1,105 research outputs found
Gyrokinetic and kinetic particle-in-cell simulations of guide-field reconnection. I: Macroscopic effects of the electron flows
In this work, we compare gyrokinetic (GK) and fully kinetic Particle-in-Cell
(PIC) simulations of magnetic reconnection in the limit of strong guide field.
In particular, we analyze the limits of applicability of the GK plasma model
compared to a fully kinetic description of force free current sheets for finite
guide fields (). Here we report the first part of an extended comparison,
focusing on the macroscopic effects of the electron flows. For a low beta
plasma (), it is shown that both plasma models develop magnetic
reconnection with similar features in the secondary magnetic islands if a
sufficiently high guide field () is imposed in the kinetic PIC
simulations. Outside of these regions, in the separatrices close to the X
points, the convergence between both plasma descriptions is less restrictive
(). Kinetic PIC simulations using guide fields
reveal secondary magnetic islands with a core magnetic field and less energetic
flows inside of them in comparison to the GK or kinetic PIC runs with stronger
guide fields. We find that these processes are mostly due to an initial shear
flow absent in the GK initialization and negligible in the kinetic PIC high
guide field regime, in addition to fast outflows on the order of the ion
thermal speed that violate the GK ordering. Since secondary magnetic islands
appear after the reconnection peak time, a kinetic PIC/GK comparison is more
accurate in the linear phase of magnetic reconnection. For a high beta plasma
() where reconnection rates and fluctuations levels are reduced,
similar processes happen in the secondary magnetic islands in the fully kinetic
description, but requiring much lower guide fields ().Comment: 18 pages, 13 figures. Revised to match with the published version in
Physics of Plasma
Magnetism of the LTT phase of Eu doped La_{2-x}Sr_xCuO_4
The ESR signal of Gd spin probes (0.5 at %) as well as the static normal
state susceptibility of Eu (J(Eu^{3+})=0) doped La_{2-x-y}Sr_xEu_yCuO_4 reveal
pronounced changes of the Cu magnetism at the structural transition from the
orthorhombic to the low temperature tetragonal phase for all
non-superconducting compositions. Both a jumplike decrease of \chi as well as
the ESR data show an increase of the in-plane magnetic correlation length in
the LTT phase. From the Gd^{3+} ESR linewidth we find that for specific Eu and
Sr concentrations in the LTT phase the correlation length increases up to more
than 100 lattice constants and the fluctuation frequency of the CuO_2 spin
system slows down to 10^{10}- 10^{11}sec^{-1}. However, there is no static
order above T ~ 8K in contrast to the LTT phase of Nd doped La_{2-x}Sr_xCuO_4
with pinned stripe correlations.Comment: 7 pages, RevTex, 3 eps figures. To appear in the Proceedings of the
International Conference "Stripes, Lattice Instabilities and High Tc
Superconductivity", (Rome, Dec. 1996
TaIrTe4 a ternary Type-II Weyl semi-metal
In metallic condensed matter systems two different types of Weyl fermions can
in principle emerge, with either a vanishing (type-I) or with a finite
(type-II) density of states at the Weyl node energy. So far only WTe2 and MoTe2
were predicted to be type-II Weyl semi-metals. Here we identify TaIrTe4 as a
third member of this family of topological semi-metals. TaIrTe4 has the
attractive feature that it hosts only four well-separated Weyl points, the
minimum imposed by symmetry. Moreover, the resulting topological surface states
- Fermi arcs connecting Weyl nodes of opposite chirality - extend to about 1/3
of the surface Brillouin zone. This large momentum-space separation is very
favorable for detecting the Fermi arcs spectroscopically and in transport
experiments
Why of (CaFeAs)PtAs is twice as high as (CaFePtAs)PtAs
Recently discovered (CaFePtAs)PtAs and
(CaFeAs)PtAs superconductors are very similar materials
having the same elemental composition and structurally similar superconducting
FeAs slabs. Yet the maximal critical temperature achieved by changing Pt
concentration is approximately twice higher in the latter. Using angle-resolved
photoemission spectroscopy(ARPES) we compare the electronic structure of their
optimally doped compounds and find drastic differences. Our results highlight
the sensitivity of critical temperature to the details of fermiology and point
to the decisive role of band-edge singularities in the mechanism of high-
superconductivity
Phonon Anomalies, Orbital-Ordering and Electronic Raman Scattering in iron-pnictide Ca(Fe0.97Co0.03)2As2: Temperature-dependent Raman Study
We report inelastic light scattering studies on Ca(Fe0.97Co0.03)2As2 in a
wide spectral range of 120-5200 cm-1 from 5K to 300K, covering the tetragonal
to orthorhombic structural transition as well as magnetic transition at Tsm ~
160K. The mode frequencies of two first-order Raman modes B1g and Eg, both
involving displacement of Fe atoms, show sharp increase below Tsm.
Concomitantly, the linewidths of all the first-order Raman modes show anomalous
broadening below Tsm, attributed to strong spin-phonon coupling. The high
frequency modes observed between 400-1200 cm-1 are attributed to the electronic
Raman scattering involving the crystal field levels of d-orbitals of Fe2+. The
splitting between xz and yz d-orbital levels is shown to be ~ 25 meV which
increases as temperature decreases below Tsm. A broad Raman band observed at ~
3200 cm-1 is assigned to two-magnon excitation of the itinerant Fe 3d
antiferromagnet.Comment: Accepted for Publication in JPC
Plasmons and Interband Transitions of CaSrCuO investigated by Electron Energy-Loss Spectroscopy
Electron energy-loss spectroscopy studies have been performed in order to get
a deeper insight into the electronic structure and elementary excitations of
the two-leg ladder system CaSrCuO. We find a strong
anisotropy of the loss function for momentum transfers along the a and
c-crystallographic axis, and a remarkable linear plasmon dispersion for a
momentum transfer parallel to the legs of the ladders. The investigated
spectral features are attributed to localized and delocalized charge-transfer
excitations and the charge carrier plasmon. The charge carrier plasmon position
and dispersion in the long wave-length limit agree well with expectations based
upon the band structure of the two-leg ladder, while the observed quasi-linear
plasmon dispersion might be related to the peculiar properties of underdoped
cuprates in general.Comment: 16 pages, 8 figure
First measurements of the index of refraction of gases for lithium atomic waves
We report here the first measurements of the index of refraction of gases for
lithium waves. Using an atom interferometer, we have measured the real and
imaginary part of the index of refraction for argon, krypton and xenon, as
a function of the gas density for several velocities of the lithium beam. The
linear dependence of with the gas density is well verified. The total
collision cross-section deduced from the imaginary part is in very good
agreement with traditional measurements of this quantity. Finally, as predicted
by theory, the real and imaginary parts of and their ratio
exhibit glory oscillations
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