1,385 research outputs found
Nernst effect in the electron-doped cuprates
We calculate the normal state Nernst signal in the cuprates resulting from a
reconstruction of the Fermi surface due to spin density wave order. An order
parameter consistent with the reconstruction of the Fermi surface detected in
electron-doped materials is shown to sharply enhance the Nernst signal close to
optimal doping. Within a semiclassical treatment, the obtained magnitude and
position of the enhanced Nernst signal agrees with Nernst measurements in
electron-doped cuprates.Comment: 9 pages, 5 figures, revised version as accepted by Phys. Rev. B,
changed several citations and reference
Full- & Reduced-Order State-Space Modeling of Wind Turbine Systems with Permanent-Magnet Synchronous Generator
Wind energy is an integral part of nowadays energy supply and one of the
fastest growing sources of electricity in the world today. Accurate models for
wind energy conversion systems (WECSs) are of key interest for the analysis and
control design of present and future energy systems. Existing control-oriented
WECSs models are subject to unstructured simplifications, which have not been
discussed in literature so far. Thus, this technical note presents are thorough
derivation of a physical state-space model for permanent magnet synchronous
generator WECSs. The physical model considers all dynamic effects that
significantly influence the system's power output, including the switching of
the power electronics. Alternatively, the model is formulated in the -
and -reference frame. Secondly, a complete control and operation
management system for the wind regimes II and III and the transition between
the regimes is presented. The control takes practical effects such as input
saturation and integral windup into account. Thirdly, by a structured model
reduction procedure, two state-space models of WECS with reduced complexity are
derived: a non-switching model and a non-switching reduced-order model. The
validity of the models is illustrated and compared through a numerical
simulation study.Comment: 23 pages, 11 figure
Superconducting Gap and Pseudogap in Bi-2212
We present results of Raman scattering experiments in differently doped
Bi-2212 single crystals. Below Tc the spectra show pair-breaking features in
the whole doping range. The low frequency power laws confirm the existence of a
-wave order parameter. In the normal state between Tc and T* =
200K we find evidence for a pseudogap in B2g symmetry. Upon doping its effect
on the spectra decreases while its energy scale appears to be unchanged.Comment: 2 pages, 1 EPS figure; LT22 Proceedings to appear in Physica
Signatures of nematic quantum critical fluctuations in the Raman spectra of lightly doped cuprates
We consider the lightly doped cuprates YCaBaCuO
and LaSrCuO (with ,0.04), where the presence of a
fluctuating nematic state has often been proposed as a precursor of the stripe
(or, more generically, charge-density wave) phase, which sets in at higher
doping. We phenomenologically assume a quantum critical character for the
longitudinal and transverse nematic, and for the charge-ordering fluctuations,
and investigate the effects of these fluctuations in Raman spectra. We find
that the longitudinal nematic fluctuations peaked at zero transferred momentum
account well for the anomalous Raman absorption observed in these systems in
the channel, while the absence of such effect in the channel
may be due to the overall suppression of Raman response at low frequencies,
associated with the pseudogap. While in YCaBaCuO the
low-frequency lineshape is fully accounted by longitudinal nematic collective
modes alone, in LaSrCuO also charge-ordering modes with finite
characteristic wavevector are needed to reproduce the shoulders observed in the
Raman response. This different involvement of the nearly critical modes in the
two materials suggests a different evolution of the nematic state at very low
doping into the nearly charge-ordered state at higher doping.Comment: 12 pages with 10 figures, to appear in Phys. Rev. B 201
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