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Electronic Transport in Topological Insulator Hybrid Devices

Final Report Summary - TI HYBRID DEVICES (Electronic Transport in Topological Insulator Hybrid Devices)

The use of quantum devices as an experimental platform has so far yielded a wealth of physics observed in low-dimensional systems (2D and 1D), superconducting devices and, more recently, devices consisting of layered materials such as graphene. With the emergence of new material classes, such as topological insulators and correlated states, there is a significant experimental drive to develop fabrication methods for integrating such materials into quantum devices. Under the CIG program, I have set out to build a research group with the focus of fabricating van-der-Waals (vdW) devices, and their measurement at low temperatures.
In setting up my research group at the Hebrew University, I set out to utilize the expanding device technology capabilities to continue using devices as experimental platforms. I chose to focus on the “van-der-Waals transfer technique”, which I learned during my post-doctoral work in MIT, where I developed device fabrication techniques for probing topological insulators (TIs) using electronic transport. The vdW transfer technique is a new fabrication method allowing for vertical stacking of atomically thin layers. This technique allows for table-top fabrication of novel devices, utilizing minuscule quantities of high quality raw material which can be obtained from a variety of sources. My projects include the integration of topological insulator devices, tunneling devices, and graphene devices.
Following my hiring on July 2013, my lab space became available late 2013 and first equipment was purchased early 2014. During 2015 and 2016, I have purchased and installed 2 cryostats, and have set up a glove box for inert atmosphere device fabrication. My lab now includes 1 post-doc, 2 Ph.D. students, and 4 M.Sc. students. I have instructed over 10 undergraduates for research projects and participate in the training of high-school students.
Tunneling in vdW devices
In this project, the objective is the development of a device-based tunneling spectroscopy technology using an ex-situ fabrication of tunnel barriers, applicable for broad range of target systems. I focus on layered vdW materials, which presently attract attention due to advances in device-fabrication technologies and progress in material synthesis. Their layered structure results in a tendency to cleave along atomically flat surfaces – making them amenable for ex-situ deposition of on-demand high-quality planar tunnel barriers, and the creation of atomically flat interfaces.
In this project, we developed vdW-based devices using NbSe2 as the superconducting target material and ultrathin MoS2 as tunnel barrier. The devices are fabricated in my lab using our glove-box transfer setup to avoid oxidation, and electrodes are patterned using instrumentation present at the Hebrew University unit for nano-fabrication. We then measure the devices using our own cryostats or with the assistance of collaborators in Orsay (Charis Quay and Marco Aprili). The main result, shows a very clean tunneling spectrum, from which are able to probe the multi-band nature of NbSe2, and the sub-gap vortex bound states. The results of this study are now under review. They are reported in http://arxiv.org/abs/1703.07677
Graphene-TI interfaces
Stacking materials with atomically precise interfaces may cause various types of effects. From the perspective of topological insulator physics, we are interested to study what type of proximity is afforded by the topological surface state – according to theory, a range of possible outcomes are possible for graphene stacked on a TI, including an enhancement of the spin-orbit term and induced topology. Motivated by such predictions, I have been studying graphene-TI hybrids since my post-doc, and have found graphene to be weakly coupled to the TI surface state. In my lab, the project was further advanced by fabricating graphene-TI devices of very small mismatch angle. Our results show that when graphene and TI are close to perfect relative orientation, an unusual excitation spectrum emerges. This spectrum is associated with either a very strong doping of the graphene, or with a band-folding of the TI surface state. These results, recently accepted for publication in Physical Review B, suggest that the graphene-TI hybrid has a number of very interesting degrees of freedom which can be further explored, most notably the effect of the superlattice at small orientation angles.
Outlook
My lab is now continuing its drive towards refining the 2 types of devices discussed above. We are interested in carrying out tunneling spectroscopy of sophisticated hybrid devices, involving topological insulators. The Career Integration Grant has been extremely useful in providing the initial funds to hire students and fund consumables in these studies.