Choosing PhD advisor in a field different from your undergrad research? I am applying to grad schools now, in physics dept. I have been doing my undergrad research on high energy and astrophysics. But recently I have started to gain a lot of interest in condensed matter physics. If I get accepted to grad school, I will be selected on the basis of my massive undergrad research which is mostly focused on high energy physics. But, I want to do my PhD on condensed matter. But one of my main concern is finding an advisor. I am worried that the professors in top tier universities may not comply with it.

Is it possible to persuade advisors in condensed matter to guide me? (especially in a top tier university)

If yes, how easy is it ?

I am not so sure about it....But is this thing really common among grad students?
You are talking about what you did as an undergraduate. When talking to a potential supervisor, you have to remember that you are still going to be a student. You will be learning things. A supervisor will see your previous research and your potential for additional research.

As an undergraduate, I'm pretty sure that you did not become a world expert in astrophysics. So it's not like that you will have "wasted" knowledge, now that you will switch to a different topic.

Also, it's not like you're changing from biology to physics. You are staying within the realm of physics, and any skills that you gained doing your ("unrelated") undergraduate research will only benefit you when doing your graduate research.