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#### Superconductivity: between "one" and "two"

Publications: Interaction of vortices in superconductors with kappa close to 1/sqrt(2) Theory of superconductors with kappa close to 1/sqrt(2) I. Luk'yanchuk; Phys. Rev. B 63, 174504 (2001)As was first shown by Bogomolnyi, the critical Ginzburg-Landau (GL) parameter k5kappa=1/sqrt2 at which a superconductor changes its behavior from type I to type II, is the special highly degenerate point where Abrikosov vortices do not interact and therefore all vortex states have the same energy. Developing a secular perturbation theory, we studied how this degeneracy is lifted when k is slightly different from 1/sqrt2 or when the GL theory is extended to the higher terms in T2Tc . We constructed a simple secular functional that depends only on few experimentally measurable phenomenological parameters and therefore is quite efficient to study the vortex state of superconductor in this transitional region of k . On this base, we calculated such vortex state properties as critical fields, energy of the normal-superconductor interface, energy of the vortex lattice, vortex interaction energy, etc., and compared them with previous results that were based on bulky solutions of GL equations. |

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