I am told that in ferromagnets, time reversal symmetry is broken. However, I don't know any hamiltonian terms in solid that can break time reversal symmetry. So is there a hamiltonian term I don't know or is there any subtlety in ferromagnets?
I don't understand this. Can you show me an example?In fact, the spontaneously broken symmetries cannot be represented as operators in a Hilbert space, as all matrix elements involving two different ground states vanish. That's the formal definition of a spontaneously broken symmetry.