Pressure-Induced Quantum Critical and Multicritical Points in a Frustrated Spin Liquid

The quantum spin-liquid compound (C sub(4)H sub(12)N sub(2)) Cu sub(2)Cl sub(6) is studied by muon spin relaxation under hydrostatic pressures up to 23.6 kbar. At low temperatures, pressure-induced incommensurate magnetic order is detected beyond a quantum critical point at P sub(c) ~ 4.3 kbar. An additional phase transition to a different ordered phase is observed at P sub(1) ~ 13.4 kbar. The data indicate that the high-pressure phase may be a commensurate one. The established (P-T) phase diagram reveals the corresponding pressure-induced multicritical point at P sub(1), T sub(1) = 2.0 K.