16 research outputs found
Effect of electron-phonon interaction on the shift and attenuation of optical phonons
Using the Boltzmann equation for electrons in metals, we show that the
optical phonons soften and have a dispersion due to screening in agreement with
the results reported recently [M. Reizer, Phys. Rev. B {\bf 61}, 40 (2000)].
Additional phonon damping and frequency shift arise when the electron--phonon
interaction is properly included.Comment: 4 pages, late
The Upper Critical Field Hc2 in Advanced Superconductors with Anisotropic Energy Spectrum
A brief review of works on the microscopic theory of determining the upper
critical field in two-band isotropic and anisotropic superconductors is given.
The research is based on a set of the Ginzburg-Landau equations for the order
parameters in a magnetic field that are studied in terms of the classical
approach to a superconducting system in a magnetic field. Two inequivalent
energy bands with different topology of Fermi surface cavities overlapping on
the Fermi surface are discussed. The cases of the direction of the external
magnetic field H\to// the (ab) plane and H\to // the crystallographic c axis
are studied. The equations for determining Hc2(ab) and Hc2(c) for a pure
superconductor and a superconductor doped with electrons and holes are derived.
The analytical solutions to these equations in the vicinity of the
superconducting transition temperature (Tc - T<<Tc) and in the vicinity of zero
(T<<Tc) are found. The temperature and impurity dependences of the upper
critical fields Hc2(ab) and Hc2(c), as well as the anisotropy coefficient
\gammaH, are studied. The resulting theory is applied to determine the
dependences of the above magnetic characteristics of intermetallic compound
MgB2. The theory agrees qualitatively with experimental data.Comment: 24 pages, 6 fi
Electronic Collective Modes and Superconductivity in Layered Conductors
A distinctive feature of layered conductors is the presence of low-energy
electronic collective modes of the conduction electrons. This affects the
dynamic screening properties of the Coulomb interaction in a layered material.
We study the consequences of the existence of these collective modes for
superconductivity. General equations for the superconducting order parameter
are derived within the strong-coupling phonon-plasmon scheme that account for
the screened Coulomb interaction. Specifically, we calculate the
superconducting critical temperature Tc taking into account the full
temperature, frequency and wave-vector dependence of the dielectric function.
We show that low-energy plasmons may contribute constructively to
superconductivity. Three classes of layered superconductors are discussed
within our model: metal-intercalated halide nitrides, layered organic materials
and high-Tc oxides. In particular, we demonstrate that the plasmon contribution
(electronic mechanism) is dominant in the first class of layered materials. The
theory shows that the description of so-called ``quasi-two-dimensional
superconductors'' cannot be reduced to a purely 2D model, as commonly assumed.
While the transport properties are strongly anisotropic, it remains essential
to take into account the screened interlayer Coulomb interaction to describe
the superconducting state of layered materials.Comment: Final version (minor changes) 14 pages, 6 figure
Specific Heat Discontinuity in Impure Two-Band Superconductors
The Ginzburg-Landau coefficients, and the jump of the specific heat are
calculated for a disordered two-band superconductor. We start with the analysis
of a more general case arbitrary anisotropy. While the specific heat
discontinuity at the critical temperature T_c decreases with increasing
disorder, its ratio to the normal state specific heat at T_c increases and
slowly converges to the isotropic value. For a strong disorder the deviation
from the isotropic value is proportional to the elastic electron scattering
time. In the case of a two-band superconductor we apply a simplified model of
the interaction independent on momentum within a band. In the framework of this
model all thermodynamic values can be found explicitly at any value of the
scattering rate. This solution explains the sample dependence of the specific
heat discontinuity in MgB_2 and the influence of the disorder on the critical
temperature.Comment: New results relate to two-band superconductors, 9 pages, 2 figure
Ginzburg-Landau theory of vortices in a multi-gap superconductor
The Ginzburg-Landau functional for a two-gap superconductor is derived within
the weak-coupling BCS model. The two-gap Ginzburg-Landau theory is, then,
applied to investigate various magnetic properties of MgB2 including an upturn
temperature dependence of the transverse upper critical field and a core
structure of an isolated vortex. Orientation of vortex lattice relative to
crystallographic axes is studied for magnetic fields parallel to the c-axis. A
peculiar 30-degree rotation of the vortex lattice with increasing strength of
an applied field observed by neutron scattering is attributed to the multi-gap
nature of superconductivity in MgB2.Comment: 11 page
Challenging the nature of low-energy plasmon excitations in CaC 6
The nature of low energy plasmon excitations plays an important role in
understanding the low energy electronic properties and coupling mechanism of
different superconducting compounds such as CaC. Recent ab-initio studies
predict a charge carrier intraband plasmon in keeping with a low energy
acoustic plasmon. Here, we have studied the low-energy electronic excitations
of CaC using high-resolution electron energy-loss spectroscopy in
transmission at low temperatures. The analysis of the core-level excitations
leads to the conclusion that hybridization between graphite and calcium states
plays an essential role in this graphite intercalated compound. Regarding the
low energy plasmon excitation, we observe the formation of an intraband (charge
carrier) plasmon with a negative dispersion at about 3.5 eV in sound agreement
with the theory. Finally, a weak excitation around 1.2 eV with an almost linear
dispersion relation can be observed as predicted for an acoustic plasmon that
may mediate the superconducting coupling in CaC. However its optical limit
at ~1 eV challenges the theoretical predictions and safely rules out an
electronic superconducting coupling mechanism in CaC.Comment: 5 figure