转角双层石墨烯在应变下的光电导率

Optical conductivity of twisted bilayer graphene under heterostrain

理论研究了转角双层石墨烯在施加不同单轴应变下的能带结构和光电导率,用连续模型分别计算了转角为1.05°和1.47°的转角双层石墨烯在应变下的能带、态密度以及光电导率,发现这些量随应变的变化是连续且显著的.通过对能带的分析以及光电导率的测量能够获得应变对平带产生的实际影响,这为今后实验对应变与平带的研究打下基础;此外样品往往受到具有空间不均匀性的应变作用,测量其局域的光电导率便能够估计应变的空间分布大小;同时应变对能带的调制为原位调控转角双层石墨烯的强关联、拓扑以及量子效应提供了思路.

Twisted bilayer graphene(TBG)is a two-dimensional material composed of two layers stacked at a certain angle.When the twisted angle decreases,the lattice mismatch between two layers produces moirépattern at a long wavelength which significantly modifies the low-energy band structure.In particular,when the twisted angle is close to the so-called“magic angle”,two moiréflat bands are formed near a charge neutral point due to the strong interlayer coupling.These flat bands with high density of states are essential in realizing superconductivity and correlated insulating states.More recently,the magic angle TBG combining an hBN system has exhibited spin-valley polarization when 3/4 of flat bands are filled,thereby providing an ideal platform to achieve quantum anomalous Hall states.Whether it is TBG system or TBG-hBN system,the flat band becomes a crucial condition for discovering so rich physical connotations.Besides the twisted angle,the strain gives an alternative way to modulate flat bands.It has been reported that applying heterostrain in magic angle TBG can makes flat moiréband tunable;strain can also generate flat bands in non-magic angle TBG.Moreover,the reconstruction of TBG due to the strain gives rise to a serial of novel physical phenomena such as topological protected soliton and photonic crystal.Another reason for studying strain effect is that the strain is ubiquitous in the fabrication progress.The strain can also be controlled via piezoelectric substrate which makes possible the in situ modulation of correlated states,topology and quantum effect.Our work is to study the heterostrain effect in TBG band structure and optical conductivity by using a continuum model.Although the resulting conduction band and valence bands keep connected through Dirac points protected by the C2 symmetry,their separation increases significantly when heterostrain is applied while the Dirac point is also shifted.The optical conductivity is characterized by a series of peaks associated with van Hove singularities,and the peak energies are systematically shifted with the strain amplitude.These changes show that the heterostrain exerts a great influence on electron property of TBG.