Counterflow leads to roton spectra in locally interacting superfluids

TL;DR

This paper demonstrates that roton spectra can emerge in superfluids with only local interactions through the nonlinear dynamics of counterflowing condensates, broadening the understanding of excitation spectra in quantum fluids.

Contribution

It introduces a novel mechanism for roton spectrum emergence in locally interacting superfluids via counterflow dynamics, without requiring nonlocal interactions.

Findings

Roton spectra can arise in local-interaction superfluids through counterflow dynamics.

The mechanism is based on classical nonlinear field equations.

Potential for realizing roton dynamics in various coupled nonlinear systems.

Abstract

The dynamics of strongly interacting quantum fluids such as Helium II are fundamentally distinct from those of dilute, contact-interacting atomic Bose-Einstein condensates. Most dramatically, superfluids with finite-range interactions can support excitations with a roton-like dispersion, exhibiting a minimum at finite wavenumber. Here we introduce a mechanism through which roton spectra can emerge in superfluids without any nonlocal interactions, instead resulting from the collective excitations of two counterflowing, zero-range interacting condensates. As our mechanism relies only on the nonlinear dynamics of classical fields, this work opens the door to the realization of roton dynamics in the broad class of physical systems governed by coupled nonlinear-Schr\"odinger equations.