750 research outputs found
Self-Consistent Two-Gap Description of MgBâ‚‚ Superconductor
A self-consistent two-gap Îł-model is used to quantitatively describe several thermodynamic properties of MgB2 superconductor. The superconducting coupling matrix, vij, was obtained from the fitting of the superfluid density in the entire superconducting temperature range. Using this input, temperature-dependent superconducting gaps, specific heat, and upper critical fields were calculated with no adjustable parameters and compared with the experimental data as well as with the first-principles calculations. The observed agreement between fit and data shows that Îł-model provides adequate quantitative description of the two-gap superconductivity in MgB2 and may serve as a relatively simple and versatile self-consistent description of the thermodynamic quantities in multi-gap superconductors
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
Magnetic Penetration Depth in Unconventional Superconductors
This topical review summarizes various features of magnetic penetration depth
in unconventional superconductors. Precise measurements of the penetration
depth as a function of temperature, magnetic field and crystal orientation can
provide detailed information about the pairing state. Examples are given of
unconventional pairing in hole- and electron-doped cuprates, organic and heavy
fermion superconductors. The ability to apply an external magnetic field adds a
new dimension to penetration depth measurements. We discuss how field dependent
measurements can be used to study surface Andreev bound states, nonlinear
Meissner effects, magnetic impurities, magnetic ordering, proximity effects and
vortex motion. We also discuss how penetration depth measurements as a function
of orientation can be used to explore superconductors with more than one gap
and with anisotropic gaps. Details relevant to the analysis of penetration
depth data in anisotropic samples are also discussed.Comment: topical review, 57 pages, 219 reference
Electronic Structure, Electron-Phonon Coupling, and Multiband Effects in MgB2
We review the current situation in the theory of superconducting and
transport properties of MgB2. First principle calculations of of the electronic
structure and electron-phonon coupling are discussed and compared with the
experiment. We also present a brief description of the multiband effects in
superconductivity and transport, and how these manifest themselves in MgB2. We
also mention some yet open questions.Comment: Physica C, in pres
Resonant and crossover phenomena in a multiband superconductor tuning the chemical potential near a band edge
Resonances in the superconducting properties, in a regime of crossover from
BCS to mixed Bose-Fermi superconductivity, are investigated in a two-band
superconductor where the chemical potential is tuned near the band edge of the
second mini-band generated by quantum confinement effects. The shape resonances
at T=0 in the superconducting gaps (belonging to the class of Feshbach-like
resonances) is manifested by interference effects in the superconducting gap at
the first large Fermi surface when the chemical potential is in the proximity
of the band edge of the second mini-band. The case of a superlattice of quantum
wells is considered and the amplification of the superperconducting gaps at the
3D-2D Fermi surface topological transition is clearly shown. The results are
found to be in good agreement with available experimental data on a
superlattice of honeycomb boron layers intercalated by Al and Mg spacer layers.Comment: 13 pages, 9 image
Full-gap superconductivity robust against disorder in heavy-fermion CeCu2Si2
A key aspect of unconventional pairing by the antiferromagnetic
spin-fluctuation mechanism is that the superconducting energy gap must have
opposite sign on different parts of the Fermi surface. Recent observations of
non-nodal gap structure in the heavy-fermion superconductor CeCuSi were
then very surprising, given that this material has long been considered a
prototypical example of a superconductor where the Cooper pairing is
magnetically mediated. Here we present a study of the effect of controlled
point defects, introduced by electron irradiation, on the temperature-dependent
magnetic penetration depth in CeCuSi. We find that the
fully-gapped state is robust against disorder, demonstrating that low-energy
bound states, expected for sign-changing gap structures, are not induced by
nonmagnetic impurities. This provides bulk evidence for -wave
superconductivity without sign reversal.Comment: 5 pages, 4 figures + Supplemental Material (1 page, 1 figure). Will
appear in Phys. Rev. Let
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