In the last few years there has been an explosive development in materials science. It began with the theoretical prediction of a new class of three dimensional (3D) topological insulators (TIs) which are fully gapped in the bulk, and with unusual gapless protected 2D Dirac surface states. This protection arises from the linear energy-momentum dispersion, with the surface states near the Fermi surface residing on a single Dirac cone. If realized, these systems could be the Holy Grail in the fields of spintronics and fault-tolerant quantum computing. However, access to this 2D quantum matter is a challenge, owing to the difficulty of separating surface contribution from the finite conductivity of the bulk. In this MWN project supported by the Division of Materials Research, swift (~ MeV range) electron and / or proton beams will be utilized to create controlled disorder by (a) tuning the bulk carrier density and Fermi level across the Dirac point, and (b) reducing bulk conductivity by forcing Anderson localization. The former will result in charge compensation in a bulk TI. The later will test recent theoretical ideas of Quantized Anomalous Hall Effect (QAHE) and, even more remarkably, (c) test a recent prediction of a Topological Anderson Insulator - a nontrivial quantum phase with quantized conductance obtained by introducing disorder in a metal with strong spin-orbit interaction. Determining the precise dose at which this occurs should establish a new large-scale route to achieving intrinsic quantum transport of the topological surfaces states.
NON-TECHNICAL SUMMARY: The ability to control the quantum mechanical properties of materials is one of the forefront challenges for material scientists and condensed matter physicists. In the past few decades several bottom-up approaches striving to engineer new materials (thereby controlling their properties) at the atomic level have been devised to achieve this goal. In this proposal a top-down material modification approach will be utilized in which a controlled amount of defects will be introduced to alter the electrical properties of topological insulators (TI's) - a newly discovered class of materials which promises to offer a robust platform for quantum computing. Owing to the unique properties of TI's disorder introduced by particle irradiation will only affect the unwanted / parasitic electrical properties of these materials, while simultaneously enhancing the desired quantum properties that will foster the realization of revolutionary electronic devices and the synthesis of novel states of matter. This proposal is an international collaboration between the condensed matter physics group of the PI at The City College of New York (CCNY) - CUNY, and groups at Ecolé Polytechnique in Palaiseau, France, with unique expertise in swift particle irradiation techniques and advanced optical spectroscopy. This collaboration combines the complementary technical strengths of materials science with particle beam technology to control and tune key electronic properties of the newly discovered functional materials class. CCNY is a federally recognized minority serving institution. Ecole Polytechnique is one of the leading institutes of higher education in France, with a decidedly international outlook. The research program will have a great educational impact on the students involved, both graduate and undergraduate, for it will facilitate international visits and exchanges that will broaden their educational range and provide training in a wide spectrum of materials synthesis and experimental characterization techniques in a collaborative spirit.
