Investigation of the magnetic susceptibility of the disordered BEC system NiCl$_{0.85}$Br$_{0.15}$-4SC(NH$_{2})_{2}$ at ultralow-temperatures

We report measurements of the magnetic susceptibility of a disordered BEC system of magnons for single crystals of NiCl$_{0.85}$Br$_{0.15}$-4SC(NH$_{2})_{2}$ (with 15{\%} Cl atoms replaced by Br). NiCl$_{0.85}$Br$_{0.15}$-4SC(NH$_{2})_{2}$ is a potential candidate for a Bose glass (BG) phase of the spins adjacent to a region of Bose-Einstein condensation (BEC). The BG to BEC phase is the bosonic analog of a metal-insulator transition for fermions. The measurements were carried out for temperatures down to 1mK and for applied magnetic fields up to 14.5T. The results show that the critical fields $H_{c}$ do not obey the conventional 3D universality class for a BEC, $H_{c}(T)$ -- $H_{c}(0) \quad \sim $ $T^{\alpha }$, where $\alpha $ = 1.5 [1]. The values of $\alpha $ changes from $\alpha $ = 0.52 for T $>$ 300 mK to $\alpha $ = 0.91 for T $<$ 250 mK and then again at 70$\sim $90mK to $\alpha $ = 0.48 for T$<$ 70 mK, indicating a crossover to possible BG behavior.