Raman scattering of Tl(Cu1-xMgx)Cl3

We have studied magnetic excitations in the coupled spin dimer system Tl(Cu1-xMgx)Cl3 by means of Raman scattering. A broad two-magnon Raman band extending from 2Δ to about 150cm-1 is observed at low temperatures, where Δ is the spin-gap energy. With increasing nonmagnetic impurity Mg, the intensity of the broad Raman band weakens and its onset energy, 2Δ, shifts to higher energy. The quasi-elastic scattering appears and the onset of the two-magnon Raman band gets smeared when x=0.05. The Raman spectrum hardly changes when a magnetic field is applied, which indicates a creation of a pair of triplets whose total S is zero in the second-order Raman process