Ferromagnetic Superconductor Research Articles

Fermi surface and Lifshitz transitions of a ferromagnetic superconductor under external magnetic fields

Lifshitz transitions are being increasingly recognized as significant in a wide variety of strongly correlated and topological materials, and understanding the origin and influence of Lifshitz transitions is leading to deeper understanding of key aspects of magnetic, transport, or quantum critical behavior. In the ferromagnetic superconductor UCoGe, a magnetic field applied along the c axis has been shown to induce a series of anomalies in both transport and thermopower that may be caused by Lifshitz transitions. The need to understand the subtleties of the relationship between magnetism, superconductivity and a heavy-electron Fermi surface in the ferromagnetic superconductors makes it important to explore if and why a series of magnetic-field-induced Lifshitz transitions occurs in UCoGe. Here we report magnetic susceptibility measurements of UCoGe, performed at temperatures down to 45 mK and magnetic fields (μ0H||c) up to 30 T. We observe a series of clearly defined features in the susceptibility, and multiple sets of strongly field-dependent de Haas-van Alphen oscillations, from which we extract detailed field dependence of the quasiparticle properties. We complement our experimental results with density functional theory band structure calculations, and include a simple model of the influence of magnetic field on the calculated Fermi surface. By comparing experimental and calculated results, we determine the likely shape of the Fermi surface and identify candidate Lifshitz transitions that could correspond to two of the features in susceptibility. We connect these results to the development of magnetization in the system. Published by the American Physical Society 2024

Circuit quantum electrodynamics detection of induced two-fold anisotropic pairing in a hybrid superconductor–ferromagnet bilayer

Hybrid systems represent one of the frontiers in the study of unconventional superconductivity and are a promising platform to realize topological superconducting states. These materials are challenging to probe using many conventional measurement techniques because of their mesoscopic dimensions, and therefore require new experimental probes so that they can be successfully characterized. Here, we demonstrate a probe that enables us to measure the superfluid density of micrometre-size superconductors using microwave techniques drawn from circuit quantum electrodynamics. We apply this technique to a superconductor–ferromagnet bilayer and find that the proximity-induced superfluid density is two-fold anisotropic within the plane of the sample. It also exhibits power-law temperature scaling that is indicative of a nodal superconducting state. These experimental results are consistent with the theoretically predicted signatures of induced triplet pairing with a nodal p-wave order parameter. Moreover, we observe modifications to the microwave response at frequencies near the ferromagnetic resonance, suggesting a coupling between the spin dynamics and induced superconducting order in the ferromagnetic layer. Our experimental technique can be employed more widely, for example to study fragile unconventional superconductivity in low-dimensional materials such as van der Waals heterostructures.

Correlation-driven topological Kondo superconductors

Searching for topological superconductors that host topological charge-neutral Majorana zero-modes at edges has become a central problem in condensed matter research due to their potential applications for quantum computations. Meanwhile, electron correlations in solid-state materials enhance quantum fluctuations and give rise to various quantum many-body phases. Whether these electron correlations alone would lead to topological superconductivity is a fundamentally important open problem. Here, we theoretically find the correlation-driven topological superconductivity in a class of Kondo lattice materials. Therein, the odd-parity Kondo hybridization mediates ferromagnetic spin-spin coupling and leads to spin-triplet pairing between local moments. Triplet p±ip′\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$p\\pm i{p}^{{\\prime} }$$\\end{document}-wave topological superconductivity with Majorana zero modes at edges is reached when Kondo hybridization co-exists with the triplet pairings. Our results offer a detailed understanding of the experimental observations on UTe2, a ferromagnetic heavy-electron triplet superconductor. Our approach to topological superconductivity shows advantages over the heterostructure approach by proximity effect.

Size-Dependence and High Temperature Stability of Radial Vortex Magnetic Textures Imprinted by Superconductor Stray Fields.

