Wu Fengcheng, Das Sarma Sankar
Condensed Matter Theory Center and Joint Quantum Institute, Department of Physics, University of Maryland, College Park, Maryland 20742, USA.
Phys Rev Lett. 2020 Jan 31;124(4):046403. doi: 10.1103/PhysRevLett.124.046403.
We present a microscopic theory for collective excitations of quantum anomalous Hall ferromagnets (QAHF) in twisted bilayer graphene. We calculate the spin magnon and valley magnon spectra by solving Bethe-Salpeter equations and verify the stability of QAHF. We extract the spin stiffness from the gapless spin wave dispersion and estimate the energy cost of a skyrmion-antiskyrmion pair, which is found to be comparable in energy with the Hartree-Fock gap. The valley wave mode is gapped, implying that the valley polarized state is more favorable compared to the valley coherent state. Using a nonlinear sigma model, we estimate the valley ordering temperature, which is considerably reduced from the mean-field transition temperature due to thermal excitations of valley waves.
我们提出了一种关于扭曲双层石墨烯中量子反常霍尔铁磁体（QAHF）集体激发的微观理论。我们通过求解贝特 - 萨尔皮特方程来计算自旋磁振子和能谷磁振子谱，并验证了QAHF的稳定性。我们从无隙自旋波色散中提取出自旋刚度，并估计了一个斯格明子 - 反斯格明子对的能量成本，发现其能量与哈特里 - 福克能隙相当。能谷波模式是有能隙的，这意味着与能谷相干态相比，能谷极化态更有利。使用非线性西格玛模型，我们估计了能谷有序温度，由于能谷波的热激发，该温度比平均场转变温度大幅降低。
