Relativistic Fluctuations and Anomalous Darwin Terms in Superconductors

The anomalous Darwin term, one of the recently derived relativistic corrections to the conventional theory of superconductivity, is analysed in detail. A new derivation of this term, much simpler than that given originally, is presented and used to interpret the term physically, in terms of fluctuations of the relativistic wave packet describing the paired electrons. Several puzzling features of the original derivation find a simple explanation in terms of this reinterpretation. The question of observability of the anomalous Darwin term, and its relation to the conventional Darwin term and to earlier proposed superconducting Darwin terms are also discussed.