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  • 1. Babaev, Egor
    et al.
    Carlstrom, J.
    Silaev, Mihail
    KTH, School of Engineering Sciences (SCI), Physics, Statistical Physics.
    Speight, J. M.
    Type-1.5 superconductivity in multicomponent systems2017In: Physica. C, Superconductivity, ISSN 0921-4534, E-ISSN 1873-2143, Vol. 533, p. 20-35Article in journal (Refereed)
    Abstract [en]

    In general a superconducting state breaks multiple symmetries and, therefore, is characterized by several different coherence lengths i = 1,..., N. Moreover in multiband material even superconducting states that break only a single symmetry are nonetheless described, under certain conditions by multi component theories with multiple coherence lengths. As a result of that there can appear a state where some coherence lengths are smaller and some are larger than the magnetic field penetration length A: xi(1) <= xi(2)...<root 2 lambda < xi(M) <=... (N). That state was recently termed "type-1.5" superconductivity. This breakdown of type-1/type-2 dichotomy is rather generic near a phase transition between superconducting states with different symmetries. The examples include the transitions between U(1) and U(1) x U(1) states or between U(1) and U(1) x Z(2) states. The later example is realized in systems that feature transition between s-wave and s + is states. The extra fundamental length scales have many physical consequences. In particular in these regimes vortices can attract one another at long range but repel at shorter ranges. Such a system can form vortex clusters in low magnetic fields. The vortex clustering in the type 1.5 regime gives rise to many physical effects, ranging from macroscopic phase separation in domains of different broken symmetries, to unusual transport properties. Prepared for the proceedings of Vortex IX conference, Rhodes 12-17 September 2015.

  • 2.
    Babaev, Egor
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Statistical Physics.
    Carlström, J.
    Silaev, Mihail
    KTH, School of Engineering Sciences (SCI), Physics, Statistical Physics.
    Speight, J. M.
    Type-1.5 superconductivity2017In: Superconductors at the Nanoscale: From Basic Research to Applications, Walter de Gruyter GmbH , 2017, p. 133-164Chapter in book (Other academic)
  • 3. Bessarab, Pavel F.
    et al.
    Mueller, Gideon P.
    Lobanov, Igor S.
    Rybakov, Filipp N.
    KTH, School of Engineering Sciences (SCI), Physics, Statistical Physics.
    Kiselev, Nikolai S.
    Jonsson, Hannes
    Uzdin, Valery M.
    Blugel, Stefan
    Bergqvist, Lars
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Delin, Anna
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Lifetime of racetrack skyrmions2018In: Scientific Reports, ISSN 2045-2322, E-ISSN 2045-2322, Vol. 8, article id 3433Article in journal (Refereed)
    Abstract [en]

    The skyrmion racetrack is a promising concept for future information technology. There, binary bits are carried by nanoscale spin swirls-skyrmions-driven along magnetic strips. Stability of the skyrmions is a critical issue for realising this technology. Here we demonstrate that the racetrack skyrmion lifetime can be calculated from first principles as a function of temperature, magnetic field and track width. Our method combines harmonic transition state theory extended to include Goldstone modes, with an atomistic spin Hamiltonian parametrized from density functional theory calculations. We demonstrate that two annihilation mechanisms contribute to the skyrmion stability: At low external magnetic field, escape through the track boundary prevails, but a crossover field exists, above which the collapse in the interior becomes dominant. Considering a Pd/Fe bilayer on an Ir(111) substrate as a well-established model system, the calculated skyrmion lifetime is found to be consistent with reported experimental measurements. Our simulations also show that the Arrhenius pre-exponential factor of escape depends only weakly on the external magnetic field, whereas the pre-exponential factor for collapse is strongly field dependent. Our results open the door for predictive simulations, free from empirical parameters, to aid the design of skyrmion-based information technology.

