Out-of-equilibrium dynamics across the first-order quantum transitions of one-dimensional quantum Ising models

Abstract: We study the out-of-equilibrium dynamics of one-dimensional quantum Ising models in a transverse field $g$, driven by a time-dependent longitudinal field $h$ across their {\em magnetic} first-order quantum transition at $h=0$, for sufficiently small values of $|g|$. We consider nearest-neighbor Ising chains of size $L$ with periodic boundary conditions. We focus on the out-of-equilibrium behavior arising from Kibble-Zurek protocols, in which $h$ is varied linearly in time with time scale $t_s$, i.e., $h(t)=t/t_s$. The system starts from the ground state at $h_i\equiv h(t_i)<0$, where the longitudinal magnetization $M$ is negative. Then it evolves unitarily up to positive values of $h(t)$, where $M(t)$ becomes eventually positive. We identify several scaling regimes characterized by a nontrivial interplay between the size $L$ and the time scale $t_s$, which can be observed when the system is close to one of the many avoided level crossings that occur for $h\ge 0$. In the $L\to\infty$ limit, all these crossings approach $h=0^+$, making the study of the thermodynamic limit, defined as the limit $L\to\infty$ keeping $t$ and $t_s$ constant, problematic. We study such limit numerically, by first determining the large-$L$ quantum evolution at fixed $t_s$, and then analyzing its behavior with increasing $t_s$. Our analysis shows that the system switches from the initial state with $M<0$ to a positively magnetized state at $h = h_(t_s)>0$, where $h_(t_s)$ decreases with increasing $t_s$, apparently as $h_*\sim 1/\ln t_s$. This suggests the existence of a scaling behavior in terms of the rescaled time $\Omega = t \ln t_s/t_s$. The numerical results also show that the system converges to a nontrivial stationary state in the large-$t$ limit, characterized by an energy significantly larger than that of the corresponding homogeneously magnetized ground state.