Simple de Sitter Solutions

We present a framework for de Sitter model building in type IIA string theory, illustrated with specific examples. We find metastable dS minima of the potential for moduli obtained from a compactification on a product of two Nil three-manifolds (which have negative scalar curvature) combined with orientifolds, branes, fractional Chern-Simons forms, and fluxes. As a discrete quantum number is taken large, the curvature, field strengths, inverse volume, and four dimensional string coupling become parametrically small, and the de Sitter Hubble scale can be tuned parametrically smaller than the scales of the moduli, KK, and winding mode masses. A subtle point in the construction is that although the curvature remains consistently weak, the circle fibers of the nilmanifolds become very small in this limit (though this is avoided in illustrative solutions at modest values of the parameters). In the simplest version of the construction, the heaviest moduli masses are parametrically of the same order as the lightest KK and winding masses. However, we provide a method for separating these marginally overlapping scales, and more generally the underlying supersymmetry of the model protects against large corrections to the low-energy moduli potential.Comment: 37 pages, harvmac big, 4 figures. v3: small correction

Point-contact spectroscopy of superconducting energy gap in DyNi2B2C

The superconducting energy gap in DyNi2B2C has been investigated using a point-contact technique based on the Andreev reflection from a normal (N)–superconductor (S) boundary, where N is Ag. The observed differential resistance dV/dI is well described by the Blonder–Tinkham–Klapwijk (BTK) theory based on the BSC density of states with zero broadening parameter. Typically, the intensity of the gap structure amounts to several percentage of the normal state resistance, which is an order of magnitude less than predicted by the theory. For DyNi2B2C with Tc<TN (the Neel temperature), we found gap values satisfying the ratio of 2Δ0/kBTc=3.63±0.05 similar to other superconducting nickel-borocarbides, both nonmagnetic and magnetic with Tc≥TN. The superconducting gap nonlinearity is superimposed on the antiferromagnetic structure in dV/dI(V) which is suppressed at the magnetic field of the order of 3 T applied nominally in the ab-plane and temperature ≥11 K. The observed superconducting properties depend on the exact composition and structure at the surface of the crystal.© Elsevie

Vortex phase diagrams in R3Rh4Sn13 (R = Yb,Ca)

A comparative study of vortex phase diagrams in two weakly pinned low Tc superconductors, Ca3Rh4Sn13 (CaRhSn) and Yb3Rh4Sn13 (YbRhSn), is presented. In CaRhSn, we can witness a disorder-driven transition via the phenomenon of second magnetization peak whose field-temperature dependence is analogous to that often discussed in the context of a Bragg glass (BG) to vortex glass (VG) transition.© Elsevie

The second magnetization peak and the peak effect phenomenon in the superconductor Ca3Rh4Sn13

We report on the observation of the anomalous second magnetization peak (SMP) and the peak effect (PE) in juxtaposition to each other in the isothermal magnetization hysteresis loops at T ≤ 4 K in a weakly pinned single crystal of an isotropic superconductor Ca3Rh4Sn13 (Tc≈8.2 K). The position of the SMP does not display any temperature variation, whereas the PE appearing at the edge of the irreversibility line progressively moves towards lower fields as temperature increases. At T≈5 K, the PE nearly swamps the SMP, and above this temperature only the PE can be observed. If the SMP and the PE can be construed as representing changes in the spatial order of the flux line lattice (FLL), the apparent merger of loci of the peak fields of the SMP and the PE in the neighborhood of H≈15 kOe at a reduced temperature t [=T/Tc(0)] of about 0.6 could imply the existence of a multi-critical point.© Elsevie

High resolution neutron diffraction investigation of vortex lattice transitions in YNi2B2C

High resolution, neutron small angle scattering studies have been performed to investigate the low field (B &lt; 250 mT) region of the phase diagram for the vortex lattice in the superconducting state of YNi2B2C. The data present clear evidence for a vortex lattice reorientation transition from a state with the diagonal of the rhombic unit cell along a [110] direction to a [100] direction. Above this transition the lattice distorts under the influence of the applied field until the apex angle becomes constant at some higher field. For B parallel to c a square lattice configuration is formed. These experiments confirm qualitatively many of the predictions of a general model based on nonlocal corrections to the London model as applied to this and similar materials.</p

High resolution neutron diffraction investigation of vortex lattice transitions in YNi2B2C

High resolution, neutron small angle scattering studies have been performed to investigate the low field (B &lt; 250 mT) region of the phase diagram for the vortex lattice in the superconducting state of YNi2B2C. The data present clear evidence for a vortex lattice reorientation transition from a state with the diagonal of the rhombic unit cell along a [110] direction to a [100] direction. Above this transition the lattice distorts under the influence of the applied field until the apex angle becomes constant at some higher field. For B parallel to c a square lattice configuration is formed. These experiments confirm qualitatively many of the predictions of a general model based on nonlocal corrections to the London model as applied to this and similar materials.</p