Periodic Reporting for period 4 - Topo2DEG (Topological states in superconducting two-dimensional electron gases)
Berichtszeitraum: 2023-09-01 bis 2024-08-31
Superconductors and semiconductors are crucial components of modern information technology, with applications ranging from computing to sensing technologies. This project aimed to create and study new electronic devices by combining superconducting and semiconducting elements, with a particular focus on understanding the quantum mechanical behavior of electrons at extremely low temperatures. The hybridization of these materials is attracting considerable scientific interest due to the potential to realize topological states of matter. Such states hold promise not only for fundamental physics but also for next-generation quantum computing, where robust qubits can revolutionize information processing.
A key objective was to explore regimes where characteristic energy scales—Fermi energy, spin-orbit interaction, superconducting gap, and Zeeman splitting—are comparable. This led to the discovery of unique physical phenomena in largely uncharted territories. Our approach diverged from the conventional use of semiconductor nanowires by employing hybrid two-dimensional electron gases (2DEGs) where superconductors are grown in situ and matched to the semiconductor lattice. This innovative platform enabled us to design more complex and less disordered devices, surpassing the limitations of nanowire-based systems and allowing for more versatile experimental setups.
A key objective was to explore regimes where characteristic energy scales—Fermi energy, spin-orbit interaction, superconducting gap, and Zeeman splitting—are comparable. This led to the discovery of unique physical phenomena in largely uncharted territories. Our approach diverged from the conventional use of semiconductor nanowires by employing hybrid two-dimensional electron gases (2DEGs) where superconductors are grown in situ and matched to the semiconductor lattice. This innovative platform enabled us to design more complex and less disordered devices, surpassing the limitations of nanowire-based systems and allowing for more versatile experimental setups.
Collaborating with Prof. Wegscheider’s group at ETH Zurich, we developed high-quality InAs/Al heterostructures, screening over 26 wafers and achieving electron mobilities consistently above 60,000 cm²/Vs. This collaboration also led to breakthroughs in understanding epitaxial growth, including interfaces between Al and Nb, which are crucial for achieving stable hybrid systems. These advancements are expected to enhance device reproducibility and performance in future applications.
We conducted extensive studies on planar Josephson junctions fabricated from these heterostructures. These experiments unveiled new phenomena related to phase dynamics in the presence of large in-plane magnetic fields and interactions with microwave radiation. The insights gained are essential for realizing topological states, particularly by clarifying how spin-orbit interactions and orbital magnetic fields influence superconducting properties and switching currents.
Expanding on theoretical proposals, we explored multiterminal Josephson junctions, focusing on three-lead and four-lead configurations. These complex devices required the integration of superconducting loops and flux-bias lines, combined with precise DC filtering. The results allowed us to map the density of states in a synthetic two-dimensional phase space, demonstrating the formation of novel Andreev bands and controlled Andreev molecules. In systems with strong spin-orbit coupling, we observed spin-splitting and parity transitions, aligning with theoretical models. This work could lead to a new class of quantum devices where information is stored in spin states but manipulated using superconducting qubit technology.
Additionally, we developed flip-chip packaging techniques to combine microwave resonators with planar Josephson junctions on III-V materials, using vacuum-mediated inductive coupling. This integration enabled high-resolution measurements of Andreev spectra and fast dynamics in InAs and Ge-based systems. The approach is highly promising for future experiments targeting the spectroscopy of Floquet-Andreev states and exploring new material platforms.
Furthermore, the project fostered significant interdisciplinary collaboration, enabling new studies on the effects of high-energy electrons on mesoscopic superconductors and the microscopic origins of the superconducting diode effect in conventional materials.
We conducted extensive studies on planar Josephson junctions fabricated from these heterostructures. These experiments unveiled new phenomena related to phase dynamics in the presence of large in-plane magnetic fields and interactions with microwave radiation. The insights gained are essential for realizing topological states, particularly by clarifying how spin-orbit interactions and orbital magnetic fields influence superconducting properties and switching currents.
