Special Transition and Extraordinary Phase on the Surface of a Two-Dimensional Quantum Heisenberg Antiferromagnet

TL;DR

This study investigates the surface critical behavior of a 2D quantum Heisenberg antiferromagnet, revealing a unique special transition and an extraordinary phase with long-range order, distinct from classical models.

Contribution

It uncovers a new surface universality class and critical exponents for the 2D quantum Heisenberg model, expanding understanding of surface critical phenomena in quantum systems.

Findings

Discovery of a direct special transition from ordinary to extraordinary phase on the surface.

The extraordinary phase exhibits long-range antiferromagnetic order on the surface.

Identification of new critical exponents indicating a novel surface universality class.

Abstract

Continuous phase transitions exhibit richer critical phenomena on the surface than in the bulk, because distinct surface universality classes can be realized at the same bulk critical point by tuning the surface interactions. The exploration of surface critical behavior provides a window looking into higher-dimensional boundary conformal field theories. In this work, we study the surface critical behavior of a two-dimensional (2D) quantum critical Heisenberg model by tuning the surface coupling strength, and discover a direct special transition on the surface from the ordinary phase into an extraordinary phase. The extraordinary phase has a long-range antiferromagnetic order on the surface, in sharp contrast to the logarithmic decaying spin correlations in the 3D classical O(3) model. The special transition point has a new set of critical exponents, $y_{s}=0.86(4)$ and $\eta_{\parallel}=-0.33(1)$, which are distinct from the special transition of the classical O(3) model and indicate a new surface universality class of the 3D O(3) Wilson-Fisher theory.