Superconducting orbitronics in hybrid systems

Quantum mechanics was initially developed to understand the laws of nature at the scale of single particles. However, it turned out to also provide a very accurate description of the behavior of bulk materials: intriguing phenomena such as superconductivity and magnetism have a fully quantum-mechanical origin. We are now entering an era where radically new technologies based on quantum theory are in reach. Examples are quantum computing, where information is coded in quantum bits (qubits), and spin-based information processing, where one uses the quantum-mechanical "spin" of electrons to carry information. Hybrid structures consisting of different materials that host different quantum phenomena are attracting much attention in this context, as they could allow "engineering" of quantum properties, potentially yielding, e.g., highly tunable elements for superconducting electric circuits and new devices such as noise-tolerant qubits. One quantum-mechanical property that only recently started attracting attention is the orbital angular momentum of electrons propagating through solids. An interesting difference between spin and orbital angular momentum is that whereas the magnitude of spin is equal for all electrons, orbital angular momentum does not have an upper bound. Therefore, it should be possible to obtain more efficient transport of angular momentum using the orbital degree of freedom rather than spin. In this project, we explore the potential of hybrid systems consisting of such orbitally active materials and superconductors. We will first develop the basic theory to understand the interplay between orbital angular momentum dynamics and superconductivity. We will then assess the potential of hybrid superconducting orbitronics to yield new phenomena and enable quantum-technological functionalities, such as electrically controlled unconventional superconductivity, large thermoelectric effects, tunable superconducting diodes, and protected qubit architectures.

Besides spin, electrons can also carry orbital angular momentum, and the emerging field of orbitronics studies the behavior of this orbital degree of freedom and its interaction with charge and spin in solid-state devices. The interplay between superconductivity and non-trivial orbital physics remains largely unexplored, and in this project we will fill this knowledge gap by developing a theoretical framework to describe orbitally polarized superconducting heterostructures, both in and out of equilibrium. We will first develop the understanding of the proximity effects, i.e., the “leakage” of superconducting correlations and orbital polarization across the interfaces in a heterostructure. Then we apply this understanding to proximity-induced superconductivity in the p-type valence band of a semiconductor, where the low-energy carriers (holes) have an effective J=3/2 angular momentum. Semiconductor-based hole gases form an attractive platform for various quantum-technological devices, and we will use our theory to investigate the detailed potential of hole-based superconductor-semiconductor hybrids for applications such as qubit-qubit coupling of spin qubits and the hosting of Andreev spin qubits. In the field of spintronics, it is known that spin magnetoresistance and spin Hall effects can be enhanced by adding superconducting elements. Using our non-equilibrium theory, we will investigate the idea that substituting quantum spin by the orbital degree of freedom could lead to even more efficient low-dissipation information transfer using superconductors. We will also apply our theory to describe the potentially large thermoelectric properties of superconducting orbitronic heterostructures, which is highly relevant in the context of cryogenic caloritronics. The question of how superconductivity adapts to an environment with non-trivial orbital polarization, both in and out of equilibrium, is an interesting and urgent one which this project aims to answer.