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Phase diagram of a superconductor-insulator transition that appears in short and thin superconducting wires was investigated. A set of wires of various lengths is used for the first time. The boundary line we can draw across the diagram in the (R_N, R_N / L) coordinate plane representation that separates exactly the superconducting and the insulating samples:
Fig.4 in http://xxx.lanl.gov/PS_cache/arxiv/pdf/0707/0707.4532v1.pdf
The expression for this empirical phase boundary is R_N / L = (2*R_Q - R_N) / 112 nm (here R_Q = R_K / 4 = 6.45 kOhms). Wires above this line happen to be insulating, while the wires below the line are all superconducting.
This line has two intercepts: (1) with the abscissa at R_N = 2*R_Q and (2) with the ordinate at R_N / L = 2 * R_Q / 112 nm. The first intercept just means all wires with R_N >2*R_Q are insulating. The second intercept means all wires with R_N / L >= 2R_Q / 112 nm are insulating. Te first conduction can be explained by the localization of electronic states and the second one – by the surface effects causing superconductivity suppression.
A detailed discussion of the SIT phase diagram is given in