Critical quench dynamics of random quantum spin chains: Ultra-slow relaxation from initial order and delayed ordering from initial disorder

TL;DR

This paper investigates the non-equilibrium dynamics of the random transverse-field Ising chain after quantum quenches, revealing ultra-slow relaxation and delayed ordering behaviors characterized by logarithmic scaling and finite-size effects.

Contribution

It provides a detailed analysis of the relaxation dynamics at criticality for different initial states using free fermionic techniques, highlighting new scaling laws and the role of rare events.

Findings

Logarithmically slow relaxation from ordered states with size-dependent saturation

Delayed magnetization increase from disordered states with size-dependent delay time

Finite-size scaling governed by a universal variable involving system size and time

Abstract

By means of free fermionic techniques combined with multiple precision arithmetic we study the time evolution of the average magnetization, $\overline{m}(t)$, of the random transverse-field Ising chain after global quenches. We observe different relaxation behaviors for quenches starting from different initial states to the critical point. Starting from a fully ordered initial state, the relaxation is logarithmically slow described by $\overline{m}(t) \sim \ln^a t$, and in a finite sample of length $L$ the average magnetization saturates at a size-dependent plateau $\overline{m}_p(L) \sim L^{-b}$; here the two exponents satisfy the relation $b/a=\psi=1/2$. Starting from a fully disordered initial state, the magnetization stays at zero for a period of time until $t=t_d$ with $\ln t_d \sim L^{\psi}$ and then starts to increase until it saturates to an asymptotic value $\overline{m}_p(L) \sim L^{-b'}$, with $b'\approx 1.5$. For both quenching protocols, finite-size scaling is satisfied in terms of the scaled variable $\ln t/L^{\psi}$. Furthermore, the distribution of long-time limiting values of the magnetization shows that the typical and the average values scale differently and the average is governed by rare events. The non-equilibrium dynamical behavior of the magnetization is explained through semi-classical theory.