970 research outputs found
Bosonic Spectral Function and The Electron-Phonon Interaction in HTSC Cuprates
In Part I we discuss accumulating experimental evidence related to the
structure and origin of the bosonic spectral function in high-temperature
superconducting (HTSC) cuprates at and near optimal doping. Some global
properties of the spectral function, such as number and positions of peaks, are
extracted by combining optics, neutron scattering, ARPES and tunnelling
measurements. These methods give convincing evidence for strong electron-phonon
interaction (EPI) with the coupling constant between 1-3 in cuprates near
optimal doping. Here we clarify how these results are in favor of the
Eliashberg-like theory for HTSC cuprates near optimal doping. In Part II we
discuss some theoretical ingredients - such as strong EPI, strong correlations
- which are necessary to explain the experimental results related to the
mechanism of d-wave pairing in optimally doped cuprates. These comprise the
Migdal-Eliashberg theory for EPI in strongly correlated systems which give rise
to the forward scattering peak. The latter is further supported by the weakly
screened Madelung interaction in the ionic-metallic structure of layered
cuprates. In this approach EPI is responsible for the strength of pairing while
the residual Coulomb interaction (by including spin fluctuations) triggers the
d-wave pairing.Comment: 59 pages, 38 figures, review articl
Ab initio calculations of the physical properties of transition metal carbides and nitrides and possible routes to high-Tc
Ab initio linear-response calculations are reported of the phonon spectra and
the electron-phonon interaction for several transition metal carbides and
nitrides in a NaCl-type structure. For NbC, the kinetic, optical, and
superconducting properties are calculated in detail at various pressures and
the normal-pressure results are found to well agree with the experiment.
Factors accounting for the relatively low critical temperatures Tc in
transition metal compounds with light elements are considered and the possible
ways of increasing Tc are discussed.Comment: 19 pages, 7 figure
Field-Dependent Critical Current in Type-II Superconducting Strips: Combined Effect of Bulk Pinning and Geometrical Edge Barrier
Recent theoretical and experimental research on low-bulk-pinning
superconducting strips has revealed striking dome-like magnetic-field
distributions due to geometrical edge barriers. The observed magnetic-flux
profiles differ strongly from those in strips in which bulk pinning is
dominant. In this paper we theoretically describe the current and field
distributions of a superconducting strip under the combined influence of both a
geometrical edge barrier and bulk pinning at the strip's critical current Ic,
where a longitudinal voltage first appears. We calculate Ic and find its
dependence upon a perpendicular applied magnetic field Ha. The behavior is
governed by a parameter p, defined as the ratio of the bulk-pinning critical
current Ip to the geometrical-barrier critical current Is0. We find that when p
> 2/pi and Ip is field-independent, Ic vs Ha exhibits a plateau for small Ha,
followed by the dependence Ic-Ip ~ 1/Ha in higher magnetic fields.Comment: 4 pages, 2 figures, Fig. 1 revised, submitted to Phys. Rev.
Anomalous Hall effect for the phonon heat conductivity in paramagnetic dielectric
The theory of anomalous Hall effect for the heat transfer in a paramagnetic
dielectric, discovered experimentally in [1], is developed. The appearance of
the phonon heat flux normal to both the temperature gradient and the magnetic
field is connected with the interaction of magnetic ions with the crystal field
oscillations. In crystals with an arbitrary phonon spectrum this interaction
creates the elliptical polarization of phonons. The kinetics related to phonon
scattering induced by the spin-phonon interaction determines an origin of the
off-diagonal phonon density matrix. The combination of the both factors is
decisive for the phenomenon under consideration.Comment: 5 pages; typos and abstract correcte
Nanoengineered Curie Temperature in Laterally-Patterned Ferromagnetic Semiconductor Heterostructures
We demonstrate the manipulation of the Curie temperature of buried layers of
the ferromagnetic semiconductor (Ga,Mn)As using nanolithography to enhance the
effect of annealing. Patterning the GaAs-capped ferromagnetic layers into
nanowires exposes free surfaces at the sidewalls of the patterned (Ga,Mn)As
layers and thus allows the removal of Mn interstitials using annealing. This
leads to an enhanced Curie temperature and reduced resistivity compared to
unpatterned samples. For a fixed annealing time, the enhancement of the Curie
temperature is larger for narrower nanowires.Comment: Submitted to Applied Physics Letters (minor corrections
Manifestation of superfluidity in an evolving Bose-condensed gas
We study the generation of excitations due to an ''impurity''(static
perturbation) placed into an oscillating Bose-condensed gas in the
time-dependent trapping field. It is shown that there are two regions for the
position of the local perturbation. In the first region the condensate flows
around the ''impurity'' without generation of excitations demonstrating
superfluid properties. In the second region the creation of excitations occurs,
at least within a limited time interval, revealing destruction of
superfluidity. The phenomenon can be studied by measuring the damping of
condensate oscillations at different positions of the ''impurity''
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