Type-1.5 superconductivity: recent experimental evidence and candidate materials

The widely-held consensus for a long time was that all superconducting materials, discovered during a century of research, belonged to one of two categories proposed by Ginzburg and Landau. Type-1 superconductors (where the magnetic field penetration length is smaller than the coherence length) or type-2 (where the magnetic field penetration length is larger than the coherence length). About a decade ago we proposed that there could be superconductors which fall outside this dichotomy. The proposed state (termed recently “type-1.5”) is characterized by multiple coherence lengths some of which are larger and some smaller than the magnetic field penetration length. It was predicted to lead to unconventional magnetic properties. I will focus on new substantial experimental evidence that this state is realized in certain materials and will discuss that microscopic physics of certain compounds should lead to essentially generic breakdown of the type-1/type-2 dichotomy.