Demler, EugeneSachdev, SubirZhang, Ying2016-08-262001Demler, Eugene, Subir Sachdev, and Ying Zhang. 2001. “Spin-Ordering Quantum Transitions of Superconductors in a Magnetic Field.” Physical Review Letters 87 (6) (July 19). doi:10.1103/physrevlett.87.067202.0031-9007http://nrs.harvard.edu/urn-3:HUL.InstRepos:27989891We argue that recent neutron scattering measurements by Lake et al. [Science 291, 1759 (2001)] of the spin excitation spectrum of La22dSrdCuO4 in a magnetic field can be understood in terms of proximity to a phase with co-existing superconductivity and spin density wave order. We present a general theory for such quantum transitions, and argue that their low energy spin fluctuations are controlled by a singular correction from the superflow kinetic energy, acting in the region outside the vortex cores. We propose numerous experimental tests of our theory.en-USJournal Article2016-08-2610.1103/PhysRevLett.87.067202