99 research outputs found
Electronic crystals: an experimental overview
This article reviews the static and dynamic properties of spontaneous
superstructures formed by electrons. Representations of such electronic
crystals are charge density waves and spin density waves in inorganic as well
as organic low dimensional materials. A special attention is paid to the
collective effects in pinning and sliding of these superstructures, and the
glassy properties at low temperature. Charge order and charge
disproportionation which occur in organic materials resulting from correlation
effects are analysed. Experiments under magnetic field, and more specifically
field-induced charge density waves are discussed. Properties of meso- and
nanostructures of charge density waves are also reviewed.Comment: 255 pages, 247 figure
Surface Charge Density Wave Transition in NbSe
The two charge-density wave (CDW) transitions in NbSe %at wave numbers at
and , occurring at the surface were investigated by
scanning tunneling microscopy (STM) on \emph{in situ} cleaved
plane. The temperature dependence of first-order CDW satellite spots, obtained
from the Fourier transform of the STM images, was measured between 5-140 K to
extract the surface critical temperatures (T). The low T CDW transition
occurs at T=70-75 K, more than 15 K above the bulk TK while at
exactly the same wave number. %determined by x-ray diffraction experiments.
Plausible mechanism for such an unusually high surface enhancement is a
softening of transverse phonon modes involved in the CDW formation.% The large
interval of the 2D regime allows to speculate on % %the special
Berezinskii-Kosterlitz-Thouless type of the surface transition expected for
this incommensurate CDW. This scenario is checked by extracting the temperature
dependence of the order % %parameter correlation functions. The regime of 2D
fluctuations is analyzed according to a Berezinskii-Kosterlitz-Thouless type of
surface transition, expected for this incommensurate 2D CDW, by extracting the
temperature dependence of the order parameter correlation functions.Comment: 5 pages, 2 figure
Ni–W diffusion barrier: Its influence on the oxidation behaviour of a β-(Ni,Pt)Al coated fourth generation nickel-base superalloy
A Ni–W base diffusion barrier (DB) has been developed to limit interdiffusion between a fourth generation Ni-base superalloy (MCNG) and a Pt-modified nickel aluminide bondcoat. After long term oxidation, the DB layer permits to reduce the Al depletion in the coating and to delay the phase transformations in the coating. But despite this result, the oxidation behaviour of the system with DB is slightly worse than without the DB. This difference may be caused by the addition of S and/orWin the coating of the system with the DB. The DB layer also delays the Secondary Reaction Zone (SRZ) formation. Nevertheless, the propagation of the SRZ is similar in systems with and without a DB, with growth kinetics which are driven by interdiffusion
Effect of platinum on the growth rate of the oxide scale formed on cast nickel aluminide intermetallic alloys
Thermo-Gravimetric Analysis ( TGA) and Scanning Electron M icroscopy ( SEM ) data of the initial stages of oxidation of Ni50Al50 and Ni40Pt10Al50 alloys of low and high sulfur content at 900◦ C and 1100◦ C are reported. The results show that the addition of Pt promotes the growth of the transient θ- Al2O3 oxide scale. This effect is particularly sensitive in the initial stages of oxidation at 1100◦ C where Pt considerably increases the total mass gain. It is attenuated in the presence of a high sulfur content in the alloy, indicating a competitive effect of Pt and S on the segregation of Al. The slower θ-to- α transition observed in the presence of Pt leads to an extended lifetime of the θ phase layer, which is proposed to be benecial to the relaxation of the stresses created by the growth of α-Al2O3
Spontaneous Breaking of Isotropy Observed in the Electronic Transport of Rare-Earth Tritellurides
International audienceWe show that the isotropic conductivity in the normal state of rare-earth tritelluride RTe3 compounds is broken by the occurrence of theunidirectional charge density wave (CDW) in the (a, c) plane below the Peierls transition temperature. In contrast with quasi-one-dimensional systems, the resistivity anomaly associated with the CDW transition is strong in the direction perpendicular to the CDW wave vector Q (a axis) and very weak in the CDW wave vector Q direction (c axis). We qualitatively explain this result by calculating the electrical conductivity for the electron dispersion with momentum-dependent CDW gap as determined by angle-resolved photoemission spectroscopy. Similar measurements of in-plane conductivity may uncover the gap anisotropy in other compounds for which angle-resolved photoemission spectroscopy is not available
(93)Nb NMR spin echo spectroscopy in single crystal NbSe(3)
International audienceWe report electric field induced phase displacements of the charge density wave (CDW) in a single crystal of NbSe(3) using (93)Nb NMR spin-echo spectroscopy. CDW polarizations in the pinned state induced by unipolar and bipolar pulses are linear and reversible up to at least E = (0.96)E(T). The polarizations have a broad distribution extending up to phase angles of order 60 degrees for electric fields close to threshold. No evidence for polarizations in excess of a CDW wavelength or for a divergence in polarization near ET are observed. The results are consistent with elastic depinning models, provided that the critical regime expected in large systems is not observable
Sol–gel thermal barrier coatings: Optimization of the manufacturing route and durability under cyclic oxidation
A new promising and versatile process based on the sol–gel transformation has been developed to deposit yttria-stabilised thermal barrier coatings. The non-oriented microstructure with randomly structured pore network, resulting from the soft chemical process, is expected to show satisfactory thermo-mechanical behaviour when the TBC is cyclically oxidized. First stage of the research consists of optimizing the processing route to generate homogeneous microstructure and controlled surface roughness. The objective is to reduce, as much as possible, the size and depth of the surface cracks network inherent to the process. Indeed, the durability of the TBC when cyclically oxidized strongly depends on the sharpness of those cracks that concentrate thermo-mechanical stresses and generate detrimental propagation resulting in spallation. Cyclic oxidation tests are performed using a cyclic oxidation rig instrumented with CCD cameras to monitor in a real time basis the mechanism of crack propagation and spallation. The impact of various parameters either directly related to the processing route, e.g. the intimate microstructure of the TBC and the TBC thickness, or to the thermal loading, e.g. the oxidation temperature and the cumulated hot time, on the durability of the TBC is investigate
Finite dimensional quantizations of the (q,p) plane : new space and momentum inequalities
We present a N-dimensional quantization a la Berezin-Klauder or frame
quantization of the complex plane based on overcomplete families of states
(coherent states) generated by the N first harmonic oscillator eigenstates. The
spectra of position and momentum operators are finite and eigenvalues are
equal, up to a factor, to the zeros of Hermite polynomials. From numerical and
theoretical studies of the large behavior of the product of non null smallest positive and largest eigenvalues, we infer
the inequality (resp. ) involving, in suitable
units, the minimal () and maximal () sizes of
regions of space (resp. momentum) which are accessible to exploration within
this finite-dimensional quantum framework. Interesting issues on the
measurement process and connections with the finite Chern-Simons matrix model
for the Quantum Hall effect are discussed
Development of a NiW in-situ diffusion barrier on a fourth generation nickel-base superalloy
A diffusion barrier based on a NiW electrolytic coating has been developed to limit interdiffusion between a Ni-base superalloy (MCNG) and a β-NiAl bondcoating. Isothermal oxidation tests of 50h at 1100°C confirmed that W-rich layer formed with NiW coating modifies the oxidation behaviour of the bondcoat and limits interdiffusion. The diffusion barrier reduced β-NiAl γ’-Ni3Al transformation in the bondcoating and prevented SRZ formation
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