  • 4.
    Ergül, Adem
    et al.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics. Stockholm University, Sweden.
    Weissl, Thomas
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Johansson, Jan
    Lidmar, Jack
    KTH, School of Engineering Sciences (SCI), Physics, Statistical Physics.
    Haviland, David B.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Spatial and temporal distribution of phase slips in Josephson junction chains2017In: Scientific Reports, ISSN 2045-2322, E-ISSN 2045-2322, Vol. 7, article id 11447Article in journal (Refereed)
    Abstract [en]

    The Josephson effect, tunnelling of a supercurrent through a thin insulator layer between two superconducting islands, is a phenomena characterized by a spatially distributed phase of the superconducting condensate. In recent years, there has been a growing focus on Josephson junction devices particularly for the applications of quantum metrology and superconducting qubits. In this study, we report the development of Josephson junction circuit formed by serially connecting many Superconducting Quantum Interference Devices, SQUIDs. We present experimental measurements as well as numerical simulations of a phase-slip center, a SQUID with weaker junctions, embedded in a Josephson junction chain. The DC transport properties of the chain are the result of phase slips which we simulate using a classical model that includes linear external damping, terminating impedance, as well as internal nonlinear quasiparticle damping. We find good agreement between the simulated and the experimental current voltage characteristics. The simulations allow us to examine the spatial and temporal distribution of phase-slip events occurring across the chains and also the existence of travelling voltage pulses which reflect at the chain edges.

  • 5.
    Garaud, Julien
    et al.
    KTH, School of Engineering Sciences (SCI), Physics.
    Corticelli, Alberto
    KTH, School of Engineering Sciences (SCI), Physics.
    Silaev, Mihail
    Babaev, Egor
    KTH, School of Engineering Sciences (SCI), Physics, Statistical Physics.
    Properties of dirty two-band superconductors with repulsive interband interaction: Normal modes, length scales, vortices, and magnetic response2018In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 98, no 1, article id 014520Article in journal (Refereed)
    Abstract [en]

    Disorder in two-band superconductors with repulsive interband interaction induces a frustrated competition between the phase-locking preferences of the various potential and kinetic terms. This frustrated interaction can result in the formation of an s + is superconducting state that breaks the time-reversal symmetry. In this paper we study the normal modes and their associated coherence lengths in such materials. We especially focus on the consequences of the soft modes stemming from the frustration and time-reversal symmetry breakdown. We find that two-band superconductors with such impurity-induced frustrated interactions display a rich spectrum of physical properties that are absent in their clean counterparts. It features a mixing of Leggett's and Anderson-Higgs modes, and a soft mode with diverging coherence length at the impurity-induced second-order phase transition from s +/- / s ++ states to the s + is state. Such a soft mode generically results in long-range attractive intervortex forces that can trigger the formation of vortex clusters. We find that, if such clusters are formed, their size and internal flux density have a characteristic temperature dependence that could be probed in muon-spin-rotation experiments. We also comment on the appearance of spontaneous magnetic fields due to spatially varying impurities.

  • 6. Garaud, Julien
    et al.
    Silaev, Mihail
    KTH, School of Engineering Sciences (SCI), Physics, Statistical Physics.
    Babaev, Egor
    KTH, School of Engineering Sciences (SCI), Physics, Statistical Physics.
    Microscopically derived multi-component Ginzburg-Landau theories for s plus is superconducting state2017In: Physica. C, Superconductivity, ISSN 0921-4534, E-ISSN 1873-2143, Vol. 533, p. 63-73Article in journal (Refereed)
    Abstract [en]

    Starting with the generic Ginzburg-Landau expansion from a microscopic N-band model, we focus on the case of a 3-band model which was suggested to be relevant to describe some iron-based superconductors. This can lead to the so-called s + is superconducting state that breaks time-reversal symmetry due to the competition between different pairing channels. Of particular interest in that context, is the case of an interband dominated pairing with repulsion between different bands. For that case we consider in detail the relevant reduced two-component Ginzburg-Landau theory. We provide detailed analysis of the ground state, length scales and topological properties of that model. Prepared for the proceedings of Vortex IX conference in Rhodes (Sept. 2015).

