Relaxation and aging in quantum spin systems

The nonthermal quantum relaxation of the magnetization under nonequilibrium initial conditions is studied for the transverse Ising chain and for the o ne-dimensional isotropic XY model. In the absence of a heat bath the transv erse Ising system exhibits an oscillating relaxation with a decaying amplit ude. At the critical transverse field some stationary properties become ind ependent of the initial state. The isotropic XY chain with a spatially inho mogeneous initial state shows a slow, modulated algebraic decay of the magn etization. Analysis of the two-time autocorrelation function reveals the ph enomenon of aging.