Couple of weeks ago I applied for a PhD position. Last friday, I got an email saying that I was selected as one of the best 10% of the candidates and that they want to progress further with my candidacy, if I'm still interested. I replied that I'm happy to hear that and that I'm still interested. However, up until now, I didn't hear anything back from them. Is this normal? I thought they would send me an invitation for an interview or something as soon as possible. Can I email them asking about the next steps in the application procedure, or should I just wait?