Gauge transformations and Galilean covariance in nonlinear gauge-coupled quantum fluids

Galilean boosts and superfluidity of resonantly driven polariton fluids in the presence of an incoherent reservoir

We theoretically investigate how the presence of a reservoir of incoherent excitations affects the superfluidity properties of resonantly driven polariton fluids. While in the absence of reservoir the two cases of a defect moving in a fluid at rest and of a fluid flowing against a static defect are linked by a formal Galilean transformation, here the reservoir defines a privileged reference frame attached to the semiconductor structure and causes markedly different features between the two settings. The consequences on the critical velocity for superfluidity are highlighted and compared to exp…

Thouless泵理论的伽利略协变性

Galilei covariance of the theory of Thouless pumps

本文研究Thouless量子泵理论在伽利略参考系变换下的协变性质，通过矢量丛变换和Bloch理论刻画两类电荷的变换关系，并利用体-边对应原理在Büttiker散射理论中实现该变换定律。

相互作用的空间调制玻色-爱因斯坦凝聚体中的超流分数

Superfluid fraction in an interacting spatially modulated Bose-Einstein condensate

本文理论和实验研究了外部一维周期势打破平移不变性导致的稀薄玻色-爱因斯坦凝聚体超流密度猝灭，通过Leggett界限和声速各向异性一致测定超流分数，并揭示了两体相互作用对超流性的重要作用。

On the hydrodynamics of Bose-condensed fluids subject to density-dependent gauge potentials

When the energy functional of a Bose-condensed state of matter features an effective gauge potential which depends on the density $ρ$ of the condensate, the kinetic energy density of the matter field becomes nonlinear in $ρ$ and additional flow-dependent terms enter the wave equation for the phase of the condensate wavefunction. To begin with, we consider a certain class of density-dependent `single-component' gauge potentials, and later extend this class to encompass more general `multi-component' potentials. The nonlinear flow terms are cast into the general form of an inner-product between …

Superfluid Density of a Spin-orbit Coupled Bose Gas

We discuss the superfluid properties of a Bose-Einstein condensed gas with spin-orbit coupling, recently realized in experiments. We find a finite normal fluid density $ρ_n$ at zero temperature which turns out to be a function of the Raman coupling. In particular, the entire fluid becomes normal at the transition point from the zero momentum to the plane wave phase, even though the condensate fraction remains finite. We emphasize the crucial role played by the gapped branch of the elementary excitations and discuss its contributions to various sum rules. Finally, we prove that an independent d…

From Classical Fields to Two-Fluid Model of Superfluidity: Emergent Kinetics and Local Gauge Transformations

The first successful macroscopic theory for the motion of superfluid helium was that of Lev Landau (1941) in which the fluid is modelled phenomenologically as an interpenetrating mixture of a superfluid and a normal fluid. It has later been shown that Landau's two-fluid model can be rigorously derived from a one-fluid model within the classical fields approximation. Assuming a separation of scales exists between the slowly varying, large-scale, background (condensate) field, and the short rapidly evolving excitations, a full description of the kinetics between the condensate and the thermal cl…