Triplet excitations in low-Hc spin-gap systems KCuCl3 and TlCuCl3:: An inelastic neutron scattering study -: art. no. 132415

Elementary excitations in valence bond magnets have a finite spin gap Delta=gmu(B)H(c) to well-defined triplet states. The degeneracy of the triplet states is lifted in the presence of an external field, according to the Zeeman interaction term. The energy and intensity of the excitation spectra in S=1/2 valence bond KCuCl3 and TlCuCl3 are investigated by inelastic neutron scattering at finite external fields. Experimental observations along representative directions of reciprocal space are addressed up to H/H(c)similar to0.6 and H/H(c)similar to0.9, respectively. A comprehensive analysis of the split Zeeman modes is reported, which shows excellent agreement with first principles. The obtained results extend former characterizations at H=0 and are of importance in the context of the field-driven quantum criticality realized in the sister compounds KCuCl3 and TlCuCl3.