Disorder-induced Revival of the Bose-Einstein Condensation in Ni(Cl$_{1-x}$Br$_x$)$_2$-4SC(NH$_2$)$_2$ at High Magnetic Fields

TL;DR

This paper theoretically explores how disorder from Br-doping induces a revival of Bose-Einstein condensation in a disordered antiferromagnetic material under high magnetic fields, revealing a complex phase diagram with impurity-controlled long-range order.

Contribution

It uncovers a disorder-induced resurgence of phase coherence at high magnetic fields, contrasting with Bose-glass behavior, and highlights the role of impurities and interchain couplings in stabilizing long-range order.

Findings

Disorder causes a revival of Bose-Einstein condensation at high fields.

Impurities extend the phase coherence region in field-temperature space.

The phenomenon differs from previously observed Bose-glass physics.

Abstract

Building on recent NMR experiments [Phys. Rev. Lett. 118, 067203], we theoretically investigate the high magnetic field regime of the disordered quasi-one-dimensional $S=1$ antiferromagnetic material Ni(Cl$_{1-x}$Br$_x$)$_2$-4SC(NH$_2$)$_2$. The interplay between disorder, chemically controlled by Br-doping, interactions, and the external magnetic field, leads to a very rich phase diagram. Beyond the well-known antiferromagnetically ordered regime, analog of a Bose condensate of magnons, which disappears when $H\ge 12.3$ T, we unveil a resurgence of phase coherence at higher field $H\sim 13.6$ T, induced by the doping. Interchain couplings stabilize finite temperature long-range order whose extension in the field - temperature space is governed by the concentration of impurities $x$. Such a "mini-condensation" contrasts with previously reported Bose-glass physics in the same regime, and should be accessible to experiments.