Electronic Raman scattering in HgBa_{2}Ca_{2}Cu_{3}O_{8+\delta} single crystals. Analysis of the superconducting state

TL;DR

This study uses electronic Raman scattering on HgBa2Ca2Cu3O8+δ single crystals to reveal a complex anisotropic superconducting gap with two maxima and nodes outside standard directions, challenging pure d-wave symmetry models.

Contribution

It provides the first detailed analysis of electronic Raman spectra in this compound, identifying two distinct superconducting gap maxima and evidence for anisotropic gap structure with nodes outside conventional directions.

Findings

Two superconducting gap maxima at 6.4 and 9.4 kBTc

Pure electronic spectra free of phonon interference

Evidence for an anisotropic gap with nodes outside standard directions

Abstract

Electronic Raman scattering measurements have been performed on $HgBa_{2}Ca_{2}Cu_{3}O_{8+\delta} $ single crystals in the superconducting state. Pure electronic Raman spectra with no phonon structures hindering the analysis of the electronic continuum have been obtained. As a consequence, the spectra in the pure $B_{1g} $ and $B_{2g} $ symmetries are directly and reliably analyzed and the pure $A_{1g} $ contribution can be easily identified. Below the critical temperature $T_{c}, $ two electronic structures at $2\Delta \sim 6.4 k_{B}T_{c} $ and $2\Delta \sim 9.4 $ $k_{B}T_{c} $ are clearly seen. Both are observed simultaneously in pure $A_{1g} $ symmetry, the highest energy one being located at the energy of the $B_{1g} $ maximum. These two maxima disappear at $T_{c} $ and do not soften significantly as the temperature is raised up to $T_{c}. $ The low energy frequency dependence of the $B_{1g} $ electronic response is strongly linear, for various excitation lines in the 476.5 to 647.1 nm range. Such experimental data cannot be reconciled with a pure $d_{x^{2}-y^{2}} $ symmetry. Instead, they strongly advocate in favor of an anisotropic superconducting gap with two distinct gap maxima and of nodes existing outside the [110] and [1,$\bar{1}$,0] directions in {\bf k}-space. We discuss in detail the simplest order parameter compatible with our experimental findings.