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 NbSe3_3

The two charge-density wave (CDW) transitions in NbSe$_3$ %at wave numbers at $\bm{q_1}$ and $\bm{q_2}$, occurring at the surface were investigated by scanning tunneling microscopy (STM) on \emph{in situ} cleaved $(\bm{b},\bm{c})$ 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$_s$). The low T CDW transition occurs at T$_{2s}$=70-75 K, more than 15 K above the bulk T$_{2b}=59$K 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 $N$ behavior of the product $\lambda\_m(N) \lambda\_M(N)$ of non null smallest positive and largest eigenvalues, we infer the inequality $\delta\_N(Q) \Delta\_N(Q) = \sigma\_N \overset{<}{\underset{N \to \infty}{\to}} 2 \pi$ (resp. $\delta\_N(P) \Delta\_N(P) = \sigma\_N \overset{<}{\underset{N \to \infty}{\to}} 2 \pi$) involving, in suitable units, the minimal ($\delta\_N(Q)$) and maximal ($\Delta\_N(Q)$) 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