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 $T=1.5\mathrm{}\hspace{0.5em}\mathrm{K},$ the magnetic excitation spectrum is dominated by a single propagating mode with a gap, $\Delta =1\mathrm{}\hspace{0.5em}\mathrm{meV},$ and bandwidth of $\approx 1.8\hspace{0.5em}\mathrm{meV}$ in the $\mathrm{}\left(h0l\right)\mathrm{}$ plane. The mode has no dispersion along the $b^{*}$ direction indicating that neighboring $\mathbf{a}-\mathbf{c}$ 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 $H_{c1\mathrm{}}\approx 7.5\hspace{0.5em}\mathrm{T}.$ Analysis of the neutron-scattering data reveals the important exchange interactions and indicates that some of these are highly frustrated.