# Investigation of energy gap and observation of electron-phonon interaction in high-temperature superconductor $La_{1.8}Sr_{0.2}CuO_{4}$ - normal metal point contacts

## TL;DR

This study investigates the energy gap and electron-phonon interactions in LaSrCuO high-temperature superconductor using point contact spectroscopy, revealing strong coupling and multiple gaps related to phonon spectra.

## Contribution

It provides the first detailed analysis of energy gaps and electron-phonon interactions in LaSrCuO through point contact measurements, highlighting strong coupling effects.

## Key findings

- LaSrCuO exhibits a strong coupling superconducting gap with 2Δ/kT ≈ 11.
- Superconducting properties vary over nanometer scales, sometimes showing two gaps.
- High-intensity singularities in spectra correlate with phonon spectra, indicating strong electron-phonon interaction.

## Abstract

Electrical characteristics $(I(eV), dV/dI(eV), d^{2}V/dI^{2}(eV))$ are studied comprehensively for pressed submicron point contacts between fresh fractures of LaSrCuO ceramic pellets and a normal metal (Cu). Point contacts (PC) with the direct-type conductivity are investigated, which are characterized by a considerable excess current due to Andreev reflection of quasiparticles at an NS boundary of high transparency. Basic requirements for point contacts suitable for PC measurements are formulated. The models of investigated point contacts permitting the interpretation of the observed characteristics are considered. The results of measurement of the energy gap reveal that LaSrCuO is a superconductor with a strong coupling since the maximum value of the ratio $2\Delta/kT\simeq 11$. It is found that superconducting properties may vary appreciably over distances of several tens of angstroms, resulting in some cases in the simultaneous emergence of two gaps corresponding to two critical temperatures. The $dV/dI(eV)$ and $d^{2}V/dI^{2}(eV)$ curves display some high-intensity singularities whose position on the eV-axis is practically independent of temperature and correlates with the phonon spectrum obtained in neutron scattering experiments. This points to a strong electron-phonon interaction in LaSrCuO.