Students who get admission for Int-PhD in Physics at institutes like IISc, IISER Pune, or IISER Kolkata score decent marks in IIT-JAM. You should have a rank under 300 if you want admission at IISc. The required cutoff for IISER Pune remains under 500. For IISER Kolkata, you will need a rank under 700. 

Here is an interview experience shared by a senior researcher at IISER