  • 7. Jin, Chiming
    et al.
    Li, Zi-An
    Kovacs, Andras
    Caron, Jan
    Zheng, Fengshan
    Rybakov, Filipp N.
    KTH, School of Engineering Sciences (SCI), Physics, Statistical Physics. Ural federal university, Russian Federation.
    Kiselev, Nikolai S.
    Du, Haifeng
    Bluegel, Stefan
    Tian, Mingliang
    Zhang, Yuheng
    Farle, Michael
    Dunin-Borkowski, Rafal E.
    Control of morphology and formation of highly geometrically confined magnetic skyrmions2017In: Nature Communications, ISSN 2041-1723, E-ISSN 2041-1723, Vol. 8, article id 15569Article in journal (Refereed)
    Abstract [en]

    The ability to controllably manipulate magnetic skyrmions, small magnetic whirls with particle-like properties, in nanostructured elements is a prerequisite for incorporating them into spintronic devices. Here, we use state-of-the-art electron holographic imaging to directly visualize the morphology and nucleation of magnetic skyrmions in a wedge-shaped FeGe nanostripe that has a width in the range of 45-150 nm. We find that geometrically-confined skyrmions are able to adopt a wide range of sizes and ellipticities in a nanostripe that are absent in both thin films and bulk materials and can be created from a helical magnetic state with a distorted edge twist in a simple and efficient manner. We perform a theoretical analysis based on a three-dimensional general model of isotropic chiral magnets to confirm our experimental results. The flexibility and ease of formation of geometrically confined magnetic skyrmions may help to optimize the design of skyrmion-based memory devices.

  • 8. Meng, Qingyou
    et al.
    Varney, Christopher N.
    Fangohr, Hans
    Babaev, Egor
    KTH, School of Engineering Sciences (SCI), Physics, Statistical Physics.
    Phase diagrams of vortex matter with multi-scale inter-vortex interactions in layered superconductors2017In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 29, no 3, article id 035602Article in journal (Refereed)
    Abstract [en]

    It was recently proposed to use the stray magnetic fields of superconducting vortex lattices to trap ultracold atoms for building quantum emulators. This calls for new methods for engineering and manipulating of the vortex states. One of the possible routes utilizes type-1.5 superconducting layered systems with multi-scale inter-vortex interactions. In order to explore the possible vortex states that can be engineered, we present two phase diagrams of phenomenological vortex matter models with multi-scale inter-vortex interactions featuring several attractive and repulsive length scales. The phase diagrams exhibit a plethora of phases, including conventional 2D lattice phases, five stripe phases, dimer, trimer, and tetramer phases, void phases, and stable low-temperature disordered phases. The transitions between these states can be controlled by the value of an applied external field.

  • 9.
    Sellin, Karl
    KTH, School of Engineering Sciences (SCI), Physics, Statistical Physics.
    Structure formation, phase transitions and drag interactions in multicomponent superconductors and superfluids2018Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Superconductivity and superfluidity are some of the most funda-mental and important phenomena of modern physics. However, muchtheoretical work for such systems so far has been restricted to the one-component case. For multicomponent systems, the spectrum of possible topological defects, their structure formation and associated phasetransitions, can all be much richer than in the one-component case, motivating theoretical studies of multicomponent systems.

    In this thesis, the structure formation of vortices with complicated interactions due to multicomponent effects are considered using point-particle Monte Carlo simulations. Besides the triangular vortex latticesfound for one-component type-2 superconducting vortices, it is found that a rich plethora of structural phases is possible for vortices in mul-ticomponent systems.

    Since vortices play a key role in phase transitions, the problem of phase transitions in multicomponent systems is also studied in thisthesis. It could be expected that U(1) lattice London superconductorscan only have a continuous “inverted-XY” phase transition by a Peskin-Dasgupta-Halperin duality argument for the one-component case. Itis discussed here that the non-trivial internal structure of vortices in multicomponent U(1) London superconductors can instead lead to a first-order phase transition, which is supported by large-scale parallel tempering Monte Carlo simulations. Even for such systems, wherein the ground state vortex lines are axially symmetric, thermally induced splitting of composite vortices into fractional vortices can lead to a phase separation of vortex tangles, rendering the superconducting phase transition first-order.

    A similar phase separation can occur for two-component superconductors with an Andreev-Bashkin drag interaction, for which a phase separation can occur even in the ground state: the drag can cause com-posite vortices to decay into attractively interacting skyrmions. Suchdrag interactions can to a large extent influence phase transitions, rotational response and vortex structures in multicomponent systems. Thisthesis thus finishes with microscopic calculations of such an Andreev-Bashkin drag interaction in an extended Bose-Hubbard model of two-species bosons in an optical lattice, using worm quantum Monte Carlosimulations. Dependencies of the drag interaction on boson-boson in-teractions and properties of the optical lattice are characterized, andpaired phases (where only co- or counter-flow states occur) are observed.

