Last weekend, I attended MOSS 2, where Malte Zimmermann gave a talk on the two German modal particles “doch” and “schon” (you can find the abstract here). He focussed on stressed variants of these particles and argued for two hypotheses. (i) The stressed variants consist of the unstressed particle plus verum focus. (ii) The meaning of the particle “schon” is based on its temporal adverbial counterpart.
While I have not much to say regarding hypothesis (ii), I already said something regarding (i). In a paper based on a talk I gave at a fest-conference for Harald Weydt in Bern in 2009, I defended basically the same claim for accented “JA”. The paper is titled “Betonte Modalpartikeln und Verumfokus” and appeared at the end of 2010 in the volume “40 Jahre Partikelforschung” edited by Elke Hentschel and Theo Harden. I uploaded a manuscript version of the article which you can find here. The picture links to the official page for more information on the volume.