Nonequilibrium Steady State in a Quantum System: One-Dimensional Transverse Ising Model with Energy Current

We study the nonequilibrium steady states of an Ising chain in a transverse field h by investigating the effect of a field λ, which drives the current of energy. The zero-temperature, h- λ phase diagram is determined exactly, and it is found that the energy current appears continuously above a threshold λ>λc(h). The long-range magnetic order (present for h<hc,λ<λc) is destroyed in the current-carrying state where the correlations are characterized by power-law, oscillatory decay. The mechanism which generates the power-law correlations in the current-carrying state is discussed.