Field-induced soft-mode quantum phase transition in La$_{1.855}$Sr$_{0.145}$CuO$_{4}$

Abstract

Inelastic neutron-scattering experiments on the high-temperature superconductor La$_{1.855}$Sr$_{0.145}$CuO$_{4}$ reveal a magnetic excitation gap $Δ$ that decreases continuously upon application of a magnetic field perpendicular to the CuO$_2$ planes. The gap vanishes at the critical field required to induce long-range incommensurate antiferromagnetic order, providing compelling evidence for a field-induced soft-mode driven quantum phase transition.