Spin-resonance modes of the spin-gap magnet TlCuCl₃

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Three kinds of magnetic resonance signals were detected in crystals of the spin-gap magnet TlCuCl₃. First, we have observed the microwave absorption due to the excitation of the transitions between the singlet ground state and the excited triplet states. This mode has the linear frequency-field dependence corresponding to the previously known value of the zero-field spin-gap of 156 GHz and to the closing of spin-gap at the magnetic field Hc of about 50 kOe. Second, the thermally activated resonance absorption due to the transitions between the spin sublevels of the triplet excitations was found. These sublevels are split by the crystal field and external magnetic field. Finally, we have observed antiferromagnetic resonance absorption in the field-induced antiferromagnetic phase above the critical field Hc. This resonance frequency is strongly anisotropic with respect to the direction of the magnetic field.