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Register a new userObservation of Josephson-like tunneling junction characteristics and positive magnetoresistance in Oxygen deficient Nickelate films of $Nd_{0.8}Sr_{0.2}NiO_{3-delta}$
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Nickelate films have recently attracted broad attention due to the observation of superconductivity in the infinite layer phase of $Nd_{0.8}Sr_{0.2}NiO_2$ (obtained by reducing Sr doped $NdNiO_3$ 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 $Nd_{0.8}Sr_{0.2}NiO_{3-delta}$ films. At high temperatures, variable range hopping is observed while at low temperatures a novel tunneling behavior is found where 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 tunneling junction characteristic with Josephson-like current is striking, and seems to support the existence of superconductivity in our samples.
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Oxygen vacancies play a crucial role in the control of the electronic, magnetic, ionic, and transport properties of functional oxide perovskites. Rare earth nickelates (RENiO$_{3-x}$) have emerged over the years as a rich platform to study the interplay between the lattice, the electronic structure, and ordered magnetism. In this study, we investigate the evolution of the electronic and magnetic structure in thin films of RENiO$_{3-x}$, using a combination of X-ray absorption spectroscopy and imaging, resonant X-ray scattering, and extended multiplet ligand field theory modeling. We find that oxygen vacancies modify the electronic configuration within the Ni-O orbital manifolds, leading to a dramatic evolution of long-range electronic transport pathways despite the absence of nanoscale phase separation. Remarkably, magnetism is robust to substantial levels of carrier doping, and only a moderate weakening of the $(1/4, 1/4, 1/4)_{pc}$ antiferromagnetic order parameter is observed, whereas the magnetic transition temperature is largely unchanged. Only at a certain point long-range magnetism is abruptly erased without an accompanying structural transition. We propose the progressive disruption of the 3D magnetic superexchange pathways upon introduction of point defects as the mechanism behind the sudden collapse of magnetic order in oxygen-deficient nickelates. Our work demonstrates that, unlike most other oxides, ordered magnetism in RENiO$_{3-x}$ is mostly insensitive to carrier doping. The sudden collapse of ordered magnetism upon oxygen removal may provide a new mechanism for solid-state magneto-ionic switching and new applications in antiferromagnetic spintronics.
Infinite-layer Nd1-xSrxNiO2 thin films with Sr doping level x from 0.08 to 0.3 were synthesized and investigated. We found a superconducting dome to be between 0.12 and 0.235 which is accompanied by a weakly insulating behaviour in both underdoped and overdoped regimes. The dome is akin to that in the electron-doped 214-type and infinite-layer cuprate superconductors. For x higher than 0.18, the normal state Hall coefficient ($R_{H}$) changes the sign from negative to positive as the temperature decreases. The temperature of the sign changes monotonically decreases with decreasing x from the overdoped side and approaches the superconducting dome at the mid-point, suggesting a reconstruction of the Fermi surface as the dopant concentration changes across the center of the dome.
The newly found superconductivity in infinite-layer nickelate superconducting films has attracted much attention, because their crystalline and electronic structures are similar to high-$T_c$ cuprate superconductors. The upper critical field can provide much information on superconductivity, but detailed experimental data are still lacking in these films. Here we present temperature and angle dependence of resistivity measured under different magnetic fields ($H$) in Nd$_{0.8}$Sr$_{0.2}$NiO$_{2}$ thin films. The onset superconducting transition occurs at about 16.2 K at 0 T. Temperature dependent upper critical fields determined by using a criterion very close to the onset transition show a clear negative curvature near the critical transition temperature, which is explained as the consequence of the paramagnetically limited effect on superconductivity. The temperature dependent anisotropy of the upper critical field is obtained from resistivity data, which yields a value decreasing from 3 to 1.2 with lowering temperature. This can be explained by a variable contribution from the orbital limit effect on upper critical field. The angle dependent resistivity at a fixed temperature and different magnetic fields cannot be scaled to one curve, which deviates from the prediction of the anisotropic Ginzburg-Landau theory. However, at low temperatures, the increased resistivity by magnetic field can be scaled by the parameter $H^beta |costheta|$ ($1<beta<6$) with $theta$ the angle enclosed between $c$-axis and the applied magnetic field. As the first detailed study on the upper critical field of the nickelate thin films, our results clearly indicate a small anisotropy and paramagnetically limited effect of superconductivity in nickelate superconductors.
The recent observation of superconductivity in infinite-layer nickelate Nd$_{0.8}$Sr$_{0.2}$NiO$_{2}$ has received considerable attention. Despite the many efforts to understand the superconductivity in infinite-layer nickelates, a consensus on the underlying mechanism for the superconductivity has yet to be reached, partly owing to the challenges with the material synthesis. Here, we report the successful growth of superconducting infinite-layer Nd$_{0.8}$Sr$_{0.2}$NiO$_{2}$ films by pulsed-laser deposition and soft chemical reduction. The details on growth process will be discussed.
We have studied fundamental properties of weak-link Sr2RuO4/Sr2RuO4 Josephson junctions fabricated by making a narrow constriction on superconducting Sr2RuO4 films through laser micro-patterning. The junctions show a typical overdamped behavior with much higher critical current density, compared with those previously reported for bulk Sr2RuO4/s-wave superconductor junctions. Observed magnetic field and temperature dependences of the Josephson critical current suggest that the chiral p-wave is unlikely for the superconducting symmetry, encouraging further theoretical calculations of the Sr2RuO4/Sr2RuO4 type junctions.
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