202 research outputs found
Microwave Penetration Depth and Quasiparticle Conductivity in PrFeAsO_1-y Single Crystals : Evidence for a Full-Gap Superconductor
In-plane microwave penetration depth and quaiparticle
conductivity at 28 GHz are measured in underdoped single crystals of the
Fe-based superconductor PrFeAsO ( K) by using a
sensitive superconducting cavity resonator. shows flat
dependence at low temperatures, which is incompatible with the presence of
nodes in the superconducting gap . The temperature dependence
of the superfluid density demonstrates that the gap is non-zero
() all over the Fermi surface. The microwave
conductivity below exhibits an enhancement larger than the coherence
peak, reminiscent of high- cuprate superconductors.Comment: 4 pages, 3 figures. Version accepted for publication in Phys. Rev.
Lett. For related results of hole-doped 122 system, see arXiv:0810.350
Correlation-driven electronic nematicity in the Dirac semimetal BaNiS2
In BaNiS2 a Dirac nodal-line band structure exists within a two-dimensional
Ni square lattice system, in which significant electronic correlation effects
are anticipated. Using scanning tunneling microscopy, we discover signs of
correlated-electron behavior, namely electronic nematicity appearing as a pair
of C2-symmetry striped patterns in the local density-of-states at ~60 meV above
the Fermi energy. In observations of quasiparticle interference, as well as
identifying scattering between Dirac cones, we find that the striped patterns
in real space stem from a lifting of degeneracy among electron pockets at the
Brillouin zone boundary. We infer a momentum-dependent energy shift with d-form
factor, which we model numerically within a density wave equation framework
that considers spin-fluctuation-driven nematicity. This suggests an unusual
mechanism driving the nematic instability, stemming from only a small
perturbation to the Fermi surface, in a system with very low density of states
at the Fermi energy. The Dirac points lie at nodes of the d-form factor, and
are almost unaffected by it. These results highlight BaNiS2 as a unique
material in which Dirac electrons and symmetry-breaking electronic correlations
coexist.Comment: 11 pages, 5 figures (plus 6 pages, 4 figures
Lower Critical Fields of Superconducting PrFeAsO Single Crystals
We have studied the lower critical fields H_{c1} of superconducting iron
oxipnictide PrFeAsO_{1-y} single crystals for H parallel and perpendicular to
the ab-planes. Measurements of the local magnetic induction at positions
straddling the sample edge by using a miniature Hall-sensor array clearly
resolve the first flux penetration from the Meissner state. The temperature
dependence of H_{c1} for H || c is well scaled by the in-plane penetration
depth without showing any unusual behavior, in contrast to previous reports.
The anisotropy of penetration lengths at low temperatures is estimated to be ~
2.5, which is much smaller than the anisotropy of the coherence lengths. This
is indicative of multiband superconductivity in this system, in which the
active band for superconductivity is more anisotropic. We also point out that
the local induction measured at a position near the center of the crystal,
which has been used in a number of reports for the determination of H_{c1},
might seriously overestimate the obtained H_{c1}-value.Comment: 7 pages, 7 figures, accepted for publication in Phys. Rev.
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
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