Neutron scattering study of a quasi-two-dimensional spin- 1/2 dimer system: Piperazinium hexachlorodicuprate under hydrostatic pressure

We report inelastic neutron scattering study of a quasi-two-dimensional S= 1 2 dimer system piperazinium hexachlorodicuprate under hydrostatic pressure. The spin gap Δ becomes softened with the increase of the hydrostatic pressure up to P=9.0kbar. The observed threefold degenerate triplet excitation at P=6.0kbar is consistent with the theoretical prediction and the bandwidth of the dispersion relation is unaffected within the experimental uncertainty. At P=9.0kbar the spin gap is reduced to Δ=0.55meV from Δ=1.0meV at ambient pressure.