1,689 research outputs found
Suppression and restoration of superconductivity in PrBa2Cu3O7
I review the principal experimental findings about superconductivity,
suppression thereof, and related properties of RE_{1-x}Pr_xBa_2Cu3O7 in a
decision-tree manner, eliminating the models substantially incompatible with
established experimental facts, eventually focussing on the pf\sigma
hybridization models and listing on a verbal level the interpretations of the
existing experiments as they emerge from such models.Comment: Talk given at the HTS-99 conference, Miami, 1999. Requires
aipproc.sty, epsfig. V.3, correcte
SrVOFeAs as Compared to Other Fe-based Superconductors
One of the most popular scenarios for the superconductivity in Fe-based
superconductors (FeBSC) posits that the bosons responsible for electronic
pairing are spin-fluctuations with a wave vector spanning the hole Fermi
surfaces (FSs) near and the electron FSs near M points. So far all
FeBSC for which neutron data are available do demonstrate such excitations, and
the band structure calculations so far were finding quasi-nested FSs in all
FeBSC, providing for a peak in the spin susceptibility at the desired wave
vectors. However, the newest addition to the family, SrVOFeAs, has
been calculated to have a very complex FS with no visible quasi-nesting
features. It was argued therefore that this material does not fall under the
existing paradigm and calls for revisiting our current ideas about what is the
likely cause of superconductivity in FeBSC. In this paper, I show that the
visible complexity of the FS is entirely due to the V-derived electronic
states. Assuming that superconductivity in SrVOFeAs, as in the
other FeBSC, originates in the FeAs layers, and the superconducting electrons
are sensitive to the susceptibility of the FeAs electronic subsystem, I
recalculate the bare susceptibility, weighting the electronic states with their
Fe character, and obtain a susceptibility that fully supports the existing
quasi-nesting model.Comment: Journal reference adde
Impurity scattering in highly anisotropic superconductors and interband sign reversal of the order parameter
We discuss various mechanisms that can lead to interband sign reversal of the
order parameter in a multiband superconductor. In particular, we generalize
Abrikosov-Gor'kov solution of the problem of weakly coupled superconductor with
magnetic and nonmagnetic impurities on the case of arbitary order parameter
anisotropy, including extreme cases as pairing or interband sign reversal
of the order parameter, and show that interband scattering by magnetic
impurities can stabilize an interband sign-reversal state. We discuss a
possibility of such state in YBaCuO in the context of various
experiments: Josephson tunneling, neutron scattering, isotope effect
measurements.Comment: 8 pages, 1 psfig. To be published in materials of 1996 SPIE
conference "Spectroscopic Studies of Superconductors". This is a summary of
papers cond-mat/9501117, cond-mat/9501118, cond-mat/9502025,
cond-mat/9504076. Besides, we derive a formula for Tc suppression by magnetic
and nonmagnetic impurities for arbitrary anisotrop
Competition between Electron-Phonon coupling and Spin Fluctuations in superconducting hole-doped BiOCuS
BiOCuS is a band insulator that becomes metallic upon hole doping.
Superconductivity was recently reported in doped BiOCuS and attributed
to spin fluctuations as a pairing mechanism. Based on first principles
calculations of the electron-phonon coupling, we argue that the latter is very
strong in this material, and probably drives superconductivity, which is
however strongly depressed by the proximity to magnetism. We find however that
BiOCuS is a quite unique compound where both a conventional
phonon-driven and an unconventional triplet superconductivity are possible, and
compete with each other. We argue that, in this material, it should be possible
to switch from conventional to unconventional superconductivity by varying such
parameters as doping or pressure
Competitions in layered ruthenates: ferro- vs. antiferromagnetism and triplet vs. singlet pairing
Ru based perovskites demonstrate an amazing richness in their magnetic
properties, including 3D and quasi-2D ferromagnetism, antiferromagnetism, and
unconventional superconductivity. Tendency to ferromagnetism, stemming from the
unusually large involvement of O in magnetism in ruthenates, leads to
ferromagnetic spin fluctuations in Sr2RuO4 and eventually to p-wave
superconductivity. A related compound Ca2RuO4 was measured to be
antiferromagnetic, suggesting a possibility of antiferromagnetic fluctuations
in Sr2RuO4 as well. Here we report first principles calculations that
demonstrate that in both compounds the ferro- and antiferromagnetic
fluctuations coexist, leading to an actual instability in Ca2RuO4 and to a
close competition between p-wave and d-wave superconducting symmetries in
Sr2RuO4. The antiferromagnetism in this system appears to be mostly related
with the nesting, which is the strongest at Q=(2pi/3,2pi/3,0). Surprisingly,
for the Fermiology of Sr2RuO4 the p-wave state wins over the d-wave one
everywhere except in close vicinity of the antiferromagnetic instability. The
most stable state within the d-wave channel has vanishing order parameter at
one out of three Fermi surfaces in Sr2RuO4, while in the p channel its
amplitude is comparable at all three of them.Comment: 4 Revtex pages with 4 embedded postscript figure. Some figures are
color, but should look OK in B&W as wel
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