We show that an out-of-equilibrium percolation transition occurs after quenching ferromagnetic Ising-like systems across their magnetic first-order transitions. As a paradigmatic example, we consider a two-dimensional Ising system driven across its low-temperature first-order transition line by a quench of the magnetic field
h from
hi<0 to
h>0. In the thermodynamic limit and for finite values of
h, the post-quench evolution under a purely relaxational dynamics is characterized by a dynamic transition at a finite critical time
tc(h) from the metastable negatively magnetized phase to the positive one, marked by the percolation of the largest clusters of positive and negative spins. This out-of-equilibrium percolation transition displays a finite-size scaling behavior as in the standard random-percolation case. However, while the fractal dimension of the percolating clusters is consistent with the random-percolation value, the exponent controlling the approach to criticality differs and depends on
h. We also show that the percolation critical behavior is related to the spinodal-like behavior of the magnetization in the small-
h limit, which implies that the percolation time
tc(h) exhibits a spinodal-like exponential dependence on
h.
We study the out-of-equilibrium scaling behavior of two-dimensional and three-dimensional Ising systems, when they are slowly driven across their magnetic first-order transitions at low temperature T<T_{c}, where T_{c} is the temperature of their continuous transition. We consider Kibble-Zurek (KZ) protocols in which a spatially homogenous magnetic field h varies as h(t)=t/t_{s} with a timescale t_{s}. The KZ dynamics starts from negatively magnetized configurations equilibrated at h_{i}<0 and stops at a positive value of h where the configurations acquire a positive average magnetization. We consider the Metropolis and the heat-bath dynamics, which are two specific examples of a purely relaxational dynamics. We focus on two different dynamic regimes. We consider the out-equilibrium finite-size scaling (OFSS) limit in which the system size L and the timescale t_{s} diverge simultaneously, keeping an appropriate combination fixed. Then, we analyze the KZ dynamics in the thermodynamic limit (TL), obtained by taking first the L→∞ limit at fixed t and t_{s}, and then considering the scaling behavior in the large-t_{s} limit. Our numerical results provide evidence of OFSS, as predicted by general scaling arguments. The results in the TL show the emergence of spinodal-like behaviors: The passage from the negatively magnetized phase to the positively magnetized one occurs at positive values h_{*}>0 of the magnetic field, which decrease as h_{*}∼1/(lnt_{s})^{κ}, with κ=2 and κ=1 in two and three dimensions, respectively, for t_{s}→∞. We identify σ≡t(lnt)^{κ}/t_{s} as the relevant scaling variable associated with the KZ dynamics in the TL.
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
ts, i.e.,
h(t)=t/ts. The system starts from the ground state at
hi≡h(ti)<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
ts, which can be observed when the system is close to one of the many avoided level crossings that occur for
h≥0. In the
L→∞ limit, all these crossings approach
h=0+, making the study of the thermodynamic limit, defined as the limit
L→∞ keeping
t and
ts constant, problematic. We study such limit numerically, by first determining the large-
L quantum evolution at fixed
ts, and then analyzing its behavior with increasing
ts. Our analysis shows that the system switches from the initial state with
M<0 to a positively magnetized state at
h=h∗(ts)>0, where
h∗(ts) decreases with increasing
ts, apparently as
h∗∼1/lnts. This suggests the existence of a scaling behavior in terms of the rescaled time
Ω=tlnts/ts. 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.
We study the out-of-equilibrium spinodal-like dynamics of three-dimensional q-state Potts systems driven across their thermal first-order transition in the thermodynamic limit, by a relaxational (heat-bath) dynamics. During the evolution, the inverse temperature β increases linearly with time, as δβ(t)≡β(t)-β_{fo}∼t/t_{s}, where β_{fo} is the inverse temperature at the transition point, t is the time, and t_{s} is a timescale. The dynamics starts at t_{i}<0 from an ensemble of disordered configurations equilibrated at an inverse temperature β(t_{i})<β_{fo} and ends at positive values of t, corresponding to β(t)>β_{fo} in the ordered phase (this is analogous to a standard Kibble-Zurek protocol). The time-dependent energy density shows an out-of-equilibrium scaling behavior in the large-t_{s} limit, in terms of the scaling variable σ≡t(lnt)^{κ}/t_{s}. The exponent κ turns out to be consistent with κ=3/2 (with good accuracy), which is the value obtained by assuming that the initial nucleation of ordered regions is the relevant mechanism providing the largest timescale. This scaling behavior implies a spinodal-like phenomenon close to the transition point: the passage from the disordered to the ordered phase, composed of large ordered regions of different color, occurs at δβ(t)=δβ_{*}>0, which decreases as δβ_{*}∼(lnt_{s})^{-κ} in the large-t_{s} limit.
We study the out-of-equilibrium scaling behavior of two-dimensional and three-dimensional Ising systems, when they are slowly driven across their {\em magnetic} first-order transitions at low temperature
T0 of the magnetic field, which decrease as
h∗∼1/(lnts)κ, with
κ=2 and
κ=1 in two and three dimensions, respectively, for
ts→∞. We identify
σ≡t(lnt)κ/ts as the relevant scaling variable associated with the KZ dynamics in the TL.
We study the out-of-equilibrium spinodal-like dynamics of three-dimensional
q-state Potts systems driven across their thermal first-order transition in the thermodynamic limit, by a relaxational (heat-bath) dynamics. During the evolution, the inverse temperature
β increases linearly with time, as
δβ(t)≡β(t)−βfo∼t/ts, where
βfo is the inverse temperature at the transition point,
t is the time and
ts is a time scale. The dynamics starts at
ti<0 from an ensemble of disordered configurations equilibrated at inverse temperature
β(ti)<βfo and ends at positive values of
t, when the system is ordered (this is analogous to a standard Kibble-Zurek protocol). The time-dependent energy density shows an out-of-equilibrium scaling behavior in the large-
ts limit, in terms of the scaling variable
t(lnt)κ/ts. The corresponding exponent turns out to be consistent with
κ=3/2 (with a good accuracy), which is the value obtained by assuming that the initial nucleation of ordered regions provides the relevant mechanism for the passage from one phase to the other. The scaling behavior implies a spinodal-like phenomenon close to the transition point: the passage from the disordered to the ordered phase, composed of large ordered regions of different color, occurs at
δβ(t)=δβ∗>0, where
δβ∗ decreases as
1/(lnts)3/2 in the large-
ts limit.