9 research outputs found
Optical sum rule in metals with a strong interaction
The restricted optical sum rule and its dependence on the temperature, a
superconducting gap and the cutoff energy have been investigated. As known this
sum rule depends on the cutoff energy and the relaxation rate even for a
homogeneous electron gas interacting with impurities or phonons. It is shown
here that additional dependence of the spectral weight on a superconducting gap
is very small in this model and this effect disappears totally when the
relaxation rate is equal zero. The model metal with a single band is considered
in details. It is well known that for this model there is the dependence of the
sum rule on the temperature and the energy gap even in the case when the
relaxation is absent. This dependence exists due to the smearing of the
electron distribution function and it is expressed in the terms of Sommerfeld
expansion. Here it is shown that these effects are considerably smaller than
that of related with the relaxation rate if the band width is larger than the
average phonon frequency. It is shown also that the experimental data about the
temperature dependence of the spectral weight for the high- materials can be
successfully explained in the framework approach based on the temperature
dependence of the relaxation rateComment: 13 pages, 7 figures, the talk given on Internatinal coference on
theoretical physics, april 11-16,2005, Mosco
Observation of the Holstein shift in high superconductors with thermal modulation reflectometry
We use the experimental technique of thermal modulation reflectometry to
study the relatively small temperature dependence of the optical conductivity
of superconductors. Due to a large cancellation of systematic errors, this
technique is shown to a be very sensitive probe of small changes in
reflectivity. We analyze thermal modulation reflection spectra of single
crystals and epitaxially grown thin films of YBaCuO and
obtain the function in the normal state, as well as
the superconductivity induced changes in reflectivity. We present detailed
model calculations, based on the Eliashberg-Migdal extension of the BCS model,
which show good qualitative and quantitative agreement with the experimental
spectra. VSGD.93.12.thComment: 6 pages, figures on request. Revtex, version 2, Materials Science
Center Internal Report Number VSGD.93.12.t
Far-infrared electrodynamics of superconducting Nb: comparison of theory and experiment
Complex conductivity spectra of superconducting Nb are calculated from the
first principles in the frequency region around the energy gap and compared to
the experimental results. The row experimental data obtained on thin films can
be precisely described by these calculations.Comment: 4 pages, 3 eps figures incl. Accepted to Solid State Commu
Electromagnetic response of superconductors and optical sum rule
The interrelation between the condensation energy and the optical sum rules
has been investigated. It has been shown that the so called 'partial' sum rule
violation is related mainly to a temperature dependence of the relaxation rate
rather than to the appearance of superconductivity itself. Moreover, we
demonstrate that the experimental data on the temperature dependence of the
optical sum rule can be explained rather well by an account of strong
electron-phonon interaction.Comment: 16 pages, 1 figure. Submitted to Solid State Communication
Optical Sum Rule anomalies in the High-Tc Cuprates
We provide a brief summary of the observed sum rule anomalies in the
high-T cuprate materials. A recent issue has been the impact of a
non-infinite frequency cutoff in the experiment. In the normal state, the
observed anomalously high temperature dependence can be explained as a `cutoff
effect'. The anomalous rise in the optical spectral weight below the
superconducting transition, however, remains as a solid experimental
observation, even with the use of a cutoff frequency.Comment: 4 pages, 2 figures, very brief review of optical sum rule anomal
Optical absorption in the strong coupling limit of Eliashberg theory
We calculate the optical conductivity of superconductors in the
strong-coupling limit. In this anomalous limit the typical energy scale is set
by the coupling energy, and other energy scales such as the energy of the
bosons mediating the attraction are negligibly small. We find a universal
frequency dependence of the optical absorption which is dominated by bound
states and differs significantly from the weak coupling results. A comparison
with absorption spectra of superconductors with enhanced electron-phonon
coupling shows that typical features of the strong-coupling limit are already
present at intermediate coupling.Comment: 10 pages, revtex, 4 uuencoded figure
Optical Sum Rule in Finite Bands
In a single finite electronic band the total optical spectral weight or
optical sum carries information on the interactions involved between the charge
carriers as well as on their band structure. It varies with temperature as well
as with impurity scattering. The single band optical sum also bears some
relationship to the charge carrier kinetic energy and, thus, can potentially
provide useful information, particularly on its change as the charge carriers
go from normal to superconducting state. Here we review the considerable
advances that have recently been made in the context of high oxides, both
theoretical and experimental.Comment: Review article accepted for publication in J. Low Temp. Phys. 29
pages, 33 figure