1,364 research outputs found
Multiorbital Spin Susceptibility in a Magnetically Ordered State - Orbital versus Excitonic Spin Density Wave Scenario
We present a general theory of multiorbital spin waves in magnetically
ordered metallic systems. Motivated by the itinerant magnetism of iron-based
superconductors, we compare the magnetic excitations for two different
scenarios: when the magnetic order either sets in on the on-site orbital level;
or when it appears as an electron-hole pairing between different bands of
electron and hole character. As an example we treat the two-orbital model for
iron-based superconductors. For small magnetic moments the spin excitations
look similar in both scenarios. Going to larger interactions and larger
magnetic moments, the difference between both scenarios becomes striking. While
in the excitonic scenario the spin waves form a closed structure over the
entire Brillouin zone and the particle-hole continuum is gapped, the spin
excitations in the orbital scenario can be treated as spin waves only in a
close vicinity to the ordering momenta. The origin of this is a gapless
electronic structure with Dirac cones which is a source of large damping. We
analyze our results in connection with recent neutron scattering measurements
and show that certain features of the orbital scenario with multiple order
parameters can be observed experimentally.Comment: 12 pages, 7 figure
Spin excitations in layered antiferromagnetic metals and superconductors
The proximity of antiferromagnetic order in high-temperature superconducting
materials is considered a possible clue to the electronic excitations which
form superconducting pairs. Here we study the transverse and longitudinal spin
excitation spectrum in a one-band model in the pure spin density wave (SDW)
state and in the coexistence state of SDW and the superconductivity. We start
from a Stoner insulator and study the evolution of the spectrum with doping,
including distinct situations with only hole pockets, with only electron
pockets and with pockets of both types. In addition to the usual spin-wave
modes, in the partially gapped cases we find significant weight of low-energy
particle-hole excitations. We discuss the implications of our findings for
neutron scattering experiments and for theories of Cooper-pairing in the
metallic SDW state.Comment: (14 pages, 6 figures
Quasiparticle interference in iron-based superconductors
We systematically calculate quasiparticle interference (QPI) signatures for
the whole phase diagram of iron-based superconductors. Impurities inherent in
the sample together with ordered phases lead to distinct features in the QPI
images that are believed to be measured in spectroscopic imaging-scanning
tunneling microscopy (SI-STM). In the spin-density wave phase the rotational
symmetry of the electronic structure is broken, signatures of which are also
seen in the coexistence regime with both superconducting and magnetic order. In
the superconducting regime we show how the different scattering behavior for
magnetic and non-magnetic impurities allows to verify the symmetry of
the order parameter. The effect of possible gap minima or nodes is discussed.Comment: 19 pages, 7 figure
Dynamical spin susceptibility and the resonance peak in the pseudogap region of the underdoped cuprate superconductors
We present a study of the dynamical spin susceptibility in the pseudogap
region of the high-T cuprate superconductors. We analyze and compare the
formation of the so-called resonance peak, in three different ordered states:
the -wave superconducting (DSC) phase, the -density wave (DDW)
state, and a phase with coexisting DDW and DSC order. An analysis of the
resonance's frequency and momentum dependence in all three states reveals
significant differences between them. In particular, in the DDW state, we find
that a nearly dispersionless resonance excitation exists only in a narrow
region around . At the same time, in the coexisting DDW and
DSC state, the dispersion of the resonance peak near is significantly
changed from that in the pure DSC state. Away from , however, we
find that the form and dispersion of the resonance excitation in the coexisting
DDW and DSC state and pure DSC state are quite similar. Our results demonstrate
that a detailed experimental measurement of the resonance's dispersion allows
one to distinguish between the underlying phases - a DDW state, a DSC state, or
a coexisting DDW and DSC state - in which the resonance peak emerges.Comment: 9 pages, 9 figure
Possible isotope effect on the resonance peak formation in high-T cuprates
Starting from the three-band Hubbard Hamiltonian we derive an effective
model including electron-phonon interaction of quasiparticles with
