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

The quantum spin-liquid compound (C₄H₁₂N₂)Cu₂Cl₆ 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_c ∼ 4.3 kbar. An additional phase transition to a different ordered phase is observed at P₁ ∼ 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 P₁, T₁ = 2.0 K.