Instability of the solitary wave solutions for the genenalized derivative Nonlinear Schrodinger equation in the critical frequency case

We study the stability theory of solitary wave solutions for the generalized derivative nonlinear Schrodinger equation $$ ipartial_{t}u+partial_{x}^{2}u+i|u|^{2sigma}partial_x u=0. $$ The equation has a two-parameter family of solitary wave solutions of the form begin{align*} phi_{omega,c}(x)=varphi_{omega,c}(x)exp{big{ ifrac c2 x-frac{i}{2sigma+2}int_{-infty}^{x}varphi^{2sigma}_{omega,c}(y)dybig}}. end{align*} Here $ varphi_{omega,c}$ is some real-valued function. It was proved in cite{LiSiSu1} that the solitary wave solutions are stable if $-2sqrt{omega }<c <2z_0sqrt{omega }$, and unstable if $2z_0sqrt{omega }<c <2sqrt{omega }$ for some $z_0in(0,1)$. We prove the instability at the borderline case $c =2z_0sqrt{omega }$ for $1<sigma<2$, improving the previous results in cite{Fu-16-DNLS} where $3/2<sigma<2$.