Single-crystal growth of Pb₂V₃O₉ and the Bose-Einstein condensed state of triplons studied by thermal conductivity, specific heat and magnetization measurements

We have succeeded in growing large-size single crystals of the quasi-one-dimensional S 1/2 spin-dimer system Pb₂V₃O ₉ by the floating-zone method, and also investigated the thermal conductivity, specific heat and magnetization in magnetic fields. In high magnetic fields, it has been found that the suppression of the thermal conductivity by the application of magnetic field is relaxed. This behavior is caused by the enhancement of the thermal conductivity due to triplons and/or phonons owing to the extension of the mean free path of triplons and/or phonons in the Bose-Einstein condensed (BEC) state of field-induced triplons. We have estimated the critical field H_c(T) between the BEC state of triplons and the gapped normal state from the thermal conductivity, specific heat and magnetization measurements and obtained the value of the critical exponent oslash; ∼ 2.0 in [H_c(T) - H_c(0)] T^ø using H_c(T)'s at low temperatures.