Detection of an anisotropy of the superconducting gap in Bi2Sr2CaCu2O8+δ single crystals by Raman and tunneling spectroscopy

Abstract

Abstract We report on polarized Raman spectra and tunneling measurements on the (010)-and (001)-planes of Bi2Sr2CaCu2O8+δ single crystals in the temperature range from 5 K up to room temperature. The Raman continuum of electronic excitations in the normal state of the Bi-2212 compound is redistributed below Tc. In the Ag symmetry, a superconducting gap is detected, which scales with Tc for different crystals and which is strongly anisotropic for different scattering configurations. While in the Y(Z, Z)Ȳ scattering geometry the gap energy amounts to 2ΔZZ(0)/kBTc=3.5±0.1, a value of 2ΔXX(0)/kBTc=5.7±0.2 is found for the gap energy in the Y(X, X) Ȳ scattering geometry. Tunneling measurements confirm the existence of the gap anisotropy in the Bi-2212 superconductor and yield gap values of 2Δ(0)ZZ/kBTc=3.5±0.2 and 2Δ(0)xx/kBTc=5.8±0.3. The temperature dependence of the gaps has been measured by means of Raman spectroscopy. In Y(Z, Z) Ȳ scattering geometry at T/Tc=0.96, the gap is reduced by less than 10% of its value at 5 K. This reduction amounts to 30% for the gap in Y(X, X) Ȳ scattering geometry. Consequently, the temperature dependence of both gaps differs strongly from the BCS behavior.