Solution
= Solution
At g=1, $\epsilon_k=4J|\sin(k/2)|\sim2J|k|$. The gap closes at zero momentum and $E_0/N=-4J/\pi$. With unit lattice spacing the critical velocity is $2J/\hbar$.
= Solution
At g=1, $\epsilon_k=4J|\sin(k/2)|\sim2J|k|$. The gap closes at zero momentum and $E_0/N=-4J/\pi$. With unit lattice spacing the critical velocity is $2J/\hbar$.