Swirling spin textures, including topologically nontrivial states, such as skyrmions, chiral domain walls, and magnetic vortices, have garnered significant attention within the scientific community due to their appeal from both fundamental and applied points of view. However, their creation, controlled manipulation, and stability are typically constrained to certain systems with specific crystallographic symmetries, bulk or interface interactions, and/or a precise stacking sequence of materials. Recently, a new approach has shown potential for the imprint of magnetic radial vortices in soft ferromagnetic compounds making use of the stray field of YBa2Cu3O7-δ superconducting microstructures in ferromagnet/superconductor (FM/SC) hybrids at temperatures below the superconducting transition temperature (TC). Here, we explore the lower size limit for the imprint of magnetic radial vortices in square and disc shaped structures as well as the persistence of these spin textures above TC, with magnetic domains retaining partial memory. Structures with circular geometry and with FM patterned to smaller radius than the superconductor island facilitate the imprinting of magnetic radial vortices and improve their stability above TC, in contrast to square structures where the presence of magnetic domains increases the dipolar energy. Micromagnetic modeling coupled with a SC field model reveals that the stabilization mechanism above TC is mediated by microstructural defects. Superconducting control of swirling spin textures, and their stabilization above the superconducting transition temperature by means of defect engineering holds promising prospects for shaping superconducting spintronics based on magnetic textures.

Anomalous Josephson Effect in a Planar Hybrid Structure with the Spin–Orbit Coupling

The way of formation of controlled phase inhomogeneity in a hybrid structure consisting of a short Josephson junction between two superconducting thin films, with one electrode partially coated with a ferromagnetic insulator, has been theoretically studied. The joint action of spin splitting and the Rashba spin–orbit coupling at the superconductor–ferromagnet interface leads to the generation of a spontaneous supercurrent, which changes the transport properties of the junction. The critical current and the current–phase relation of this hybrid structure have been calculated; it has been shown that this structure can be used to form an anomalous φ0 Josephson junction with the phase shift φ0 smoothly varying over a wide range.

Interplay between metastable magnetic domains and superconducting vortices in a ferromagnet–superconductor hybrid

Interplay between metastable magnetic domains and superconducting vortices in a ferromagnet–superconductor hybrid

The Type-II/Type-I Crossover in Dirty Ferromagnetic Superconductors.

In this work, we investigate the intertype (IT) domain in strongly disordered ferromagnetic superconductors with a Curie temperature lower than the superconducting critical temperature. In such unique materials, the coexistence of superconductivity and ferromagnetism allows for the exploration of both unconventional superconductivity and the interplay between magnetism and superconductivity. The study utilizes an extended Ginzburg-Landau model for the dirty limit to calculate the boundaries of the IT domain, which is characterized by a complex vortex-vortex interaction and exotic vortex configurations. The analysis reveals that the IT domain in dirty ferromagnetic superconductors is not qualitatively different from that in the clean case and remains similarly large, suggesting that disorder does not hinder the exploration of the rich variety of IT superconductivity in ferromagnetic superconductors. This work expands our understanding of the interplay between superconductivity and magnetism in complex materials and could guide future experimental studies.

Non-Abelian Landau-Ginzburg theory of ferromagnetic superconductivity and mixing between U(1) and SU(2) gauge bosons

