Disorder-Induced Revival of the Bose-Einstein Condensation in Ni(Cl1-xBrx)(2)-4SC(NH2)(2) at High Magnetic Fields

摘要

Building on recent NMR experiments [A. Orlova et al., Phys. Rev. Lett. 118, 067203 (2017).], we theoretically investigate the high magnetic field regime of the disordered quasi-one-dimensional S = 1 antiferromagnetic material Ni(Cl1-xBrx)(2)-4SC(NH2)(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, an analog of a Bose condensate of magnons, which disappears when H >= 12.3 T, we unveil a resurgence of phase coherence at a higher field H similar to 13.6 T, induced by the doping. Interchain couplings stabilize the finite temperature long-range order whose extension in the field-temperature space is governed by the concentration of impurities x. Such a "minicondensation" contrasts with previously reported Bose-glass physics in the same regime and should be accessible to experiments.