Attractive vortices

Superconductors are usually categorized as being either type-I or type-II, depending on their behavior under a magnetic field. Now, researchers at Katholieke Universiteit Leuven report in Physical Review Letters that they have found evidence for a superconducting phase that spans these categories, showing coexisting type-I and type-II behavior [1]. The fascinating phenomenon of superconductivity was discovered in 1911 in Leiden by Heike KamerlinghOnnes when he observed that mercury completely loses its electrical resistivity when cooled down to the temperature of liquid helium, 4.2 K. In 1913 he was awarded the Nobel Prize in Physics for his discovery. However, it would take almost 50 years until the superconducting state was explained by the microscopic theory of John Bardeen, Leon Neil Cooper, and Robert Schrieffer (BCS theory, Nobel Prize 1972) as being due to the formation of electron pairs that can move through the crystal without being scattered by the atoms. Long before BCS theory, however, some very successful phenomenological theories of superconductors had been conceived: in 1935 Fritz and Heinz London introduced the London depth λ, the distance to which an applied magnetic field can penetrate the surface of a superconductor, and in 1953 Brian Pippard introduced the Pippard length ξP. The most powerful tool, however, is the Ginzburg-Landau (GL) theory, conceived in 1950 by Lev Landau and Vitali Ginzburg. They introduced a similar penetration depth λ as London, over which the magnetic field can vary spatially, and a similar coherence length ξ as Pippard, over which the complex GL function ψ can vary. Both λ(T) and ξ(T) diverge when the temperature T approaches the transition temperature Tc, but their ratio κ = λ/ξ, is a constant material parameter called the GL parameter [2, 3]. These three phenomenological theories can be derived from the BCS theory in particular cases. This was shown by Lev Gor’kov in 1959, who derived the GL theory for the case T ≈ Tc.