Transitions at T>Tcsin underdoped crystals ofYBa2Cu3O7−xsobserved by resonant Raman scattering

Abstract

It has been shown that under a particular resonant condition it is possible to observe Raman interband electronic scattering between quasidegenerate bands associated with the apical oxygen around the S point of the Brillouin zone in ${\mathrm{YBa}}_{2}$ ${\mathrm{Cu}}_{3}$ ${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{x}}$ (1-2-3).mThe identification of the near-IR (1.16 eV) excited Raman-scattering structure observed between 200 and 700 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ with the transition between these quasidegenerate bands at the Fermi level, which have neither ${\mathrm{CuO}}_{2}$ nor chain character, and the vicinity of the energy of this transition with the value of the superconducting gap, has caused us to investigate the temperature dependence of the Raman electronic scattering.mWe have analyzed two single crystals underdoped with different ${\mathrm{T}}_{\mathrm{c}}$ . The integrated intensity of the electronic background versus temperature shows a similar behavior for both samples. Singularities are observed not only at ${\mathrm{T}}_{\mathrm{c}}$ but also at two temperatures higher than ${\mathrm{T}}_{\mathrm{c}}$ at T=1.${6\mathrm{f}\mathrm{T}}_{\mathrm{c}}$ and ${\mathrm{T}}_{\mathrm{D}}$ that correspond to the appearance of a pseudogap for underdoped 1-2-3 with the same ${\mathrm{T}}_{\mathrm{c}}$ .