71 research outputs found
Forward Electron-Phonon Scattering and HTS
Tunneling and point contact spectroscopy show clear phonon features and
together with optic measurements give strong support that the electron-phonon
interaction (EPI) is large in HTS oxides. Strong correlations in HTS oxides
renormalize the EPI (and interaction with impurities) so that the forward
scattering peak (FSP) develops for small hole doping \delta<<1. The FSP
mechanism explains important properties of the normal and superconducting
state.Comment: 6 pages, 5 figure
Cosmological Neutrino Background Revisited
We solve the Boltzmann equation for cosmological neutrinos around the epoch
of the electron-positron annihilation in order to verify the freeze-out
approximation and to compute accurately the cosmological neutrino distribution
function. We find the radiation energy density to be about 0.3% higher than the
one predicted by the freeze-out approximation. As a result, the spectrum of the
Cosmic Microwave Background anisotropies changes by 0.3-05%, depending on the
angular scale, and the amplitude of the mass fluctuations on scales below about
100 h^{-1} Mpc decreases by about 0.2-0.3%.Comment: An error is corrected, figure revised; submitted to Ap
Band filling and interband scattering effects in MgB: C vs Al doping
We argue, based on band structure calculations and Eliashberg theory, that
the observed decrease of of Al and C doped MgB samples can be
understood mainly in terms of a band filling effect due to the electron doping
by Al and C. A simple scaling of the electron-phonon coupling constant
by the variation of the density of states as function of electron
doping is sufficient to capture the experimentally observed behavior. Further,
we also explain the long standing open question of the experimental observation
of a nearly constant gap as function of doping by a compensation of the
effect of band filling and interband scattering. Both effects together generate
a nearly constant gap and shift the merging point of both gaps to higher
doping concentrations, resolving the discrepancy between experiment and
theoretical predictions based on interband scattering only.Comment: accepted by PR
Developing a strategy for the development of small enterprises in the energy sector of the economy
As a part of the study, the problems of forming the investment strategy of energy companies were considered. Possible ways of choosing the most rational forms of investment in the assets and a comprehensive assessment of the strategy’s effectiveness were proposed. A methodology for the qualitative and quantitative assessment of the investment strategy for the development of an enterprise has been developed by analyzing the Cobb-Douglas production function. In the paper, when solving specific problems, general economic methods of analysis, probabilistic and statistical methods, the method of expert assessments, and methods of system and comparative analysis were used. The practical application of the proposed assessment methodology is considered on the example of comparing two options for the investment strategy of an enterprise operating in the field of energy processing
Interband superconductivity: contrasts between BCS and Eliashberg theory
The newly discovered iron pnictide superconductors apparently present an
unusual case of interband-channel pairing superconductivity. Here we show that,
in the limit where the pairing occurs within the interband channel, several
surprising effects occur quite naturally and generally: different
density-of-states on the two bands lead to several unusual properties,
including a gap ratio which behaves inversely to the ratio of
density-of-states; the weak-coupling limit of the Eliashberg and the BCS
theory, commonly taken as equivalent, in fact predict qualitatively different
dependence of the and ratios on coupling
constants. We show analytically that these effects follow directly from the
interband character of superconductivity. Our results show that in the
interband-only pairing model the maximal gap ratio is as
strong-coupling effects act only to reduce this ratio. This suggests that if
the large experimentally reported gap ratios (up to a factor 2) are correct,
the pairing mechanism must include more intraband interaction than is usually
assumed.Comment: 4 pages, 3 figure
Evidence for strong electron-phonon coupling and polarons in the optical response of La_{2-x}Sr_xCuO_4
The normal state optical response of La_{2-x}Sr_xCuO_4 is found to be
consistent with a simple multi-component model, based on free carriers with
strong electron-phonon interaction, localized polaronic states near 0.15 eV and
a mid-infrared band at 0.5 eV. Normal state reflectance and absorbance of
La_{1.83}Sr_{0.17}CuO_4 are investigated and their temperature dependence is
explained. Both, the ac and dc response are recovered and the quasi-linear
behavior of the optical scattering rate up to 3000- 4000 cm^{-1} is found to be
consistent with strong electron-phonon interaction, which also accounts for the
value of T_c. Although not strictly applicable in the superconducting state,
our simple model accounts for the observed penetration depth and the optical
response below T_c can be recovered by introducing a small amount of additional
carriers. Our findings suggest that the optical response of La_{2-x}Sr_xCuO_4
could be explained both, in the normal and superconducting state, by a simple
multi-fluid model with strong electron-phonon interaction if the gap symmetry
and the temperature dependence of the 0.5 eV mid-infrared band are adequately
taken into account.Comment: 22 pages, REVTeX, 12 figures in ps-fil
The optical response of Ba_{1-x}K_xBiO_3: Evidence for an unusual coupling mechanism of superconductivity?
