Condensation of quasiparticles and density modulation beyond the superfluid critical velocity

TL;DR

This paper explores how a constant external velocity influences the ground state of a bosonic quasiparticle system, revealing a transition from superfluid to a density-modulated phase with quasiparticle condensation at a critical velocity.

Contribution

It introduces a model showing quasiparticle and particle condensation at nonzero wave vectors beyond a critical velocity, leading to density modulations and bifurcation phenomena.

Findings

Quasiparticle condensation occurs at a nonzero wave vector beyond critical velocity.

Density modulations emerge due to combined condensates at specific wave vectors.

At higher velocities, the system bifurcates into two pure condensates with no density modulation.

Abstract

We investigate the effect of a constant external velocity field on the ground state of a bosonic quasiparticle Hamiltonian. Below a critical velocity the ground state is a quasiparticle vacuum, corresponding to a pure superfluid phase at zero temperature. Beyond the critical velocity energy minimization leads to a macroscopic condensation of quasiparticles at a nonzero wave vector k_v parallel to the velocity v. Simultaneously, physical particles also undergo a condensation at k_v and, to a smaller extent, at -k_v. Together with the BEC at k=0, the three entangled condensates give rise to density modulations of wave vectors k_v and 2k_v. For larger |v| our model predicts a bifurcation of k_v with corresponding two pure condensates and no density modulation.