414 research outputs found
Impurity effects on optical response in a finite band electronic system coupled to phonons
The concepts, which have traditionally been useful in understanding the
effects of the electron--phonon interaction in optical spectroscopy, are based
on insights obtained within the infinite electronic band approximation and no
longer apply in finite band metals. Impurity and phonon contributions to
electron scattering are not additive and the apparent strength of the coupling
to the phonon degrees of freedom is substantially reduced with increased
elastic scattering. The optical mass renormalization changes sign with
increasing frequency and the optical scattering rate never reaches its high
frequency quasiparticle value which itself is also reduced below its infinite
band value
Effect of disorder on the far-infrared conductivity and on the microwave conductivity of two-band superconductors
We consider the far-infrared and the microwave conductivities of a two-band
superconductor with non-magnetic impurities. The strong coupling expressions
for the frequency and temperature dependent conductivity of a two-band
superconductor are developed assuming isotropic bands and interactions. Our
numerical results obtained using realistic interaction parameters for MgB
are compared with experiments on this compound. We find that the available
experimental results for the far-infrared conductivity of MgB are
consistent with multi-band superconductivity in the presence of a sufficiently
strong interband impurity scattering. On the other hand, our numerical results
for the microwave conductivity in the superconducting state indicate that the
experimental results obtained on samples with the highest transition
temperature are consistent with a low interband impurity scattering
rate but depend sensitively on the ratio of the total scattering rates in the
two bands. For the -band scattering rate not greater than
the -band scattering rate there is a single, broad,
low-temperature (at about 0.5) coherence peak in the microwave
conductivity. For =4--7 a high-temperature (at
about 0.9) coherence peak is dominant, but there is also a
low-temperature peak/shoulder resulting from the contribution of the -band
carriers to the microwave conductivity. For 1
only the high-temperature coherence peak should be observable.Comment: 11 pages, 6 figure
Effect of disorder on the NMR relaxation rate in two-band superconductors
We calculate the effect of nonmagnetic impurity scattering on the
spin-lattice relaxation rate in two-band superconductors with the s-wave
pairing symmetry. It is found that for the interaction parameters appropriate
for MgB2 the Hebel-Slichter peak is suppressed by disorder in the limit of
small interband impurity scattering rate. In the limit of strong impurity
scattering, when the gap functions in the two bands become nearly equal, the
single-band behavior is recovered with a well-defined coherence peak just below
the transition temperature.Comment: 6 pages, 4 figure
Phase Diagram of CeCoIn_5 in the Vicinity of H_{c2} as Determined by NMR
We report ^{115}In nuclear magnetic resonance (NMR) measurements in the
heavy-fermion superconductor CeCoIn_5 as a function of temperature in different
magnetic fields applied parallel to the plane. The
measurements probe a part of the phase diagram in the vicinity of the
superconducting critical field H_{c2} where a possible inhomogeneous
superconducting state, Fulde-Ferrel-Larkin-Ovchinnikov (FFLO), is stabilized.
