What are heavy fermions?

I'm surprised that I haven't written about heavy fermion s as a separate post before, so here we go. (It's a break from thinking about science and politics, anyway.) I've written before about " effective mass " for electronic excitations in solids ( wiki page here ).  From classical physics, we are used to the idea that inertial mass \(m\) is the ratio between an external force \(\mathbf{F}\) and the acceleration \(\mathbf{a}\) of some object, \(\mathbf{F} = m\mathbf{a}\), which is also the rate