François Gautier

Electronic structure, magnetism and growth of ultrathin films of transition metals

The conditions which determine the growth mode (Frank-van der Merwe, Stranski-Krastanov, Volmer-Weber) on a perfect surface are obtained in terms of the electronic structure of ultrathin films and interfaces. The various contributions (elastic, structural, chemical, magnetic, etc.) to the spreading energy and to the interface energy are obtained at zero Kelvin for overlayers of a transition metal A on a transition, metal B. We use a tight-binding model and the recursion method to get general trends for all transition metal pairs (A, B) but the electronic structure derived from this simple model agrees when available with the ab-initio (LSDA) calculations. The spreading energy results from a delicate balance between its contributions but the wetting of the substrate by a monolayer (λ1 < 0) is generally obtained when the thermodynamical criterion λA< λB is satisfied. Finally the stability of an overlayer with respect to the formation of two-dimensional ordered compounds is investigated.

Theoretical investigations of the magnetic behaviour of Cr monolayers deposited on a Fe(001) substrate: Role of a mono-atomic step

Abstract In this paper we present a theoretical study of the magnetic properties of n Cr monolayers (ML) deposited on a Fe(001) substrate. We use the real space recursion method in a tight binding framework to determine the electronic structure of non-periodic systems involving a large number of inequivalent atoms. The aim of this work is to investigate the possibility to obtain a magnetic defect in the Cr layer by frustrating the plane to plane antiferromagnetic (AF) order. First, we study the magnetism of n perfect Cr monolayers (1 ≤ n ≤ 10) deposited on a perfect (001) Fe substrate and we show that a magnetic defect can be obtained for n ≥ 6 ML but it is clearly less stable than the solution without defect. We show also that, after the deposition of 3 Fe monolayers onto the Cr, the magnetic defect is obtained for much smaller Cr thicknesses ( n ≥ 2). Then, we consider a Fe substrate presenting a mono-atomic step. We study the magnetic moments distributions during the growth process of Cr monolayers assuming that the growth starts at the step. We show that (i) the perturbation due to the step is limited to the vicinity of the step and has a small extension in the plane of the Cr layers, and (ii) a wall is generated in the Cr layer by the step and splits clearly the Cr layer into two domains. Finally, we determine the structure of collinear walls in chromium and we discuss their role on the magnetic properties.

Magnetic couplings in Fe/Mn and Fe/Cr superlattices

Abstract In this paper, we point out the importance of the spacers magnetic order with regard to the properties of ferromagnetic/ antiferromagnetic superlattices at T = 0 K. We discuss (i) the magnetic properties of superlattices based on ferromagnetic (Fe) and antiferromagnetic (Cr and Mn) layers in the itinerant magnetism framework, (ii) the relation between the interfacial, interlayer and intralayer couplings and the magnetic moment distributions and (iii) the role of the magnetic frustrations. The interlayer magnetic couplings are found to oscillate with the parity of n but the magnitude of these oscillations decreases more rapidly with an Mn spacer than with a Cr spacer.

Electronic structure and interlayer magnetic couplings in metallic superlattices

Abstract The general trends for the magnetic interlayer couplings are calculated in a tight binding model from the electronic structure of perfect model superlattices A m B n (Fe-V, Co-Pd, Co-Ru and Fe-Cr). Different features are deduced for non-magnetic, nearly ferromagnetic and antiferromagnetic spacers B; they are discussed in relation with ab initio calculations and experimental results.

An investigation of magnetic structures and phase transitions in NiS2−xSex by 61 Ni-Mössbauer spectroscopy

Abstract We report on an investigation of mixed pyrite-structure compounds NiS2-xSex by 61 Ni-Mossbauer spectroscopy in conjunction with other methods. From the results we deduce the occurrence of noncollinear antiferromagnetic ordering in the metallic phase for 0.47 ⩽ x ⪅ 1.0 and a significant change of the magnetic structure with the replacement of S by Se in the semiconducting phase.

