Could someone tell me about what I must have as a knowledge to study condensed matter physics ?
I think having a reasonable base of quantum mechanics helps you to study condense matter physics more conveniently.
To go to graduate school for CM? It's a large and diverse field of physics. You'll need the basic core curriculum in physics, elective classes in statistical mechanics, fluids, E&M, chemistry, materials science, programming, or computational physics are useful areas of further study. There is no "you did this class, so now you're good to go to study all of condensed matter physics!" It just doesn't work that way.
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