Hole concentration and phonon renormalization in Ca-doped YBa_2Cu_3O_y (6.76

In order to access the overdoped regime of the YBa 2 Cu 3 O y phase diagram, 2 % Ca is substituted for Y in YBa 2 Cu 3 O y ( y = 7.00,6.93,6.88,6.76). Raman scattering studies have been carried out on these four single crystals. Measurements of the superconductivity-induced renormalization in frequency (∆ ω ) and linewidth (∆2 γ ) of the 340 cm − 1 B 1 g phonon demonstrate that the magnitude of the renormalization is directly related to the hole concentration ( p ), and not simply the oxygen content. The changes in ∆ ω with p imply that the superconducting gap (∆ max ) decreases monotonically with increasing hole concentration in the overdoped regime, and ∆ ω falls to zero in the underdoped regime. The linewidth renormalization ∆2 γ is negative in the underdoped regime, crossing over at optimal doping to a positive value in the overdoped state.