Transition from Bose glass to a condensate of triplons in Tl _1-xK_xCuCl₃

We report the magnetic-field-induced Bose glass-Bose-Einstein condensate (BEC) transition of triplons in Tl_1-xK_xCuCl₃ 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]T_N^φ near the quantum critical point H_c∼3 T. The critical exponent . tends to reach a value smaller than 1/2 with decreasing fitting window in contrast to φ→3/2 for the standard BEC in a pure system. At sufficiently low temperatures, the ESR line shape for H≃H_c is intermediate between Gaussian and Lorentzian functions. This implies the localization of triplons for H<H_c at T=0.