Field induced non-BEC transitions in frustrated magnets

TL;DR

This paper investigates how frustration and magnetic fields cause deviations from the Bose-Einstein condensation paradigm in certain spin systems, revealing a Lifshitz transition and a novel bosonic liquid state.

Contribution

It demonstrates the breakdown of the BEC description in frustrated magnets under magnetic fields, introducing a Lifshitz transition and identifying a new correlated bosonic liquid phase.

Findings

Lifshitz transition occurs due to frustration-driven suppression of Nél order.

Behavior near the Lifshitz point is controlled by a multicritical point.

Evidence for a frustration and field-driven bosonic liquid state.

Abstract

Frustrated spin-systems have traditionally proven challenging to understand, owing to a scarcity of controlled methods for their analyses. By contrast, under strong magnetic fields, certain aspects of spin systems admit simpler and universal description in terms of hardcore bosons. The bosonic formalism is anchored by the phenomenon of Bose-Einstein condensation (BEC), which has helped explain the behaviors of a wide range of magnetic compounds under applied magnetic fields. Here, we focus on the interplay between frustration and externally applied magnetic field to identify instances where the BEC paradigm is no longer applicable. As a representative example, we consider the antiferromagnetic $J_1 - J_2 - J_3$ model on the square lattice in the presence of a uniform external magnetic field, and demonstrate that the frustration-driven suppression of the N\'{e}el order leads to a Lifshitz transition for the hardcore bosons. In the vicinity of the Lifshitz point, the physics becomes unmoored from the BEC paradigm, and the behavior of the system, both at and below the saturation field, is controlled by a Lifshitz multicritical point. We obtain the resultant universal scaling behaviors, and provide strong evidence for the existence of a frustration and magnetic-field driven correlated bosonic liquid state along the entire phase boundary separating the N\'{e}el phase from other magnetically ordered states.