Well neither did I, until today. Neither does Wikipedia.
Who is he?
Well, Today I found two arXiv papers of Jeroen van Dongen:
Emil Rupp, Albert Einstein and the canal ray experiments on wave-particle duality: Scientific fraud and theoretical bias
The interpretation of the Einstein-Rupp experiments and their influence on the history of quantum mechanics
It seems that
In 1926 Emil Rupp published a number of papers on the interference properties of light emitted by canal ray sources. These articles, particularly one paper that came into being in close collaboration with Albert Einstein, drew quite some attention as they probed the wave versus particle nature of light. They also significantly propelled Rupp’s career, even though that from the outset they were highly controversial.
In 1935 Rupp very publicly retracted no less than five of his scientific publications from the previous year. The articles dealt with such subjects as the polarization of electrons and the artificial production of positrons.2 Rupp published his retraction in a short notice that appeared in the Zeitschrift für Physik. He stated that his withdrawals were the result of an illness and supplied a medical opinion—by a “Dr. E. Freiherr von Gebsattel” — in support of his claim:
"Dr. Rupp had been ill since 1932 with an emotional weakness (psychasthenia) linked
to psychogenic semiconsciousness. During this illness, and under its influence, he has, without being himself conscious of it, published papers on physical phenomena [...] that have the character of ‘fictions.’ It is a matter of the intrusion of dreamlike states into the area of his scientific activity".
Well, it seems to be a very interesting part of physics history. One that was curiously absent from most accounts. For example, Abraham Pais biography of Einstein, The Lord is Subtle, does not mention Rupp at all. The only other publication mentioning Rupp I was able to find at short notice was A. P French, The Strange Case of Emil Rupp (unfortunately not available freely). The abstract of paper by French reads:
Physics has seldom had to deal with claims of alteration or fabrication of data, such as have troubled biological and medical research in recent years. The case of Emil Rupp in the 1930s was, however, a notable exception. The present paper revisits this case, adding in certain areas to earlier accounts of Rupp and his work. The case is not without significance, because Rupp's publications carried considerable weight during a historically important era of 20th-century physics.
My usual source, Google Scholar lists 8 papers by Emil Rupp (half of this list being US Patents.
I wonder why is this case so hushed up? If, indeed, this is a case of fraud in physics, is it a sign of shame by physicists that they - within the most concrete of natural sciences - let through the fraud?
Perhaps You can supply with more links?