P. Häussler

Preparation and properties of thin polycrystalline MnSi1.73 films

Abstract Thin MnSi1.73 films were prepared by a DC sputter method in a high vacuum deposition chamber. As target material a MnSi2 silicide target was used. Films were deposited onto quartz substrates at different temperatures (330–875 K) without any annealing steps after the sputter process. The composition of the samples was determined by Rutherford backscattering spectroscopy. Within the detection limit of this method no contamination in the films was found. The crystalline structure of the films was analyzed by transmission electron microscopy. At substrate temperatures of 530 K always a tetragonal crystalline phase of MnSi1.73 was observed. The specific resistivity and the Hall mobility of the samples were determined in the temperature range from 300 to 25 K. The results of the electrical measurements verify the low crystallization temperature of the films. The character of the electrical transport in the crystalline films is strongly influenced by the preparation temperature. With rising substrate temperature the sign of the temperature coefficient of resistivity at room temperature changes from negative to positive values. At room temperature Hall mobilities in the region of 1.4 cm2 V−1 s−1 were obtained. The temperature dependence of the electrical transport properties can be interpreted in the picture of a degenerated semiconductor. With optical measurements at room temperature an indirect gap of 0.46 eV was determined. The structure of the transmission spectra was influenced by the different heat treatments in the sample preparation.

24 electron cluster formulas as the ‘molecular’ units of ideal metallic glasses

It is known that ideal metallic glasses fully complying with the Hume-Rothery stabilization mechanism can be expressed by a universal cluster formula of the form [cluster](glue atom)1 or 3. In the present work, it is shown, after a re-examination of the cluster-resonance model, that the number of electrons per unit cluster formula, e/u, is universally 24. The cluster formulas are then the atomic as well as the electronic structural units, mimicking the ‘molecular’ formulas for chemical substances. The origin of different electron number per atom ratios e/a is related to the total number of atoms Z in unit cluster formula, e/a = 24/Z. The 24 electron formulas are well confirmed in typical binary and ternary bulk metallic glasses.

Concept of resonances in disordered metallic matter

A new unique description of a class of amorphous (and liquid) alloys is presented. We treat these alloys as composed of three interacting subsystems, namely electrons, static structure and dynamic exitations. Resonances between these subsystems determine the stability and give rise to new quasiparticles. Electronic as well as dynamic transport anomalies arise.

On the formation of quasicrystals and the evolution of transport anomalies

We report on a (quasi)continuous structural evolution of the amorphous to the quasicrystalline state and the accompanied evolution of electronic transport anomalies. Both phases are known to be Hume-Rothery-stabilized. The amorphous phase shows, in the mean around any ad-atom, spherical-periodic short- and medium-range order causing a broad pseudogap at E F and relatively small deviations of electronic transport to the free-electron behaviour. During annealing, already in the amorphous state, angular correlation and planar order arises. This r-related order matches the spherical periodic order and improves it. The planary order causes, additional to the broad, a sharp pseudogap at E F . Both pseudogaps together, their concentration and T-dependences, as well as their evolution during annealing, are able to explain all the unusual electronic transport properties of the conductivity, the thermopower and the Hall-coefficient for 50 K

Photoelectron spectroscopic investigations of thin FexSi100−x films

Photoelectron spectroscopic methods (UPS, XPS) were employed to study the electronic properties of FexSi100−x films (0 ≤ x ≤ 75) in the amorphous and polycrystalline state. The amorphous samples were prepared by vapour condensation on sapphire substrates held at room temperature (RT). The polycrystalline samples were obtained by annealing the amorphous films at 950 K. An insulator-to-metal transition in the amorphous films has been detected for Fe content between 10 and 20 at%, as indicated by the development of a step-like photoelectron intensity at the Fermi level. The semiconducting β-FeSi2 phase is observable in the polycrystalline films, as seen by the disappearance of the step-like Fermi edge in the UPS spectra. A line shape analysis of the Fe 2p core levels is used to obtain additional information about the electronic structure of the different phases. Our results will be compared to data taken on FeSi interface layers by other authors.