We propose an effective theory of non-Abelian superconductivity, an SU(2)xU(1) extension of the Abelian Landau-Ginzburg theory, which could be viewed as an effective theory of ferromagnetic superconductivity made of spin-up and spin-down doublet Cooper pair. Just like the Abelian Landau-Ginzburg theory it has the U(1) electromagnetic interaction, but the new ingredient is the non-Abelian SU(2) gauge interaction between the spin doublet Cooper pairs. A remarkable feature of the theory is the mixing between the U(1) gauge boson and the diagonal part of the SU(2) gauge boson. After the mixing it has a massless non-Abelian gauge boson (the massless s-boson) and massive gauge boson (the massive photon), in addition to the massive off-diagonal non-Abelian gauge bosons (the massive s-bosons) which induce the spin-flip interaction between the spin up and down components of the Cooper pair. So, unlike the ordinary Landau-Ginzburg theory it has a long range interaction mediated by the massless s-boson, which could be responsible for the long range magnetic order and spin waves observed in ferromagnetic superconductors. The theory is characterized by three scales. In addition to the correlation length fixed by the mass of the Higgs field it has two different penetration lengths, the one fixed by the mass of the photon (which generates the well known Meissner effect) and the other fixed by the mass of the off-diagonal gauge bosons (which determines the range of the spin flip interaction). The non-Abelian structure of the theory naturally accommodates new topological objects, the non-Abrikosov quantized spin vortex (as well as the well known Abrikosov vortex) and non-Abelian spin monopole. We discuss the physical implications of the non-Abelian Landau-Ginzburg theory.

Current-driven motion of magnetic topological defects in ferromagnetic superconductors

Current-driven motion of magnetic topological defects in ferromagnetic superconductors

Spin-dependent resonant Andreev tunneling in hybrid ferromagnetic metal–magnetic quantum dot–superconductor nanostructures

There exists a variety of theoretical proposals to transform states induced by magnetic nanoparticles inside a superconducting gap into Majorana fermion states. The main challenge in this route is a conclusive proof and undoubted distinguishing between topologically trivial subgap Andreev bound states and topologically nontrivial magnetically polarized Majorana bound states. This motivated us to investigate a nonequilibrium electrons tunneling through a ferromagnetic normal metal–magnetic quantum dot–s-wave superconductor (F-mQD-SC) nanostructure, where the mQD’s discrete levels are spin splitted. By using the Keldysh Green’s function method, the expressions for a tunnel current and probability of the Andreev reflection (AR) versus energy are derived and studied. We find that the system’s resonant ARs conductance exhibits different kinds of peaks depending on a spin splitting of the mQD levels, the spin polarization magnitude of the F-lead current, the gate voltage, and an external magnetic field magnitude. The nanostructure’s conductance versus a bias voltage exhibits extra peaks which at some combination of its parameters can mimic ones expected for Majorana modes in a topological superconducting state. The distinguishing transport characteristics of a F-mQD-SC nanoscale structure being in non-topological state are discussed. We suggest that the results obtained can provide helpful clarification for understanding recent experiments in superconductor–ferromagnet hybrid nanostructures with topologically protected excitations.

All-JJ Logic Based on Bistable JJs

The all-JJ logic family is a promising area and power efficient, scalable single flux quantum (SFQ) technology for application to exascale supercomputers. All-JJ superconductive logic is based on a superconductor-ferromagnet-superconductor (SFS) bistable JJ, enabling nanometer feature sizes in VLSI complexity superconductive systems. In this paper, a mechanical analogy is proposed to describe the dynamic behavior of these bistable JJs. Novel all-JJ logic gates, such as TFF, DFF, OR, AND, and NOT gates, are presented here. All-JJ circuits are composed of bistable JJs, standard JJs, and bias currents, requiring no large inductors within the storage loops. All-JJ logic cells exhibit less delay and power than standard SFQ cells with the same critical current density. All-JJ systems can operate at high frequencies due to the small capacitance of the SFS JJ. A complex all-JJ circuit from the suite of ISCAS'85 benchmark circuits is also characterized. This complex all-JJ circuit exhibits less delay and power as compared to standard SFQ logic. The bias current in a conventional benchmark circuit and all-JJ benchmark circuit is, respectively, 22 mA and 13 mA. The delay of each cell within the conventional benchmark circuit and all-JJ benchmark circuit is, respectively, approximately 20 and 8 ps. A parasitic inductance in series with the JJs disturbs the current distribution within the all-JJ circuits while degrading the margins. To suppress the effects of this parasitic inductance on SFS JJs, small linear inductors are added to manage the current distribution and improve parameter margins.

