Physikalisch-Technische Bundesanstalt

In addition to the great number of isolated research projects concerned with the fundamentals of metrology, some cross-sectional projects involving various divisions have been established at PTB. They cover a wide field, from detailed problems of metrology to fundamental research in solid-state and material physics. Some of them are presented in the following: 

Wanted: a successor to the original kilogram 

The "Avogadro" project, in which members of four PTB divisions cooperate, is one of the focal points of experimental work. The unit of mass is the only base unit which to this date is materialized by the prototype kilogram All other base units can be traced back to fundamental constants or to the properties of atoms or molecules. The aim of the Avogadro project therefore is to arrive at a redefinition of the unit of mass on the basis of atomic masses, and the project coordinates investigations in sub-areas carried out in different PTB departments. One of these investigations concerns the determination of the number of a certain kind of atom in a volume 1 kg in mass. The measuring method is based on a very precise determination of the Avogadro constant N_A on selected, highly pure silicon crystals. This task has led to further research and development work within the framework of length and angle measurements on an atomic scale. 

On the trail of electrons 

The exact characterization of interfaces plays an important part in many issues of solid-state physics and technology. In addition to direct microscopy on an atomic scale, two other complementary methods are applied: X-ray and neutron reflectometry. In the case of the X-ray reflectometer equipped with a rotary anode, the intensity of the reflected beam and thus the distribution of the electron density inside the specimen are measured in a wide angular range, far beyond total reflection. Thermal neutrons serve as a probe for the kind and distribution of the atomic nuclei in the same specimen. This makes a complete investigation of the light elements and of neighbouring elements of the periodic system possible, whose differentiation is much more difficult with the aid of X-ray photons. Both methods allow conclusions to be drawn regarding the density profile at the interface and the surface roughness. Multi-layer structures, for example in semiconductor technique, can be quantitatively characterized by these methods. 

Quantum Hall effect  

The projects of Division Q (Scientific and Technical Cross-sectional Tasks), which concentrate on theoretical problems, cover above all research work aimed at a better understanding of the quantum Hall effect (QHE). The QHE has meanwhile developed into the de facto standard of the ohm, although it is far from having been theoretically explained in every detail. It is not even undisputed whether the von Klitzing constant may nevertheless slightly deviate from h/e in the real solid. PTB cooperates with many other research teams worldwide to treat topics such as the frequency dependence of the QHE and the influence of disorder and of interaction among electrons. Here, the spin relaxation in an interacting 2-d electron gas was investigated for the first time. For this purpose, relaxation equations for the electron’s spins were derived from the microscopic theory. In contrast to the Bloch equations known from magnetic resonance, the relaxation equations here are non-linear and their solutions depend very strongly on filling factor and temperature. Thus, a relaxation behaviour is found which decays as 1/t instead of following the usual exponential law.