855 research outputs found
Resonant Excitation of Shear Alfv\'en Perturbations by Trapped Energetic Ions in a Tokamak
A new analytic expression is derived for the resonant drive of high n
Alfvenic modes by particles accelerated to high energy by Ion Cyclotron
Resonance Heating. This derivation includes finite orbit effects, and the
formalism is completely non-perturbative. The high-n limit is used to calculate
the complex particle response integrals along the orbits explicitly. This new
theory is applied to downward sweeping Alfven Cascade quasimodes completing the
theory of these modes, and making testable predictions. These predictions are
found to be consistent with experiments carried out on the Joint European Torus
[P. H. Rebut and B. E. Keen, Fusion Technol. 11, 13 (1987)].Comment: 31 pages, 6 figure
Optical-conductivity sum rule in cuprates and unconventional charge density waves: a short review
We begin with an overview of the experimental results for the temperature and
doping dependences of the optical-conductivity spectral weight in cuprate
superconductors across the whole phase diagram. Then we discuss recent attempts
to explain the observed behavior of the spectral weight using reduced and full
models with unconventional charge-density waves.Comment: 17 pages, RevTeX4, 4 EPS figures; Invited paper for a special issue
of Low Temperature Physics dedicated to the 20th anniversary of HTS
Alfvén Eigenmodes in shear reversed plasmas
Experiments on JT-60U and JET have shown that plasma configurations with shear reversal are prone to the excitation of unusual Alfvén eigenmodes by energetic particles. These modes emerge outside the TAE frequency gap, where one might expect them to be strongly damped. The modes often appear in bunches and they exhibit a quasi-periodic pattern of predominantly upward frequency sweeping (Alfvén Cascades) as the safety factor q changes in time. This work presents a theory that explains the key features of the observed unusual modes including their connection to TAE’s as well as the modifications of TAE’s themselves near the shear reversal point. The developed theory has been incorporated into a reduced numerical model and verified with full geometry codes. JET experimental data on Alfvén spectroscopy have been simulated to infer the mode numbers and the evolution of qmin in the discharge. This analysis confirms the values of q that characterize the internal transport barrier triggering in reversed shear plasmas
The a-axis optical conductivity of detwinned ortho-II YBa_2Cu_3O_6.50
The a-axis optical properties of a detwinned single crystal of
YBa_2Cu_3O_6.50 in the ortho II phase (Ortho II Y123, T_c= 59 K) were
determined from reflectance data over a wide frequency range (70 - 42 000
cm^-1) for nine temperature values between 28 and 295 K. Above 200 K the
spectra are dominated by a broad background of scattering that extends to 1 eV.
Below 200 K a shoulder in the reflectance appears and signals the onset of
scattering at 400 cm^-1. In this temperature range we also observe a peak in
the optical conductivity at 177 cm^-1. Below 59 K, the superconducting
transition temperature, the spectra change dramatically with the appearance of
the superconducting condensate. Its spectral weight is consistent, to within
experimental error, with the Ferrell-Glover-Tinkham (FGT) sum rule. We also
compare our data with magnetic neutron scattering on samples from the same
source that show a strong resonance at 31 meV. We find that the scattering
rates can be modeled as the combined effect of the neutron resonance and a
bosonic background in the presence of a density of states with a pseudogap. The
model shows that the decreasing amplitude of the neutron resonance with
temperature is compensated for by an increasing of the bosonic background
yielding a net temperature independent scattering rate at high frequencies.
This is in agreement with the experiments.Comment: 13 pages 16 figure
On the universal AC optical background in graphene
The latest experiments have confirmed the theoretically expected universal
value of the ac conductivity of graphene and have revealed
departures of the quasiparticle dynamics from predictions for the Dirac
fermions in idealized graphene. We present analytical expressions for the ac
conductivity in graphene which allow one to study how it is affected by
interactions, temperature, external magnetic field and the opening of a gap in
the quasiparticle spectrum. We show that the ac conductivity of graphene does
not necessarily give a metrologically accurate value of the von Klitzing
constant , because it is depleted by the electron-phonon interaction. In
a weak magnetic field the ac conductivity oscillates around the universal value
and the Drude peak evolves into a peak at the cyclotron frequency.Comment: 18 pages, 4 figures; v2: to match New J. Phys. (Focus on Graphene
issue
Deformation quantization of linear dissipative systems
A simple pseudo-Hamiltonian formulation is proposed for the linear
inhomogeneous systems of ODEs. In contrast to the usual Hamiltonian mechanics,
our approach is based on the use of non-stationary Poisson brackets, i.e.
corresponding Poisson tensor is allowed to explicitly depend on time. Starting
from this pseudo-Hamiltonian formulation we develop a consistent deformation
quantization procedure involving a non-stationary star-product and an
``extended'' operator of time derivative , differentiating
the -product. As in the usual case, the -algebra of physical
observables is shown to admit an essentially unique (time dependent) trace
functional . Using these ingredients we construct a complete and
fully consistent quantum-mechanical description for any linear dynamical system
with or without dissipation. The general quantization method is exemplified by
the models of damped oscillator and radiating point charge.Comment: 14 pages, typos correcte
Ward identity and optical-conductivity sum rule in the d-density wave state
We consider the role of the Ward identity in dealing with the transport
properties of an interacting system forming a d-wave modulated charge-density
wave or staggered flux phase. In particular, we address this issue from the
point of view of the restricted optical-conductivity sum rule. Our aim is to
provide a controlled approximation for the current-current correlation function
which allows us also to determine analytically the corresponding sum rule. By
analyzing the role of the vertex functions in both the microscopic interacting
model and in the effective mean-field Hamiltonian, we propose a non-standard
low-energy sum-rule for this system. We also discuss the possible applicability
of these results for the description of cuprate superconductors in the
pseudogap regime.Comment: Revised version, accepted for publication in Phys. Rev.
Neutron Resonance Spectroscopy of 117Sn from1 eV to 1.5 keV
Parity violation has been studied recently for neutron resonances in 117Sn. The neutron resonance spectroscopy is essential for the analysis of the parity violation data. We have measured neutron resonances in 117Sn for neutron energies from 1 to 1500 eV using the time-of-flight method and the (n,γ) reaction. The sample was enriched to 87.6% 117Sn. Neutron scattering and radiative widths were determined, and orbital angular momentum assignments were made with a Bayesian analysis. The s-wave and p-wave strength functions and average level spacings were determined
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