Quantum friction effects in superfluids

Monday, April 24, 2017 - 2:30am

Low-temperature ordered states of matter  with spontaneously broken symmetry host collective excitations - Goldstone modes  - that are coupled to the order parameter field.  The collective excitations act on the order parameter as a thermal bath and give rise to its non-trivial dynamics that include effects like "quantum friction," a stochastic Langevin force and effective Brownian motion. In this talk, I will discuss new theoretical and experimental results and ideas on such quantum friction effects in superfluids  and magnetic systems. In the first part of my talk, I will focus on quantum dynamics of topological textures, in particular solitons/domain walls, propagating in low-dimensional superfluids/magnets.  An effective quasi-classical equations of motion will be derived. It will be shown that interestingly the familiar Ohmic friction is absent in the integrable setup, but the equations contain non-Markovian dissipative forces [1]. I will explain a way to restore and control the Ohmic friction force and related Brownian motion of topological textures, as was demonstrated in a recent experiment [2]. In the second part of the talk, I will explain our general geometric theory of fluctuations and dissipation in non-equilibrium superfluids [3]. The theory relies on the observation that collective excitations in a moving superfluid satisfy the wave equation in a curved space time, with the metric determined by the underlying superflow. This leads to a compact and elegant geometric formulation of two-fluid hydrodynamics that suggests interesting observable effects from synthetic Hawking radiation  to synthetic lensing of phonons/magnons. Possible experimental platforms (in superfluids, superconductors, and magnets) to observe these exotic phenomena will be discussed.
[1] D. Efimkin, J. Hofmann,  and V. Galitski, "Non-Markovian quantum friction of bright solitons in superfluids," Phys. Rev. Lett. 116, 225301 (2016)
[2] L. Ayckock et al., "Brownian motion of solitons," Proc. Nat. Acad. Sciences 114, 2503 (2017)
[3] A. Keser and V. Galitski, "Analogue Stochastic Gravity in Strongly-Interacting BECs," arXiv:1612.08980

3 Le Conte Hall
Joint Quantum Institute, University of Maryland