787 research outputs found
Multiband Superconductivity in Spin Density Wave Metals
We study the emergence of multiband superconductivity with - and wave
symmetry on the background of spin density wave (SDW). We show that the SDW
coherence factors renormalize the momentum dependence of the superconducting
(SC) gap, yielding a SC state with an \emph{unconventional} s-wave symmetry.
Interband Cooper pair scattering stabilizes superconductivity in both
symmetries. With increasing SDW order, the s-wave state is more strongly
suppressed than the d-wave state. Our results are universally applicable to
two-dimensional systems with a commensurate SDW.Comment: 4 pages, 3 figure
Spin susceptibility in bilayered cuprates: resonant magnetic excitations
We study the momentum and frequency dependence of the dynamical spin
susceptibility in the superconducting state of bilayer cuprate superconductors.
We show that there exists a resonance mode in the odd as well as the even
channel of the spin susceptibility, with the even mode being located at higher
energies than the odd mode. We demonstrate that this energy splitting between
the two modes arises not only from a difference in the interaction, but also
from a difference in the free-fermion susceptibilities of the even and odd
channels. Moreover, we show that the even resonance mode disperses downwards at
deviations from . In addition, we demonstrate that there
exists a second branch of the even resonance, similar to the recently observed
second branch (the -mode) of the odd resonance. Finally, we identify the
origin of the qualitatively different doping dependence of the even and odd
resonance. Our results suggest further experimental test that may finally
resolve the long-standing question regarding the origin of the resonance peak.Comment: 8 pages, 5 figure
Polaron Effects on Superexchange Interaction: Isotope Shifts of , , and in Layered Copper Oxides
A compact expression has been obtained for the superexchange coupling of
magnetic ions via intermediate anions with regard to polaron effects at both
magnetic ions and intermediate anions. This expression is used to analyze the
main features of the behavior of isotope shifts for temperatures of three types
in layered cuprates: the Neel temperatures (), critical temperatures of
transitions to a superconducting state (), and characteristic temperatures
of the pseudogap in the normal state ().Comment: 4 pages, 1 figur
Orbital ordering in charge transfer insulators
We discuss a new mechanism of orbital ordering, which in charge transfer
insulators is more important than the usual exchange interactions and which can
make the very type of the ground state of a charge transfer insulator, i.e. its
orbital and magnetic ordering, different from that of a Mott-Hubbard insulator.
This purely electronic mechanism allows us to explain why orbitals in
Jahn-Teller materials typically order at higher temperatures than spins, and to
understand the type of orbital ordering in a number of materials, e.g.
K_2CuF_4, without invoking the electron-lattice interaction.Comment: 4 pages, 2 figure
Gap Structure of the Spin-Triplet Superconductor Sr2RuO4 Determined from the Field-Orientation Dependence of Specific Heat
We report the field-orientation dependent specific heat of the spin-triplet
superconductor Sr2RuO4 under the magnetic field aligned parallel to the RuO2
planes with high accuracy. Below about 0.3 K, striking 4-fold oscillations of
the density of states reflecting the superconducting gap structure have been
resolved for the first time. We also obtained strong evidence of multi-band
superconductivity and concluded that the superconducting gap in the active
band, responsible for the superconducting instability, is modulated with a
minimum along the [100] direction.Comment: 4 pages, 4 figure
Electronic theory for itinerant in-plane magnetic fluctuations in NaCoO
Starting from {\it ab-initio} band structure for NaCoO, we derive the
single-electron energies and the effective tight-binding description for the
bands using a projection procedure. We find that due to the presence
of the next-nearest-neighbor hoppings a local minimum in the electronic
dispersion close to the point of the first Brillouin zone forms.
Therefore, in addition to a large Fermi surface an electron pocket close to the
point emerges at high doping concentrations. The latter yields the new
scattering channel resulting in a peak structure of the itinerant magnetic
susceptibility at small momenta. This indicates itinerant in-plane
ferromagnetic state above certain critical concentration , in agreement
with neutron scattering data. Below the magnetic susceptibility shows a
tendency towards the antiferromagnetic fluctuations. We estimate the value of
within the rigid band model and within the Hubbard model
with infinite on-site Coulomb repulsion consistent with the experimental phase
diagram.Comment: 4 pages, 4 figures; LDA calculations were done with Na in the
symmetric 2d position contrary to the 6h position in a previous version of
this pape
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Ultrafast modulation of the chemical potential in BaFe2As2 by coherent phonons
Time- and angle-resolved extreme ultraviolet photoemission spectroscopy is used to study the electronic structure dynamics in BaFe2As2 around the high-symmetry points Γ and M. A global oscillation of the Fermi level at the frequency of the A1g(As) phonon mode is observed. It is argued that this behavior reflects a modulation of the effective chemical potential in the photoexcited surface region that arises from the high sensitivity of the band structure near the Fermi level to the A1g(As) phonon mode combined with a low electron diffusivity perpendicular to the layers. The results establish a novel way to tune the electronic properties of iron pnictides: coherent control of the effective chemical potential. The results further suggest that the equilibration time for the effective chemical potential needs to be considered in the ultrafast electronic structure dynamics of materials with weak interlayer coupling. © 2014 American Physical Society
London penetration depth in the tight binding approximation: Orthorhombic distortion and oxygen isotope effects in cuprates
We present a simple derivation of an expression for the superfluid density in superconductors with the tight binding energy
dispersion. The derived expression is discussed in detail because of its
distinction from the known expressions for ordinary superconductors with
parabolic energy dispersion. We apply this expression for the experimental data
analysis of the isotope effect in London penetration depth parameter in the BiSrCuO and YBaCuO family compounds near optimal doping, taking into
account the orthorhombic distortion of crystal structure, and estimate the
isotopic change of hopping parameters from the experimental data. We point out
that temperature behaviour is very sensitive to the ratio and estimate this quantity for a number of compounds.Comment: 10 pages, 4 figure
Simulations of electromagnetic effects in high frequency capacitively coupled discharges using the Darwin approximation
The Darwin approximation is investigated for its possible use in simulation
of electromagnetic effects in large size, high frequency capacitively coupled
discharges. The approximation is utilized within the framework of two different
fluid models which are applied to typical cases showing pronounced standing
wave and skin effects. With the first model it is demonstrated that Darwin
approximation is valid for treatment of such effects in the range of parameters
under consideration. The second approach, a reduced nonlinear Darwin
approximation-based model, shows that the electromagnetic phenomena persist in
a more realistic setting. The Darwin approximation offers a simple and
efficient way of carrying out electromagnetic simulations as it removes the
Courant condition plaguing explicit electromagnetic algorithms and can be
implemented as a straightforward modification of electrostatic algorithms. The
algorithm described here avoids iterative schemes needed for the divergence
cleaning and represents a fast and efficient solver, which can be used in fluid
and kinetic models for self-consistent description of technical plasmas
exhibiting certain electromagnetic activity
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