7 research outputs found
Non-equilibrium Berezinskii-Kosterlitz-Thouless Transition in a Driven Open Quantum System
The Berezinskii-Kosterlitz-Thouless mechanism, in which a phase transition is
mediated by the proliferation of topological defects, governs the critical
behaviour of a wide range of equilibrium two-dimensional systems with a
continuous symmetry, ranging from superconducting thin films to two-dimensional
Bose fluids, such as liquid helium and ultracold atoms. We show here that this
phenomenon is not restricted to thermal equilibrium, rather it survives more
generally in a dissipative highly non-equilibrium system driven into a
steady-state. By considering a light-matter superfluid of polaritons, in the
so-called optical parametric oscillator regime, we demonstrate that it indeed
undergoes a vortex binding-unbinding phase transition. Yet, the exponent of the
power-law decay of the first order correlation function in the (algebraically)
ordered phase can exceed the equilibrium upper limit -- a surprising
occurrence, which has also been observed in a recent experiment. Thus we
demonstrate that the ordered phase is somehow more robust against the quantum
fluctuations of driven systems than thermal ones in equilibrium.Comment: 11 pages, 9 figure
Probing fermionic condensates by fast-sweep projection onto Feshbach molecules
Fast-sweep projection onto Feshbach molecules has been widely used as a probe of fermionic condensates. By determining the exact dynamics of a pair of atoms in time-varying magnetic fields, we calculate the number of condensed and noncondensed molecules created after fast magnetic field sweeps from the BCS to the Bose-Einstein condensate side of the resonances in K-40 and Li-6, for different sweep rates and a range of initial and final fields. We discuss the relation between the initial fermionic condensate fraction and the molecular condensate fraction measured after the sweep