95 research outputs found
Fluctuations in superconducting rings with two order parameters
Starting from the Ginzburg-Landau energy functional, we discuss how the
presence of two order parameters and the coupling between them influence a
superconducting ring in the fluctuative regime. Our method is exact, but
requires numerical implementation. We also study approximations for which some
analytic expressions can be obtained, and check their ranges of validity. We
provide estimates for the temperature ranges where fluctuations are important,
calculate the persistent current in magnesium diboride rings as a function of
temperature and enclosed flux, and point out its additional dependence on the
cross-section area of the ring. We find temperature regions in which
fluctuations enhance the persistent currents and regions where they inhibit the
persistent current. The presence of two order parameters that can fluctuate
independently always leads to larger averages of the order parameters at Tc,
but only for appropriate parameters this yields larger persistent current. In
cases of very different material parameters for the two coupled condensates,
the persistent current is inhibited
Intercalant-Driven Superconductivity in YbC and CaC
Recently deiscovered superconductivity in YbC and CaC at temperatures
substantially higher than previously known for intercalated graphites, raised
several new questions: (1) Is the mechanism considerably different from the
previously known intercalated graphites? (2) If superconductivity is
conventional, what are the relevant phonons? (3) Given extreme similarity
between YbC and CaCa, why their critical temperatures are so different?
We address these questions on the basis of first-principles calculations and
conclude that coupling with intercalant phonons is likely to be the main force
for superconductivity in YbC and CaC, but not in alkaline-intercalated
compounds, and explain the difference in by the ``isotope effect'' due to
the difference in Yb and Ca atomic masses.Comment: 4 pages, embedded postscript figire
Is the term "type-1.5 superconductivity" warranted by Ginzburg-Landau theory?
It is shown that within the Ginzburg-Landau (GL) approximation the order
parameters Delta1(r, T) and Delta2(r, T) in two-band superconductors vary on
the same length scale, the difference in the zero-T coherence lengths xi0_i
~vF_i/Delta_i(0), i = 1, 2 notwithstanding. This amounts to a single physical
GL parameter kappa and the classic GL dichotomy: kappa < 1/sqrt(2) for type-I
and kappa > 1/sqrt(2) for type-II.Comment: 5 pages, revised and extended version; previous title "Two-band
superconductors near Tc" change
NMR relaxation time in a clean two-band superconductor
We study the spin-lattice relaxation rate of nuclear magnetic resonance in a
two-band superconductor. Both conventional and unconventional pairing
symmetries for an arbitrary band structure in the clean limit are considered.
The importance of the inter-band interference effects is emphasized. The
calculations in the conventional case with two isotropic gaps are performed
using a two-band generalization of Eliashberg theory.Comment: 9 pages, 3 figure
Field dependence of the vortex core size probed by scanning tunneling microscopy
We study the spatial distribution of the density of states (DOS) at zero bias N(r) in the mixed state of single and multigap superconductors. We provide an analytic expression for N(r) based on deGennes' relationship between DOS and the order parameter that reproduces well scanning tunneling microscopy (STM) data in several superconducting materials. In the single gap superconductor β-Bi2Pd, we find that N(r) is governed by a length scale ξH = √φ0/2πH, which decreases in rising fields. The vortex core size C, defined via the slope of the order parameter at the vortex center, C ∝ (d%/dr|r→0)−1, differs from ξH by a material dependent numerical factor. The new data on the tunneling conductance and vortex lattice of the 2H-NbSe1.8S0.2 show the in-plane isotropic vortices, suggesting that substitutional scattering removes the in-plane anisotropy found in the two-gap superconductor 2H-NbSe2. We fit the tunneling conductance of 2H-NbSe1.8S0.2 to a two gap model and calculate the vortex core size C for each band. We find that C is field independent and has the same value for both bands. We also analyze the two-band superconductor 2H-NbS2 and find the same result. We conclude that, independently of the magnetic field induced variation of the order parameter values in both bands, the spatial variation of the order parameter close to the vortex core is the same for all bands
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.
Electron-lattice interaction and its impact on high Tc superconductivity
In this Colloquium, the main features of the electron-lattice interaction are
discussed and high values of the critical temperature up to room temperature
could be provided. While the issue of the mechanism of superconductivity in the
high Tc cuprates continues to be controversial, one can state that there have
been many experimental results demonstrating that the lattice makes a strong
impact on the pairing of electrons. The polaronic nature of the carriers is
also a manifestation of strong electron-lattice interaction. One can propose an
experiment that allows an unambiguous determination of the intermediate boson
(phonon, magnon, exciton, etc.) which provides the pairing. The
electron-lattice interaction increases for nanosystems, and this is due to an
effective increase in the density of states
Superfluid density and specific heat within self-consistent scheme for two-band superconductor
The two gaps in a two-band clean s-wave superconductor are evaluated
self-consistently within the quasiclassical Eilenberger weak-coupling formalism
with two in-band and one inter-band pairing potentials. Superfluid density,
free energy and specific heat are given in the form amenable for fitting the
experimental data. Well-known two-band MgB and VSi superconductors are
used to test the developed approach. The pairing potentials obtained from the
fit of the superfluid density data in MgB crystal were used to calculate
temperature-dependent specific heat, . The calculated compares
very well with the experimental data. Advantages and validity of this, what we
call the "-model", are discussed and compared with the commonly used
empirical (and \textit{not self-consistent}) "-model". Correlation
between the sign of the inter-band coupling and the signs of the two order
parameters is discussed. Suppression of the critical temperature by the
inter-band scattering is evaluated and shown to be severe for the inter-band
repulsion as compared to the attraction. The data on a strong suppression
in MgB crystals by impurities suggest that the order parameters on two
effective bands of this material may have opposite signs, i.e., may have the
structure similar to the current proposals in iron-based pnictide
superconductors
Re-entrant localization of single particle transport in disordered Andreev wires
We study effects of disorder on the low energy single particle transport in a
normal wire surrounded by a superconductor. We show that the heat conductance
includes the Andreev diffusion decreasing with increase in the mean free path
and the diffusive drift produced by a small particle-hole asymmetry,
which increases with increasing . The conductance thus has a minimum as a
function of which leads to a peculiar re-entrant localization as a
function of the mean free path.Comment: 4 pages, 2 figure
- …