| 1. |
- Biswas, P. K., et al.
(author)
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Coexistence of type-I and type-II superconductivity signatures in ZrB12 probed by muon spin rotation measurements
- 2020
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In: Physical Review B. - : AMER PHYSICAL SOC. - 2469-9950 .- 2469-9969. ; 102:14
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Journal article (peer-reviewed)abstract
- Superconductors usually display either type-I or type-II superconductivity and the coexistence of these two types in the same material, for example, at different temperatures, is rare in nature. We employed the muon spin rotation (mu SR) technique to unveil the superconducting phase diagram of the dodecaboride ZrB12 and obtained clear evidence of both type-I and type-II characteristics. Most important, we found a region showing unusual behavior where the usually mutually exclusive mu SR signatures of type-I and type-II superconductivity coexist. We reproduced that behavior in theoretical modeling that required taking into account multiple bands and multiple coherence lengths, which suggests that material has one coherence length larger and another smaller than the magnetic field penetration length (the type-1.5 regime). At stronger fields, a footprint of the type-II mixed state showing square flux-line lattice was also obtained using neutron diffraction.
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| 2. |
- Benfenati, Andrea, et al.
(author)
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Vortex nucleation barrier in superconductors beyond the Bean-Livingston approximation : A numerical approach for the sphaleron problem in a gauge theory
- 2020
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In: Physical Review B. - : AMER PHYSICAL SOC. - 2469-9950 .- 2469-9969. ; 101:22
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Journal article (peer-reviewed)abstract
- The knowledge of vortex nucleation barriers is crucial for applications of superconductors, such as single-photon detectors and superconductor-based qubits. Contrarily to the problem of finding energy minima and critical fields, there are no controllable methods to explore the energy landscape, identify saddle points, and compute associated barriers. Similar problems exist in high-energy physics where the saddle-point configurations are called sphalerons. Here, we present a generalization of the string method to gauge field theories, which allows the calculation of energy barriers in superconductors. We solve the problem of vortex nucleation, assessing the effects of the nonlinearity of the model, complicated geometry, surface roughness, and pinning.
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| 3. |
- Bessarab, Pavel F., et al.
(author)
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Lifetime of racetrack skyrmions
- 2018
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In: Scientific Reports. - : Springer. - 2045-2322. ; 8
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Journal article (peer-reviewed)abstract
- 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.
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| 4. |
- Bommer, Jouri D. S., et al.
(author)
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Spin-Orbit Protection of Induced Superconductivity in Majorana Nanowires
- 2019
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In: Physical Review Letters. - : AMER PHYSICAL SOC. - 0031-9007 .- 1079-7114. ; 122:18
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Journal article (peer-reviewed)abstract
- Spin-orbit interaction (SOI) plays a key role in creating Majorana zero modes in semiconductor nanowires proximity coupled to a superconductor. We track the evolution of the induced superconducting gap in InSb nanowires coupled to a NbTiN superconductor in a large range of magnetic field strengths and orientations. Based on realistic simulations of our devices, we reveal SOI with a strength of 0.15-0.35 eV angstrom. Our approach identifies the direction of the spin-orbit field, which is strongly affected by the superconductor geometry and electrostatic gates.
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| 5. |
- Di Fratta, Giovanni, et al.
(author)
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Variational principles of micromagnetics revisited
- 2020
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In: SIAM Journal on Mathematical Analysis. - : Society for Industrial & Applied Mathematics (SIAM). - 0036-1410 .- 1095-7154. ; 52:4, s. 3580-3599
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Journal article (peer-reviewed)abstract
- We revisit the basic variational formulation of the minimization problem associated with the micromagnetic energy, with an emphasis on the treatment of the stray field contribution to the energy, which is intrinsically nonlocal. Under minimal assumptions, we establish three distinct variational principles for the stray field energy: a minimax principle involving magnetic scalar potential and two minimization principles involving magnetic vector potential. We then apply our formulations to the dimension reduction problem for thin ferromagnetic shells of arbitrary shapes.
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| 6. |
- Du, Haifeng, et al.
