arXiv subjects

Jacques Villain

Publications and source records attributed to Jacques Villain.

7 recordsLinked to original sources

arXiv2018

Jacques Friedel and the physics of metals and alloys

This is an introduction to the theoretical physics of metals for students and physicists from other specialities. Certain simple consequences of the Fermi statistics in pure metals are first addressed, namely the Peierls distortion, Kohn anomalies and the Labbé-Friedel distortion. Then the physics of dilute alloys is discussed. The analogy with nuclear collisions was a fruitful starting point, which suggested one should analyse the effects of impurities in terms of a scattering problem with the introduction of phase shifts. Starting from these concepts, Friedel derived a theory of the resistivity of alloys, and a celebrated sum rule relating the phase shifts at the Fermi level to the number of electrons in the impurity, which turned out to play a prominent role later in the context of correlated impurities, as for instance in the Kondo effect. Friedel oscillations are also an important result, related to incommensurate magnetic structures. It is shown how they can be derived in various ways: from collision theory, perturbation theory, self-consistent approximations and Green's function methods. While collision theory does not permit to take the crystal structure into account, which is responsible for electronic bands, those effects can be included in other descriptions, using for instance the tight binding approximation.

cond-mat.mtrl-sci↗

arXiv2011

In a nanoscience lab

The production, observation and manipulation of very small objects is a tour de force, but these objects, which could infiltrate anywhere without being seen, may arouse suspicion. To assess the situation at best, we describe the activity of a nanoscience research institution, some of the methods used there, the spirit of its researchers and their attitude towards risk.

physics.soc-ph↗

arXiv2009

Symétrie et théorie des groupes à travers la physique

Physical properties of matter are tightly related with the kind of symmetry of the medium. Group theory is a systematic tool, though not always easy to handle, to exploit symmetry properties, for instance to find the eigenvectors and eigenvalues of an operator. Certain properties (optical activity, piezoelectricity...) are forbidden in molecules or crystals of high symmetry. A few theorems (Noether, Goldstone...) establish general relations between physical properties and symmetry. Applications of group theory to condensed matter physics, elementary particle physics, quantum mechanics, electromagnetism are reviewed. Group theory is not only a tool, but also a beautiful construction which casts insight into natural phenomena. ----- Les propriétés de la matière sont liées au type de symétrie qui y règne. La théorie des groupes est un outil systématique, mais pas toujours commode, pour exploiter cette symétrie, notamment quand il faut trouver les vecteurs propres et valeurs propres d'un opérateur. Certaines propriétés (pouvoir rotatoire optique, piézoélectricité...) sont interdites dans des cristaux ou molécules de haute symétrie. Quelques théorèmes (Noether, Goldstone...) énoncent des relations générales entre les propriétés physiques et la symétrie. On passe en revue quelques applications de la théorie des groupes à la physique de la matière condensée, à la physique des particules élémentaires, à la mécanique quantique, à l'électromagnétisme. La théorie des groupes n'est pas seulement un outil, mais aussi une belle construction qui permet de mieux comprendre la nature.

cond-mat.stat-mech↗

arXiv2005

Square root relaxation: two possible mechanisms

Magnetic relaxation in large spin molecular paramagnets is often found to behave as the square root of time at short times t. This behaviour was explained by Prokofiev & Stamp as arising from dipole interactions between molecular spins. However, as observed by Miyashita & Saito, the same behaviour can arise from a different mechanism which, in the present work, is related to hyperfine interactions. The Miyashita-Saito scheme is found to be possible at short times if the nuclear longitudinal spin relaxation is very slow. In the case of moderately slow nuclear spin relaxation, the electronic magnetization variation is initially proportional to t, then to the square root of t and finally to decay exponentially. This behaviour may be mostly expected in dilute systems.

cond-mat.stat-mech↗

arXiv1999

Slow dynamics of stepped surfaces

Two kinds of configurations involving steps on surfaces are reviewed. The first one results from an initially planar vicinal surface, i.e. slightly deviating from a high-symmetry (001) or (111) orientation. In some cases, these surfaces separate into domains of different orientations by a mechanism which is very similar to phase separation in mixtures. The domain size initially increases with time and goes to a finite limit, whose value is related to elastic phenomena. The second kind of configurations results from making grooves in a high-symmetry surface. The surface smoothes out and takes an intermediate shape with facet-like hills and valleys, which are the source of a controversy which we try to clarify as much as possible.

cond-mat.stat-mech↗

arXiv1999

Instabilities in crystal growth by atomic or molecular beams

The planar front of a growing a crystal is often destroyed by instabilities. In the case of growth from a condensed phase, the most frequent ones are diffusion instabilities, which will be but briefly discussed in simple terms in chapter II. The present review is mainly devoted to instabilities which arise in ballistic growth, especially Molecular Beam Epitaxy (MBE). The reasons of the instabilities can be geometric (shadowing effect), but they are mostly kinetic or thermodynamic. The kinetic instabilities which will be studied in detail in chapters IV and V result from the fact that adatoms diffusing on a surface do not easily cross steps (Ehrlich-Schwoebel or ES effect). When the growth front is a high symmetry surface, the ES effect produces mounds which often coarsen in time according to power laws. When the growth front is a stepped surface, the ES effect initially produces a meandering of the steps, which eventually may also give rise to mounds. Kinetic instabilities can usually be avoided by raising the temperature, but this favours thermodynamic instabilities. Concerning these ones, the attention will be focussed on the instabilities resulting from slightly different lattice constants of the substrate and the adsorbate. They can take the following forms. i) Formation of misfit dislocations (chapter VIII). ii) Formation of isolated epitaxial clusters which, at least in their earliest form, are `coherent' with the substrate, i.e. dislocation-free (chapter X). iii) Wavy deformation of the surface, which is presumably the incipient stage of (ii) (chapter IX). The theories and the experiments are critically reviewed and their comparison is qualitatively satisfactory although some important questions have not yet received a complete answer.

cond-mat.mtrl-sci↗

arXiv1997

Rotational Reconstruction of Sapphire (0001)

The structure of the $(\sqrt{31}\times \sqrt{31})R\pm9^\circ$ reconstructed phase on sapphire (0001) surface is investigated by means of a simulation based on the energy minimization. The interaction between Al adatoms is described with the semi-empirical many-body Sutton-Chen potential, corrected for the charge transfer between the metallic overlayer and the substrate. The interactions between the Al adatoms and sapphire substrate are described with a simple three-dimensional potential field which has the hexagonal periodicity of sapphire surface. Our energy analysis gave evidence that the structure which is observed at room temperature is in fact a frozen high-temperature structure. In accordance with the X-ray scattering, a hexagonal domain pattern separated by domain walls has been found. The Al adatoms, distributed in two monolayers, are ordered and isomorphic to metallic Al(111) in the domains and disordered in the domain walls. The main reason for the rotational reconstruction is the lattice misfit between the metallic Al and sapphire.

cond-mat.mtrl-sci↗