We have analysed optical reflectivity data for Ba_{1-x}K_xBiO_3 in the
far-infrared region using Migdal-Eliashberg theory and found it inconsistent
with standard electron-phonon coupling: Whereas the superconducting state data
could be explained using moderate coupling, \lambda=0.7, the normal state
properties indicate \lambda \le 0.2. We have found that such behaviour could be
understood using a simple model consisting of weak standard electron-phonon
coupling plus weak coupling to an unspecified high energy excitation near 0.4
eV. This model is found to be in general agreement with the reflectivity data,
except for the predicted superconducting gap size. The additional high energy
excitation suggests that the dominant coupling mechanism in Ba_{1-x}K_xBiO_3 is
not standard electron-phonon.Comment: 5 pages REVTex, 5 figures, 32 refs, accepted for publication in Phys.
Rev.
Development of Wolffia arrhiza as a Producer for Recombinant Human Granulocyte Colony-Stimulating Factor
To date, the expression of recombinant proteins in transgenic plants is becoming a powerful alternative to classical expression methods. Special efforts are directed to the development of contained cultivation systems based on cell culture or rhyzosecretion, which reliably prevents the heterologous DNA releasing into the environment. A promising object for the development of such systems is the tiny aquatic plant of Wolffia arrhiza, which can be used as a dipped culture in bioreactors. Herein we have expressed the human granulocyte colony-stimulating factor (hG-CSF) in nuclear-transformed Wolffia. The nucleotide sequence of hG-CSF was optimized for expression in Wolffia and cloned into the vector pCamGCSF downstream of double CaMV 35S promoter. Wolffia plants were successfully transformed and 34 independent transgenic lines with hG-CSF gene were obtained, PCR and Southern blot analysis confirmed the transgenic origin of these lines. Western blot analysis revealed accumulation of the target protein in 33 transgenic lines. Quantitative ELISA of protein extracts from these lines showed hG-CSF accumulation up to 35.5 mg/kg of Wolffia fresh weight (0.194% of total soluble protein). This relatively high yield holds promise for the development of Wolffia-based expression system in strictly controlled format to produce various recombinant proteins
Thermodynamics of Two - Band Superconductors: The Case of MgB
Thermodynamic properties of the multiband superconductor MgB have often
been described using a simple sum of the standard BCS expressions corresponding
to - and -bands. Although, it is \textit{a priori} not clear if
this approach is working always adequately, in particular in cases of strong
interband scattering. Here we compare the often used approach of a sum of two
independent bands using BCS-like -model expressions for the specific
heat, entropy and free energy to the solution of the full Eliashberg equations.
The superconducting energy gaps, the free energy, the entropy and the heat
capacity for varying interband scattering rates are calculated within the
framework of two-band Eliashberg theory. We obtain good agreement between the
phenomenological two-band -model with the Eliashberg results, which
delivers for the first time the theoretical verification to use the
-model as a useful tool for a reliable analysis of heat capacity data.
For the thermodynamic potential and the entropy we demonstrate that only the
sum over the contributions of the two bands has physical meaning.Comment: 27 pages, 10 figures, 1 table, submitted to Phys. Rev.
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