Microscopic gauge-invariant theory of thec-axis infrared response of bilayer cuprate superconductors and the origin of the superconductivity-induced absorption bands

Abstract

We report on results of our theoretical study of the c-axis infrared conductivity of bilayer high-Tc cuprate superconductors using a microscopic model involving the bilayer-split (bonding and antibonding) bands. An emphasis is on the gauge-invariance of the theory, which turns out to be essential for the physical understanding of the electrodynamics of these compounds. The description of the optical response involves local (intra-bilayer and inter-bilayer) current densities and local conductivities. The local conductivities are obtained using a microscopic theory, where the quasiparticles of the two bands are coupled to spin fluctuations. The coupling leads to superconductivity and is described at the level of generalized Eliashberg theory. Also addressed is the simpler case of quasiparticles coupled by a separable and nonretarded interaction. The gauge invariance of the theory is achieved by including a suitable class of vertex corrections. The resulting response of the model is studied in detail and an interpretation of two superconductivity-induced peaks in the experimental data of the real part of the c-axis conductivity is proposed. The peak around 400 cm −1 is attributed to a collective mode of the intra-bilayer regions, that is an analogue of the BogolyubovAnderson mode playing a crucial role in the theory of the longitudinal response of superconductors.For small values of the bilayer splitting, its nature is similar to that of the transverse plasmon of the phenomenological Josephson superlattice model. The peak around 1000 cm −1 is interpreted as a pair breaking-feature that is related to the electronic coupling through the spacing layers separating the bilayers.

Pseudogap and precursor superconductivity in underdoped cuprate high temperature superconductors: A far-infrared ellipsometry study

“…[60]. The justification of the relation ω SC ≈ 2Δ SC involves the mode around 1000 cm −1 that occurs right above ω SC in the spectra at T < T c and likely corresponds to an interband (bonding-antibonding) pair-breaking peak whose coherence factor decreases with decreasing bilayer coupling [61,62]. This has been [11].…”

Evidence for precursor superconducting pairing aboveTcin underdoped cuprates from an analysis of the in-plane infrared response

“…Two reasons are: (i) the c-axis response of Y-123 is a fairly complex quantity due to the specific bilayer structure of this compound. Its interpretation therefore requires a detailed understanding of the c-axis electrodynamics of the bilayer compounds [16][17][18][19][20][21][22][23]. (ii) UD cuprates are known to exhibit ordered phases distinct from superconductivity, in particular, charge modulations have been reported [24,25], that set in at temperatures comparable to T ons .…”

Temperature dependence of the superconductivity-induced collective mode in the c-axis infrared spectra of bilayer cuprate superconductors

“…The maxima correspond to a pair-breaking (bonding-antibonding) peak and a collective mode (CM), respectively. As discussed in [1], the CM can be viewed as an analogue of the Bogoljubov-Anderson mode occurring in the longitudinal response of homogeneous superconductors, see Fig. 1c and d. For small values of the bilayer splitting, its nature is similar to that of the transverse plasmon introduced in Ref.…”

“…max ¼ 45 meV, the values of other input parameters are the same as in Section III B of Ref. [1]. Above T c of 89 K, the spectra display a broad maximum centered around 600 cm À1 due to the interband transitions.…”

“…The important issue of the origin of the superconductivity-induced modes in the spectra of the c-axis infrared response of bilayer HTCS has been recently addressed using a microscopic theory [1]. Its elements are: (a) the local (intra-bilayer -''bl" and inter-bilayer -''int") fields, current densities, conductivities, and an extension of the multilayer formula of Ref.…”