Raman Scattering from the Superconducting Phase: Electronic Excitations and Phonon Renormalization Effects

Electron-Phonon Interaction and Strong Correlations in High-Temperature Superconductors: One can not avoid the unavoidable

“…In any case this analysis confirm that the EPI of some Raman modes in HTSC oxides is strong. To this point, very recent Raman scattering measurements on the (Cu,C) − 1234 compound with T c = 117 K reveal strong superconductivity induced phonon self-energy effects [5]. The A 1g phonons at 235 cm −1 and 360 cm −1 (note ω pl < 2∆ 0 ), which involve vibrations of the plane oxygen with some admixture of Ca displacements, exhibit pronounced Fano line shape (in the normal and superconducting state) with the following interesting properties in the superconducting state: (i) the phonon intensity is increased substantially; (ii) both phonons soften; (iii) the phonon line width (of both phonons) increases dramatically below T c passing through a maximum slightly below T c , and decreases again at low T but remaining broader than immediately below T c .…”

“…The data from [82] in the semiconductor range and ρ a−b (T ) ≫ ρ Cu (T ); (3) ρ a−b (T ) ∼ T for T > T c , which deviates at T > (800 − 1000) K and saturates at even higher temperatures, depending on samples etc. ; (4) ρ a−b varies from ρ a−b (T ) ∼ T (with small residual resistivity) in optimally doped systems being ρ a−b (T ) ∼ T 3/2 in overdoped systems, as experiments on La 2−x Sr x CuO 4 show [89]; (5) In most samples of HTSC oxides the c-axis resistivity ρ c (T ) shows a non-metallic behavior especially in samples with huge anisotropy along the c-axis, growing by decreasing temperature, i.e. (dρ c (T )/dT ) < 0, being superconducting below T c .…”

“…λ > 1. Let us quote some of them: (1) The superconductivity induced phonon renormalization [3], [36], [4], [5] is much larger in HTSC oxides than in LT SC superconductors. This is partially due to the larger value of ∆/E F in HTSC than in LT SC; (2) the line-shape in the phonon Raman scattering is very asymmetric (Fano line), which points to a substantial interaction of the lattice with some quasiparticle (electronic liquid) continuum.…”

“…At the same time there is a dramatic increase of the line-width immediately below T c , while above T c the line-shape is strongly asymmetric. A substantial phonon renormalization was obtained in (Cu,C)Ba 2 Ca 3 Cu 4 O 10+x [5]; (3) the large isotope coefficients (α O > 0.4) in Y BCO away from the optimal doping [105] and α O ≈ 0.15 − 0.2 in the optimally doped La 1.85 Sr 0.15 CuO 4 . At the same time one has α O ≈ α Cu making α ≈ 0.25 − 0.3.…”