Elucidating the mechanism of millimeter-long transport of photogenerated carriers in topological insulators

非技术摘要：当光被半导体等材料吸收时，自由电子和空穴的数量增加，导致导电性增加。这种现象被称为光电导性。了解光载流子如何传输电荷是太阳能电池和光探测器等许多重要应用的关键。在过去的十年里，一种新的令人着迷的材料被发现，称为拓扑绝缘体。这些材料在表面上具有独特的电子性质，可能会在技术上得到应用。首席研究员的研究小组最近证明了在低温下，光生载流子在拓扑绝缘体中的异常长距离传输。这表明了一种称为激子凝聚的量子态的形成。如果得到证实，这种量子态在量子计算中具有潜在的用途。这个项目的目的是通过各种实验技术来研究这一现象，并了解其背后的物理机制。该项目还在快速发展的量子和自旋电子器件研究领域教育和培训本科生和研究生，并提供针对K-12学生的外展活动。技术摘要：PI最近对三维拓扑绝缘体(TIS)的实验研究发现了不寻常的非局部光电流。值得注意的是，局域光生载流子可以沿着本征掺Sb的Bi2Se3纳米带传输一毫米以上，然后在高达40K的温度下复合，并伴随着高达60%的内量子效率。观察到光生载流子的高度无耗散输运意味着形成了超流激子凝聚体。这个项目的目标是阐明这种高度非局域光电流产生的物理机制。一套创新的空间、时间和光谱分辨技术被应用来实现这一目标。研究了空间分辨的螺旋度相关的光电流和表面磁光克尔效应，以了解光生载流子在TI表面的自旋极化和自旋-动量耦合。红外光电流谱、角度分辨光电子能谱(ARPES)和扫描隧道谱(STS)将被用来为表面Dirac锥中的激子空隙打开提供证据。研究了比Bi2Se_3更薄、更本征的样品和拓扑材料，以达到更高的临界温度。如果得到证实，拓扑激子凝聚有望提供一种新的令人兴奋的范例，利用手性超流激子走向高温量子逻辑和新型自旋电子器件。这一奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

NON-TECHNICAL SUMMARY:When light is absorbed by a material such as a semiconductor, the number of free electrons and holes increases, resulting in increased electrical conductivity. This phenomenon is known as photoconductivity. Understanding how photocarriers transport charge is key to many important applications such as solar cells and light detectors. A new category of fascinating materials, called topological insulators, has been discovered over the past decade. These materials have unique electronic properties at the surface and may find technological applications. The principal investigator’s research group has recently demonstrated unusual long-distance transport of photogenerated charge carriers in topological insulators at low temperatures. This suggests the formation of a quantum state known as an exciton condensate. If confirmed, this quantum state has potential uses in quantum computing. The aim of this project is to study this phenomena by a variety of experimental techniques and understand the physical mechanism behind it. This project also educates and trains undergraduate and graduate students in the rapidly advancing research area of quantum and spintronic devices and offers outreach activities targeting K-12 students.TECHNICAL SUMMARY:The PI’s recent experimental studies of three-dimensional topological insulators (TIs) have revealed unusual non-local photocurrent. Remarkably, the locally photogenerated carriers can transport over a millimeter along the intrinsic Sb-doped Bi2Se3 nanoribbons before recombination at up to 40 K, accompanied by an internal quantum efficiency as high as 60%. The observation of highly dissipationless transport of photogenerated carriers implies the formation of superfluidic exciton condensate. The goal of this project is to elucidate the physical mechanism of this highly nonlocal photocurrent generation. A suite of innovative spatially, temporally, and spectrally resolved techniques are applied to achieve this goal. Spatially resolved helicity-dependent photocurrent and the surface magneto-optical Kerr effect is investigated to understand the spin polarization and spin-momentum coupling of the photogenerated carriers at the TI surface. Infrared photocurrent spectroscopy, angle-resolved photoemission spectroscopy (ARPES), and scanning tunneling spectroscopy (STS) will be performed to provide evidence for excitonic gap opening in the surface Dirac cone. Thinner and more intrinsic samples and topological materials beyond Bi2Se3 are studied to achieve higher critical temperatures. If confirmed, topological exciton condensation is expected to provide a new and exciting paradigm exploiting chiral superfluidic excitons towards high temperature quantum logic and novel spintronic devices.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.