摘要
考虑外加磁场导致的量子化朗道能级以及邻近效应诱导产生的交换作用,采用无规相近似(Random-phase approximation,RPA)下的介电函数对单层二硫化钼(Monolayer molybdenum disulfide,ML-MoS2)的纵向磁光电导率进行理论研究。探究了磁场、邻近效应诱导产生的交换作用等因素对纵向磁光电导率的影响。在太赫兹(Terahertz, THz)频段,可以看到由导带内电子跃迁所贡献的两个磁光吸收峰。在可见光频段,可以观察到从价带到导带电子跃迁所贡献的多个磁光吸收峰。研究结果表明,邻近效应诱导产生的交换作用和磁场强度对纵向磁光电导率有重要的影响,单层二硫化钼可应用于可见光到太赫兹频段的自旋电子学和谷电子学磁光器件。
近年来,以单层二硫化钼(Monolayer Molybdenum Disulfide,ML-MoS2)为代表的单层过渡金属硫化物(Monolayer Transition Metal Dichalcogenides, ML-TMDs)以其优异的物理性
当在垂直于二维电子气系统平面方向外加均匀磁场将构成二维量子化朗道系统,并在该系统将能观测到许多有趣的物理现
随着对二维电子气材料磁光吸收特性实验研究的不断增加,对相关理论研究的需求也日益突出。在本论文中,将对单层二硫化钼的磁光电导率进行理论计算。在此之前对单层二硫化钼等新型二维电子气材料的磁光性质研究大多采用Kubo公式、玻尔兹曼方程等方法。本文利用RPA理论下的介电函数和磁光电导之间的关系分析得到纵向磁光电导率。在磁场以及由衬底引入的邻近诱导交换作用下,研究自旋和谷极化对n-型单层二硫化钼中的朗道能级(Landau Levels, LLs)和磁光电导率的影响。
在本研究中,我们考虑
, | (1) |
其中,为费米速度,和分别为谷和自旋指数。磁场下正则动量并取朗道规范。泡利矩阵表示赝自旋,, 为2×2的单位矩阵。为带隙参数,/分别为导带/价带本征自旋轨道耦合参数。和分别是邻近诱导的交换作用下导带和价带中电子所受的有效塞曼场。此外,和具有相同的形式,其中j=z, v分别表示自旋和谷塞曼场,玻尔磁子, ,为自由电子g因子。将哈密顿量对角化后,可以得到能量本征值(朗道能级):
, | (2) |
这里,n为朗道指数,分别表示导带和价带,回旋频率,是磁场长度。,。由
, | (3) |
, | (4) |
在

图1 理论模型所考虑的样品和实验测量示意图
Fig. 1 Schematic diagram of theoretical consideration of the sample and experimental measurement
, | (6) |
其中,是库仑势的二维傅里叶变换,为高频介电函数。密度-密度关联函数由下式给出:
, | (7) |
其中,是电子-电子交互作用项的形状因子,为所考虑系统的面积。
在本研究中,将n-型单层二硫化钼置于一个能引起邻近效应诱导产生的交换作用的介电衬底上。在数值计算过程中,采用了典型的单层二硫化钼参

图2 单层二硫化钼随磁场强度变化的朗道能级以及载流子浓度时对应的费米能级,有效塞曼场强度为和。顶部和底部分别为导带和价带
Fig. 2 The LLs and Fermi level as a function of magnetic field strength B for a carrier density in ML-MoS2. The effective Zeeman fields are and . The top and bottom panels correspond to the conduction and valence bands, respectively
根据

图3 在一定温度、电子浓度和有效塞曼场,不同磁场强度下n-型单层二硫化钼导带内不同朗道能级间电子跃迁所贡献的纵向磁光电导率随光子能量的变化
Fig. 3 The magneto-optical conductivity induced via the transitions processes within the conduction band LLs as a function of photon energy at fixed temperature, electron density, and effective Zeeman fields for different magnetic field strengths in n-type ML-MoS2
为了探究邻近效应诱导产生的交换作用对纵向磁光电导率的影响,

图4 在一定温度、电子浓度以及磁场强度,不同有效塞曼场下n-型单层二硫化钼导带内不同朗道能级间电子跃迁所贡献的纵向磁光电导率随光子能量的变化
Fig. 4 The magneto-optical conductivity induced via the transitions processes within the conduction band LLs as a function of photon energy at fixed temperature, electron density, and magnetic field strength for different effective Zeeman fields in n-type ML-MoS2

图5 在一定温度、电子浓度、有效塞曼场以及磁场强度下,从满占据的价带到电子浓度的导带电子跃迁前6个通道所贡献的纵向磁光电导率随光子能量的变化
Fig. 5 The magneto-optical conductivity induced via the transitions from the occupied valence band to conduction band with an electron density as a function of photon energy at a fixed temperature, electron density, effective Zeeman fields, and magnetic field strength. The magneto-optical absorptions from the first six transition channels are indicated
在本项研究中,考虑外加磁场导致的量子化朗道能级以及邻近效应诱导产生的交换作用,通过低能区单电子哈密顿量和RPA近似下的介电函数,研究了单层二硫化钼中电子的朗道能级以及n-型单层二硫化钼导带内、价带到导带间的电子跃迁所贡献的纵向磁光电导率。进一步探究了磁场、邻近效应诱导产生的交换作用对纵向磁光电导率的影响。研究表明,邻近效应诱导产生的交换作用能够提升单层二硫化钼的自旋和谷极化;对于n-型单层二硫化钼样品,导带内电子跃迁纵向磁光电导率谱在太赫兹频段出现两个吸收峰;从满占据的价带到电子浓度的导带间的电子跃迁贡献的纵向磁光电导率谱在可见光频段出现一系列吸收峰;磁场强度和邻近效应诱导产生的交换作用均能有效地调节磁光吸收峰的位置。研究结果表明,单层二硫化钼作为一个新型二维半导体材料,可应用于太赫兹到可见光频段的自旋电子学和谷电子学磁光材料和器件,且可通过改变外加磁场和邻近效应诱导产生的交换作用的方法来调控材料的磁光响应性质,进而拓展材料潜在的器件应用领域。
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