N. Cavadini, Ch. Rüegg, A. Furrer, H. U. Güdel, K. Krämer, H. Mutka, A. Wildes, K. Habicht and P. Vorderwisch
Triplet modes in a quantum spin liquid across the critical field
Int. J. Mod. Phys. B 16, 3302-3305 (2002)                 

Abstract: S = 1/2 TlCuCl₃ is a magnetic insulator with a singlet ground state and a finite spin gap to triplet excited states. At finite fields, the degeneracy of the triplet states is lifted according to the Zeeman interaction term. Field-induced magnetic ordering occurs in TlCuCl₃ at the critical field H_c 6T, when the Zeeman interaction overcomes the spin gap at the antiferromagnetic zone center. A detailed characterization of the elementary excitations realized at H > H_c is obtained by means of inelastic neutron scattering on single crystals. The general framework of field-induced quantum criticality in a three-dimensional spin liquid is illustrated on the complete dynamic range for the first time.