  • 10. Sellin, Karl
    et al.
    Babaev, Egor
    KTH, School of Engineering Sciences (SCI), Physics, Statistical Physics.
    Superfluid drag in the two-component Bose-Hubbard modelManuscript (preprint) (Other academic)
    Abstract [en]

    In multicomponent superfluids and superconductors, co- and counter-flows of components have in general different properties. It was discussed in 1975 by Andreev and Bashkin, in the context of He3/He4 superfluid mixtures, that inter-particle interactions produce a dissipationless drag. The drag can be understood as a superflow of one component induced by phase gradients of the other component. Importantly the drag can be both positive (entrainment) and negative (counter-flow). The effect is known to be of crucial importance for many properties of diverse physical systems ranging from the dynamics of neutron stars, rotational responses of Bose mixtures of ultra-cold atoms to magnetic responses of multicomponent superconductors. Although there exists a substantial literature that includes the drag interaction phenomenologically, much fewer regimes are covered by quantitative studies of the microscopic origin of the drag and its dependence on microscopic parameters. Here we study the microscopic origin and strength of the drag interaction in a quantum system of two-component bosons on a lattice with short-range interaction. By performing quantum Monte-Carlo simulations of a two-component Bose-Hubbard model we obtain dependencies of the drag strength on the boson-boson interactions and properties of the optical lattice. Of particular interest are the strongly-correlated regimes where the ratio of co-flow and counter-flow superfluid stiffnesses can diverge, corresponding to the case of saturated drag.

  • 11.
    Sellin, Karl
    et al.
    KTH, School of Engineering Sciences (SCI), Physics.
    Babaev, Egor
    KTH, School of Engineering Sciences (SCI), Physics, Statistical Physics.
    Superfluid drag in the two-component Bose-Hubbard model2018In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 97, no 9, article id 094517Article in journal (Refereed)
    Abstract [en]

    In multicomponent superfluids and superconductors, co- and counterflows of components have, in general, different properties. A. F. Andreev and E. P. Bashkin [Sov. Phys. JETP 42, 164 (1975)] discussed, in the context of He-3/He-4 superfluid mixtures, that interparticle interactions produce a dissipationless drag. The drag can be understood as a superflow of one component induced by phase gradients of the other component. Importantly, the drag can be both positive (entrainment) and negative (counterflow). The effect is known to have crucial importance for many properties of diverse physical systems ranging from the dynamics of neutron stars and rotational responses of Bose mixtures of ultracold atoms to magnetic responses of multicomponent superconductors. Although substantial literature exists that includes the drag interaction phenomenologically, only a few regimes are covered by quantitative studies of the microscopic origin of the drag and its dependence on microscopic parameters. Here we study the microscopic origin and strength of the drag interaction in a quantum system of two-component bosons on a latticewith short-range interaction. By performing quantum Monte Carlo simulations of a two-component Bose-Hubbard model we obtain dependencies of the drag strength on the boson-boson interactions and properties of the optical lattice. Of particular interest are the strongly correlated regimes where the ratio of coflow and counterflow superfluid stiffnesses can diverge, corresponding to the case of saturated drag.

  • 12.
    Silaev, Mihail
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Statistical Physics.
    Garaud, Julien
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Statistical Physics. KTH, School of Engineering Sciences (SCI), Theoretical Physics, Condensed Matter Theory.
    Babaev, Egor
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Statistical Physics.
    Phase diagram of dirty two-band superconductors and observability of impurity-induced s plus i s state2017In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 95, no 2, article id 024517Article in journal (Refereed)
    Abstract [en]

    We investigate the phase diagram of dirty two-band superconductors. This paper primarily focuses on the properties and observability of the time-reversal symmetry-breaking s + is superconducting states, which can be generated in two-band superconductors by interband impurity scattering. We show that such states can appear in two distinct ways. First, according to a previously discussed scenario, the s + is state can form as an intermediate phase at the impurity-driven crossover between s(+/-) and s(++) states. We show that there is a second scenario where domains of the s + is state exists in the form of an isolated dome inside the s(+/-) domain, completely detached from the transition between s(+/-) and s(++) states. We demonstrate that in both cases the s + is state generated by impurity scattering exists in an extremely small interval of impurity concentrations. Although this likely precludes direct experimental observation of the s + is state formation due to this mechanism, this physics leads to the appearance of a region inside both the s(+/-) and s(++) domains with unusual properties due to softening of normal modes.