Experimental study of A0 and T1 modes of the concert harp

String instruments are usually composed of a set of strings, a soundboard, and a soundbox with sound holes, which is generally designed to increase the sound level by using the acoustic resonances of the cavity. In the case of the harp, the soundbox and especially the sound holes are primarily designed to allow access to the strings for their mounting. An experimental modal analysis, associated to measurements of the acoustic velocity in the holes, shows the importance of two particular modes labeled A0 and T1 as it was done for the guitar and the violin. Their mode shapes involve coupled motions of the soundboards bending and of the oscillations of the air pistons located in the sound holes. The A0 mode is found above the frequency of the lowest acoustically significant structural mode T1. Thus, the instrument does not really take advantage of the soundbox resonance to increase its radiated sound in low frequencies. However, contribution of mode A0 is clearly visible in the response of the instrument, confirming the importance of the coupling between the soundboard and the cavity.

Multi-point vibrometer based on high-speed digital in-line holography

This paper describes a digital holographic setup based on in-line holography and a high-speed recording to get a multipoint vibrometer. The use of a high-speed sensor leads to specificities that enable the in-line configuration to be used. The case of transient vibrations is investigated through a full simulation of the holographic process. The simulation shows that the first instants are critical since distortion may occur, resulting in errors in the phase measurement. Experimental results are provided by exciting an aluminum beam with a transient signal. A comparison with the velocity measured by a pointwise vibrometer is provided. Frequency response functions are extracted and the experimental results confirm the ability of the method to provide full-field contactless measurements at the high-speed time scale evolution of the vibration.

Evaluation of surface acoustic waves on the human skin using quasi-time-averaged digital Fresnel holograms

This paper proposes a first attempt to visualize and analyze the vibrations induced by a bone-conduction device and propagating at the surface of the skin of a human face. The method is based on a new approach in a so-called quasi-time-averaging regime, resulting in the retrieval of the vibration amplitude and phase from a sequence of digital Fresnel holograms recorded with a high image rate. The design of the algorithm depends on the ratio between the exposure time and the vibration period. The results show the propagation of vibrations at the skin surface, and quantitative analysis is achieved by the proposed approach.

Macro parameters describing the mechanical behavior of classical guitars

Since the 1960s and 1970s, researchers have proposed simplified models using only a few parameters to describe the vibro-acoustical behavior of string instruments in the low-frequency range. This paper presents a method for deriving and estimating a few important parameters or features describing the mechanical behavior of classical guitars over a broader frequency range. These features are selected under the constraint that the measurements may readily be made in the workshop of an instrument maker. The computations of these features use estimates of the modal parameters over a large frequency range, made with the high-resolution subspace ESPRIT algorithm (Estimation of Signal Parameters via Rotational Invariant Techniques) and the signal enumeration technique ESTER (ESTimation of ERror). The methods are applied to experiments on real metal and wood plates and numerical simulations of them. The results on guitars show a nearly constant mode density in the mid- and high-frequency ranges, as it is found for a flat panel. Four features are chosen as characteristic parameters of this equivalent plate: Mass, rigidity, characteristic admittance, and the mobility deviation. Application to a set of 12 guitars indicates that these features are good candidates to discriminate different classes of classical guitars.

Quality assessment of combined quantization-shot-noise-induced decorrelation noise in high-speed digital holographic metrology

this paper discusses on the influence of decorrelation noise induced by quantization and shot-noise when recording digital holograms at very high frame rate. A criterion based on the coherence factor of the hologram phase difference is proposed. The main parameters of interest are the ratio between the reference and the object waves and the sensor dynamics, depending on the photo-electron capacity of pixels. The study is based on a full numerical simulation of the holographic process, which provides useful rules. This leads to define the optimal conditions for recording at very-high frame rate with minimization of the decorrelation noise. Experimental results obtained with frame rate at 50kHz confirm the proposed approach.