Possible Néel orderings of antiferromagnetically coupled spins on a kagomé lattice are studied using linearspin-wave theory and high-temperature expansions. Spin-wave analysis, applied to q=0 (three spins per magnetic unit cell) and to √3 × √3 (nine spins per cell) Néel orderings yield identical excitation spectra with twofold-degenerate linear modes and a dispersionless zero-energy mode. This dispersionless mode is equivalent to an excitation localized to an arbitrary hexagon of nearest-neighbor spins. Second-(J 2) and third-(J 3) neighbor interactions are shown to stabilize the q=0 state for J 2 >J 3 and the √3 × √3 state for J 2 3. A hightemperature expansion of the spin-spin susceptibility χ αβ (q) is performed to order 1/T 8 , for n-component, classical spins with nearest-neighbor interactions only. To order 1/T 7 the largest eigenvalue of the susceptibility matrix is found to be independent of wave vector with an eigenvector that corresponds to the dispersionless mode of the ordered phase. This degeneracy is removed at order 1/T 8. For n=0, the q=0 mode is favored; for n=1, the band is flat; and, for n>1, the maximum susceptibility is found for a √3 × √3 excitation. Similar results are found for the three-dimensional pyrochlore lattice. The high-temperature expansion is used to interpret experimental data for the uniform susceptibility and powder-neutron-diffraction spectrum for the kagomé-lattice system SrCr 8−x Ga 4+x O 19 .

No abstract

We show that a superconducting vortex in underdoped high Tc superconductors could have an antiferromagnetic core. This type of vortex configuration arises as a topological solution in the recently constructed SO(5) nonlinear σ model and in Landau Ginzburg theory with competing antiferromagnetic and superconducting order parameters. Experimental detection of this type of vortex by µSR and neutron scattering is proposed.PACS numbers: 74.20. De, 74.25.Dw, 74.25.Ha One of the most striking properties of high T c superconductivity is the close proximity between the antiferromagnetic (AF) and the superconducting (SC) phases. While there are a number of theories [1] linking the microscopic origin of high T c SC to antiferromagnetic correlations, it is natural to ask if the close proximity between these two phases could have any macroscopic manifestations. Recently, a unified theory [2] of AF and d-wave SC in the cuprates has been constructed based on an SO(5) symmetry. In this theory, the AF and the d-wave SC order parameters are unified into a five dimensional vector (n 1 , n 2 , n 3 , n 4 , n 5 ) called a superspin. The AF order parameters N α correspond to the (n 2 , n 3 , n 4 ) components, while the real and imaginary parts of the SC order parameter ∆ correspond to the (n 1 , n 5 ) components. It was shown that the chemical potential induces a first order superspin-flop transition where the superspin abruptly changes direction from AF to SC.The SO(5) theory predicts a spin triplet pseudo Goldstone boson associated with the spontaneous breaking of SO(5) symmetry in the SC phase [3,2]; these can be identified with the recently observed resonant neutron scattering peaks in superconducting YBCO [4]. Physically, these modes corresponds to Gaussian fluctuations of the superspin. However, it was noted [2] that the SO(5) theory also admits a special class f topological solutions called meron configurations. In this configuration, the superspin lies inside the SC plane far away from the origin, and the SC phase winds around the origin by 2π. As the origin is approached from the radial direction, the superspin lifts up from the SC plane into the AF sphere in order to minimize the energy cost of winding the SC phase. The result is a SC vortex with an AF core [5].The existence of SC vortices with AF cores leads to non-trivial macroscopic consequence which we shall explore in this paper. We first present detailed analytic and numerical solutions of the SO(5) vortex. We also study a more general Landau-Ginzburg (LG) theory obtained from the SO(5) theory by relaxing the constraint on the magnitude of the superspin. This theory describes AF and SC order parameters in competition with each other. We show that even if the SC state wins in the bulk, under appropriate conditions a nonvanishing AF component can occur inside a SC vortex core. The nature of the condition leads us to conclude that a SC vortex with AF core should only be realized in underdoped high T c superconductors, not in the overdoped ones. We believe that the natu...

No abstract

No abstract