Investigation of energy gap and observation of electron-phonon interaction in high-temperature superconductor La1.8Sr0.2CuO4 - normal metal point contacts

Electrical characteristics (I(eV), dV/dI(eV), d2V/dI2(eV)) are studied comprehensively for pressed submicron point contacts between fresh fractures of LaSrCuO ceramic pellets and a normal metal (Cu). The point contacts (PC) with a direct-type conductivity are examined which are characterized by a substantial excess current caused by the Andreev reflection of quasi-particles at the NS boundary of high transparency. Prinicipal requirements to the contacts are specified which permit us to assess their applicability to PC measurements. Models of the contacts examined are considered; they may be used to interpret the observed characteristics. The experimental results on energy gap suggest that LaSrCuO is a strong coupled superconductor: a maximum value of the ratio 2Δ/kTc is about 11. It is found that for distances of a few tens of Angstrom the superconducting properties can vary appreciably resulting in some cases in simultaneous initiation of two gaps which are followed by two critical temperatures. The dV/dI(eV) and d2V/dI2(eV) curves display some singularities of high intensity whose positions in the eV-axis are essentially independent of temperature and correlate well with the phonon spectrum from the neutron scattering experiments. These facts are indicative of a strong electron-phonon interaction in LaSrCuO