Periodic Reporting for period 4 - MajoranaTopIn (Majorana Fermions in Topological Insulator Platforms)
Reporting period: 2021-11-01 to 2022-10-31
Majorana fermions are exotic quasiparticles to appear in topological superconductors, 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 topological quantum computation. However, currently available platforms to materialize Majorana fermions are limited, and the existing platforms have respective drawbacks. This project focused on topological insulators as a promising platform and tried to build the materials basis as well as the device fabrication technologies necessary for the Majorana-fermion research. Once the topological quantum computation based on Majorana fermions becomes a reality following the achievements of this project, it would revolutionize the future information technologies. During the project period, we have succeeded in fabricating bulk-insulating topological-insulator nanowires and establishing the methodology to efficiently induce superconductivity in such nanowires, which are expected to be highly promising platform to generate and manipulate Majorana fermions.
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.
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.
All the major results mentioned above (epitaxial TI/superconductor interface, direct confirmation of the subband structure in bulk-insulating TI nanowires, superconducting In-doped SnTe epitaxial thin films, efficient proximitization of a TI nanowire, bulk-insulating TI-based Josephson junction, gigantic magnetochiral anisotropy in gated TI nanowires, magnetic-field-resilient superconducting qubits) are beyond the state of the art. These results will serve in future experiments to nail down the appearance of Majorana fermions in TI-based devices.