Since the first report on the superconducting InN with a narrow band gap in 2000, several reports on it have been published.  C. T. Liang et al. reported the superconductivity of InN as huge positive magnetoresistance (APL 90, 172101), D. C. Ling et al. reported it as intrinsic properties (pss(b) 244, 4594), and Kadir et al. reported it as extrinsic properties (SSC 146, 361).  Recently Tiras et al. reported that p-type InN becomes a superconductor at T_c=3.97 K (APL 94, 142108). It indeed should be decided whether the superconductivity is intrinsic or extrinsic. The superconductivity of InN is characterized by its low carrier concentration.  Diamond and Si which are boron doped become superconductors when the carrier concentration exceeds ~5x10²¹cm^-3.  On the other hand InN with a carrier concentration of 4x10¹⁷cm^-3 shows superconductivity below 3 K, which indicates that the electronic structure of InN is the key issue to understand the origin of the superconductivity. 

Here we report on the Meissner effect of InN.  Samples with a carrier concentration as low as 4x10¹⁷cm^-3 show Meissner effect.  They show an upward curvature of the upper critical field expressed by H_c2(T)~exp(-T/T₀), where T₀ is the specific temperature for the superconductive transition.  This curvature is explained by a micro-Josephson junction mechanism, which was introduced for the explanation of irreversible line of high T_c superconductors.  The excellent fitting of the theoretical curve to the experimental data corroborates the conclusion that the transition is Josephson coupling due to the presence of superconducting clusters with a transition temperature T_c1 higher than the bulk transition temperature T_c0. The transition at T_c1 (~3 K) is attributed to the resistive onset temperature where the superconductivity occurs in clusters separated from each other by ~1 μm, which is estimated from T₀, while T_c0 (~0.5 K) is the temperature at which superconducting coherence forms through Josephson coupling between the superconducting clusters.  It is found that the flux motion between T_c1 and T_c0 is reversible, which means that no vortices have formed within the clusters.  

From these results we propose a mechanism of the superconductivity in terms of residual carriers produced by 4d semicore electrons originating from the second nearest neighbor In-In interactions, which is the cause of the micro-Josephson junctions.

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1. Superconductivity of InN