(author)
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Interaction of Individual Skyrmions in a Nanostructured Cubic Chiral Magnet
- 2018
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In: Physical Review Letters. - : American Physical Society. - 0031-9007 .- 1079-7114. ; 120:19
-
Journal article (peer-reviewed)abstract
- We report direct evidence of the field-dependent character of the interaction between individual magnetic skyrmions as well as between skyrmions and edges in B20-type FeGe nanostripes observed by means of high-resolution Lorentz transmission electron microscopy. It is shown that above certain critical values of an external magnetic field the character of such long-range skyrmion interactions changes from attraction to repulsion. Experimentally measured equilibrium inter-skyrmion and skyrmion-edge distances as a function of the applied magnetic field shows quantitative agreement with the results of micromagnetic simulations. The important role of demagnetizing fields and the internal symmetry of three-dimensional magnetic skyrmions are discussed in detail.
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| 7. |
- Hoffmann, Markus, et al.
(author)
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Skyrmions, Antiskyrmions and Bobbers : Novel particles in chiral magnets
- 2019
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In: The 10th International Conference on Metamaterials, Photonic Crystals and Plasmonics, META 2019. - : META Conference. ; , s. 677-678
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Conference paper (peer-reviewed)abstract
- Chiral magnets are an emerging class of topological matter harbouring localized and topologically protected vortexlike magnetic textures called skyrmions, which are currently under intense scrutiny as a new entity for information storage and processing. Chirality in magnetism is introduced through the chiral symmetry breaking Dzyaloshinskii-Moriya interaction, which arizes through the spin-orbit interaction in magnets with structural bulk and interface inversion asymmetry. We have shown that chiral magnets cannot only host skyrmions but also antiskyrmions and chiral bobbers as localized magnetization particles. We present a multiscale approach relating the electronic structure to an atomistic spin-lattice model and further to micromagnetic models by means of densityfunctional calculations, derive practical criteria for their occurrence and coexistence with skyrmions and minimizing the energy on a mesoscopic scale applying varying spinrelaxation methods. We conjecture on possible materials for antiskyrmions and give experimental evidence for bobbers. The technology potential of having two distinctly different stable particles will be discussed.
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| 8. |
- Jin, Chiming, et al.
(author)
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Control of morphology and formation of highly geometrically confined magnetic skyrmions
- 2017
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In: Nature Communications. - : Nature Publishing Group. - 2041-1723. ; 8
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Journal article (peer-reviewed)abstract
- 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.
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| 9. |
- Kuchkin, Vladyslav M., et al.
(author)
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Geometry and symmetry in skyrmion dynamics
- 2021
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In: Physical Review B. - : American Physical Society (APS). - 2469-9950 .- 2469-9969. ; 104:16
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Journal article (peer-reviewed)abstract
- The uniform motion of chiral magnetic skyrmions induced by a spin-transfer torque displays an intricate dependence on the skyrmions' topological charge and shape. We reveal surprising patterns in this dependence through simulations of the Landau-Lifshitz-Gilbert equation with Zhang-Li torque and explain them through a geometric analysis of Thiele's equation. Our results provide a universal geometrical understanding of the dependence of the skyrmion's velocity and Hall angle on the skyrmion's topological charge, shape, and orientation. The generality of our approach suggests the validity of our results for exchange-frustrated magnets, bubble materials, and other materials.
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| 10. |
- Kuchkin, Vladyslav M., et al.
(author)
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Heliknoton in a film of cubic chiral magnet
- 2023
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In: Frontiers in Physics. - : FRONTIERS MEDIA SA. - 2296-424X. ; 11
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Journal article (peer-reviewed)abstract
- Cubic chiral magnets exhibit a remarkable diversity of two-dimensional topological magnetic textures, including skyrmions. However, the experimental confirmation of topological states localized in all three spatial dimensions remains challenging. In this paper, we investigate a three-dimensional topological state called a heliknoton, which is a hopfion embedded into a helix or conic background. We explore the range of parameters at which the heliknoton can be stabilized under realistic conditions using micromagnetic modeling, harmonic transition state theory, and stochastic spin dynamics simulations. We present theoretical Lorentz TEM images of the heliknoton, which can be used for experimental comparison. Additionally, we discuss the stability of the heliknoton at finite temperatures and the mechanism of its collapse. Our study offers a pathway for future experimental investigations of three-dimensional topological solitons in magnetic crystals.
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