optical phonons. Within the effective Hamiltonian we analyze the influence of
electronic correlations and electron-phonon interaction on the dynamical spin
susceptibility in layered cuprates. We find a huge isotope effect on the
resonance peak in the magnetic spin susceptibility, ,
seen by inelastic neutron scattering. It results from both the electron-phonon
coupling and the electronic correlation effects taken into account beyond
random phase approximation(RPA) scheme. We find at optimal doping the isotope
coeffiecient which can be further tested
experimentally.Comment: revised version, new figure is added. Phys. Rev. B 69, 0945XX (2004);
in pres
Dynamical charge susceptibility in layered cuprates: the influence of screened inter-site Coulomb repulsion
The analytical expression for dynamical charge susceptibility in layered
cuprates has been derived in the frame of singlet-correlated band model beyond
random-phase-approximation (RPA) scheme. Our calculations performed near
optimal doping regime show that there is a peak in real part of the charge
susceptibility at {\bf Q} = (, ) at strong
enough inter-site Coulomb repulsion. Together with the strong maximum in the Im
at 15 meV it confirms the formation of low-energetic
plasmons or charge fluctuations. This provides a jsutification that these
excitations are important and together with a spin flcutuations can contribute
to the Cooper pairing in layered cuprates. Analysing the charge susceptibilitiy
with respect to an instability we obtain a new plasmon branch, , along the Brillouin Zone. In particular, we have found that it goes to
zero near {\bf Q}
Unconventional superconductivity and magnetism in SrRuO and related materials
We review the normal and superconducting state properties of the
unconventional triplet superconductor SrRuO with an emphasis on the
analysis of the magnetic susceptibility and the role played by strong
electronic correlations. In particular, we show that the magnetic activity
arises from the itinerant electrons in the Ru -orbitals and a strong
magnetic anisotropy occurs () due to spin-orbit
coupling. The latter results mainly from different values of the -factor for
the transverse and longitudinal components of the spin susceptibility (i.e. the
matrix elements differ). Most importantly, this anisotropy and the presence of
incommensurate antiferromagnetic and ferromagnetic fluctuations have strong
consequences for the symmetry of the superconducting order parameter. In
particular, reviewing spin fluctuation-induced Cooper-pairing scenario in
application to SrRuO we show how p-wave Cooper-pairing with line nodes
between neighboring RuO-planes may occur.
We also discuss the open issues in SrRuO like the influence of
magnetic and non-magnetic impurities on the superconducting and normal state of
SrRuO. It is clear that the physics of triplet superconductivity in
SrRuO is still far from being understood completely and remains to be
analyzed more in more detail. It is of interest to apply the theory also to
superconductivity in heavy-fermion systems exhibiting spin fluctuations.Comment: short review article. Annalen der Physik, vol. 13 (2004), to be
publishe
Superconducting phase diagram of itinerant antiferromagnets
We study the phase diagram of the Hubbard model in the weak-coupling limit
for coexisting spin-density-wave order and spin-fluctuation-mediated
superconductivity. Both longitudinal and transverse spin fluctuations
contribute significantly to the effective interaction potential, which creates
Cooper pairs of the quasi-particles of the antiferromagnetic metallic state. We
find a dominant -wave solution in both electron- and hole-doped
cases. In the quasi-spin triplet channel, the longitudinal fluctuations give
rise to an effective attraction supporting a -wave gap, but are overcome by
repulsive contributions from the transverse fluctuations which disfavor
-wave pairing compared to . The sub-leading pair instability is
found to be in the -wave channel, but complex admixtures of and are
not energetically favored since their nodal structures coincide. Inclusion of
interband pairing, in which each fermion in the Cooper pair belongs to a
different spin-density-wave band, is considered for a range of electron dopings
in the regime of well-developed magnetic order. We demonstrate that these
interband pairing gaps, which are non-zero in the magnetic state, must have the
same parity under inversion as the normal intraband gaps. The self-consistent
solution to the full system of five coupled gap equations give intraband and
interband pairing gaps of structure and similar gap magnitude. In
conclusion, the gap dominates for both hole and electron doping
inside the spin-density-wave phase.Comment: 14 pages, 9 figure
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