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arXiv 2103.11583cond-mat.mes-hallcond-mat.quant-gasmath-phmath.MP
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Kähler几何与Chern绝缘体:拓扑与量子度规之间的关系

Kähler geometry and Chern insulators: Relations between topology and the quantum metric

Bruno Mera, Tomoki Ozawa

摘要:

我们从Kähler几何的角度研究了Chern绝缘体,Kähler几何是光滑流形的几何,它具有由辛形式、可积几乎复结构和黎曼度量组成的相容三元组。费米投影仪,即投影到被占据的能带上的投影仪,提供了一个到凯勒流形的映射。在量子态的目标空间上,占据带的量子度规和Berry曲率分别与黎曼度规和辛形式有关。我们发现参数空间关于量子度规的最小体积是$π|\mathcal{C}|$,其中$\mathcal{C}$是第一个陈数.我们确定的条件下,实现最小体积的布里渊区和扭转角空间。当量子度规具有从量子态空间中继承的Kähler流形结构时,布里渊区的体积最小,量子度规处处非简并。如果扭角环面的量子体积是最小的,则两个参数空间都具有从量子态空间继承的凯勒流形的结构。这些条件被证明是与分数陈绝缘体的稳定性。对于两个波段的系统,布里渊区的体积自然是最小的贝里曲率是无处不在的非负或非正的,我们还展示了如何后者,在这种情况下是成比例的量子体积形式,必然有零由于拓扑约束。

英文摘要:

We study Chern insulators from the point of view of Kähler geometry, i.e. the geometry of smooth manifolds equipped with a compatible triple consisting of a symplectic form, an integrable almost complex structure and a Riemannian metric. The Fermi projector, i.e. the projector onto the occupied bands, provides a map to a Kähler manifold. The quantum metric and Berry curvature of the occupied bands are then related to the Riemannian metric and symplectic form, respectively, on the target space of quantum states. We find that the minimal volume of a parameter space with respect to the quantum metric is $π|\mathcal{C}|$, where $\mathcal{C}$ is the first Chern number. We determine the conditions under which the minimal volume is achieved both for the Brillouin zone and the twist-angle space. The minimal volume of the Brillouin zone, provided the quantum metric is everywhere non-degenerate, is achieved when the latter is endowed with the structure of a Kähler manifold inherited from the one of the space of quantum states. If the quantum volume of the twist-angle torus is minimal, then both parameter spaces have the structure of a Kähler manifold inherited from the space of quantum states. These conditions turn out to be related to the stability of fractional Chern insulators. For two-band systems, the volume of the Brillouin zone is naturally minimal provided the Berry curvature is everywhere non-negative or nonpositive, and we additionally show how the latter, which in this case is proportional to the quantum volume form, necessarily has zeros due to topological constraints.

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