Deconfined classical criticality in the anisotropic quantum spin-$\frac{1}{2}$ XY model on the square lattice

TL;DR

This paper demonstrates that thermal fluctuations transform a first-order quantum phase transition in the anisotropic quantum XY model into a deconfined classical critical point, revealing novel critical behavior.

Contribution

It uncovers how thermal fluctuations smooth a zero-temperature quantum transition into a classical critical point with deconfined excitations in the 2D XY model.

Findings

Quantum phase transition is smoothed by thermal fluctuations.

Deconfined classical criticality emerges at finite temperature.

The transition involves competing Ising ordered phases.

Abstract

The anisotropic quantum spin-1/2 XY model on a linear chain was solved by Lieb, Schultz, and Mattis in 1961 and shown to display a continuous quantum phase transition at the O(2) symmetric point separating two gapped phases with competing Ising long-range order. For the square lattice, the following is known. The two competing Ising ordered phases extend to finite temperatures, up to a boundary where a transition to the paramagnetic phase occurs, and meet at the O(2) symmetric critical line along the temperature axis that ends at a tricritical point at the Berezinskii-Kosterlitz-Thouless transition temperature where the two competing phases meet the paramagnetic phase. We show that the first-order zero-temperature (quantum) phase transition that separates the competing phases as a function of the anisotropy parameter is smoothed by thermal fluctuations into deconfined classical criticality.