I'm applying from Europe to US or US-styled Ph.D. programs and I'm wondering about who to choose for referees.
I graduated 2 years ago and my undergraduate professors don't remember me well, or at least I don't think so. In those two years I have worked as a research assistant in one lab, with three different post-docs. Should I diversify the source of my references and try to get references from my undergraduate institution or should I just opt for the people that I've been working with?
I think the latter group knows me better and has, generally, a better opinion of me and my work. Then again, I'm afraid that the single source of references will be suspicious.
I mentioned having studied in Europe because the undergraduate research options are typically limited, compared to American institutions where research experience is strongly encouraged. I only did a research thesis during my undergrad.