Lev Landau (USSR, 1908-1968, Nobel in 1962) was probably the most important Russian physicist of the 20th century. His significant contributions concern so many domains that it is difficult to make a list: quantum physics, magnetism, phase transition, understanding metals, plasma, quantum electrodynamics, neutrinos, etc. He is famous for getting, among other prizes, the Nobel Prize for his model of helium superfluidity.
Although they did not discover the original mechanism of superconductivity, (Bardeen, Cooper and Schrieffer did a couple years later), Landau and his colleague Ginzburg (USSR, 1916-2009, Nobel in 2003) gave a “phenomenological” approach of superconductivity. Inspired by London, they described the superconductor as a quantum wave. But above all, they developed a theory according to which the superconducting state is more orderly than the non-superconducting state, the wave and its amplitude enabling the measurement of this order. From energetic and thermodynamic arguments, Landau and Ginzburg intuited the two equations that should follow this order parameter. With these two equations, the two Russians could predict the behaviour of a superconductor depending on temperature, the effect of a magnetic field on the superconductor, the distance necessary for the superconducting order to set up (referred to as “coherence length”), etc. We can see how powerful their approach is: from a simple intuition and without actually understanding the original mechanism of superconductivity, physicists can still develop a theory with new results.
That is how one of their young colleagues, Alexei Abrikosov (USSR, 1929-), used these two equations in 1952 and predicted a curious behaviour: according to his calculations, if some superconductors were exposed to strong enough magnetic fields, they should become actual magnetic sieves enabling the magnetic field to penetrate through little tubes called vortices, around which would whirl an electric supercurrent. These vortices should even organize themselves in triangular networks. Twelve years later, vortices were observed in experiments and they have been one of the essential means to understand superconductivity and its applications ever since. These vortices help us understand why a magnet levitating above a superconductor seems to be pinned by the superconductor.