Atomically precise epitaxial structures are unique systems for tunneling spectroscopy that minimize extrinsic effects of disorder. We present a systematic tunneling spectroscopy study, over a broad doping, temperature, and bias range, in epitaxial $c$-axis $\mathrm{L}{\mathrm{a}}_{2-x}\mathrm{S}{\mathrm{r}}_x\mathrm{Cu}{\mathrm{O}}_4/\mathrm{L}{\mathrm{a}}_2\mathrm{Cu}{\mathrm{O}}_4/\mathrm{L}{\mathrm{a}}_{2-x}\mathrm{S}{\mathrm{r}}_x\mathrm{Cu}{\mathrm{O}}_4$ heterostructures. The behavior of these superconductor/insulator/superconductor (SIS) devices is unusual. Down to 20 mK there is complete suppression of $c$-axis Josephson critical current with a barrier of only 2 nm of $\mathrm{L}{\mathrm{a}}_2\mathrm{Cu}{\mathrm{O}}_4$, and the zero-bias conductance remains at 20–30% of the normal-state conductance, implying a substantial population of in-gap states. Tunneling spectra show greatly suppressed coherence peaks. As the temperature is raised, the superconducting gap fills in rather than closing at $T_c$. For all doping levels, the spectra show an inelastic tunneling feature at ∼80 meV, suppressed as T exceeds $T_c$. These nominally simple epitaxial cuprate junctions deviate markedly from expectations based on the standard Bardeen-Cooper-Schrieffer theory.

• Received 9 January 2020
• Accepted 9 June 2020

DOI:https://doi.org/10.1103/PhysRevB.101.224512