Special values of Kloosterman sums and binomial bent functions

Let $p\ge 7$, $q=p^m$. $K_q(a)=\sum_{x\in \mathbb{F}_{p^m}} \zeta^{\mathrm{Tr}^m_1(x^{p^m-2}+ax)}$ is the Kloosterman sum of $a$ on $\mathbb{F}_{p^m}$, where $\zeta=e^{\frac{2\pi\sqrt{-1}}{p}}$. The value $1-\frac{2}{\zeta+\zeta^{-1}}$ of $K_q(a)$ and its conjugate have close relationship with a class of binomial function with Dillon exponent. This paper first presents some necessary conditions for $a$ such that $K_q(a)=1-\frac{2}{\zeta+\zeta^{-1}}$. Further, we prove that if $p=11$, for any $a$, $K_q(a)\neq 1-\frac{2}{\zeta+\zeta^{-1}}$. And for $p\ge 13$, if $a\in \mathbb{F}_{p^s}$ and $s=\mathrm{gcd}(2,m)$, $K_q(a)\neq 1-\frac{2}{\zeta+\zeta^{-1}}$. In application, these results explains some class of binomial regular bent functions does not exits.