Frustration-induced two-dimensional quantum disordered phase in piperazinium hexachlorodicuprate

Piperazinium hexachlorodicuprate is shown to be a frustrated quasi-two-dimensional quantum Heisenberg antiferromagnet with a gapped spectrum. Zero-field inelastic neutron scattering and susceptibility and specific-heat measurements as a function of applied magnetic field are presented. At T=1.5 K, the magnetic excitation spectrum is dominated by a single propagating mode with a gap, Δ=1 meV, and bandwidth of ≈1.8 meV in the (h0l) plane. The mode has no dispersion along the b* direction indicating that neighboring a-c planes of the triclinic structure are magnetically decoupled. The heat capacity shows a reduction of the gap as a function of applied magnetic field in agreement with a singlet-triplet excitation spectrum. A field-induced ordered phase is observed in heat capacity and magnetic susceptibility measurements for magnetic fields greater than H_cl≈7.5 T. Analysis of the neutron-scattering data reveals the important exchange interactions and indicates that some of these are highly frustrated.