Seminarium Fizyki Materii Skondensowanej

I am planning to discuss several recent findings (theoretical andexperimental) in the dynamics of an impurity particle injected into aquantum liquid:(i) The momentum distribution of the impurity in one dimension subjectto a constant external force exhibits characteristic Bragg reflectionsat the edge of an emergent Brillouin zone. As a consequence, theimpurity exhibits periodic dynamics that is interpreted as Blochoscillations, which arise even though the quantum liquid istranslationally invariant.(ii) The impurity injected into a one-dimensional liquid with someinitial momentum sheds only a part of it to the background gas, andforms a correlated state that no longer decays in time; furthermore,if the initial momentum is large enough, the impurity undergoeslong-lived oscillations. The value of the impurity's velocity atinfinite time lies between zero and the speed of sound in the gas, andis determined by the injection protocol. This way, the impurity'sfrictionless motion is a dynamically emergent phenomenon whosedescription goes beyond accounting for the kinematic constraints ofLandau's approach to superfluidity.(iii) The impurity traveling through a weakly interactingthree-dimensional Bose-Einstein condensate (BEC) of ultra-cold atomsshows a phase transition in the ground state and far-from-equilibriumproperties of the system as a function of the impurity's velocity.Above a critical initial velocity, the impurity's final velocityreaches a fundamental kinematic boundary, the BEC's speed of sound.For weak and intermediate impurity-BEC interactions, the critical lineis determined by the effective mass of the polaron. This quantumtransition can be interpreted as a dynamical Cherenkov effect; theimpurity-BEC interaction causes the dynamics of the system to appearas if the bare impurity is propagating through a BEC whose speed ofsound is altered by an effective refractive index.