\begin{pmatrix} b_{0,\mathbf{k}}+b^{\dagger}_{0,-\mathbf{k}} \\ b_{0,\mathbf{k}}-b^{\dagger}_{0,-\mathbf{k}} \\ b_{0,\mathbf{k}}+b^{\dagger}_{0,-\mathbf{k}} \\ b_{0,\mathbf{k}}-b^{\dagger}_{0,-\mathbf{k}} \end{pmatrix}