Influence of macroscopic magnetic inhomogeneity on the superconducting proximity effect in MgB2/La2/3Ca1/3MnO3 nanocomposite

As theory predicts, long-extended triplet superconducting correlations are realized in superconductor/ferromagnet (SC/F) structures in the presence of magnetization inhomogeneity at the SC–F interface. In this report, the external magnetic field effect on triplet superconducting correlations in the nanocomposite of proximity coupled s-wave SC, MgB2, and half-metallic ferromagnet (hmF), La2/3Ca1/3MnO3, has been investigated experimentally. The results obtained bring out clearly that the nanocomposite’s superconducting characteristics significantly correlate with the La2/3Ca1/3MnO3 nanoparticles bulk magnetization order. In particular, the dependence of the point contacts’ conductance characteristics on the external magnetic field demonstrates direct spectroscopic signatures that the nanoscale magnetization inhomogeneity plays an essential role in the generation of a long-extended triplet superconductivity in SC/hmF proximity coupled nanostructures. The results obtained prove the impact of nanoscale magnetic nonhomogeneity on the generation of the spin-triplet Cooper pairing in SC/hmF structures.

Anisotropic spin-current spectroscopy of ferromagnetic superconducting gap symmetries

We develop a microscopic theory of tunneling spin transport at the magnetic interface between a ferromagnetic insulator (FI) and a ferromagnetic superconductor (FSC) driven by ferromagnetic resonance. We show that the spin susceptibilities of the FSC can be extracted from the spin currents by tuning the easy axis of the FI, and thus the spin currents can be a probe for the symmetries of the spin-triplet Cooper pairing. Our results will offer a route to exploiting the synergy of magnetism and superconductivities for spin devices.

Weinberg-Salam model as non-Abelian Landau-Ginzburg theory of superconductivity

We propose that the Weinberg-Salem model well known as the standard model of the electroweak unification could also describe the non-Abelian ferromagnetic superconductivity made of two-component (spin-up and spin-down) Cooper pair, which could be viewed as the SU(2)xU(1) extension of the Abelian Landau-Ginzburg theory. In this interpretation the two gauge couplings of SU(2) and U(1) (and thus the Weinberg's mixing angle) are completely fixed by the electromagnetic coupling e. And the massless photon in the standard model plays the role of the massless non-Abelian spinon which induces the long range magnetic order and the spin waves in non-Abelian ferromagnetic superconductors. Moreover, the massive Z boson plays the role of the massive photon which generates the well known Meissner effect in ordinary superconductors, and the W boson becomes the charged massive spinon whose mass determines the range of the non-Abelian spin flip interaction in the ferromagnetic superconductors. We discuss the physical implications of the non-Abelian Landau-Ginzburg theory.

Triplet odd-frequency superconductivity in hybrid superconductor–ferromagnet structures

We present an overview of the contributions made by Konstantin B. Efetov and the authors to the field of triplet odd-frequency superconductivity in hybrid ferromagnet–superconductor structures (Bergeret et al., 2005) and the repercussions they had on experimental and theoretical research. Konstantin B. Efetov passed away in August 2021. We hope the present short review gives a faithful testimony of an important part of his scientific legacy.

Strongly correlated itinerant magnetism near superconductivity in NiTa4Se8

Metallic ferromagnets with strongly interacting electrons often exhibit remarkable electronic phases such as ferromagnetic superconductivity, complex spin textures, and nontrivial topology. In this report, we discuss the synthesis of a layered magnetic metal $\mathrm{Ni}{\mathrm{Ta}}_{4}{\mathrm{Se}}_{8}$ (or ${\mathrm{Ni}}_{1/4}\mathrm{Ta}{\mathrm{Se}}_{2}$) with a Curie temperature of 58 Kelvin. Magnetization data and density functional theory calculations indicate that the nickel atoms host uniaxial ferromagnetic order of about $0.7{\ensuremath{\mu}}_{B}$ per atom, while an even smaller moment is generated in the itinerant tantalum conduction electrons. Strong correlations are evident in flat bands near the Fermi level, a high heat capacity coefficient, and a high Kadowaki-Woods ratio. Density functional theory calculations suggest that electron and hole Fermi surfaces in the ferromagnetic phase are associated with opposite spin polarization. When the system is diluted of magnetic ions, the samples become superconducting below about 2 Kelvin. We discuss possible mechanisms for superconductivity in this family.

