During the period of this project, we obtained state-of-the-art results to realize proximity-induced superconductivity in the surface states of topological insulators (TIs), with focus on bulk-insulating materials. The main results concerns both materials and devices. In parallel, we have developed magnetic-field-resilient superconducting qubits as a preparation for realizing Majorana qubits.
On the materials front, we have discovered a novel route to realize a very good TI/superconductor interface; namely, we found that the Pd metal deposited on the surface of the MBE-grown (Bi,Sb)2Te3 thin film (which is a bulk-insulating TI material) leads to a self-formation of superconducting PdTe2 layer through diffusion of Pd into (Bi,Sb)2Te3, and the resulting TI/SC interface shows a high transparency. As another major achievement, we have succeeded in growing high-quality bulk-insulating TI nanowires, in which we found that the resistance of the wire shows peculiar gate-voltage-dependent oscillations that reflect the quantum-confined subband structure realized in the nanowire. Also, we have succeeded in the molecular beam epitaxy (MBE) growth of thin films of the candidate topological superconductor In-doped SnTe for the first time; in the tunnel-junction devices made on these films, we have elucidated that in the superconducting state of this material, the topological surface states of its parent material, SnTe, present a proximity-induced 2D superconductivity, which is expected to harbor a Majorana fermion in the vortex core.
On the device front, we have developed a novel route to realize proximity-induced superconductivity in bulk-insulating TI nanowires by utilizing the Pd-diffusion process mentioned above; with this method, we can have a TI nanowire laterally sandwiched by PdTe2 superconductors, leaving the top surface available for gate tuning or tunnel contacts. Another major achievement is that we have succeeded in fabricating Josephson junctions on a bulk-insulating TI flake with a record-high TI/superconductor interface transparency of 0.8; in these 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. In addition, we discovered that the peculiar spin-momentum-locked subband structure realized in TI nanowires leads to the largest magnetochiral anisotropy observed to date when a gate-voltage is applied to the nanowire; this result demonstrates that the electronic structure necessary for generating Majorana fermions is actually realized in TI nanowires.
As an achievement along the activities to realize Majorana qubits, we were able to fabricate a superconducting transmon qubit which can sustain a coherence time of the order of 1 micro-second in a strong magnetic field of 1 T; since we plan to use a transmon qubit for the readout of Majorana qubits which requires a high magnetic field, this achievement is an important step towards the long-term goal of building Majorana qubits.
All these results have been published with an open access. They form the foundation of the future research to nail down the generation of Majorana fermions and to actually realize topological Majorana qubits.
