Slow quenches in XXZ spin-chains -- the role of Galilean invariance breaking

Abstract

We study a XXZ spin-chain in a gapless Tomonaga-Luttinger liquid (TLL) phase with time dependent anisotropy of spin exchange interactions. To begin we focus on a linear ramp of $J_z$, starting at XX point and slowly increasing towards the anti-ferromagnetic Heisenberg point. Although the problem of a linear ramp in the TLL has been recently under intense scrutiny in a perturbative \emph{g-ology} framework, an aspect that has been overlooked so far is the role of the Galilean invariance breaking. We find that, although the differential equation that needs to be solved to find time evolution of the system is substantially more complicated, in some cases exact analytic solutions can be given. We obtain them for the linear ramp in the limit of small $J_z$ as well as $J_z\rightarrow 1$, and for such protocols that are tailored to keep the Galilean invariance breaking term constant for every $J_z$. We point out the features of dynamics during the quench that stays unaltered, and those that need to be taken with care when Galilean invariance breaking is present. We are able to deduce that it is the shape of the propagating front that is affected in the most pronounced way.