Magnetic susceptibility investigation of Bose-glass state in Ni0.85Cd0.15Cl2-4SC(NH2)2 at ultra-low temperatures

We report measurements of the AC susceptibility of a site-diluted quantum magnet Ni0.85Cd0.15Cl2-4SC(NH2)2 (15% Cd-doped dichloro-tetrakis-thiourea-Nickel, or Cd-DTN) down to 10 mK Below a crossover temperature Tcr ≍ 100 ∼ 200mK, we find that the critical fields Hc for Bose-Einstein condensation obey the scaling relation |Hc(T)–Hc(0)| ∼ Tα, with a novel and universal scaling exponent α ≍ 0.9, which is in agreement with numerical results from a theoretical model. Our findings provide strong evidence of the existence of a Bose glass phase in Cd-DTN, and they display a quantitative signature of the transition between a Bose glass and a Bose Einstein condensate.