Pseudogap and Superconducting Gap in the Electronic Raman Spectra of Underdoped Cuprates

Abstract

Raman spectra of ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}x}$ and ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}({\mathrm{Ca}}_{0.62}{\mathrm{Y}}_{0.38}){\mathrm{Cu}}_{2}{\mathrm{O}}_{8+\ensuremath{\delta}}$ with ${T}_{c}\ensuremath{\cong}{0.65T}_{c}^{\mathrm{max}}$ in the underdoped regime of the phase diagram are studied as a function of temperature and polarization. At ${B}_{2g}$ $(\mathrm{xy})$ symmetry a reduction of spectral weight by 10% for frequencies less than $700{\mathrm{cm}}^{\ensuremath{-}1}$ or ${15k}_{B}{T}_{c}$ is found below approximately 200 K. Below ${T}_{c}$, a superconducting gap opens up which closely resembles that observed at higher doping levels. It is compatible with ${d}_{{x}^{2}{\ensuremath{-}y}^{2}}$ pairing and its amplitude $2{\ensuremath{\Delta}}_{0}$ can be estimated to be ${8k}_{B}{T}_{c}$.