  • 13.
    Silaev, Mihail
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Statistical Physics.
    Vargunin, Artjom
    Vortex motion and flux-flow resistivity in dirty multiband superconductors2016In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 94, no 22, article id 224506Article in journal (Refereed)
    Abstract [en]

    The conductivity of vortex lattices in multiband superconductors with high concentration of impurities is calculated based on microscopic kinetic theory at temperatures significantly smaller than the critical one. Both the limits of high and low fields are considered, when the magnetic induction is close to or much smaller than the critical field strength IIc2, respectively. It is shown that in contrast to single-band superconductors, the resistive properties are not universal but depend on the pairing constants and ratios of diffusivities in different bands. The low-field magnetoresistance can strongly exceed the Bardeen-Stephen estimation in a quantitative agreement with experimental data for the two-band superconductor MgB2.

  • 14.
    Svistunov, Boris
    et al.
    KTH.
    Babaev, Egor
    KTH, School of Engineering Sciences (SCI), Physics, Statistical Physics.
    Prokofev, N. V.
    KTH.
    Superfluid states of matter2015Book (Other academic)
    Abstract [en]

    Covers the State of the Art in Superfluidity and Superconductivity Superfluid States of Matter addresses the phenomenon of superfluidity/superconductivity through an emergent, topologically protected constant of motion and covers topics developed over the past 20 years. The approach is based on the idea of separating universal classical-field superfluid properties of matter from the underlying system’s “quanta.” The text begins by deriving the general physical principles behind superfluidity/superconductivity within the classical-field framework and provides a deep understanding of all key aspects in terms of the dynamics and statistics of a classical-field system. It proceeds by explaining how this framework emerges in realistic quantum systems, with examples that include liquid helium, high-temperature superconductors, ultra-cold atomic bosons and fermions, and nuclear matter. The book also offers several powerful modern approaches to the subject, such as functional and path integrals. Comprised of 15 chapters, this text: Establishes the fundamental macroscopic properties of superfluids and superconductors within the paradigm of the classical matter field Deals with a single-component neutral matter field Considers fundamentals and properties of superconductors Describes new physics of superfluidity and superconductivity that arises in multicomponent systems Presents the quantum-field perspective on the conditions under which classical-field description is relevant in bosonic and fermionic systems Introduces the path integral formalism Shows how Feynman path integrals can be efficiently simulated with the worm algorithm Explains why nonsuperfluid (insulating) ground states of regular and disordered bosons occur under appropriate conditions Explores superfluid solids (supersolids) Discusses the rich dynamics of vortices and various aspects of superfluid turbulence at T → 0 Provides account of BCS theory for the weakly interacting Fermi gas Highlights and analyzes the most crucial developments that has led to the current understanding of superfluidity and superconductivity Reviews the variety of superfluid and superconducting systems available today in nature and the laboratory, as well as the states that experimental realization is currently actively pursuing. 

