F. Cyrot-Lackmann

High Resistivity and Diamagnetism in AlPdMn Icosahedral Phase

We present here the first results on transport and magnetic properties in the new stable icosahedral Al1-x-yPdxMny phases (x/y = 22/7.5, 21/8.5, 19/10). These melt-spun samples are of high structural quality even in the as-quenched state and present high-resistivity values which depend strongly on the nominal composition and structural quality with ρ4K ~ 10 000 μΩ cm in annealed Al70.5Pd22Mn7.5 and 1600 μΩ cm in as-quenched Al71Pd19Mn10. Moreover the system becomes more diamagnetic as its resistivity increases.

Electronic structure and hybridization effects in the compounds Al2Ru and Ga2Ru

Abstract The electronic structure of the compounds Al2Ru and Ga2Ru has been calculated self-consistently by using LMTO-ASA (Linear Muffin Tin Orbitals in the A Atomic Sphere Approximation) method. The calculation strongly indicates that a quasi-semiconductor gap at the Fermi level occurs inside the d states of Ru for both compounds. The models existing in the litterature are discussed and compared to our results and the origin of this quasi-semiconducting behaviour is found to be essentially due to effects of sp (Al,Ga) - d (Ru) hybridization.

On the local densities of states on flat and stepped Pt surfaces

Abstract The electronic structure of the d band of both flat and stepped Pt surfaces is investigated within the tight-binding approximation, using a moment method. A sharp surface virtual bound state peak is found in the local density of states at the protruding edge of the stepped surfaces and the symmetry of states near the Fermi level are found to be rather dependent on the geometry of the surface. Possible connections with experiments are briefly discussed.

ON ELECTRONIC PROPERTIES OF Al//4Mn QUASICRYSTALS IN MELT-SPUN Al-Mn ALLOYS

We present the resitivity and structural studies of the melt-spun Al-Mn alloys (with 10-18 at% Mn) that contain more than 50 vol% of the quasicrystalline (qX) Al//4Mn phase. The resistivity varies little with temperature up to approximately 600 K where it reveals structural transformations in the samples. Its magnitude is directly proportional to the overal percentage of Mn in each alloy. Simple theoretical considerations enable us to estimate the resistivity of the qX phase to be approximately 150 mu OMEGA . cm. High resitivity and high (meta)stability of quasicrystals can be understood within Friedel model of virtual bound states.

Heats of formation of binary transition metal alloys

Abstract The heats of formation of disordered transition metal alloys are related to moments of the density of states within an analytical tight-binding model for the d band. The difference in bandwidth and in energy levels between the metals are two dominant effects on alloying. The d charge transfer is evaluated in a self consistent way from the knowledge of the partial densities of states.

On the electronic structure of ferromagnetic transition metal surfaces

Abstract The electronic structure at the surface of ferromagnetic Ni and Fe is studied within a rigid exchange splitting model, in the tight-binding approximation. The surface resonances existing in the paramagnetic state are split. The magnetic moment is found to be the same at the surface and in the bulk in agreement with recent experiments. It is shown to be distributed almost spherically, except for (100)Ni where it points towards nearest neighbours.

Electronic properties and the role of d states in stable quasicrystals

Abstract We discuss the electronic structure of quasicrystals containing transition elements by considering the effects of the diffraction of conduction states by Bragg planes and of their coupling with localized d orbitals. We calculate the consequences of these two terms on transport properties, densities of states, magnetism and negative valency of transition atoms, and compare our predictions with ab initio calculations and experiments.

The new AlPdRe icosahedral phase: Towards universal electronic behaviour for quasicrystals?

Abstract Electronic properties have been measured for different new compositions close to Al 70.5 Pd 22 Re 7.5 of the icosahedral phase. They exhibit the same exceptional behaviours that were previously found for other stable quasicrystalline phases exept for the resistivity magnitude which is much higher that what is observed for the parent AlPdMn icosahedral phase. For high quality quasicrystals, the conductivity reaches the same order of magnitude as the one for doped semiconductors. The conductivity σ(T) increases rapidly with temperature. σ(T) curves appear to be parallel for all i-AlPdRe samples, although conductivity values strongly varies with the composition and the structural quality. High field magnetoconductivity indicates the presence of quantum interference effects including strong electron-electron interactions. All these features strengthen the concept of universal electronic behaviour for these complex phases, with particularly enhanced anomalous behaviour for i-AlPdRe.

Atomic dynamics of icosahedral Al62Cu25.5Fe12.5 and tetragonal Al70Cu20Fe10: A comparative study using inelastic neutron scattering

Abstract The total dynamic structure factor S ( Q , ω ) and the generalized vibrational density of states G ( ω ) have been determined for icosahedral Al 62 Cu 25.5 Fe 12.5 and, for comparison, for tetragonal Al 70 Cu 20 Fe 10 using inelastic cold neutron scattering. The strongly structured spectra expected on theoretical grounds for the icosahedral alloy were not found in this investigation with a sample at this best concentration known so far for icosahedral AlCuFe. Instead, we found a broad intensity distribution in S ( Q , ω ) and a broad two-band structure in G ( ω ) for the icosahedral alloy, not very different from what one would expect for a disordered solid. In contrast, the dynamic structure factor and the phonon density of states of tetragonal AlCuFe measured under identical experimental conditions are very sharply structured in the whole energy region covered in this experiment.

Diamagnetism of quasicrystals due to Bragg's reflexions

We point out that in quasicrystals, one has to take into account a large but finite number of effective Braggs reflexions out of the dense set. This leads to small gaps on the Fermi surface (some tenths of meV), much narrower than the usual Hume-Rothery ones (0.2-0.5 eV) invoked for explaining their stability. Small pockets of electrons and holes with very anisotropic effective masses are thus existing. The very low effective masses in some directions for electron pockets are responsible for the diamagnetism of quasicrystals. As these gaps are very sensitive to impurities, phasons, so is the diamagnetism.