Nonequilibrium Quantum Phase Transitions
Aditi Mitra, NYU

An important set of questions in condensed matter relate to the effect of
nonequilibrium drives on systems near quantum critical points. Simple scaling
analysis indicate that response to an external drive is highly nonlinear in
the vicinity of a quantum critical point, so that a linear response Kubo
formula treatment is usually invalid.

In this talk I will present a microscopic treatment for the effect of current
flow on a ferromagnetic quantum critical point and show that current flow has
two important effects.  One is to induce decoherence which affects the system
in ways rather similar to temperature.  Second it causes a symmetry breaking
which leads to new terms in the critical theory. In the disordered phase and
for Ising systems decoherence is the dominant effect, with inversion symmetry
breaking giving only subleading corrections to scaling. On the other hand for
models with continuous rotation symmetry, current flow can lead to dynamical
instabilities of the ordered phase.