Superconducting Quantum Dot Reveals Exotic Electron Behaviour and Potential for New Devices

Researchers are increasingly exploring indium antimonide nanosheet quantum dots as potential building blocks for topological quantum devices. Xingjun Wu, Ji-Yin Wang (Beijing Academy of Quantum Information Sciences), and Haitian Su, et al. demonstrate a superconductor-coupled quantum dot fabricated within an InSb nanosheet, exhibiting strong spin-orbit coupling and tunable coupling to superconducting leads. Their transport measurements reveal a few-electron regime characterised by Coulomb diamonds and a superconducting gap, alongside Kondo correlations and a novel doublet-singlet quantum phase transition evidenced by changes in Andreev bound state behaviour. This work significantly advances understanding of quantum phenomena in two-dimensional materials and highlights the potential of InSb nanosheet quantum dots for realising advanced quantum technologies.

This device, constructed using a novel bilayer gate technique, allows precise control over the quantum dot’s coupling to superconducting leads and its electron occupancy.

Transport measurements reveal distinct Coulomb diamond features exhibiting alternating even-odd sizes, alongside prominent conductance lines indicative of a few-electron, superconductor-coupled regime. The study demonstrates a significant advancement in planar quantum dot fabrication, overcoming material and fabrication challenges previously hindering the creation of such devices.
The fabricated quantum dot exhibits both a large g-factor and strong spin-orbit coupling, characteristics crucial for manipulating electron spin. At odd electron occupation, clear Kondo signatures emerge, including a zero-bias peak that splits under a magnetic field and diminishes logarithmically with increasing temperature.

These observations confirm the presence of strong electron correlations within the quantum dot and its interaction with the superconducting electrodes. Furthermore, the research details a doublet-singlet quantum phase transition, evidenced by a change in Andreev bound state behaviour as the coupling strength is adjusted.

This transition, from crossing to anticrossing of Andreev bound states, directly correlates with the quantum phase transition within the Josephson junction formed by the superconductor-quantum dot system. The bilayer gate architecture provides enhanced tunability of the dot size and electron occupation, facilitating access to the few-electron regime essential for quantum phenomena.

By avoiding multi-step processing on the InSb nanosheet, the researchers mitigated potential material degradation and interface issues, ensuring high-quality device performance. These findings underscore the potential of InSb nanosheet quantum dots for realising topological quantum devices and exploring novel quantum phenomena.

InSb nanosheet device fabrication utilising bilayer electrostatic gates and superconducting contacts

A molecular beam epitaxy technique facilitates the growth of high-quality InSb nanosheets, forming the basis of this study. These nanosheets were then transferred onto a Si/SiO2 substrate pre-patterned with bilayer metallic electrostatic gates consisting of 2.5-nm-thick titanium and 6.5-nm-thick gold, subsequently capped with a 10-nm-thick alumina film deposited via atomic layer deposition.

Superconducting electrodes, each 125nm wide and separated by 150nm, were fabricated directly onto the InSb nanosheet using standard electron-beam lithography following removal of any native surface oxide. The fabrication process deliberately avoided multi-step processing on the InSb nanosheet to prevent potential material degradation and interface deterioration between the InSb and the superconductor.

A bilayer gate architecture defines the quantum dot, offering enhanced tunability of dot size and electron occupation compared to single-layer designs. Specifically, the plunger gate adjusts the electrochemical potential of the dot, while left and right barrier gates modulate tunnel couplings to the superconducting leads.

Control and cutoff gates further refine the quantum dot confinement. Transport spectroscopy measurements were then performed on the fabricated device within a dilution refrigerator maintaining a base temperature of 25 mK, with devices mounted in a plane parallel to the applied magnetic field. A positive voltage applied to the plunger gate accumulates electrons, forming the quantum dot, while negative voltages on the barrier, cutoff, and control gates create confining potential barriers. Differential conductance measurements, displaying Coulomb diamond-shaped features, were obtained as a function of plunger gate voltage and source-drain voltage at specific gate voltages of (-0.43, -0.6, -0.18, 0) V for the left barrier, right barrier, cutoff, and control gates respectively.

Kondo physics and charge degeneracy in an indium antimonide quantum dot coupled to superconductivity

Researchers have realized a quantum dot within an indium antimonide nanosheet, demonstrating a platform for exploring topological superconductivity. The fabricated device incorporates a quantum dot-superconductor junction defined by a bottom bilayer gate, allowing for tuning of the dot’s coupling to superconducting leads.

Transport measurements revealed Coulomb diamond-shaped differential conductance features exhibiting even-odd alternating sizes, alongside pronounced conductance lines associated with the superconducting gap, confirming a few-electron, superconductor-coupled regime. At odd electron occupation, Kondo signatures emerged, including a zero-bias peak that splits upon application of a magnetic field and is logarithmically suppressed at elevated temperatures.

Temperature-dependent measurements indicated a higher Kondo temperature at energy levels closer to a diamond’s charge degeneracy point. This observation suggests enhanced Kondo correlations in specific regions of the Coulomb blockade. Furthermore, modulating the superconductor-quantum dot coupling strength induced a transition of Andreev bound states from crossing to anticrossing characteristics.

This transition directly corresponds to a doublet-singlet quantum phase transition within the quantum dot Josephson junction, signifying a change in the system’s quantum state. The bilayer gate architecture provided enhanced tunability of the dot size and electron occupation, facilitating access to the few-electron regime crucial for observing these effects. The fabrication process avoided multi-step processing on the InSb nanosheet, preventing potential material degradation and interface deterioration between the InSb and the superconductor.

Quantum Phenomena and Superconducting Coupling in Planar Indium Antimonide Nanoscale Josephson Junctions

Researchers have fabricated a superconducting quantum dot (QD) Josephson junction within a planar indium antimonide (InSb) nanosheet. This device incorporates a bottom bilayer gate to define the QD and tune its coupling to superconducting electrodes, resulting in a high-quality QD with controllable superconductor coupling.

Transport measurements demonstrate that the InSb nanosheet QD possesses large g-factors and strong spin-orbit coupling, comparable to those observed in InSb nanowire QDs. The investigation revealed several quantum phenomena, including the Kondo effect and a singlet-doublet quantum phase transition, evidenced by changes in Andreev bound states as coupling strength increases.

Differential conductance measurements exhibited Coulomb diamonds with alternating sizes, alongside features indicative of a few-electron, superconducting-coupled regime. These findings highlight the importance of sub-gap bound states in the transport characteristics of mesoscopic Josephson junctions and establish InSb nanosheets as a viable platform for exploring topological physics, potentially including Majorana zero modes.

The authors acknowledge limitations stemming from interface disorder potentially induced by fabrication processes, specifically the use of ammonium sulfide etching and argon-ion cleaning, which may create soft superconducting gaps. Future device optimisation could benefit from employing *n situ epitaxial aluminium contacts to induce hard superconducting gaps. Further research will likely focus on refining fabrication techniques to minimise processing-induced disorder and fully exploring the potential of InSb nanosheets for realising topological quantum devices.