Emergent criticality in fully frustrated quantum magnets

TL;DR

This paper investigates how an applied magnetic field induces novel emergent critical phenomena in a fully frustrated quantum magnet, revealing complex phase transitions and universality classes through quantum Monte Carlo simulations.

Contribution

It demonstrates the emergence of a 4-state Potts universality class in a frustrated quantum magnet, linking microscopic models to complex critical behavior.

Findings

Identification of a novel emergent criticality driven by magnetic field.

Observation of a Berezinskii-Kosterlitz-Thouless transition at finite temperatures.

Discovery of a 4-state Potts universality class at a multicritical point.

Abstract

Phase transitions in condensed matter are often linked to exotic emergent properties. We study the fully frustrated bilayer Heisenberg antiferromagnet to demonstrate that an applied magnetic field creates a novel emergent criticality. The quantum phase diagram contains four states, the DS (singlets on every interlayer dimer bond), DTAF (all triplets with antiferromagnetic order), TC (a singlet-triplet checkerboard) and FM (saturated ferromagnet). The thermal phase diagram is dominated by a wall of discontinuities extending from the zero-field DTAF-DS transition to a quantum critical endpoint where the field drives the DTAF and TC into the FM. This first-order wall is terminated at finite temperatures by a line of critical points, where the Berezinskii-Kosterlitz-Thouless (BKT) transition of the DTAF and the thermal Ising transition of the TC also terminate. We demonstrate by quantum Monte Carlo simulations that the BKT transition does not change the Ising nature of the DTAF-DS critical line. By contrast, the combination of symmetries merging on the multicritical DTAF-TC line leads to a 4-state Potts universality not contained in the microscopic Hamiltonian, which we associate with the Ashkin-Teller model. Our results represent a systematic step in understanding emergent phenomena in quantum magnetic materials including the ``Shastry-Sutherland compound'' SrCu$_2$(BO$_3$)$_2$.