\begin{document}$\hat H = - J\sum\limits_j{} {\left( {\hat c_j^\dagger {{\hat c}_{j + 1}} + {\rm{h}}{\rm{.c}}{\rm{.}}} \right)} + 2\lambda \sum\limits_j{} {\dfrac{{\cos \left( {2{\text{π}}\alpha j} \right)}}{{1 - b\cos \left( {2{\text{π}}\alpha j} \right)}}} {\hat n_j} + \dfrac{U}{2}\sum\limits_j{} {{{\hat n}_j}\left( {{{\hat n}_j} - 1} \right)} ,$\end{document} where there exists no interaction, the system displays mobility edges at \begin{document}$b\varepsilon = 2(J - \lambda )$\end{document}, which separates the extended regime from the localized one and b = 0 is the standard Aubry-André model. By applying the perturbation method to the third order in a strong interaction case, we can induce an effective Hamiltonian for bosonic pairs. In the small b case, the bosonic pairs present the mobility edges in a simple closed expression form \begin{document}$b\left( {\dfrac{{{E^2}}}{U} - E - \dfrac{4}{E}} \right) = - 4\left(\dfrac{1}{E} + \lambda \right)$\end{document}, which is the central result of the paper. In order to identify our results numerically, we define a normalized participation ratio (NPR) \begin{document}$\eta (E)$\end{document} to discriminate between the extended properties of the many-body eigenvectors and the localized ones. In the thermodynamic limit, the NPR tends to 0 for a localized state, while it is finite for an extended state. The numerical calculations finely coincide with the analytic results for b = 0 and small b cases. Especially, for the b = 0 case, the mobility edges of the bosonic pairs are described as \begin{document}$\lambda = - 1/E$\end{document}. The extended regime and the one with the mobility edges will vanish with the interaction U increasing to infinity. We also study the scaling of the NPR with system size in both extended and localized regimes. For the extended state the NPR \begin{document}$\eta (E) \propto 1/L$\end{document} tends to a finite value with the increase of L and \begin{document}$L \to \infty $\end{document}, while for the localized case, \begin{document}$\eta (E) \propto {(1/L)^2}$\end{document} tends to zero when \begin{document}$L \to \infty $\end{document}. The \begin{document}$b \to 1$\end{document} limit is also considered. As the modulated potential approaches to a singularity when \begin{document}$b \to 1$\end{document}, the analytic expression does not fit very well. However, the numerical results indicate that the mobility edges of bosonic pairs still exist. We will try to consider the detection of the mobility edges of the bosonic pairs in the future."> - 必威体育下载

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Xu Zhi-Hao, Huangfu Hong-Li, Zhang Yun-Bo
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  • Abstract views:6666
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  • Received Date:17 December 2018
  • Accepted Date:20 February 2019
  • Available Online:01 April 2019
  • Published Online:20 April 2019

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