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 $\hat{I}”$ 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.