When we reported the observation of the superconductivity of InN in 2000 for the first time, we could not discuss its mechanism because we could not regard the superconductivity as a fundamental property of InN. At that time it was well known that the crystal growth of InN was very difficult and a minor change of the growth condition would easily produce a metal-In phase in InN. In the succeeding experiments, however, we made clear that neither the metal-In phase nor the In precipitation was the cause of the superconductivity.

We, then, considered that there was an optimum carrier density for the occurrence of the superconductivity. The scenario was as follows; when the lattice imperfection becomes large enough, but not too much, to produce high carrier density, a metal-In network will form in InN and produce superconductivity originating from the metal In; when the carrier density becomes small enough to produce a clear Fermi sphere of a degenerate semiconductor, the superconductivity will vanish. This scenario, however, was found to be wrong in the recent experiments done at GHMFL.

What is still obvious is that InN of small carrier density and with a well-defined Fermi surface shows superconductivity, and neither the electron density nor the crystal quality is the main cause of the superconductivity. We also know that the superconductivity is of the second kind and the upper-critical field determined from the magnetic field dependence of zero-resistance temperature is disturbed by the thermal and quantum fluctuations of the vortex, which suggests that the coherent length is short and the penetration depth is long. Neither the surface electron accumulation layer nor the metal-In precipitation has any contribution to the superconductivity.

Following these results, we propose a mechanism of the superconductivity of InN based on the interaction between the electrons with special distribution in wurtzite structure.

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1. Meissner effect of superconducting InN