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Abstract
Nickelate films have recently attracted broad attention due to the observation of superconductivity in the infinite layer phase of NdSrNiO (obtained by reducing Sr doped NdNiO films) and their similarity to the cuprates high temperature superconductors. Here, we report on the observation of a new type of transport in oxygen poor NdSrNiO films. At high temperatures, variable range hopping is observed while at low temperatures a novel tunneling behavior is found where a Josephson-like tunneling junction characteristic with serial resistance is revealed. We attribute this phenomenon to coupling between superconductive (S) surfaces of the grains in our Oxygen poor films via the insulating (I) grain boundaries, which yields SIS junctions in series with the normal (N) resistance of the grains themselves. The similarity of the observed conductance spectra to the tunneling junction characteristic with Josephson-like current is striking, and seems to support the existence of superconductivity in our samples.
Highlights
The recent discovery of superconductivity in Nd0.8Sr0.2NiO2 thin films [1] has brought up research of the Nickelates to the front of condense matter physics [2]
The way in which the Nd0.8Sr0.2NiO2 thin films were prepared involved laser deposition of the perovskite Nd0.8Sr0.2NiO3 films first, which were reduced by a chemical annealing process with CaH2 that changed them to the superconductive infinite layer phase [1]
The expected behavior on such a plot should be linear if variable range hopping (VRH) in 2D occurs [15,16]
Summary
The recent discovery of superconductivity in Nd0.8Sr0.2NiO2 thin films [1] has brought up research of the Nickelates to the front of condense matter physics [2]. [1] results by physical extraction of oxygen from the precursor Nd0.8Sr0.2NiO3 films by annealing at elevated temperatures under vacuum or with low oxygen pressure This led to even more insulating films and no change of phase to the superconductive infinite layer one was obtained [5,6]. Annealing under 10 mTorr oxygen increased the films resistivity at low temperatures by two order of magnitudes but did not change their structural phase These films showed variable range hopping (VRH) transport in 2D versus temperature with deviation due to tunneling conductance below about 4 K. Our films can be visualized as comprised of a network of SIS tunneling junctions connected in series with additional serial resistance of the normal (N) grains themselves
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