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arXiv 2102.09214cond-mat.dis-nnquant-ph
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非厄米型Aubry-André-Harper模型中的相变

Phase transitions in a non-Hermitian Aubry-André-Harper model

Stefano Longhi

摘要:

Aubry-André-Harper模型提供了一维晶格中非周期有序的范例,在准周期势幅值$V$的有限临界值$V_c$处显示出离域-定域相变。就系统的动力学行为而言,当测量波包扩散的量子扩散指数$δ$时,相变是不连续的,其中$δ=1$处于离域相$V<V_c$(弹道输运),$δ\simeq 1/2$处于临界点$V=V_c$(扩散输运),而$δ=0$处于定域相$V>V_c$(动力学定域)。然而,相变原来是平滑的,当一个措施,作为一个动力学变量,在晶格中的激发传输的速度$v(V)$,这是一个连续的函数的潜在振幅$V$和消失的本地化相接近。本文考虑Aubry-André-Harper模型的一个非Hermitian扩展,其中沿晶格的跳跃是不对称的,并证明了动力学定域-离域跃迁不仅在扩散指数$δ$上是不连续的,而且在弹道输运速度$v$上也是不连续的.这意味着,即使非常接近光谱相变点,晶格中也允许具有有限速度的相当违反直觉的弹道输运。此外,我们表明,弹道速度可以增加$V$增加到零以上,即令人惊讶的无序在晶格中可以导致增强的运输。

英文摘要:

The Aubry-André-Harper model provides a paradigmatic example of aperiodic order in a one-dimensional lattice displaying a delocalization-localization phase transition at a finite critical value $V_c$ of the quasiperiodic potential amplitude $V$. In terms of dynamical behavior of the system, the phase transition is discontinuous when one measures the quantum diffusion exponent $δ$ of wave packet spreading, with $δ=1$ in the delocalized phase $V<V_c$ (ballistic transport), $δ\simeq 1/2$ at the critical point $V=V_c$ (diffusive transport), and $δ=0$ in the localized phase $V>V_c$ (dynamical localization). However, the phase transition turns out to be smooth when one measures, as a dynamical variable, the speed $v(V)$ of excitation transport in the lattice, which is a continuous function of potential amplitude $V$ and vanishes as the localized phase is approached. Here we consider a non-Hermitian extension of the Aubry-André-Harper model, in which hopping along the lattice is asymmetric, and show that the dynamical localization-delocalization transition is discontinuous not only in the diffusion exponent $δ$, but also in the speed $v$ of ballistic transport. This means that, even very close to the spectral phase transition point, rather counter-intuitively ballistic transport with a finite speed is allowed in the lattice. Also, we show that the ballistic velocity can increase as $V$ is increased above zero, i.e. surprisingly disorder in the lattice can result in an enhancement of transport.

机构、总结、中文摘要均由AI分析而来,可能存在错误,请以论文正文为准。

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