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Majorana Fermions in Topological Insulator Platforms

Periodic Reporting for period 2 - MajoranaTopIn (Majorana Fermions in Topological Insulator Platforms)

Reporting period: 2018-11-01 to 2020-04-30

Majorana fermions were recently discovered in topological superconductors as exotic quasiparticles having the curious property of being their own antiparticles. They are not only interesting as novel relativistic quasiparticles, but are also useful for realizing fault-tolerant quantum computers, which would revolutionize the future information technologies. However, currently available platforms to materialize Majorana fermions are limited, and the existing platforms have respective drawbacks for actually building qubits for a scalable quantum computer. Also, various unusual properties are predicted for Majorana fermions, but few have been experimentally addressed. To make a leap in the Majorana-fermion research which is technically highly demanding, one needs to grow state-of-the-art materials and tightly combine them with mesoscopic device research. By performing such an integrated research efforts in the same laboratory and by focusing on topological insulators as the materials basis, this project aims to explore new platforms for Majorana qubits and to establish new methodologies to address peculiar physics of Majorana fermions.
The main objectives in the first half of this project were to develop the technologies for Majorana devices based on topological insulators (TIs) and to establish the necessary materials basis to realize them. During this period, there were major progresses both in the fabrication technology of TI-based Josephson junction devices and in the materials basis.
On the fabrication technology front, we have optimized the technology to fabricate TI-based Josephson junction with Al as the superconducting (SC) electrode, and our devices present a record-high transparency at the TI/SC interface. In these Josephson-junction devices, we were able to tune the chemical potential of the TI surface states to the Dirac point by back gating and to demonstrate that a finite supercurrent is maintained even at the Dirac point, which is important for the future Majorana devices.
On the materials front, we have serendipitously discovered a novel route to realize a very good TI/SC interface; namely, we found that simply depositing Pd on the surface of (Bi,Sb)2Te3 thin film (which is a bulk-insulating TI material) leads to the epitaxial self-formation of superconducting PdTe2 layer through diffusion of Pd into (Bi,Sb)2Te3, and the resulting TI/SC interface shows almost a perfect transparency. This discovery will contribute significantly to the realization of TI-based superconducting devices to address Majorana fermions in the second half of this project.
There was also an important progress in the synthesis and device fabrications of TI nanowires. Specifically, we were able to directly elucidate the peculiar subband formation of the Dirac surface states due to quantum confinement in TI nanowires through gate-controlled transport experiments across the Dirac point. The realization of the subband structure peculiar to the topological surface states is an important prerequisite for using TI nanowires for the generation of Majorana fermions through the SC proximity effect, and hence its direct confirmation is of great importance.
All three major results mentioned above (bulk-insulating TI-based Josephson junction, epitaxial TI/SC interface, and the direct confirmation of the subband structure in bulk-insulating TI nanowires) are beyond the state of the art. By taking full advantage of these results, we expect to be able to nail down unique features associated with Majorana fermions in TI-based devices and to build basic components of Majorana qubits until the end of the project.
Gate-tunable Josephson junction based on a bulk-insulating topological insulator