Voltage control of superconducting exchange interaction and anomalous Josephson effect

Exerting control of the magnetic exchange interaction in heterostructures is of both basic interest and has potential for use in spin-based applications relying on quantum effects. We here show that the sign of the exchange interaction in a spin-valve, determining whether the ferro- or antiferromagnetic configuration is favored, can be controlled via an electric voltage. This occurs due to an interplay between a nonequilibrium quasiparticle distribution and the presence of spin-polarized Cooper pairs. Additionally, we show that a voltage-induced distribution controls the anomalous supercurrent that occurs in magnetic Josephson junctions, obviating the challenging task to manipulate the magnetic texture of the system. This demonstrates that two key phenomena in superconducting spintronics, the magnetic exchange interaction and the phase shift generating the anomalous Josephson effect, can be controlled electrically. Our findings are of relevance for spin-based superconducting devices which in practice most likely have to be operated precisely by nonequilibrium effects.Comment: 4 pages, 3 figures, 10 subfigures, revised editio

Field-free nucleation of antivortices and giant vortices in non-superconducting materials

Giant vortices with higher phase-winding than $2\pi$ are usually energetically unfavorable, but geometric symmetry constraints on a superconductor in a magnetic field are known to stabilize such objects. Here, we show via microscopic calculations that giant vortices can appear in intrinsically non-superconducting materials, even without any applied magnetic field. The enabling mechanism is the proximity effect to a host superconductor where a current flows, and we also demonstrate that antivortices can appear in this setup. Our results open the possibility to study electrically controllable topological defects in unusual environments, which do not have to be exposed to magnetic fields or intrinsically superconducting, but instead display other types of order.Comment: Revised version; 4 pages manuscript, 4 pages supplemental, 6 figure

Paramagnetic Meissner effect in voltage-biased proximity systems

Conventional superconductors respond to external magnetic fields by generating diamagnetic screening currents. However, theoretical work has shown that one can engineer systems where the screening current is paramagnetic, causing them to attract magnetic flux -- a prediction that has recently been experimentally verified. In contrast to previous studies, we show that this effect can be realized in simple superconductor/normal-metal structures with no special properties, using only a simple voltage bias to drive the system out of equilibrium. This is of fundamental interest, since it opens up a new avenue of research, and at the same time highlights how one can realize paramagnetic Meissner effects without having odd-frequency states at the Fermi level. Moreover, a voltage-tunable electromagnetic response in such a simple system may be interesting for future device design.Comment: Article: 4 pages, 3 figures; Supplemental: 3 pages, no figures

Critical Temperature and Tunneling Spectroscopy of Superconductor-Ferromagnet Hybrids with Intrinsic Rashba-Dresselhaus Spin-Orbit Coupling

We investigate theoretically how the proximity effect in superconductor/ferromagnet hybrid structures with intrinsic spin-orbit coupling manifests in the density of states and critical temperature. To describe a general scenario, we allow for both Rashba and Dresselhaus type spin-orbit coupling. Our results are obtained via the quasiclassical theory of superconductivity, extended to include spin-orbit coupling in the Usadel equation and Kupriyanov--Lukichev boundary conditions. Unlike previous works, we have derived a Riccati parametrization of the Usadel equation with spin-orbit coupling which allows us to address the full proximity regime. First, we consider the density of states in both SF bilayers and SFS trilayers, where the spectroscopic features in the latter case are sensitive to the phase difference between the two superconductors. We find that the presence of spin-orbit coupling leaves clear spectroscopic fingerprints in the density of states due to its role in creating spin-triplet Cooper pairs. Unlike SF and SFS structures without spin-orbit coupling, the density of states in the present case depends strongly on the direction of magnetization. We show that the spin-orbit coupling can stabilize singlet superconductivity even in the presence of a strong exchange field $h \gg \Delta$. This leads to the possibility of a magnetically tunable minigap: changing the direction of the exchange field opens and closes the minigap. We also determine how the critical temperature $T_c$ of an SF bilayer is affected by spin-orbit coupling and demonstrate that one can achieve a spin-valve effect with a single ferromagnet. We find that $T_c$ displays highly non-monotonic behavior both as a function of the magnetization direction and the type and direction of the spin-orbit coupling, offering a new way to exert control over the superconductivity of proximity structures.Comment: 25 pages, 21 figures. Accepted for publication in Phys. Rev.

