1,635 research outputs found
Caractérisation des anneaux noethériens de séries formelles à croissance controlée. Application à la synthèse spectrale
Given a subring of the ring of formal power series defined by the growth of the coefficients, we prove a necessary and sufficient condition for it to be a noetherian ring. As a particular case, we show that the ring of Gevrey power series is a noetherian ring. Then, we get a spectral synthesis theorem for some classes of ultradifferentiable functions
Nonlinear lattice dynamics as a basis for enhanced superconductivity in YBa2Cu3O6.5
THz-frequency optical pulses can resonantly drive selected vibrational modes
in solids and deform their crystal structure. In complex oxides, this method
has been used to melt electronic orders, drive insulator to metal transitions
or induce superconductivity. Strikingly, coherent interlayer transport strongly
reminiscent of superconductivity can be transiently induced up to room
temperature in YBa2Cu3O6+x. By combining femtosecond X-ray diffraction and ab
initio density functional theory calculations, we determine here the crystal
structure of this exotic non-equilibrium state. We find that nonlinear lattice
excitation in normal-state YBa2Cu3O6+x at 100 K causes a staggered
dilation/contraction of the Cu-O2 intra/inter- bilayer distances, accompanied
by anisotropic changes in the in-plane O-Cu-O bond buckling. Density functional
theory calculations indicate that these motions cause dramatic changes in the
electronic structure. Amongst these, the enhancement in the dx2-y2 character of
the in-plane electronic structure is likely to favor superconductivity.Comment: 28 pages, including Supplemen
Diffraction based Hanbury Brown and Twiss interferometry performed at a hard x-ray free-electron laser
We demonstrate experimentally Hanbury Brown and Twiss (HBT) interferometry at
a hard X-ray Free Electron Laser (XFEL) on a sample diffraction patterns. This
is different from the traditional approach when HBT interferometry requires
direct beam measurements in absence of the sample. HBT analysis was carried out
on the Bragg peaks from the colloidal crystals measured at Linac Coherent Light
Source (LCLS). We observed high degree (80%) spatial coherence of the full beam
and the pulse duration of the monochromatized beam on the order of 11 fs that
is significantly shorter than expected from the electron bunch measurements.Comment: 32 pages, 10 figures, 2 table
Photoinduced suppression of the ferroelectric instability in PbTe
The interactions between electrons and phonons drive a large array of
technologically relevant material properties including ferroelectricity,
thermoelectricity, and phase-change behaviour. In the case of many group IV-VI,
V, and related materials, these interactions are strong and the materials exist
near electronic and structural phase transitions. Their close proximity to
phase instability produces a fragile balance among the various properties. The
prototypical example is PbTe whose incipient ferroelectric behaviour has been
associated with large phonon anharmonicity and thermoelectricity. Experimental
measurements on PbTe reveal anomalous lattice dynamics, especially in the soft
transverse optical phonon branch. This has been interpreted in terms of both
giant anharmonicity and local symmetry breaking due to off-centering of the Pb
ions. The observed anomalies have prompted renewed theoretical and
computational interest, which has in turn revived focus on the extent that
electron-phonon interactions drive lattice instabilities in PbTe and related
materials. Here, we use Fourier-transform inelastic x-ray scattering (FT-IXS)
to show that photo-injection of free carriers stabilizes the paraelectric
state. With support from constrained density functional theory (CDFT)
calculations, we find that photoexcitation weakens the long-range forces along
the cubic direction tied to resonant bonding and incipient ferroelectricity.
This demonstrates the importance of electronic states near the band edges in
determining the equilibrium structure.Comment: 9 page, 3 figure
How efficient are coronal mass ejections at accelerating solar energetic particles?
The largest solar energetic particle (SEP) events are thought to be due to particle acceleration at a shock driven by a fast coronal mass ejection (CME). We investigate the efficiency of this process by comparing the total energy content of energetic particles with the kinetic energy of the associated CMEs. The energy content of 23 large SEP events from 1998 through 2003 is estimated based on data from ACE, GOES, and SAMPEX, and interpreted using the results of particle transport simulations and inferred longitude distributions. CME data for these events are obtained from SOHO. When compared to the estimated kinetic energy of the associated coronal mass ejections (CMEs), it is found that large SEP events can extract ~10% or more of the CME kinetic energy. The largest SEP events appear to require massive, very energetic CMEs
Using Open Source Libraries in the Development of Control Systems Based on Machine Vision
The possibility of the boundaries detection in the images of crushed ore particles using a convolutional neural network is analyzed. The structure of the neural network is given. The construction of training and test datasets of ore particle images is described. Various modifications of the underlying neural network have been investigated. Experimental results are presented. © 2020, IFIP International Federation for Information Processing.Foundation for Assistance to Small Innovative Enterprises in Science and Technology, FASIEFunding. The work was performed under state contract 3170ΓC1/48564, grant from the FASIE
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