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 $T=1.5 \mathrm{K},$ the magnetic excitation spectrum is dominated by a single propagating mode with a gap, $\ensuremath{\Delta}=1 \mathrm{meV},$ and bandwidth of $\ensuremath{\approx}1.8 \mathrm{meV}$ in the $(h0l)$ plane. The mode has no dispersion along the ${b}^{*}$ direction indicating that neighboring $\mathbf{a}\ensuremath{-}\mathbf{c}$ 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 ${H}_{c1}\ensuremath{\approx}7.5 \mathrm{T}.$ Analysis of the neutron-scattering data reveals the important exchange interactions and indicates that some of these are highly frustrated.