Superconductor/Ferromagnet Heterostructures: A Platform for Superconducting Spintronics and Quantum Computation

AbstractThe interplay between superconductivity and ferromagnetism in the superconductor/ferromagnet (SC/FM) heterostructures generates many interesting physical phenomena, including spin‐triplet superconductivity, superconducting order parameter oscillation, and topological superconductivity. The unique physical properties make the SC/FM heterostructure as promising platforms for future superconducting spintronics and quantum computation applications. In this article, important research progress of SC/FM heterostructures from superconducting spintronics to quantum computation is reviewed, and it is organized as follows. First, the progress of spin current carriers in SC/FM heterostructures including Bogoliubov quasiparticles, superconducting vortex, and spin‐triplet Cooper pairs which might be used for long‐range spin transport is discussed. Then, the π Josephson junctions and their application for constructing π qubits are described. Finally, experimental signatures of Majorana states in the SC/FM heterostructures and the theoretically proposed manipulation are briefly reviewed, which could be useful to realize fault‐tolerant topological quantum computing.

Spin transport properties in ferromagnet/superconductor junctions on topological insulator

The spin-dependent Andreev reflection is investigated theoretically by analyzing the electronic transport in a thin-film topological insulator (TI) ferromagnet/superconductor (FM/SC) junction. The tunneling conductance and shot noise are calculated based on the Dirac–Bogoliubov–de Gennes equation and Blonder–Tinkham–Klapwijk theory. It is found that the magnetic gap in ferromagnet can enhance the Andreev retro-reflection but suppress the specular Andreev reflection. The gate potential applied to the electrode on top of superconductor can enhance the two types of reflections. There is a transition between the two types of reflections at which both the tunneling conductance and differential shot noise become zero. These results provide a method to realize and detect experimentally the intra-band specular Andreev reflection in thin film TI-based FM/SC structures.

Tunable magnon antibunching via degenerate three-wave mixing in a hybrid ferromagnet–superconductor system

We propose a scheme for achieving magnon antibunching in a hybrid ferromagnet–superconductor system, where the magnons excited in two yttrium iron garnet (YIG) spheres couple the different levels of a cyclic three-level superconducting qubit by eliminating two perpendicular microwave cavity modes. With the aid of the three-level system, we find that the magnon antibunching can be achieved in a weak coupling regime via the degenerate three-wave mixing process. Moreover, it is found that the magnon antibunching inside a YIG2 sphere can be effectively regulated by manipulating the YIG1 sphere, for instance, the coupling strength of the YIG1 sphere and qubit and the driving strengths of the system. This work provides an alternative scheme for constructing the single magnon source based on the ferromagnet–superconductor technology and is conducive to studying the quantum properties of magnons.

Nonvolatile memory cell using a superconducting-ferromagnetic π Josephson junction

Storage of a single magnetic flux quantum in a superconducting loop containing a Josephson junction represents a promising unit cell configuration for construction of a cryogenic memory of superconducting digital circuits. However, application of a DC bias current is required for operation of such a memory cell to maintain trapping of the flux quantum in the storage loop. In this work, we present a superconducting memory cell that uses a superconducting-magnetic π junction. The cell characteristics show flux quantum hysteresis centering at the zero-bias current. We develop a fabrication process that combines superconductor–ferromagnet–superconductor (SFS) junctions with superconductor–normal metal–superconductor (SNS) junctions. The critical current density of the SFS junctions shows a 0–π oscillation as a function of the ferromagnetic layer thickness. The formation of the π junction is confirmed further by the flux modulation curves of a superconducting quantum interference device made from SNS junctions with an additional SFS junction.