852 research outputs found
On the "spin-freezing" mechanism in underdoped superconducting cuprates
The letter deals with the spin-freezing process observed by means of NMR-NQR
relaxation or by muon spin rotation in underdoped cuprate superconductors. This
phenomenon, sometimes referred as coexistence of antiferromagnetic and
superconducting order parameters, is generally thought to result from randomly
distributed magnetic moments related to charge inhomogeneities (possibly
stripes) which exhibit slowing down of their fluctuations on cooling below
T . Instead, we describe the experimental findings as due to fluctuating,
vortex-antivortex, orbital currents state coexisting with d-wave
superconducting state. A direct explanation of the experimental results, in
underdoped YCaBaCuO and LaSrCuO,
is thus given in terms of freezing of orbital current fluctuations
Magnetic degeneracy and hidden metallicity of the spin density wave state in ferropnictides
We analyze spin density wave (SDW) order in iron-based superconductors and
electronic structure in the SDW phase. We consider an itinerant model for
Fe-pnictides with two hole bands centered at and two electron bands
centered at and in the unfolded BZ. A SDW order in such a
model is generally a combination of two components with momenta and
, both yield order in the folded zone. Neutron
experiments, however, indicate that only one component is present. We show that
or order is selected if we assume that only one hole band
is involved in the SDW mixing with electron bands. A SDW order in such 3-band
model is highly degenerate for a perfect nesting and hole-electron interaction
only, but we show that ellipticity of electron pockets and interactions between
electron bands break the degeneracy and favor the desired or
order. We further show that stripe-ordered system remains a metal for
arbitrary coupling. We analyze electronic structure for parameters relevant to
the pnictides and argue that the resulting electronic structure is in good
agreement with ARPES experiments. We discuss the differences between our model
and model of localized spins.Comment: reference list updated, typos are correcte
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}
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
Theory of magnetic excitons in the heavy-fermion superconductor
We analyze the influence of unconventional superconductivity on the magnetic
excitations in the heavy fermion compound UPdAl. We show that it leads
to the formation of a bound state at energies well below 2 at the
antiferromagnetic wave vector {\textbf Q}=. Its signature is a
resonance peak in the spectrum of magnetic excitations in good agreement with
results from inelastic neutron scattering. Furthermore we investigate the
influence of antiferromagnetic order on the formation of the resonance peak. We
find that its intensity is enhanced due to intraband transitions induced by the
reconstruction of Fermi surface sheets. We determine the dispersion of the
resonance peak near {\textbf Q} and show that it is dominated by the magnetic
exciton dispersion associated with local moments. We demonstrate by a
microscopic calculation that UPdAl is another example in which the
unconventional nature of the superconducting order parameter can be probed by
means of inelastic neutron scattering and determined unambiguously.Comment: 6 pages, 4 figure
Stable compounds in the CaO-Al2O3 system at high pressures
Using evolutionary crystal structure prediction algorithm USPEX, we showed
that at pressures of the Earth's lower mantle CaAl2O4 is the only stable
calcium aluminate. At pressures above 7.0 GPa it has the CaFe2O4-type structure
and space group Pnma. This phase is one of prime candidate aluminous phases in
the lower mantle of the Earth. We show that at low pressures 5CaO * 3Al2O3
(C5A3) with space group Cmc21, CaAl4O7 (C2/c) and CaAl2O4 (P21/m) structures
are stable at pressures of up to 2.1, 1.8 and 7.0 GPa respectively. The
previously unknown structure of the orthorhombic 'CA-III' phase is also found
from our calculations. This phase is metastable and has a layered structure
with space group P21212
Probing the Yb spin relaxation in YYbBaCuO by Electron Paramagnetic Resonance
The relaxation of Yb in YBaCuO () was studied
using Electron Paramagnetic Resonance (EPR). It was found that both electronic
and phononic processes contribute to the Yb relaxation. The phononic
part of the relaxation has an exponential temperature dependence, which can be
explained by a Raman process via the coupling to high-energy (500 K)
optical phonons or an Orbach-like process via the excited vibronic levels of
the Cu ions (localized Slonczewski-modes). In a sample with a maximum
oxygen doping =6.98, the electronic part of the relaxation follows a
Korringa law in the normal state and strongly decreases below .
Comparison of the samples with and without Zn doping proved that the
superconducting gap opening is responsible for the sharp decrease of Yb
relaxation in YBaCuO. It was shown that the electronic
part of the Yb relaxation in the superconducting state follows the same
temperature dependence as Cu and O nuclear relaxations despite
the huge difference between the corresponding electronic and nuclear relaxation
rates.Comment: 8 pages, 6 figure
Incommensurate itinerant antiferromagnetic excitations and spin resonance in the FeTeSe superconductor
We report on inelastic neutron scattering measurements that find
incommensurate itinerant like magnetic excitations in the normal state of
superconducting FeTeSe (\Tc=14K) at wave-vector
with =0.09(1). In
the superconducting state only the lower energy part of the spectrum shows
significant changes by the formation of a gap and a magnetic resonance that
follows the dispersion of the normal state excitations. We use a four band
model to describe the Fermi surface topology of iron-based superconductors with
the extended symmetry and find that it qualitatively captures the
salient features of these data.Comment: 7 pages and 5 figure
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