Rosa added At_the_band_center_the__.tex  almost 9 years ago

Commit id: 70060ffa38aba6ece958c0c1d144aa812c10f40b

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At the band center the DOS is constant: $\rho_0=1/\pi t$. There is an alternative expression for the chain DOS by   defining $\cos\phi=\frac{\omega-\epsilon_0}{2t}$. Then   \begin{equation}  g^r_{ch}(\omega)=\frac{1}{t}\left(\cos\phi-i\sin\phi\right)=\exp{-i\phi}{t}  \end{equation}  In general we get  \begin{equation}  G^{r}_{n1}=\exp{-i\phi}{t}  \end{equation}  \subsection{Impurity coupled to the semi-infinite chain}  Now we consider that the $0$ state is coupled to the semi-infinite chain with $V$ as the tunneling amplitude