I have seen a few papers using the delta symbol method (after Heath-Brown's <a href="https://doi.org/10.1515/crll.1996.481.149">*A New Form of the Circle Method*</a> and <a href="https://doi.org/10.1007/BF01231758">Duke-Friedlander-Iwaniec's paper</a>), where one creatively expands the delta symbol. Are there some examples, where this has been used successfully in the context of number fields?