Magnetic-field-induced soft mode in spin-gapped high-Tc superconductors

We present an explanation of the dynamical in-gap spin mode in La2-xSrxCuO4 induced by an applied magnetic field H as recently observed experimentally [J. Chang , Phys. Rev. Lett. 102, 177006 (2009)]. Our model consists of a phenomenological spin-only Hamiltonian, and the softening of the spin mode is caused by vortex pinning of dynamical stripe fluctuations which we model by a local ordering of the exchange interactions. The spin gap vanishes experimentally around H=7 T, which in our scenario corresponds to the field required for overlapping vortex regions.