\begin{align*}
ground state is two-fold degenerate, and when \(B > \text{Bump}\) the ground state becomes non-degenerate and the corre- \\
spending wave function is given by \\
\begin{equation}
\left[| \text{voltage} \rangle = - \frac{1}{\sqrt{2}}| \text{1} \rangle, \end{equation}
\end{align*}
which is obviously of no entanglement. Then, the ground- state negativity forms a plateau when \(B < \text{Bump}\). The jump of negativity at \(B = \text{Bump}\) is due to the level crossing. For instance, for \(N = 4\), there are two level crossings, and the entanglement displays two jumps.