Gapless quantum spin chains: multiple dynamics and conformal wavefunctions

TL;DR

This paper investigates gapless quantum spin chains, revealing emergent conformal symmetry, multiple dynamical exponents, and the impact of modifications on their spectra through analytical and numerical methods.

Contribution

It provides a continuum description of groundstates, uncovers multiple dynamical exponents, and explores the effects of adding interactions on the spectra of spin chains.

Findings

Emergent conformal-type symmetry in groundstates

Dynamical exponent z ≈ 3.2 for low-lying excitations

Multiple dynamical exponents indicating diverse dynamics

Abstract

We study gapless quantum spin chains with spin 1/2 and 1: the Fredkin and Motzkin models. Their entangled groundstates are known exactly but not their excitation spectra. We first express the groundstates in the continuum which allows for the calculation of spin and entanglement properties in a unified fashion. Doing so, we uncover an emergent conformal-type symmetry, thus consolidating the connection to a widely studied family of Lifshitz quantum critical points in 2d. We then obtain the low lying excited states via large-scale DMRG simulations and find that the dynamical exponent is z = 3.2 in both cases. Other excited states show a different z, indicating that these models have multiple dynamics. Moreover, we modify the spin-1/2 model by adding a ferromagnetic Heisenberg term, which changes the entire spectrum. We track the resulting non-trivial evolution of the dynamical exponents using DMRG. Finally, we exploit an exact map from the quantum Hamiltonian to the non-equilibrium dynamics of a classical spin chain to shed light on the quantum dynamics.