Magnetic excitations in ferro-pnictide materials controlled by a quantum critical point into hidden order

The two-orbital J1-J2 model that describes a square lattice of frustrated spin-1 iron atoms is analyzed within the linear spin-wave approximation and by exact diagonalization over a 4x4 cluster. A quantum critical point (QCP) is identified that separates hidden magnetic order at weak Hund's rule coupling from a commensurate spins density wave (cSDW) at strong Hund's rule coupling. Although the moment for cSDW order is small at the QCP, the critical linear spin-wave spectrum shows strong low-energy excitations centered at the wavenumbers that correspond to cSDW order. These disperse anisotropically. A fit to the magnetic excitation spectrum of ferro-pnictide materials obtained recently by inelastic neutron scattering measurements notably accounts for the absence of softening at the wavenumber that corresponds to Neel order.Comment: 20 pages, 8 figures, added discussion on doping, published versio

Nature of Decoupling in the Mixed Phase of Extremely Type-II Layered Superconductors

The uniformly frustrated layered XY model is analyzed in its Villain form. A decouple pancake vortex liquid phase is identified. It is bounded by both first-order and second-order decoupling lines in the magnetic field versus temperature plane. These transitions, respectively, can account for the flux-lattice melting and for the flux-lattice depinning observed in the mixed phase of clean high-temperature superconductors.Comment: 11 pages of PLAIN TeX, 1 postscript figure, published version, many change