Effect of disorder on a pressure-induced z = 1 magnetic quantum phase transition

Pressure-induced ordering close to a z = 1 quantum-critical point is studied in the presence of bond disorder in the quantum spin system (C4H12N2)Cu2(Cl1−xBrx)6 (PHCX) by means of muon-spin rotation and relaxation. As for the pure system (C4H12N2)Cu2Cl6, pressure allows PHCX with small levels of disorder (x ≤ 7.5%) to be driven through a quantum-critical point separating a low-pressure quantum paramagnetic phase from magnetic order at high pressures. However, the pressure-induced ordered state is highly inhomogeneous for disorder concentrations x > 1%. This behavior might be related to the formation of a quantum Griffiths phase above a critical disorder concentration 7.5% < xc < 15%. Br substitution increases the critical pressure and suppresses critical temperatures and ordered moment sizes.