   \sum_{{\bf k},{\bf q},\sigma}   \left(     g_{\bf q} \,      c_{{\bf k},\sigma}^{\dagger} c_{({\bf k}+{\bf q}),\sigma}      b_{\bf q}^{\dagger} +      g_{\bf q}^{*} \,     c_{({\bf k}+{\bf q}),\sigma}^{\dagger} c_{{\bf k},\sigma}     b_{\bf q}   \right)