Quantum phase slip and enhancement of superconductivity by magnetic field in NbN nanowires

Transport properties were investigated for quasi-one dimensional superconducting NbN nanowires with width w = 20nm and thickness d=20nm on 3C-SiC substrate. It has been found that 1) the resistance R shows the broad superconducting transition and the R(T) characteristic at temperatures bellow T _c∼13.6K can be well explained by the sum of the thermal activated phase slip and the quantum phase slip(QPS) of the superconducting order parameter. 2) The R(T) due to the QPS contribution is suppressed to enhance the superconductivity by the external magnetic field H. With decreasing temperature, the R(T) of short specimen with length 600nm shows nearly zero resistance in the restricted temperature region depending on the magnitude of H and recovers the QPS resistive state. 3) The critical magnetic field H _c2(T) is fit to the theoretical expression H _c2,theo (T) = ϕ ₀/[2ξ _GL (T) × w] ∝ (1 − T/T _c)^1/2, where the ξ _GL (T) is the Ginzburg-Landau superconducting coherence length. Although the relation H _c2,theo (T) ∝ (1 − T/T _c)^1/2 is satisfied, the magnitude of H _c2,theo (T) is about 5 times larger than that of the theoretical one. (paper)