Fidelity, fidelity susceptibility, and von Neumann entropy to characterize the phase diagram of an extended Harper model

Abstract

In an extended Harper model, the fidelity for the two lowest band-edge states corresponding to different model parameters, the fidelity susceptibility and the von Neumann entropy of the lowest band-edge states, and the spectrum-averaged von Neumann entropy are studied numerically. The fidelity is near one when parameters are in the same phase or same phase boundary; otherwise it is close to zero. There are drastic changes in fidelity when one parameter is at phase boundary. For the fidelity susceptibility the finite scaling analysis is performed. The critical exponents $\ensuremath{\alpha}$, $\ensuremath{\beta}$, and $\ensuremath{\nu}$ depend on system sizes for the metal-metal phase transition, while this is not so for the metal-insulator phase transition. At both phase transitions $\ensuremath{\nu}/\ensuremath{\alpha}\ensuremath{\approx}2$. The von Neumann entropy is near one for the metallic phase, while it is small for the insulating phase. There are sharp changes in the von Neumann entropy at phase boundaries. According to the variations of the fidelity, fidelity susceptibility, and the von Neumann entropy with model parameters, the phase diagram, which includes two metallic phases and one insulating phase separated by three critical lines with one bicritical point, can be completely characterized. These numerical results indicate that the three quantities are suited for revealing all the critical phenomena in the model.