Occam's Razor Cuts Down Superconductor Classifications

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Ginzburg-Landau Theory of Two-Band Superconductors: Absence of Type-1.5 Superconductivity
V. G. Kogan and J. Schmalian
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Physical Review B
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Researchers have wrestled with the question of whether or not the newest superconducting materials fit within the traditional classifications. Superconductors can be divided into Type 1 and Type 2, depending on how they behave in a magnetic field. However, magnesium diboride was discovered to have characteristics of both categories and has been dubbed “Type 1.5”. Drawing on the fundamentals of Ginzburg-Landau theory, the basis of the two original types, a combination of theory and simulation has determined that a new class is not warranted. Close to the superconducting transition temperature (where the Ginzburg-Landau theory is valid), the energy of the interface between a normal and a superconducting region is described by the same universal function for Type 1 and Type 2 superconductors and also those that behave like both. This makes the idea of Type 1.5 superconductivity unnecessary within the Ginzburg-Landau theory. The behavior of magnesium diboride can be understood in terms of Type 2 superconductivity.