Density of States and Critical Temperature in Superconductor/Ferromagnet Structures with Spin-Orbit Coupling

We present both an analytical discussion of the weak proximity regime, and a full numerical investigation of the strong proximity regime. The results show that the spin-orbit coupling leaves a clear trace in the density of states, which displays a highly nonmonotonic behaviour as a function of magnetization directions and phase differences. We also determine how the critical temperature is affected and, interestingly, demonstrate that one can achieve a spin-valve effect using a single ferromagnet. The critical temperature is found to exhibit a highly nonmonotonic behaviour, both as a function of magnetization direction and type of spin-orbit coupling. Compared to the earlier inhomogeneously magnetized structures, this offers a new way to control the superconductivity of proximity structures

Manipulating superconductivity in magnetic nanostructures in and out of equilibrium

We consider nanostructures that are constructed from superconducting, ferromagnetic, and spin–orbit-coupled materials. These structures are analyzed both theoretically and numerically, under both equilibrium and nonequilibrium conditions. Special emphasis is placed on how one can exert control over the superconducting properties of these systems. For instance, this includes developing new ways to toggle superconductivity on and off via electric or magnetic input signals, and new ways to shape the charge and spin supercurrents flowing through these systems. The thesis itself provides an introduction to how we performed our calculations, as well as a summary of some interesting research results. The main body of research consists of 15 enclosed publications, where we go into more detail on each specific project

Critical temperature and tunneling spectroscopy of superconductor-ferromagnet hybrids with intrinsic Rashba-Dresselhaus spin-orbit coupling

We investigate theoretically how the proximity effect in superconductor/ferromagnet hybrid structures with intrinsic spin-orbit coupling manifests in two measurable quantities, namely the density of states and critical temperature. To describe a general scenario, we allow for both Rashba and Dresselhaus type spin-orbit coupling. Our results are obtained via the quasiclassical theory of superconductivity, extended to include spin-orbit coupling in the Usadel equation and Kupriyanov–Lukichev boundary conditions. Unlike previous works, we have derived a Riccati parametrization of the Usadel equation with spin-orbit coupling which allows us to address the full proximity regime and not only the linearized weak proximity regime. First, we consider the density of states in both SF bilayers and SFS trilayers, where the spectroscopic features in the latter case are sensitive to the phase difference between the two superconductors. We find that the presence of spin-orbit coupling leaves clear spectroscopic fingerprints in the density of states due to its role in creating spin-triplet Cooper pairs. Unlike SF and SFS structures without spin-orbit coupling, the density of states in the present case depends strongly on the direction of magnetization. Moreover, we show that the spin-orbit coupling can stabilize spin-singlet superconductivity even in the presence of a strong exchange field h ∆. This leads to the possibility of a magnetically tunable minigap: changing the direction of the exchange field opens and closes the minigap. We also determine how the critical temperature Tc of an SF bilayer is affected by spin-orbit coupling and, interestingly, demonstrate that one can achieve a spin-valve effect with a single ferromagnet. We find that Tc displays highly non-monotonic behavior both as a function of the magnetization direction and the type and direction of the spin-orbit coupling, offering a new way to exert control over the superconductivity of proximity structures.submittedVersion© 2015 American Physical Society. This is the authors' manuscript to the article (preprint)

Josephson effect in altermagnets

The ability of magnetic materials to modify superconducting systems is an active research area for possible applications in thermoelectricity, quantum sensing, and spintronics. We consider the fundamental properties of the Josephson effect in a third class of magnetic materials beyond ferromagnets and antiferromagnets: altermagnets. We show that despite having no net magnetization, altermagnets induce $0$-$\pi$ oscillations. The decay length and oscillation period of the Josephson coupling are qualitatively different from ferromagnetic junctions and depend on the crystallographic orientation of the altermagnet. The Josephson effect in altermagnets thus serves a dual purpose: it acts as a signature that distinguishes altermagnetism from conventional (anti)ferromagnetism and offers a way to tune the supercurrent via flow direction anisotropy.Comment: 4 pages, 4 figure