Surface Superconductivity in Type I and Type II Superconductors

Abstract

CraigSaint-James and de Gennes 1 have shown theoretically that a surface superconducting layer with a critical field H c % can exist above the bulk critical field H c when the external field is parallel to the sample surface. They find that H c % = 2.39KH C where K is the Ginzburg-Landau parameter. For type II superconductors (K>1//2) this relation becomes H c % = 1.69# c 2-Experimental evidence 2 " 6 has confirmed the existence of a surface critical field H c % above i/ c 2> * or type II superconductors. For values of K between 1//2 and 1/2.39, surface critical fields are predicted for type I superconductors, although observation of H c % in type I materials has not yet been reported. We have measured the real and imaginary parts (x' and x") oi the ac susceptibility in type I and type II superconductors in both parallel and perpendicular dc magnetic fields. This technique yields much sharper transitions in E c % than have been reported and permits demonstration of the existence of surface effects in superconductors with H c % barely above H c . In a lead alloy containing only 0.1% Bi we find that H c 2 is slightly above H c . This alloy shows a reversible type I magnetization curve. In pure Pb, H c % was not detectable and the paramagnetic ac susceptibility characteristic of the intermediate state 7 in type I superconductors was observed.

Near horizon geometry of strings ending on intersecting D8/D4-branes

“…The superconformal symmetry is broken to OSp(4 * |2) ⊂ F(4; 2), while the SO(3) bosonic symmetry remains unbroken, since the Wilson line is neutral. To see this, note that a time-like Wilson line preserves an SO(4) = SO(3) × SO(3) rotational symmetry and translations in time, which in the conformal limit are enhanced to SO (1,2). This gives a full bosonic symmetry reduces the supersymmetries by half, so that the resulting theory preserves 4 Poincaré supersymmetries.…”

“…2 We identify three types of solutions. The first, given in section 5.1, we interpret as a stack of fundamental strings in the presence of D8-branes, i.e.…”

“…A magnetic impurity in a material can be viewed as a defect, facilitating the description of the Kondo effect, which involves the screening of said impurity at low temperatures [1]. If one puts a boundary in a system describing a superconductor, one may hope to describe surface superconductivity [2]. Topological insulators are constructed by joining two systems with different topological invariants along an interface [3][4][5].…”

Linear and nonlinear low-frequency electrodynamics of surface superconducting states in an yttrium hexaboride single crystal

“…A large number of experimental and theoretical studies are concentrated on the families of compact B 12 icosahedrons and B 6 octahedrons with a large diversity of electrical and magnetic characteristics. The highest critical temperatures of the transition to the superconducting state in MB 6 and MB 12 compounds were found in YB 6 with T c ≤ 8.4 K and ZrB 12 with T c ≈ 6.0 K [2]. Both materials have a highly symmetrical crystal structure (CaB 6 type for YB 6 and UB 12 type for ZrB 12 ) that can be described as boron cages in which yttrium or zirconium atoms develop large vibrational amplitudes with an Einstein-like (nearly dispersionless) lattice mode.…”

“…The highest critical temperatures of the transition to the superconducting state in MB 6 and MB 12 compounds were found in YB 6 with T c ≤ 8.4 K and ZrB 12 with T c ≈ 6.0 K [2]. Both materials have a highly symmetrical crystal structure (CaB 6 type for YB 6 and UB 12 type for ZrB 12 ) that can be described as boron cages in which yttrium or zirconium atoms develop large vibrational amplitudes with an Einstein-like (nearly dispersionless) lattice mode. In spite of some common features, the two crystals have a few distinct physical characteristics: (i) while YB 6 is a classical type-II superconductor [3], ZrB 12 (at least, for temperatures above 4.5 K) may be regarded as a textbook example of type-I superconductor [4]; (ii) while the superconducting properties are enhanced at the ZrB 12 surface [4], they are suppressed in a YB 6 surface (see our tunneling data below).…”