Triplet waves in a quantum spin liquid.

We report a neutron scattering study of the spin- 1/2 alternating bond antiferromagnet Cu(NO3)(2);2.5D(2)O for 0.06<k(B)T/J1<1.5. For k(B)T/J1<<1 the excitation spectrum is dominated by a coherent singlet-triplet mode centered at J1 = 0.442(2) meV with sinusoidal dispersion and a bandwidth of J2 = 0.106(2) meV. A complete description of the zero temperature contribution to the scattering function from this mode is provided by the single mode approximation. At finite temperatures we observe exponentially activated band narrowing and damping. The relaxation rate is thermally activated and wave vector dependent with the periodicity of the reciprocal lattice.