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

Abstract

Pressure-induced ordering close to a $z=1$ quantum critical point is studied in the presence of bond disorder in the quantum spin system (C${}_4$H${}_{12}$N${}_2$)Cu${}_2$(Cl${}_{1-x}$Br${}_x$)${}_6$ (PHCX) by means of muon-spin rotation and relaxation. As for the pure system (C${}_4$H${}_{12}$N${}_2$)Cu${}_2$Cl${}_6$, pressure allows PHCX with small levels of disorder ($x\le 7.5\%$) 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 $7.5\%<x_{\mathrm{c}}<15\%$. Br-substitution increases the critical pressure and suppresses critical temperatures and ordered moment sizes.

8 pages, 5 figures, final version

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