Localization of Spin Triplets in Quantum Dimer System Tl1-xKxCuCl3

The effect of exchange randomness on the ground state and the field-induced magnetic ordering was investigated through specific heat and magnetization measurements in Tl1-xKxCuCl3 with x≤0.22. The isostructural parent compounds TlCuCl3 and KCuCl3 are coupled spin dimer systems with a gapped singlet ground state and their field-induced antiferromagnetic ordering is described by the Bose condensation of spin triplets. Due to exchange randomness, the low-field singlet ground state turns into the magnetic state with finite susceptibility. However, the gap for the triplet excitation still remains. Well-defined field-induced phase transitions were observed in Tl1-xKxCuCl3. The relation between transition field HN(T) and temperature is represented by the power law HN(T)-Hc ∝ Tφ. Systematic decrease of the critical exponent φ with x is observed. These properties are discussed in connection with Bose gas in the presence of random potential.