1,260 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
Solving Boundary Value Problem for a Nonlinear Stationary Controllable System with Synthesizing Control
An algorithm for constructing a control function that transfers a wide class
of stationary nonlinear systems of ordinary differential equations from an
initial state to a final state under certain control restrictions is proposed.
The algorithm is designed to be convenient for numerical implementation. A
constructive criterion of the desired transfer possibility is presented. The
problem of an interorbital flight is considered as a test example and it is
simulated numerically with the presented method.Comment: 9 pages, 2 figure
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
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