Localization of spin triplets in Tl1-xKxCuCl3

The effect of exchange randomness on field-induced magnetic ordering was investigated through specific heat measurements in Tl1-xKxCuCl3 with x less than or equal to 0.22. The isostructural parent compounds TlCuCl3 and KCuCl3 are coupled spin dimer systems with a gapped ground state and their field-induced antiferromagnetic ordering is described by the Bose-Einstein condensation (BEC) of spin triplets. Well-defined field-induced phase transitions were observed in Tl1-xKxCuCl3. The critical exponent phi of the phase boundary defined by T(H) proportional to (H - H-c)(1/phi) is reevaluated in TlCuCl3 as phi = 1.67 +/- 0.07, which is close to phi(BEC) = 3/2 derived from the magnon BEC theory. For x not equal 0, the exponent phi decreases systematically with x. The phase boundary observed below 2 K for x > 0.1 approximates a linear function of temperature T. In the low-field region for H < H-c no magnetic ordering is observed despite finite susceptibility. These properties are discussed in connection with the Bose glass phase argued by Fisher et al.