251 research outputs found

    Bi_2 Sr_2 CaCu_2 O_{8+delta} Bicrystal c-Axis Twist Josephson Junctions: A New Phase-Sensitive Test of Order Parameter Symmetry

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    Li {\it et al.} [Phys. Rev. Lett. {\bf 83}, 4160 (1999)] prepared atomically clean Bi2_2Sr2_2CaCu2_2O8+δ_{8+\delta} (BSCCO) Josephson junctions between identical single crystal cleaves stacked and twisted an angle ϕ0\phi_0 about the c axis. For each bicrystal, the ratio JcJ/JcSJ_c^J/J_c^S of the c-axis twist junction critical current density to that across either single crystal part is unity, independent of ϕ0\phi_0 and the ratio AJ/ASA^J/A^S of junction areas. From extensive theoretical studies involving a variety of tunneling and superconducting order parameter (OP) forms, we conclude that the results provide strong evidence for incoherent c-axis tunneling and that the dominant OP is s-wave for TTcT\le T_c. Recently, Takano {\it et al.} [Phys. Rev. B {\bf 65}, 140315(R) (2002)] obtained results from BSCCO whisker twist junctions which also rule out a pure d-wave OP, but which are surprisingly suggestive of coherent c-axis tunneling form small Fermi surface hot spots.Comment: 5 pages, 11 figures, invited talk at LT23, to be published in Physica

    Radiation from a Josephson STAR-emitter

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    We calculate the angular dependence of the radiation-zone output power and electric polarization of stimulated terahertz amplified radiation (STAR) emitted from a dcdc voltage applied across cylindrical and rectangular stacks of intrinsic Josephson junctions. During coherent emission, a spatially uniform acac Josephson current density in the stack acts as a surface electric current density antenna source, leading to an harmonic radiation frequency spectrum, as in experiment, but absent in all cavity modesl of cylindrical mesas. Spatial fluctuations of the acac Josephson current cause its fundamental mode to lock onto the lowest finite energy cylindrical cavity mode, causing it to resonate, leading to a non-uniform magnetic surface current density radiation source, and a non-trivial combined fundamental frequency output power with linear polarization We also present a model of the superconducting substrate, and present results for rectangular mesas.Comment: 18 pages, 26 figures, submitted to PR

    Zero-field Time Correlation Functions of Four Classical Heisenberg Spins on a Ring

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    A model relevant for the study of certain molecular magnets is the ring of N=4 classical spins with equal near-neighbor isotropic Heisenberg exchange interactions. Assuming classical Heisenberg spin dynamics, we solve explicitly for the time evolution of each of the spins. Exact triple integral representations are derived for the auto, near-neighbor, and next-nearest-neighbor time correlation functions for any temperature. At infinite temperature, the correlation functions are reduced to quadrature. We then evaluate the Fourier transforms of these functions in closed form, which are double integrals. At low temperatures, the Fourier transform functions explicitly demonstrate the presence of magnons. Our exact results for the infinite temperature correlation functions in the long-time asymptotic limit differ qualitatively from those obtained assuming diffusive spin dynamics. Whether such explicitly non-hydrodynamic behavior would be maintained for large-N rings is discussed.Comment: 18 pages, 21 figure

    Enhancement of ferromagnetism by p-wave Cooper pairing in superconducting ferromagnets

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    In superconducting ferromagnets for which the Curie temperature TmT_{m} exceeds the superconducting transition temperature TcT_{c}, it was suggested that ferromagnetic spin fluctuations could lead to superconductivity with p-wave spin triplet Cooper pairing. Using the Stoner model of itinerant ferromagnetism, we study the feedback effect of the p-wave superconductivity on the ferromagnetism. Below TcT_{c}, the ferromagnetism is enhanced by the p-wave superconductivity. At zero temperature, the critical Stoner value for itinerant ferromagnetism is reduced by the strength of the p-wave pairing potential, and the magnetization increases correspondingly. More important, our results suggest that once Stoner ferromagnetism is established, TmT_m is unlikely to ever be below TcT_c. For strong and weak ferromagnetism, three and two peaks in the temperature dependence of the specific heat are respectively predicted, the upper peak in the latter case corresponding to a first-order transition.Comment: 6 pages, 6 figures, submitted to Phys. Rev.

    Is the anisotropy of the upper critical field of Sr2_2RuO4_4 consistent with a helical pp-wave state?

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    We calculate the angular and temperature TT dependencies of the upper critical field Hc2(θ,ϕ,T)H_{c2}(\theta,\phi,T) for the C4vC_{4v} point group helical pp-wave states, assuming a single uniaxial ellipsoidal Fermi surface, Pauli limiting, and strong spin-orbit coupling that locks the spin-triplet d\vec{\bf d}-vectors onto the layers. Good fits to the Sr2_2RuO4_4 Hc2,a(θ,T)H_{c2,a}(\theta,T) data of Kittaka {\it et al.} [Phys. Rev. B {\bf 80}, 174514 (2009)] are obtained. Helical states with d(k)=kxxkyy\vec{\bf d}(\vec{\bf k})=k_x\vec{\bf x}-k_y\vec{\bf y} and kyx+kxyk_y\vec{\bf x}+k_x\vec{\bf y} (or kxx+kyyk_x\vec{\bf x}+k_y\vec{\bf y} and kyxkxyk_y\vec{\bf x}-k_x\vec{\bf y}) produce Hc2(90,ϕ,T)H_{c2}(90^{\circ},\phi,T) that greatly exceed (or do not exhibit) the four-fold azimuthal anisotropy magnitudes observed in Sr2_2RuO4_4 by Kittaka {\it et al.} and by Mao {\it et al.} [Phys. Rev. Lett. {\bf 84}, 991 (2000)], respectively.Comment: 5+ pages, 4 figures, submitted as a Fast Track Communication to J. Phys. Condens. Matte

    Single-ion and exchange anisotropy effects and multiferroic behavior in high-symmetry tetramer single molecule magnets

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    We study single-ion and exchange anisotropy effects in equal-spin s1s_1 tetramer single molecule magnets exhibiting TdT_d, D4hD_{4h}, D2dD_{2d}, C4hC_{4h}, C4vC_{4v}, or S4S_4 ionic point group symmetry. We first write the group-invariant quadratic single-ion and symmetric anisotropic exchange Hamiltonians in the appropriate local coordinates. We then rewrite these local Hamiltonians in the molecular or laboratory representation, along with the Dzyaloshinskii-Moriay (DM) and isotropic Heisenberg, biquadratic, and three-center quartic Hamiltonians. Using our exact, compact forms for the single-ion spin matrix elements, we evaluate the eigenstate energies analytically to first order in the microscopic anisotropy interactions, corresponding to the strong exchange limit, and provide tables of simple formulas for the energies of the lowest four eigenstate manifolds of ferromagnetic (FM) and anitiferromagnetic (AFM) tetramers with arbitrary s1s_1. For AFM tetramers, we illustrate the first-order level-crossing inductions for s1=1/2,1,3/2s_1=1/2,1,3/2, and obtain a preliminary estimate of the microscopic parameters in a Ni4_4 from a fit to magnetization data. Accurate analytic expressions for the thermodynamics, electron paramagnetic resonance absorption and inelastic neutron scattering cross-section are given, allowing for a determination of three of the microscopic anisotropy interactions from the second excited state manifold of FM tetramers. We also predict that tetramers with symmetries S4S_4 and D2dD_{2d} should exhibit both DM interactions and multiferroic states, and illustrate our predictions for s1=1/2,1s_1=1/2, 1.Comment: 30 pages, 14 figures, submitted to Phys. Rev.
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