Expanding on theoretical proposals, we explored multiterminal Josephson junctions, focusing on three-lead and four-lead configurations. These complex devices required the integration of superconducting loops and flux-bias lines, combined with precise DC filtering. The results allowed us to map the density of states in a synthetic two-dimensional phase space, demonstrating the formation of novel Andreev bands and controlled Andreev molecules. In systems with strong spin-orbit coupling, we observed spin-splitting and parity transitions, aligning with theoretical models. This work could lead to a new class of quantum devices where information is stored in spin states but manipulated using superconducting qubit technology.
Additionally, we developed flip-chip packaging techniques to combine microwave resonators with planar Josephson junctions on III-V materials, using vacuum-mediated inductive coupling. This integration enabled high-resolution measurements of Andreev spectra and fast dynamics in InAs and Ge-based systems. The approach is highly promising for future experiments targeting the spectroscopy of Floquet-Andreev states and exploring new material platforms.
Furthermore, the project fostered significant interdisciplinary collaboration, enabling new studies on the effects of high-energy electrons on mesoscopic superconductors and the microscopic origins of the superconducting diode effect in conventional materials.
This project achieved several advancements beyond the current state of the art. The development of new epitaxial interfaces between InAs/Al and Al/Nb is a significant step forward, leading to improvements in sample quality and electron mobility. These enhancements are critical for advancing the reliability of hybrid superconducting devices. Additionally, our work on hybrid Josephson junctions with metallic weak links allowed for precise measurements of the current-phase relation under strong magnetic fields, which is essential for probing spin-orbit and Zeeman effects in hybrid systems.
We introduced a novel platform for multiterminal Josephson junctions that required sophisticated designs involving large superconducting loops and flux-bias lines. This platform enabled us to experimentally map Andreev bands in a multiterminal setup, revealing phenomena such as anisotropic band structures and spin-dependent Andreev states. These findings provide new experimental evidence for theoretical models of topological matter, with potential applications in quantum information processing and spintronics.
Our integration of high-quality superconducting resonators with hybrid materials via microwave spectroscopy allowed us to study Andreev states with unprecedented precision. This capability opens up new avenues for the exploration of non-equilibrium states and the dynamics of superconducting islands. In particular, we expect these techniques to advance the study of topological Floquet-Andreev states, which are crucial for developing robust qubits.
Expected outcomes for the remainder of the project include further optimizing our epitaxial growth techniques to enhance coherence times in hybrid systems, as well as expanding multiterminal junction experiments to study Weyl cones and other exotic topological features. Continued efforts in integrating microwave resonators with hybrid structures are anticipated to yield even higher precision in quantum state spectroscopy, ultimately enabling novel quantum devices for scalable quantum computing and sensing technologies.
We introduced a novel platform for multiterminal Josephson junctions that required sophisticated designs involving large superconducting loops and flux-bias lines. This platform enabled us to experimentally map Andreev bands in a multiterminal setup, revealing phenomena such as anisotropic band structures and spin-dependent Andreev states. These findings provide new experimental evidence for theoretical models of topological matter, with potential applications in quantum information processing and spintronics.
Our integration of high-quality superconducting resonators with hybrid materials via microwave spectroscopy allowed us to study Andreev states with unprecedented precision. This capability opens up new avenues for the exploration of non-equilibrium states and the dynamics of superconducting islands. In particular, we expect these techniques to advance the study of topological Floquet-Andreev states, which are crucial for developing robust qubits.
Expected outcomes for the remainder of the project include further optimizing our epitaxial growth techniques to enhance coherence times in hybrid systems, as well as expanding multiterminal junction experiments to study Weyl cones and other exotic topological features. Continued efforts in integrating microwave resonators with hybrid structures are anticipated to yield even higher precision in quantum state spectroscopy, ultimately enabling novel quantum devices for scalable quantum computing and sensing technologies.