Tunneling spectroscopy of c-axis epitaxial cuprate junctions

Tunnel junctions are valuable tools for electronic spectroscopy, with epitaxial structures minimizing extrinsic effects of disorder. We report tunneling spectroscopy measurements, over a broad temperature and bias range, in c-axis La_2−xSr_xCuO₄/La₂CuO₄/La_2−xSr_xCuO₄ heterostructures grown via atomic layer-by-layer molecular beam epitaxy. Above T_c of the LSCO layers, these structures show the pseudogap, while below T_c, the devices show superconductor/insulator/superconductor (SIS) response. Down to 20 mK there is no Josephson critical supercurrent I_c, indicating strongly incoherent transport. Tunneling spectra show greatly suppressed coherence peaks. The T=0 zero-bias conductance remains at approximately 20-30% that of the normal-state, implying a substantial population of unpaired carriers. As temperature is raised, the superconducting gap fills in rather than closing at T_c. The spectra show an inelastic tunneling feature at ∼0.08 eV, suppressed as T exceeds T_c. We discuss how these properties are inconsistent with standard d-wave tunneling expectations, despite the structural perfection of the epitaxial stack.