  • 15.
    Zheng, Fengshan
    et al.
    Forschungszentrum Julich, Ernst Ruska Ctr Microscopy & Spect Electrons, Julich, Germany.;Forschungszentrum Julich, Peter Grunberg Inst, Julich, Germany..
    Rybakov, Filipp N.
    KTH, School of Engineering Sciences (SCI), Physics, Statistical Physics. Russian Acad Sci, Ural Branch, MN Miheev Inst Met Phys, Ekaterinburg, Russia.;Ural Fed Univ, Ekaterinburg, Russia..
    Borisov, Aleksandr B.
    Russian Acad Sci, Ural Branch, MN Miheev Inst Met Phys, Ekaterinburg, Russia.;Ural Fed Univ, Ekaterinburg, Russia..
    Song, Dongsheng
    Tsinghua Univ, Natl Ctr Electron Microscopy Beijing, Sch Mat Sci & Engn, Beijing, Peoples R China..
    Wang, Shasha
    Chinese Acad Sci, Anhui Key Lab Condensed Matter Phys Extreme Condi, High Magnet Field Lab, Hefei, Anhui, Peoples R China.;Univ Sci & Technol China, Chinese Acad Sci, Hefei, Anhui, Peoples R China.;Nanjing Univ, Collaborat Innovat Ctr Adv Microstruct, Nanjing, Jiangsu, Peoples R China..
    Li, Zi-An
    Chinese Acad Sci, Inst Phys, Beijing, Peoples R China..
    Du, Haifeng
    Chinese Acad Sci, Anhui Key Lab Condensed Matter Phys Extreme Condi, High Magnet Field Lab, Hefei, Anhui, Peoples R China.;Univ Sci & Technol China, Chinese Acad Sci, Hefei, Anhui, Peoples R China.;Nanjing Univ, Collaborat Innovat Ctr Adv Microstruct, Nanjing, Jiangsu, Peoples R China..
    Kiselev, Nikolai S.
    Forschungszentrum Julich, Peter Grunberg Inst & Inst Adv Simulat, Julich, Germany.;JARA, Julich, Germany..
    Caron, Jan
    Forschungszentrum Julich, Ernst Ruska Ctr Microscopy & Spect Electrons, Julich, Germany.;Forschungszentrum Julich, Peter Grunberg Inst, Julich, Germany..
    Kovacs, Andras
    Forschungszentrum Julich, Ernst Ruska Ctr Microscopy & Spect Electrons, Julich, Germany.;Forschungszentrum Julich, Peter Grunberg Inst, Julich, Germany..
    Tian, Mingliang
    Chinese Acad Sci, Anhui Key Lab Condensed Matter Phys Extreme Condi, High Magnet Field Lab, Hefei, Anhui, Peoples R China.;Univ Sci & Technol China, Chinese Acad Sci, Hefei, Anhui, Peoples R China.;Nanjing Univ, Collaborat Innovat Ctr Adv Microstruct, Nanjing, Jiangsu, Peoples R China..
    Zhang, Yuheng
    Chinese Acad Sci, Anhui Key Lab Condensed Matter Phys Extreme Condi, High Magnet Field Lab, Hefei, Anhui, Peoples R China.;Univ Sci & Technol China, Chinese Acad Sci, Hefei, Anhui, Peoples R China.;Nanjing Univ, Collaborat Innovat Ctr Adv Microstruct, Nanjing, Jiangsu, Peoples R China..
    Bluegel, Stefan
    Forschungszentrum Julich, Peter Grunberg Inst & Inst Adv Simulat, Julich, Germany.;JARA, Julich, Germany..
    Dunin-Borkowski, Rafal E.
    Forschungszentrum Julich, Ernst Ruska Ctr Microscopy & Spect Electrons, Julich, Germany.;Forschungszentrum Julich, Peter Grunberg Inst, Julich, Germany..
    Experimental observation of chiral magnetic bobbers in B20-type FeGe2018In: Nature Nanotechnology, ISSN 1748-3387, E-ISSN 1748-3395, Vol. 13, no 6, p. 451-+Article in journal (Refereed)
    Abstract [en]

    Chiral magnetic skyrmions(1,2) are nanoscale vortex-like spin textures that form in the presence of an applied magnetic field in ferromagnets that support the Dzyaloshinskii-Moriya interaction (DMI) because of strong spin-orbit coupling and broken inversion symmetry of the crystal(3,4). In sharp contrast to other systems(5,6) that allow for the formation of a variety of two-dimensional (2D) skyrmions, in chiral magnets the presence of the DMI commonly prevents the stability and coexistence of topological excitations of different types(7). Recently, a new type of localized particle-like object-the chiral bobber (ChB)-was predicted theoretically in such materials(8). However, its existence has not yet been verified experimentally. Here, we report the direct observation of ChBs in thin films of B20-type FeGe by means of quantitative off-axis electron holography (EH). We identify the part of the temperature-magnetic field phase diagram in which ChBs exist and distinguish two mechanisms for their nucleation. Furthermore, we show that ChBs are able to coexist with skyrmions over a wide range of parameters, which suggests their possible practical applications in novel magnetic solid-state memory devices, in which a stream of binary data bits can be encoded by a sequence of skyrmions and bobbers.

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