低维电子系统的噪声谱研究

Electronic noise spectroscopy can not only provide necessary characterization for investigation of new electronic materials and devices, but also give valuable information on microscopic processes in the complicated many-body electron systems. Such information is impossible or difficult to obtain with conventional electron transport or other experimental techniques. In this project we plan to carry out noise spectroscopy measurements of some low dimensional electron systems under the conditions of low temperatures and high magnetic fields in order to explore the nature of the related quantum states or quantum phase transitions. The systems to be stuided will include (1) the plateau-to-plateau transitions in the quantum Hall regime, (2) the two-dimensional electron systems with the lowest Landau level half-filled, (3) the zero energy Landau level of Dirac electron systems and the related metal-insulator transition. These low dimensional systems will be based on GaAs/AlGaAs heterostructures or graphene. The proposed studies will help us to gain further insight into the low dimensional electron systems, in particular the importance of electron inhomogeneities and correlations. In addition to the basic research topics listed above, we also plan to measure the noise spectra of submicron ballistic Hall magnetometers that are based on ultrahigh mobility semiconductor two-dimensional electron systems. The goal is to study the main mechansim of noise, to optimize the material structure and the device design, and to improve the sensitivity of the Hall magnetometers.

电子噪声谱不仅能为研究新型电子材料和器件提供必备的表征，它还可为研究复杂电子体系提供常规电子输运测量所不能获取的微观信息。在本项目中，我们计划利用低温和强磁场条件下的噪声谱来研究一些重要低维电子体系中的量子态和量子相变。这包括： (1)量子霍尔平台与平台之间的转变区，(2)最低朗道能级填充数为1/2时的强相互作用可压缩二维电子系统，(3)狄拉克电子体系中的零能朗道能级及金属-绝缘体转变。具体材料体系将包括基于GaAs/AlGaAs的半导体二维电子系统和石墨烯。这些研究将有助于我们深化对这些低维电子体系中电子不均匀性和关联性的认识。除了上述基础物理研究外，我们还计划测量基于半导体超高迁移率二维电子系统的弹道型亚微米霍尔效应器件的噪声谱，目标是弄清楚产生噪声的主要微观物理机制、优化材料和器件设计和提高器件的灵敏度。

研究低维电子系统的量子输运性质和电子噪声谱对深入理解复杂电子态和实现新型器件应用有重要的意义。我们对半导体GaAs/AlGaAs二维电子系统、三维拓扑绝缘体表面上狄拉克费米子系统和铁磁外尔半金属候选材料HgCr2Se4等体系开展了电子输运和噪声谱方面的研究，主要科研成果如下：（1）利用铝delta掺杂和改变量子阱宽度等方式调控了GaAs/AlGaAs二维电子系统中的无序，观察到量子霍尔平台之间的量子相变临界指数kappa随朗道能级填充数、栅压和无序强度发生显著变化。（2）基于三维拓扑绝缘体，制备了能够用单栅和双栅电压调控的输运器件，在线宽最小达60 nm的霍尔效应器件中实现了很大磁场和温度范围内的线性响应，并且测量了不同尺寸霍尔器件的低频噪声谱。（3）对HgCr2Se4的磁学和电子输运性质进行了系统的表征，观察到数万倍的庞磁电阻以及出现在居里温度附近的噪声谱峰；基于这些实验结果，我们提出自旋关联很可能在磁极化子形成及演化过程中起重要作用，从而深化了对金属-绝缘体转变、庞磁电阻和磁极化子的理解。