Pair correlations, short-range order, and dispersive excitations in the quasi-kagome quantum magnet volborthite

We present spatial and dynamic information on the s = 1/2 distorted kagome antiferromagnet volborthite, Cu3V2O7(OD)(2) center dot 2D(2)O, obtained by polarized and inelastic neutron scattering. The instantaneous structure factor, S(Q), is dominated by nearest-neighbor pair correlations, with short-range order at wave vectors Q(1) = 0.65(3) angstrom(-1) and Q(2) = 1.15(5) angstrom(-1) emerging below 5 K. The excitation spectrum, S(Q,omega), reveals two steep branches dispersing from Q(1) and Q(2), and a flat mode omega(f) = 5.0(2) meV. The results allow us to identify the crossover at T* similar to 1 K in V-51 NMR and specific-heat measurements as the buildup of correlations at Q(1). We compare our data to theoretical models proposed for volborthite, and also demonstrate that the excitation spectrum can be explained by spin-wave-like excitations with anisotropic exchange parameters, as suggested by recent local-density calculations.