A Disordered Route to the Coulomb Quantum Spin Liquid: Random Transverse Fields on Spin Ice in Pr2Zr2O7*

Inelastic neutron scattering reveals a broad continuum of excitations in Pr$_2$Zr$_2$O$_7$, the temperature and magnetic field dependence of which indicate a continuous distribution of quenched transverse fields ($\Delta$) acting on the non-Kramers Pr$^{3+}$ crystal field ground state doublets. We show the magnetic excitations in Pr$_2$Zr$_2$O$_7$ are composed of two parts: a lower energy regime that is driven by inter-spin correlations, and a momentum transfer independent higher energy part driven by quenched transverse fields. A random phase approximation provides an excellent account of the data with a transverse field distribution $\rho(\Delta)\propto (\Delta^2+\Gamma^2)^{-1}$ where $ \Gamma=0.28(1)$~meV. Established during high temperature synthesis due to an underlying structural instability, it appears disorder in Pr$_2$Zr$_2$O$_7$ actually induces a quantum spin liquid.