Werner Buckel

Supraleitung des Galliums bei hohem Druck

A high-pressure modification of Gallium was formed at liquid helium temperature by the application of a pressure in excess of 35000 atm. It was superconducting with a transition temperature of 6·38°K. The high-pressure form remained after the pressure was removed at helium temperature. This resulted, however in a shift in transition temperature to 7·5°K. The average derivativedTc/dp was −3·10−5°K/atm.

Homogene mechanische Spannungen in aufgedampften Schichten

The internal stresses of condensed films are measured during both, condensation and annealing. Especially, materials are studied which can be stabilized in amorphous phases by quenching condensation technique. All amorphous films of Bi, Bi + Pb, Bi+Sb, Pb+Bi and Sn+Cu grow without any noticeable stress. In contrary, amorphous Sb-films arise with remarkable tensil stresses. The probably important difference between Sb and the other materials consists in the composition of the vapor. At normal evaporation conditions Sb-vapor contains almost only Sb4-molecules, whereas the other materials evaporate monoatomically. The results are discussed with regard to a modell for the origin of the stresses based on the socalled Ostwalds rule for crystal growth.

Homogene mechanische Spannungen in amorphen und kristallinen Galliumschichten

The homogeneous mechanical stresses of vacuum deposited Gallium-films have been studied during deposition and annealing. The liquid-like phase which can be frozen in by condensation onto a substrate at He-temperatures is built up almost without any homogeneous stresses, whereas all films with a crystalline structure show clearly such stresses. A metastable crystalline Ga-modification grows with tensile stresses. Compressive stresses however have been observed for the wellknown Ga-modification stable at normal conditions. To understand this rather complex behaviour the so-called “Ostwald-Rule”, an empirical rule for the crystal growth, can be applied successfully.

Widerstandsverhalten abschreckend kondensierter tellur-edelmetall-schichten

Zusammenfassung Tellur-Edelmetall-Schichten werden durch gleichzeitiges Aufdampfen der Elemente auf eine mit flussigem Helium gekuhlte Unterlage hergestellt und ihr elektrischer Widerstand in Abhangigkeit von der Temperatur gemessen. Die Schichten sind ein weiteres Beispiel dafur, dass bei der abschreckenden Kondensation aus der Dampfphase bevorzugt eine der Schmelze ahnliche Modifikation eingefroren wird.

Low temperature specific heat of quench-condensed amorphous Sn1-x Cux-films

The specific heat of Sn1-xCux-films has been measured for the concentration range 0.47<x<0.75 and in the temperature range of 0.5 K to 4.2 K. The Debye-temperature θD increases almost linearly with increasingx. The electronic band density of statesN*(EF) is about half of that given by the free-electron model, however, it increases sharply atx>0.7.

Low temperature ion beam irradiation of gallium films

Ga-films in the amorphous, the β-Ga, and the α-Ga phase have been irradiated at low temperatures with Ar-ions and He-ions at energies 275 keV and 200 keV, respectively. The α-phase can be transformed into the amorphous state by Ar-ion irradiation but not by He-ion irradiation. Rather small Ar-fluences Φ<1014 ions/cm2 are sufficient. The β-phase is stable with respect to irradiation with Ar- and He-ions at small fluences. The as-quench condensed amorphous phase is stable with respect to Ar-ion irradiation, however, it transforms into the β-phase at He-ion irradiation. The different short range order of the α- and β-phase is thought to be responsible for the different behavior at ion irradiation.

Amorphous gallium produced by low-temperature irradiation — a test of the energy spike concept

Abstract Amorphous Ga films (a-Ga) and crystalline Ga films (α-Ga) have been irradiated at low temperatures ( The as-quench condensed amorphous phase is stable with respect to Ar irradiation, however, it transforms into the β-phase by He irradiation. The crystalline α-phase can be transformed into the amorphous state by Ar irradiation, but not by He irradiation. Rather small Ar fluences φ Since the average deposited energy was the same for the Ar and He irradiation, the different results observed for these two projectiles demonstrate the importance of the energy density within cascades for attaining a crystalline-amorphous transition by irradiation.

Low temperature irradiation equipment for measurements down to 150 mK

Abstract A new keV-implantation machine has been installed, the most important parts of it being commercially available (ion source, mass separator, accelerating tube). To this machine two low temperature cryostats can be adapted: first a 4 He-cryostat (lowest attainable temperature 1.1 K) for studying changes of superconducting properties due to irradiation; and second a 3 He/ 4 -dilution refrigerator as described by Anderson [6], for studying Kondo-systems prepaed by ion implantation. Lowest attainable temperature of this cryostat is 150 mK. Technical details of the implantation machine and, in particular, of the cryostats will be described.

The influence of hydrogen on the atomic distribution function and electrical resistance of amorphous chromium films

Electrical resistivity of amorphous chromium films (20–37 nm thick) deposited in a hydrogen atmosphere (PH2=8·10−6–2·10−4 hPa) onto a substrate cooled by liquid helium down to ≈ 2 K is measured, and electron-diffraction studies have been performed immediately after the quench condensation and after annealing to different temperatures up to 300 K. The preparation method employed permits a considerable hydrogen enrichment of the films to be reached. The maximum hydrogen concentration corresponds approximately to a stoichiometric composition of CrH. It is found that as the maximum concentration is approached the atomic distribution functionG(r) changes remarkably. The interatomic distances increase considerably (by 10%) and the atomic densities decrease. It is quite possible that amorphous chromium hydride is a final state with the maximum hydrogen concentration. For films with intermediate concentration,G(r) is found to vary substantially under annealing up to ≈90 K. The electron-diffraction and electronmicroscopic data, as well as the variations in resistivity due to annealing, suggest that with annealing up to ∼90 K, a hydrogen redistribution occurs in the amorphous films, initially homogenous in concentration. We observe also distinct indications of separation into phases with increased and reduced hydrogen contents.

Extraordinary high TC's in NbZr alloys at high pressure

NbZr alloys show a steep monotonous increase of Tc in a wide pressure range. The Tc of a Nb40 Zr60 sample has been raised to 16.5 K at our highest pressure of 28 GPa. There is some indication for a maximum in the Tc-P dependence at higher pressures.