The disordered-free-moment phase: a low-field disordered state in spin-gap antiferromagnets with site dilution

TL;DR

This paper investigates a novel disordered-free-moment phase in site-diluted spin-gap antiferromagnets induced by weak magnetic fields, revealing a gapless state with unique magnetic properties and a quantum phase transition akin to superfluid-to-Bose-glass transition.

Contribution

It introduces and characterizes a new disordered-free-moment phase driven by magnetic fields in site-diluted antiferromagnets, connecting it to Bose-glass physics.

Findings

Identification of a gapless disordered-free-moment phase

Observation of pseudo-plateaus in magnetization curves

Field-driven quantum phase transition consistent with superfluid-to-Bose-glass universality

Abstract

Site dilution of spin-gapped antiferromagnets leads to localized free moments, which can order antiferromagnetically in two and higher dimensions. Here we show how a weak magnetic field drives this order-by-disorder state into a novel disordered-free-moment phase, characterized by the formation of local singlets between neighboring moments and by localized moments aligned antiparallel to the field. This disordered phase is characterized by the absence of a gap, as it is the case in a Bose glass. The associated field-driven quantum phase transition is consistent with the universality of a superfluid-to-Bose-glass transition. The robustness of the disordered-free-moment phase and its prominent features, in particular a series of pseudo-plateaus in the magnetization curve, makes it accessible and relevant to experiments.