Transition from Bose glass to a condensate of triplons in Tl1-xKxCuCl3

We report the magnetic-field-induced Bose glass-Bose-Einstein condensate (BEC) transition of triplons in Tl1-xKxCuCl3 and its critical behavior, which were investigated through specific heat and electron spin resonance (ESR) measurements. The field dependence of the BEC transition temperature T-N can be described by the power law [H - H-c] proportional to T-N(phi) near the quantum critical point H-c similar to 3 T. The critical exponent phi tends to reach a value smaller than 1/2 with decreasing fitting window in contrast to phi -> 3/ 2 for the standard BEC in a pure system. At sufficiently low temperatures, the ESR line shape for H similar or equal to H-c is intermediate between Gaussian and Lorentzian functions. This implies the localization of triplons for H < H-c at T = 0.