109 research outputs found
Anomalies of the infrared-active phonons in underdoped YBCO as an evidence for the intra-bilayer Josephson effect
The spectra of the far-infrared c-axis conductivity of underdoped YBCO
crystals exhibit dramatic changes of some of the phonon peaks when going from
the normal to the superconducting state. We show that the most striking of
these anomalies can be naturally explained by changes of the local fields
acting on the ions arising from the onset of inter- and intra-bilayer Josephson
effects.Comment: Revtex, epsf, 6 pages, 3 figures encapsulated in tex
Quantum mechanical picture of the coupling between interlayer electronic excitations and infrared active phonons in bilayer cuprate superconductors
The formula frequently used to describe the c-axis infrared response of the
coupled electron-phonon system of bilayer cuprate superconductors and providing
important insights into the physics of these materials has been originally
obtained at the level of the phenomenological multilayer model. Here we derive
it using diagrammatic perturbation theory
Microscopic gauge-invariant theory of the c-axis infrared response of bilayer cuprate superconductors and the origin of the superconductivity induced absorption bands
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 is attributed to a
collective mode of the intra-bilayer regions, that is an analogue of the
Bogolyubov-Anderson 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 is
interpreted as a pair breaking-feature that is related to the electronic
coupling through the spacing layers separating the bilayers.Comment: 18 pages, 15 figures, submitted to Phys. Rev.
Selfconsistent gauge-invariant theory of in-plane infrared response of high-Tc cuprate superconductors involving spin fluctuations
We report on results of our theoretical study of the in-plane infrared
conductivity of the high-Tc cuprate superconductors using the model where
charged planar quasiparticles are coupled to spin fluctuations. The
computations include both the renormalization of the quasiparticles and the
corresponding modification of the current-current vertex function (vertex
correction), which ensures gauge invariance of the theory and local charge
conservation in the system. The incorporation of the vertex corrections leads
to an increase of the total intraband optical spectral weight (SW) at finite
frequencies, a SW transfer from far infrared to mid infrared, a significant
reduction of the SW of the superconducting condensate, and an amplification of
characteristic features in the superconducting state spectra of the inverse
scattering rate 1/tau. We also discuss the role of selfconsistency and propose
a new interpretation of a kink occurring in the experimental low temperature
spectra of 1/tau around 1000cm^{-1}.Comment: 9 pages with 6 figures, submitted to Physical Review
Interpretation of the in-plane infrared response of the high-Tc cuprate superconductors involving spin fluctuations revisited
The in-plane infrared response of the high-Tc cuprate superconductors was
studied using the spin-fermion model, where charged quasiparticles of the
copper-oxygen planes are coupled to spin fluctuations. First, we analyzed
structures of the superconducting-state conductivity reflecting the coupling of
the quasiparticles to the resonance mode observed by neutron scattering. The
conductivity computed with the input spin susceptibility in the simple form of
the mode exhibits two prominent features: an onset of the real part of the
conductivity starting around the frequency of the mode omega_{0} and a maximum
of a related function W(omega), roughly proportional to the second derivative
of the scattering rate, centered approximately at
omega=omega_{0}+Delta_{0}/hbar, where Delta_{0} is the maximum value of the
superconducting gap. The two structures are well known from earlier studies.
Their physical meaning, however, has not been sufficiently elucidated thus far.
Our analysis involving quasiparticle spectral functions provides a clear
interpretation. Second, we explored the role played by the spin-fluctuation
continuum. Third, we investigated the temperature dependence of the
conductivity, of the intraband spectral weight, and of the effective kinetic
energy. The changes of the latter two quantities below Tc are determined by the
formation of the gap, by a feedback effect of the spin fluctuations on the
quasiparticles, and by a significant shift of the chemical potential.Comment: 20 pages, 18 figures, submitted to Physical Review
Spectroscopic distinction between the normal state pseudogap and the superconducting gap of cuprate high T_{c} superconductors
We report on broad-band infrared ellipsometry measurements of the c-axis
conductivity of underdoped RBa_{2}Cu_{3}O_{7-d} (R=Y, Nd, and La) single
crystals. Our data provide a detailed account of the spectral weight (SW)
redistributions due to the normal state pseudogap (PG) and the superconducting
(SC) gap. They show that these phenomena involve different energy scales,
exhibit distinct doping dependencies and thus are likely of different origin.
In particular, the SW redistribution in the PG state closely resembles the one
of a conventional charge- or spin density wave (CDW or SDW) system.Comment: 4 pages, 4 figure
Manifestation of pseudogap in ab-plane optical characteristics
A model in which a gap forms in the renormalized electronic density of state
(DOS) with missing states recovered just above the pseudogap , is
able to give a robust description of the striking, triangular like, peak seen
in the real part of the optical self-energy of underdoped cuprates. We use this
model to explore the effect of the pseudogap on the real part of the optical
conductivity and on the partial sum rule associated with it. An important
result is that the optical spectral weight redistributes over a much larger
frequency window than it does in the DOS.Comment: 12 pages, 3 figures. Submitted to Journal of Physics: Condensed
Matte
- …