1,277 research outputs found
Fermi-liquid based theory for the in-plane magnetic anisotropy in untwinned high-T superconductors
Using a generalized RPA-type theory we calculate the in-plane anisotropy of
the magnetic excitations in hole-doped high- superconductors. Extending
our earlier Fermi-liquid based studies on the resonance peak by inclusion of
orthorhombicity we still find two-dimensional spin excitations, however, being
strongly anisotropic. This reflects the underlying anisotropy of the hopping
matrix elements and of the resultant superconducting gap function. We compare
our calculations with new experimental data on {\it fully untwinned}
and find good agreement. Our results are in contrast
to earlier interpretations on the in-plane anisotropy in terms of stripes (H.
Mook {\it et al.}, Nature {\bf 404}, 729 (2000)), but reveal a conventional
solution to this important problem.Comment: 5 pages, 6 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
Collective magnetic excitations of symmetric magnetic states in iron-based superconductors
We study the collective magnetic excitations of the recently discovered
symmetric spin-density wave states of iron-based superconductors with
particular emphasis on their orbital character based on an itinerant
multiorbital approach. This is important since the symmetric
spin-density wave states exist only at moderate interaction strengths where
damping effects from a coupling to the continuum of particle-hole excitations
strongly modifies the shape of the excitation spectra compared to predictions
based on a local moment picture. We uncover a distinct orbital polarization
inherent to magnetic excitations in symmetric states, which provide a
route to identify the different commensurate magnetic states appearing in the
continuously updated phase diagram of the iron-pnictide family.Comment: 5+7 pages, 3+2 figure
Evidence for the formation of magnetic moments in the cuprate superconductor HgCuBaCaCuO below seen by NQR
We report pure zero field nuclear magnetic resonance (NQR) measurements on
the optimally doped three layer high--compounds HgBaCaCuO and
HgBaCaCuO(F) with 134 K. Above two Cu NQR line pairs are
observed in the spectra corresponding to the two inequivalent Cu lattice sites.
Below the Cu NQR spectra show additional lines leading to the extreme
broadened Cu NQR spectra at 4.2 K well known for the HgBaCaCuO compounds. The
spin-lattice relaxation curves follow a triple exponential function with
coefficients depend onto the saturation time (number of saturation pulses),
whereas the spin-spin relaxation curve is described by a single exponential
function. From the spin-lattice relaxation we deduced a complete removal of the
Kramers degeneracy of the Cu quadrupole indicating that the additional lines
are due to a Zeemann splitting of the Cu lines due to the spontaneous
formation of magnetic moments within the CuO layers. Below 140 K, the spectra
are well fitted by a number of 6 Cu line pairs. From the number of
the Cu lines, the position of the lines relative to each other and the complete
removal of the Kramers degeneracy we deduced an orientation of the magnetic
moments parallel to the symmetry axis of the electric field gradient tensor
with magnitudes of the order of 1000 G. We also discuss the possible
microscopic origin of the observed internal magnetic fields.Comment: 11 pages, 12 figure
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
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