Localization of Spin Triplets in Tl1-xKxCuCl3

Abstract

The effect of exchange randomness on field-induced magnetic ordering was investigated through specific heat measurements in Tl 1- x K x CuCl 3 with x ≤0.22. The isostructural parent compounds TlCuCl 3 and KCuCl 3 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 Tl 1- x K x CuCl 3 . The critical exponent φ of the phase boundary defined by T ( H ) ∝( H - H c ) 1/φ is reevaluated in TlCuCl 3 as φ=1.67 ±0.07, which is close to φ BEC =3/2 derived from the magnon BEC theory. For x ≠0, the exponent φ 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...