We have identified clear NMR signatures of two phase transitions occurring in
this part of the phase diagram. The first order phase transitions are
characterized by the sizable discontinuity of the shift. We find that a
continuous second order phase transition from the superconducting to the FFLO
state occurs at temperature below which the shift becomes temperature
independent. We have compiled the first phase diagram of CeCoIn_5 in the
vicinity of H_{c2} from NMR data and found that it is in agreement with the one
determined by thermodynamic measurements.Comment: 4 pages, submitted to Proceedings of SCES'0
Finite band inversion of ARPES in BiSrCaCuO in comparison with optics
Using a maximum entropy technique within a finite band Eliashberg formalism
we extract from recent high accuracy nodal direction angular resolved
photo-emission spectroscopy (ARPES) data in optimally doped
BiSrCaCuO (Bi2212) a quasiparticle electron-boson
spectral density. Both normal and superconducting state with d-wave gap
symmetry are treated. Finite and infinite band results are considered and
contrasted. We compare with results obtained for the related transport spectral
density which follows from a similar inversion of optical data. We discuss the
implication of our results for quasiparticle renormalizations in the antinodal
direction.Comment: 9 pages, 7 figures submitted to Physical Review
Effect of electron-phonon interaction on spectroscopies in graphene
We calculate the effect of the electron-phonon interaction on the electronic
density of states (DOS), the quasiparticle properties and on the optical
conductivity of graphene. In metals with DOS constant on the scale of phonon
energies, the electron-phonon renormalizations drop out of the dressed DOS,
however, due to the Dirac nature of the electron dynamics in graphene, the band
DOS is linear in energy and phonon structures remain, which can be emphasized
by taking an energy derivative. There is a shift in the chemical potential and
in the position in energy of the Dirac point. Also, the DOS can be changed from
a linear dependence out of value zero at the Dirac point to quadratic out of a
finite value. The optical scattering rate sets the energy scale for
the rise of the optical conductivity from its universal DC value
(expected in the simplest theory when chemical potential and temperature are
both ) to its universal AC background value . As in ordinary metals the DC conductivity remains unrenormalized
while its AC value is changed. The optical spectral weight under the intraband
Drude is reduced by a mass renormalization factor as is the effective
scattering rate. Optical weight is transferred to an Holstein phonon-assisted
side band. Due to Pauli blocking the interband transitions are sharply
suppressed, but also nearly constant, below twice the value of renormalized
chemical potential and also exhibit a phonon-assisted contribution. The
universal background conductivity is reduced below at large
energies.Comment: 22 pages, 19 figures, submitted to PR
Bloggers Behavior and Emergent Communities in Blog Space
Interactions between users in cyberspace may lead to phenomena different from
those observed in common social networks. Here we analyse large data sets about
users and Blogs which they write and comment, mapped onto a bipartite graph. In
such enlarged Blog space we trace user activity over time, which results in
robust temporal patterns of user--Blog behavior and the emergence of
communities. With the spectral methods applied to the projection on weighted
user network we detect clusters of users related to their common interests and
habits. Our results suggest that different mechanisms may play the role in the
case of very popular Blogs. Our analysis makes a suitable basis for theoretical
modeling of the evolution of cyber communities and for practical study of the
data, in particular for an efficient search of interesting Blog clusters and
further retrieval of their contents by text analysis
Nonconstant electronic density of states tunneling inversion for A15 superconductors: Nb3Sn
We re-examine the tunneling data on A15 superconductors by performing a
generalized McMillan-Rowell tunneling inversion that incorporates a nonconstant
electronic density of states obtained from band-structure calculations. For
Nb3Sn, we find that the fit to the experimental data can be slightly improved
by taking into account the sharp structure in the density of states, but it is
likely that such an analysis alone is not enough to completely explain the
superconducting tunneling characteristics of this material. Nevertheless, the
extracted Eliashberg function displays a number of features expected to be
present for the highest quality Nb3Sn samples.Comment: 11 pages, 11 figure
Control system modeling of hydraulic actuator with compressible fluid flow
U radu su tretirane Rimanove parcijalne diferencijalne jednačine u obliku izraza (1) gde je promenljiva μ definisana relacijom (2). Odgovarajući granièni uslovi definisani su u različitim oblicima, kao što su granični uslovi pritiska i protoka i to na fiksnim i na pokretnim granicama. Problem je formulisan sa ciljem da opiše kompletnu dinamiku hidrauličkog aktuatora, uključujući njegov stvarni protok i geometrijske karakteristike. U tu svrhu napravljen je poseban algoritam i odgovarajući računarski paket za simulaciju kompletne dinamike hidrauličkog aktuatora, uključujući i prisutne efekte talasa. Za rešenje problema korišćena je metoda karakteristika. Rezultati računarske simulacije dinamike hidrauličkog aktuatora predstavljeni su 3-D dijagramima.The paper treats Riemann's partial differential equations in the form (1), where variable μ is defined by relation (2). Corresponding boundary conditions are defined in various forms, such as boundary conditions of pressure and flow on the fixed and movable boundaries. Problem formulation is constructed in order to describe hydraulic actuator dynamics in a complete form, including its real flow and geometric characteristics. Special algorithm is generated and a corresponding computer package for simulation of the complete hydraulic actuator dynamics, including the existing wave effects, using the method of characteristics to obtain the desired problem solution. Results of computer simulation of hydraulic actuator dynamics are presented in 3-D diagrams
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