sense an attractive effective interaction by exchanging virtual longitudinal acoustic phonons, leading to a Bardeen-Cooper-Schrieffer (BCS) condensation state. This first-order phase transition results in dressed vacancies and vacancy pairs, with the critical temperature potentially being the transition temperature of the supersolid state. The low-temperature expressions of vacancy heat capacity and vacancy number density in the BCS condensation state are derived.