Line defects in crystals and flux pinning in superconductors Scientific work of Reiner Labusch (1935 – 2016)

Abstract

Reiner Labusch, one of the co-founders of the Conference on Extended Defects in Semiconductors in 1978, and his group made seminal contributions to a fundamental understanding of the electrical and optical properties of dislocations in semiconductors, the fluxon pinning in Type II superconductors, and the solid solution hardening of alloys.With regard to semiconductors, he developed occupation statistics and a band scheme for optical transitions of a half-filled band arising from dangling bonds in the dislocation core that could explain the carrier density in plastically deformed p- as well as n-type germanium and devised experimental setups for photoconductivity measurements avoiding the blurring effect of surface states in Ge and Si and measurements on bundles of single dislocations in CdS and Ge. He extended the band model by introducing empty bands split from the conduction band in the strain field of dislocations. This model is most convincingly confirmed for 90° partial dislocations in Ge by photoconductivity spectra on individual dislocations.Reiner Labusch's theoretical studies on superconductivity include (i) the elastic properties of the fluxoid lattice in type II superconductors, (ii) the nature and properties of fluxoid lattice dislocations and (iii) a rigorous statistical theoretical treatment of the response or, in particular the “pinning”, of the fluxoid lattice to pinning sites/defects in the crystal lattice, and (iv) on the magnetization behavior in type II superconductors of arbitrary shape. This latter rigorous analysis and numerical treatment allowed for the accurate prediction, and a contribution to the understanding of the magnetization properties in disc/platelet samples, including inter alia, the initial fluxoid penetration behavior, fluxoid pinning, and fluxoid lattice transformations relating to the “arrow-head” phenomenon, fluxoid lattice melting and the “intermediate state”.Labusch’s statistical theory of solid solution hardening is based on a fundamental understanding of the interaction of a dislocation moving through a random array of solute atoms or precipitated particles. It yielded an expression for the critical resolved shear stress as a function of the obstacle concentration, the interaction force and range of interaction between the dislocation and a single obstacle, and the dislocation line tension. The analytical result was corroborated by computer simulations which also revealed that the strengthening by extended obstacles in most alloys is beyond the ‘Friedel-limit’ and is adequately described by the ‘Labusch theory’.