R Haberkern

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

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.

Variable Fe-valence as an effect of angular correlations?

Abstract Long-range order starts to form as a spherical periodic electron-staic-structure resonance. The selforganized adjustments of the static structure to the electronic constraints, and vice versa, the electronic system via variations of the mass-density or the effective valency to the static structure, all optimize the resonance. The latter effect plays an important role in amorphous and quasicrystalline alloys containing late transition elements and aluminium.

Conduction mechanisms in some icosahedral and amorphous phases

Quasicrystalline icosahedral and amorphous phases prepared as thin films or bulk samples in the systems Al-Pd-Re, Al-Cu-Fe and Ti-Zr-Ni, have been investigated with respect to their temperature dependence of conductance and thermopower, in the range between 10 and 300 K. Various conduction mechanisms yield different exponent values in the power-law temperature dependence of conductance, T ζ . A semi-metal behaviour (ζ ≅ 1.5) is found in the amorphous phase and in the disordered icosahedral Al-Pd-Re film and Ti-Zr-Ni ribbons, as well as in icosahedral Al-Cu-Fe films. Lower ζ values found in the quasi-perfect icosahedral Al-Pd-Re bulk sample and in two of the icosahedral Al-Cu-Fe films suggest that electron localization in a hierarchy of clusters is the main conduction mechanism in these samples. The thermopower data support the existence of a pseudo-gap at the Fermi level and suggest a similar short-range order in both amorphous and icosahedral phases. The special sensitivity of the thermopower to the DOS details at E F is responsible for the strong variation of its magnitude with the degree of icosahedral order.

From the disordered via the quasicrystalline to the crystalline state

Abstract We report on structural similarities at short- and medium-range distances of the amorphous and the quasicrystalline state and their consequences on structural stability and electronic transport. In the former the nearest neighbour atoms are onion-shell-like radially ordered around any ad-atom. The spacing between the concentrical shells is up to relatively large distances unique and matches the Friedel wavelength. Their occupation by the ions is random. We interpret this in terms of a resonance, based on spherical waves, between the conduction electrons and the static structure. The spherical arrangement of the atoms plays the role of mirror-spheres for spherical electron waves at short and medium-range distances, quite similar to mirror-planes for the plane electron waves in a crystal. The conducting quasiparticles become localised as there is better spherical correlation and hence the resonance condition is fulfilled. The quasicrystals are between the disordered and the crystalline case. Radial order becomes more strict but lasts for shorter distances (cluster-like), and in addition exists only at particular sites. On the other hand, long-range orientational order, e.g. order at the spheres and between the spheres has emerged, causing planar resonances between planar electron waves and mirror-planes. Long-range periodicity is still missing. The path from the disordered via the quasicrystalline to the crystalline state seems to be triggered by the improvement of resonances between electrons and the forming static structure.

Magnetoconductivity of AlPdRe quasicrystalline films

Magnetoconductivity (MC) and zero field conductivity have been measured at low temperatures in metallic and barely insulating AlPdRe quasicrystalline films. For a metallic film, both the magnetoconductivity and conductivity can be fitted well using the 3D weak localization (WL) and electron-electron interaction (EEI) theories. The AlPdRe films exhibit very strong spin-orbit scattering with . Therefore, the inelastic scattering times can be directly extracted from the low field magnetoconductivity data taken at various temperatures, yielding . Using the fitting parameters extracted from the MC data, the zero field conductivity as a function of temperature can be described nicely using the WL and EEI theories. MC data and conductivity data obtained on barely insulating films below the metal-insulator transition cannot be explained in the framework of the WL theory.

Fabrication Of High Quality Qc Films Via The Route Of The Amorphous Phase

We discuss the preparation of thin icosahedral films (Al-Cu-Fe and Al-Pd-Re) via the route of the amorphous (a-)phase which in some aspects is a precursor to the icosahedral phase. A direct transition from the a- to the i-phase occurs for Al-Cu-Fe films at 430 C on the time scale of minutes. The resulting films are of good quality as shown by diffraction and electronic transport properties. The surface of the resulting films is very smooth.

Quasicrystalline Films and their Amorphous Precursors

Binary and ternary aluminum based amorphous precursor films have been prepared by the sequential-flash evaporation technique in the case of Al-Mn, Al-Fe, Al-Cu-Fe and Al-Pd-Mn and by co-sputtering in the case of Al-Pd-Re. For well adjusted compositions and an appropriate heat treatment films can be transformed into polycrystalline single phased quasicrystals for all of the systems mentioned above. The transition can be monitored by in situ measurements of electronic transport properties. The structure factors as a function of composition and annealing state has been measured by electron diffraction. They show a unique behavior if the composition is chosen to that of the related quasicrystal. For high annealing states of the amorphous phase the structure factor becomes more intense at the peak positions of the following i-phase. This is interpreted as a unique radial near- and medium-range order of both the i-phase and its amorphous precursor. The final transformation into the quasicrystal is sharp for Al-Cu-Fe and is broad in the case of Al-Pd-Mn and Al-Pd-Re. The transport properties of the resulting single phased films are similar to that of the related bulk samples and allow systematic studies as a function of composition. Some of the i-Al-Pd-Re films are on the insulating side of the metal-insulator transition.