Extraordinary boundary correlations at deconfined quantum critical points

TL;DR

This paper investigates the boundary behavior of the NCCP^{N-1} model at quantum critical points, revealing a new family of boundary universality classes characterized by logarithmic correlations and their implications for topological phase transitions.

Contribution

It introduces the extraordinary-log boundary correlations at deconfined quantum critical points and characterizes their dependence on N in the large-N limit.

Findings

Boundary correlations decay logarithmically at criticality.

The extraordinary-log exponent is q=N/4, defining new boundary universality classes.

Manifestation of extraordinary-log behavior in Cooper pair correlations at the QSH-SC transition.

Abstract

Recent years have seen a growing appreciation for the effects of quantum critical fluctuations on gapless boundary degrees of freedom. Here we consider the boundary dynamics of the non-compact $\mathbb{CP}^{N-1}$ (NCCP$^{N-1}$) model in two spatial dimensions, with $N$ complex boson species coupled to a fluctuating $\mathrm{U}(1)$ gauge field. These models describe quantum phase transitions beyond the Landau paradigm, such as the deconfined quantum critical point between superconducting (SC) and quantum spin Hall (QSH) phases. We show that, in a large-$N$ limit and with the bulk tuned to criticality, boundaries of the NCCP$^{N-1}$ model display logarithmically decaying, or ``extraordinary-log,'' correlations. In particular, when monopole operators exhibit quasi-long-ranged order at the boundary, we find that the extraordinary-log exponent of the NCCP$^{N-1}$ model in the large-$N$ limit is $q=N/4$, signifying a new family of boundary universality classes parameterized by $N$. In the context of the QSH -- SC transition, the quantum critical point inherits helical edge modes from the QSH phase, and this extraordinary-log behavior manifests in their Cooper pair correlations.