Superconducting Junctions
Superconducting nanojunctions are attracting enormous attention of researchers owing to their unique physical properties that make superconducting systems essential for modern technology – particularly for recently emerging quantum-computer architectures. To allow for an even greater tunability of their physical properties, and thereby offer more functionalities to potential applications, superconducting layers are often paired with magnetic components and regions that induce strong spin-orbit coupling. Together with the long-range coherence of the superconducting part, these two fundamental spin interactions – magnetic exchange and spin-orbit coupling – manifest themselves in unique spectroscopic and transport characteristics, and were furthermore predicted to induce topological superconductivity that might harbor Majorana states.
In close collaboration with experimental groups, we are performing systematic and comprehensive microscopic model calculations to unravel and further characterize the transport ramifications of the interplay between superconductivity, magnetism, and spin-orbit coupling in different multi-component junctions.
In what follows, we introduce three directions of our research on superconducting junctions.
Supercurrent diode effect
Planar Josephson junctions that are integrated into the two-dimensional electron gas (2DEG) of narrow quantum wells (e.g., InAs quantum wells) by means of the superconducting proximity effect have recently been the subject of a flurry of experimental and theoretical research activities. When combined with magnetic exchange – that could, for instance, be induced through applying an external magnetic field – in a certain way (i.e., the field needs to be aligned in the plane and perpendicular to the direction of the current), the strong intrinsic spin-orbit coupling of the 2DEG imprints a nonreciprocal character on the supercurrent transport, resulting in supercurrent magnitudes that (substantially) differ for different polarities (transport directions).
From this point of view, 2DEG magnetic Josephson junctions could act as the fully dissipationless counterparts of the yet in technological applications used semiconductor diodes – motivating to term the related effect supercurrent diode effect (SDE) – and become even more relevant to design superconducting transistors one day.
The Strunk Group in Regensburg successfully observed a large SDE in an array of more than 2,000 Josephson junctions based on an InAs quantum well for the first time ever in 2021 (in a sample provided by Manfra’s group from Purdue University), and characterized its transport properties (magnetochiral anisotropy) through state-of-the-art inductance measurements. We supported the experimental studies with tight-binding KWANT simulations that fully reproduced (both qualitatively and quantitatively) the experimental observations.
After the initial work, we developed a microscopic model that connects the SDE characteristics with the spectral properties of the underlying current-carrying Andreev bound states of the Josephson junctions. We could demonstrate that the junction may undergo current-reversing 0–π-like transitions at strong enough magnetic exchange and, most surprisingly, that also the SDE can eventually reverse. In parallel to our theoretical studies, the Strunk group measured the field-dependence of the SDE and the obtained experimental data were in good (qualitative and quantitative) agreement with our microscopic model. Moreover, the experiment confirmed the predicted reversal of the SDE at strong enough magnetic fields for the first time.
More recently, we theoretically proposed another mechanism that will give rise to a Josephson SDE in vertical three-dimensional superconductor/ferromagnet/superconductor (S/F/S) Josephson junctions. The crucial ingredients are asymmetric spin-orbit fields at the two F/S interfaces, e.g., conventional Rashba at one and unconventional (radial) Rashba spin-orbit coupling at the other interface. These fields will initially condition the preferred spin direction of the Cooper-pair electrons. However, when the magnetization of the ferromagnet (F) is perpendicular to the interfaces, these spins will precess around their initial alignment while traversing the ferromagnet. The spin-precession angle will determine the probability that the electrons can enter into the second superconductor and a finite Josephson current flows. Due to the asymmetric spin-orbit fields, the spin-precession angle will be different for right- and left-propagating electrons, imprinting a polarity dependence on the critical Josephson currents and turning the junction into a supercurrent diode. To emphasize the different physical mechanism – e.g., the spin-precession mechanism here requires out-of-plane magnetization, while the magnetic field in the planar 2DEG junctions needs to be aligned in the plane of the junctions – we termed this effect unconventional Josephson SDE.
This work has been published in:
- Physical Review B 111, L140506 (2025)
- Physical Review Letters 133, 216201 (2024)
- Nature Communications 15, 4413 (2024)
- Physical Review B 108, 054522 (2023)
- Nature Nanotechnology 18, 1266 (2023)
- Journal of Physics: Condensed Matter 34, 154005 (2022)
- Nature Nanotechnology 17, 39 (2022)
- arXiv:2303.11975
The UR press release covering our collaboration with the Strunk Group can be found here (external link, opens in a new window).
Signatures of superconducting triplet pairing in Ni–Ga bilayer junctions
In close collaboration with Moodera’s group at MIT, we performed tight-binding KWANT simulations to understand quasiparticle transport through superconducting magnetic Ni–Ga bilayers.
We showed that the observed rich, and initially extremely puzzling conductance variations (that we termed “conductance shoulders”), could serve as transport signatures of superconducting triplet pairings that are induced at the interface between the ferromagnetic Ni and the superconducting Ga films, establishing Ni–Ga bilayers as an interesting platform to further investigate long-range triplet superconductivity.
Polarized-neutron-reflectometry data provided by Dr. Valeria Lauter from Oak Ridge National Laboratory confirmed the potential presence of nonuniform ferromagnetic order (domains) around the Ni–Ga interface, which could provide the microscopic explanation for the conversion of spin-singlet into polarized spin-triplet Cooper pairs.
This work has been published in: New Journal of Physics 24, 033046 (2022) (external link, opens in a new window)
Interfacial spin-orbit coupling in ferromagnet/superconductor-like junctions
Relativistic effects like spin-orbit coupling have a tremendous impact on electrical transport through nanoscale junctions, and are therefore at the heart of numerous recent research projects within the SFB 1277 (external link, opens in a new window) at the Physics Department in Regensburg. We have been intensively studying the role of interfacial spin-orbit coupling, which is inevitably induced in all junctions through the space-inversion-symmetry breaking through thin (e.g., semiconducting) tunneling barriers, in ferromagnet/superconductor-like junctions.
The most interesting novel process that interfacial spin-orbit coupling gives rise to in superconducting junctions is the spin-flip (“unconventional”) Andreev reflection, which triggers the conversion of spin-singlet into spin-triplet Cooper pairs and thereby generates spin-polarized supercurrents with astonishing physical consequences and properties.
We elaborated on the ramifications of unconventional Andreev reflections on the charge and spin (Hall) transport properties of ferromagnet/superconductor junctions and superconductor/ferromagnet/superconductor Josephson junctions in the presence of interfacial Rashba and Dresselhaus spin-orbit couplings. In an experimental collaboration with Aliev’s group in Madrid, we demonstrated that our theory is capable of quantifying the strength of the interfacial spin-orbit couplings of Fe/V junctions through experimental conductance measurements.
We are currently continuing these studies extending our view to more advanced system geometries with more different layers and with a special focus on the distinct mechanisms that have been proposed to generate spin-polarized triplet supercurrents. Moreover, we are studying the impact of the recently predicted unconventional (radial) Rashba spin-orbit coupling on superconducting transport.
This work has been published in:
- Physical Review B 111, 054512 (2025)
- Physical Review B 104, 174504 (2021)
- Physical Review B 101, 104508 (2020)
- Physical Review Applied 13, 014030 (2020)
- Physical Review B 100, 060507(R) (2019)
- Physical Review B 98, 134511 (2018)
- Physical Review B 95, 024514 (2017)
- Physical Review Letters 115, 116601 (2015)
