SINGLE-SPIN SUPERCONDUCTIVITY - FORMULATION AND GINZBURG-LANDAU THEORY

We describe a superconducting phase that arises due to a pairing insta bility of the half-metallic antiferromagnetic (HM AFM) normal state. T his single-spin superconducting (SSS) phase contains broken time-rever sal symmetry in addition to broken gauge symmetry, the former due to t he underlying magnetic order in the normal state. A classification of normal-state symmetries leads to the conclusion that the HM AFM normal phase whose point group contains the inversion operator contains the least symmetry possible which still allows for a zero momentum pairing instability. The Ginzburg-Landau free energy for the superconducting order parameter is constructed consistent with the symmetry of the nor mal phase, electromagnetic gauge invariance and the crystallographic p oint-group symmetry including inversion. For cubic, hexagonal, and tet ragonal point groups, the possible symmetries of the superconducting p hase are classified, and the free energy is used to construct a genera lized phase diagram. We identify the leading candidate out of the poss ible SSS phases for each point group. The symmetry of the superconduct ing phase is used to determine the cases where the gap function has ge neric zeros (point or line nodes) on the Fermi surface. Such nodes alw ays occur, hence thermodynamic properties will have power-law behavior at low temperature.