Spherical periodicity, an intermediate step to long-range order

Abstract During structure formation, at a stage where long-range periodicity does not yet exist, local spherical periodicity occurs optimizing the total energy along a spherical resonance between the electronic system and the self-organizing static structure. Here, we report on the structural features of a-Ge 50 Sb 50 and a-Ga 65 Te 35 , compare both with other covalent systems of the liquid and amorphous state, and discuss them together with metallic and ionic systems. In semiconducting systems, additional to the spherical-periodic order, there are short ranging angular correlations (order at the spheres), improving the spherical resonance and hence the depth of a pseudogap at E F .

Comparison of amorphous and liquid alloys by photoelectron spectroscopy

Abstract Photoemission data of liquid AuSn alloys are presented and compared with corresponding measurements on amorphous vapour-quenched films. A fairly close general similarity between the valence bands of the two phases is observed. This result supports earlier studies of the atomic structure of the amorphous films which have been interpreted in terms of a liquid-like structure with a strong tendency to compound formation within the nearest-neighbour arrangements. Distinct deviations of spectra measured on the liquid samples compared with the amorphous phase have been observed; the Au 5d band is shifted to lower binding energies and no density-of-states minimum is observed at EF. The Au d band exhibits a continuous temperature-dependent shift in the liquid state which has been observed for the first time and which can be extrapolated to −193 °C in the amorphous phase. The origin of this temperature effect is discussed in terms of changes in the electron density and structural alterations in the short-range order at the gold site.

Composition formulae of ideal metallic glasses and their relevant eutectics established by a cluster-resonance model

Composition formulae for ideal metallic glasses are explored by combining the cluster-plus-glue-atom model with the global resonance model, termed the cluster-resonance model for short. The former model gives the [cluster]1(glue atom) x cluster formulae, stressing the local cluster order of a glassy structure; the latter model extends the local cluster order to a medium-range one by introducing spherical periodicity that relates the cluster size with Fermi vector, k F. Such a correlation allows the calculation of Fermi energy, E F, and electrochemical potential of electrons of the system from any local clusters. The cluster-resonance model also implies the equilibrium of the electrochemical potentials of electrons between different clusters so that the number of glue atoms matching one cluster (x in the cluster formula) can be determined. Examples in the Cu–Zr–Al and B–Co–Si–Ta systems are given to show the effectiveness of the proposed model and the resulting cluster formulae in interpreting multicomponent metallic-glass compositions as well as their relevant binary eutectic points.

On the structure of semiconducting amorphous systems

Abstract We present unique structural properties of amorphous semiconductors, for alloys like a-Ge x Sb 100− x , a-Ga x Te 100− x and for pure elements like a-C, a-Si, a-Ge. We observe, in all of them, a peak in the structure factor S ( K ) at scattering vectors K =2 k F , with 2 k F the diameter of the Fermi sphere (although free conducting electrons are absent) which is related to local spherical-periodic order in r -space. We discuss this structural feature as caused by an optimised resonance between the electronic and the static atomic structure (SAS) with subsequent consequences on structural stabilisation and electronic transport. In order to support the model we also present plasmon data.

Magnetoresistance of an insulating quasicrystalline AlPdRe film in large magnetic fields

A metal-insulator transition in a quasicrystalline icosahedral AlPdRe film series was recently observed. The resistance of one of the films follows an activated Mott variable-range hopping law, thus indicating insulating behaviour. The magnetoresistance (MR) ratios r = R(B,T)/R(0,T) of this insulating film exhibited large positive values at low temperatures of 93 mK and magnetic fields up to l7 T. The data are fitted using the wave function shrinkage model for insulating films, and the fits to the data above 1 K are acceptable. The low temperature MR data exhibit anomalous behaviour at high fields above 12 T, characterized by a saturation of the ratio data and followed by a turnover to smaller values. A possible explanation for the saturation of the R(B,T)/R(0,T) ratio is proposed involving a field dependence of the localization length and of the density of states, which both appear in the wave function shrinkage theory. In contrast, the weak localization theory and electron-electron interaction theory, used to describe electronic transport in metallic films, failed badly to describe the transport data of this insulating film.