Current redistribution model of anomalous resistance behaviour in superconductor-topological insulator heterostructures

Superconductor-topological insulator (TI) heterostructures are interesting due to induced p-wave superconductivity on the TI surface states near the interface. Transport studies carried out in this work on the TI surface of superconductor-TI (NbN-Bi1.95Sb0.05Se3) heterostructures have revealed anomalous resistance upturn and downturn at millimetre length scales away from the interface. Magnetotransport measurements have indicated that the anomaly is caused due to the superconducting transition of the NbN layer. The strength of the jump in resistance has been found to be strongest at the edges and the sign of the jump found to be opposite along opposite edges. Such resistance upturns and downturns have been previously reported in literature, wherein the upturns have been attributed to the antagonistic ground states of p-wave Cooper pairing and the spin-polarized TI states, and the downturns attributed to induced long-range proximity effects. However, the possibility of long-range superconducting proximity effect has been ruled out in this study through the observation of similar anomaly in NbN-Au and NbN-Al heterostructures. The present study demonstrates that the anomalies in resistance occur due to current redistribution (CRD) effects at the superconducting transition due to the geometry of the heterostructure. Results obtained from finite element analysis using COMSOL software have validated the proposed CRD model of long-range resistance anomalies in superconductor-TI and superconductor-metal heterostructures.