A. Mehdaoui

Continuous germanene layer on Al(111).

Germanene, a 2D honeycomb structure similar to silicene, has been fabricated on Al(111). The 2D germanene layer covers uniformly the substrate with a large coherence over the Al(111) surface atomic plane. It is characterized by a (3 × 3) superstructure with respect to the substrate lattice, shown by low energy electron diffraction and scanning tunnelling microscopy. First-principles calculations indicate that the Ge atoms accommodate in a very regular atomic configuration with a buckled conformation.

Defining the quality of a high‐Tc superconductor

We compare our experimental results obtained for the complex susceptibility of YBaCuO compounds with other results available in the literature concerning YBaCuO as well as other high‐temperature superconducting oxides (LaBaCuO, BiSrCaCuO) or other inhomogeneous compounds like the Chevrel phase. We discuss a phenomenological model based on Clem’s formulation [Physica C 50, 153 (1988)]. We present a plot of all the experimental collected results, which allows a classification in terms of quality for a large variety of samples.

Room-temperature ferromagnetism in single crystal Fe1.7Ge thin films of high thermal stability grown on Ge(111)

We report on the epitaxial growth of ultrathin ferromagnetic Fe1.7Ge layers on Ge(111) wafer. These single crystal intermetallic layers adopt the InNi2 (B82) crystallographic structure. They are ferromagnetic with a Curie temperature well above room temperature. The interface between the ferromagnet layer and the Ge wafer is of high perfection. Interestingly, the annealing of the sample up to 300°C alters neither the crystallographic structure, nor the interface quality, nor the magnetic properties but leads to a nearly perfect smoothening of the germanide layer surface. This high thermal robustness should open the way for the growth of fully epitaxial iron germanide/Ge hybrid structures.

Epitaxial magnetic Fe layers grown on Si(001) by means of a template method

We demonstrate the possibility of growing good-quality epitaxial Fe films on a Si(001) substrate, opening up new prospects to prepare ferromagnetic superlattices on this substrate. A template technique prevents the formation of disordered interfacial iron silicides. Transmission electron microscopy reveals that the Fe layers are fairly uniform in thickness with abrupt interfaces and in majority epitaxial relationship Fe(001)[100]//Si(001)[100]. Both the diffraction data and the observation of Moire fringes indicate that the Fe lattice is almost relaxed towards its bulk bcc phase in thick layers [≥40 monolayers (ML)] but is still strained by the substrate in the thinnest films. Magneto-optical Kerr effect measurements show that the films evaporated at normal incidence with a thickness above 4 ML are ferromagnetic at room temperature and exhibit in-plane biaxial anisotropy.

The influence of thermal treatments, fabrication methods and shapes on the magnetic transition of high temperature superconducting compounds

Abstract We discuss the problem of characterizing a superconducting transition and the question of comparing samples originating from different fabrication processes and/or obtained under various shapes or morphologies (bulk, wires, thin films, single crystals, …). We present two different methods for the analyses of the AC susceptibility (for both the real and the imaginary part) in the low field limit. Strength of flux pinning and intergrain/intragrain contributions are discussed. Both analyses are applied to different inhomogeneous superconductors like high-Tc compounds (YBCO, LCO, LSCO, LBCO, B(P)SCCO, NCCO), Chevrel phases, etc. The dynamic critical field given by such an analysis allows an estimation of the quality and/or homogeneity of the superconducting sample and is a measure of the first intergrain (or Josephson) critical field.

Flux pinning and dynamical critical fields in some YBaCuO compounds

Abstract We have studied the AC initial complex susceptibility of sintered samples of high temperature superconducting YBaCuO compounds and the importance of using very low external magnetic AC fields (h AC h ∗ c which is much lower than the first critical (static) field H c1 . The characterization and the quality of the true transition (which is field independent) can only be obtained with measuring AC fields whose values are lower than the critical AC field. The shape of the magnetic transition is otherwise strongly affected by the amplitude of the AC field and makes difficult a comparison between samples of different origins, fabricated by different processes and characterized under different conditions.

Magnetization reversal mechanisms in epitaxial Fe/Si(001) layers with twofold and fourfold magnetic anisotropies

Epitaxially grown iron thin films (thickness: 8 A < d < 120 A) on Si(0 0 1) with body centred cubic structure have been structurally and magnetically studied at room temperature. All lilms are characterized by ferromagnetic properties as soon as the thickness reaches 4 ML. In-plane uniaxial and cubic magnetic anisotropy constants are measured on samples obtained by modifying the incidence angle of evaporating iron flux. The magnetization reversal mechanisms are analyzed by combining longitudinal magneto-optical Kerr effect of both parallel and transverse magnetization components. These mechanisms are strongly influenced by the nature of the magnetic anisotropies.

Stability of B–H and B–D complexes in diamond under electron beam excitation

The substitution of hydrogen by deuterium is generally known to increase the stability of the defect passivation in semiconductors, occasionally giving rise to giant isotope effects. In this work, the stability under an electron beam irradiation of boron-hydrogen and boron-deuterium pairs in diamond are compared. The dissociation kinetics was followed in situ by cathodoluminescence spectroscopy. Surprisingly, B–H complexes are more stable than B–D complexes under electron beam at low temperature (∼100K), with a dissociation rate about twice smaller. These experimental results are coherent with a dissociation mechanism involving a cumulative vibrational excitation of the complexes.

Irreversibility line and anisotropy of magnetic melt textured Y1Ba2Cu3O7−δ studied by AC susceptibility

Abstract Magnetically melt textured YBa 2 Cu 3 O 7−δ samples have been studied by AC initial susceptibility measurements in the low field limit. The critical dynamic fields h ∗ c and the relative position of the irreversibility line and of the critical line show that magnetic melt texturing leads to the creation of strong pinning sites. The anisotropy factor A is in agreement with the value obtained from magnetization under high DC fields. The A values are lower than in single cystals, but in agreement with the important orientation effects introduced by magnetic field texturing. These values also are typical of fabrication techniques involving a melting at some stage.

Homogeneity of high and low temperature superconducting compounds in relation with magnetic characterization

Abstract We discuss the problem of characterizing a superconducting transition and the question of comparing samples originating from different fabrication processes and/or obtained under various shapes or morphologies (bulk, wires, thin films, single crystals, …). We present different methods for the analysis of the very low field a.c. susceptibility (for both the real and the imaginary part). The dynamic critical field obtained from our analysis (which can be applied to different inhomogeneous superconductors like high T c compounds, Chevrel phases, etc.…) is a measure of the quality and/or homogeneity of the superconducting sample.