单分子磁体的输运电流噪声特性研究

Electron transport through single-molecule magnet (SMM) has attracted considerable attention experimentally as well as theoretically due to the hope of applications in molecular spintronics and quantum information technology. These investigations focused on the differential conductance or average current.On the other hand, it is not enough just to know that the current-voltage characteristics of the SMM system from the fundamental research and applied point of view, and the charge fluctuation especially plays an important role in the electron transport of SMM. Full counting statistics and finite-frequency current noise can provide deep insight into the nature of transport mechanisms which are not accessible by the average current and shot noise measurements. In this project, first of all we develop an efficient fourth-order particle-number-resolved quantum master equation in the sequential and cotunneling regimes, then theoretically study the current noise properties of electron transport through a SMM based on the particle-number-resolved master equation approach. We put emphasis on the effects of the Coulomb interaction between two electrons in the lowest unoccupied molecular orbital, the transverse anisotropy, the angle between the external magnetic field and easy-axis of the SMM, the exchange coupling between the electron spin on the SMM and the SMM spin, the external gate voltage, and left-right asymmetry of the SMM-electrode coupling on the first three cumulants, namely, the average current, shot noise and skewness, and finite-frequency current noise in a SMM. We expect to explore the possible ways to enhance or suppress current noise and, in particular to enhance current super-Poissonian noise，and reveal the intrinsic quantum characteristics of the SMM through the high order current cumulants and finite-frequency current noise, which will provide the theoretical basis for SMM in the single-molecular quantum devices.

鉴于单分子磁体在分子自旋电子学和电子信息技术领域的应用，其量子输运在实验和理论上引起人们的广泛关注，但相关研究主要集中在平均电流上。另一方面，从基础研究和应用的角度来说，仅仅知道其电流特性是不够的，尤其是在单分子磁体系统中电荷涨落对其量子输运有重要影响，而电子全计数统计和有限频率噪声可以提供比平均电流和散粒噪声更多的关于系统微观机制的信息。本项目首先发展在共隧穿极限下的四阶粒子数分辨量子主方程，然后基于此方法研究单分子磁体系统的库仑相互作用、横向各向异性、易轴与磁场的夹角、传导电子与分子大自旋的耦合强度、外加门电压、电极的不对称耦合等对其前三阶电流累积矩，即平均电流、散粒噪声、偏斜度，以及有限频率电流噪声特性的影响。探索加强或抑制单分子磁体电流噪声，特别是加强电流超泊松噪声的可能方法，并揭示其内禀量子特性与高阶电流累积矩和有限频率噪声之间的关系，为其在单分子量子器件方面的应用提供理论依据。

基于单分子磁体的分子自旋电子学是半导体微加工技术的终极目标之一。因此，理解单分子磁体结的量子输运特性十分必要。特别是，无论从基础研究还是应用角度来说，仅仅知道其电流特性是不够的，尤其是在单分子磁体系统中电荷涨落对其量子输运有重要影响，而电子全计数统计和有限频率噪声可以提供比平均电流和散粒噪声更多的关于电子输运的微观机制信息。.在本项目中，首先分别在顺序隧穿极限和共隧穿极限下，建立了计算单分子体系电子全计数统计和有限频率噪声的非马尔科夫粒子数分辨量子主方程；并基于此方法，研究了单分子磁体和耦合量子点体系的电子全计数统计和有限频率噪声特性。主要结果如下：.(1)当单分子磁体与一个金属一个铁磁电极或与两个铁磁电极弱耦合时，负微分电导和电流的超泊松电流噪声可由外加的门电压调节，并且该特性敏感地依赖于源极和漏极的自旋极化率。但是，在负微分电导区域内，散粒噪声是否加强依赖于相应负微分电导的形成机制。.(2)当单分子磁体与两个铁磁电极弱耦合时，在顺序隧穿区域内，快速自旋弛豫可以压制其隧穿磁电阻；而在共隧穿区域内，其隧穿磁电阻将从一个大的正值转变到一个小的负值。.(3)通过对耦合量子点体系的研究，发现单分子体系的电流高阶累积矩敏感地依赖于其量子相干性，并且其非马尔科夫效应主要通过其量子相干性体现。这些特性可以用来定向获取系统的量子相干性信息和量子点背景原子形成的核磁场的大小和方向。.(4)单分子体系的有限频率散粒噪声可以获取其内部信息。例如，单分子磁体自旋态的翻转时间联系到其有限频率噪声特性；耦合量子点体系的自旋轨道耦合常数可以从其有限频率噪声谱提取；并联耦合量子点体系封闭的量子磁通可以通过其有限频率噪声谱观察。.上述预测的单分子磁体和耦合量子点体系的输运电流噪声特性，对于深入理解其电子输运过程非常有用，可以为其在单分子量子器件方面的应用提供理论依据。