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Register a new userLocalization of superconductivity in superconductor-electromagnet hybrids
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Added by
Alexei Yu. Aladyshkin
Publication date
2012
fields
Physics
and research's language is
English
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We investigate the nucleation of superconductivity in a superconducting Al strip under the influence of the magnetic field generated by a current-carrying Nb wire, perpendicularly oriented and located underneath the strip. The inhomogeneous magnetic field, induced by the Nb wire, produces a spatial modulation of the critical temperature T_c, leading to a controllable localization of the superconducting order parameter (OP) wave function. We demonstrate that close to the phase boundary T_c(B_ext) the localized OP solution can be displaced reversibly by either applying an external perpendicular magnetic field B_ext or by changing the amplitude of the inhomogeneous field.
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We studied experimentally the effect of a stripe-like domain structure in a ferromagnetic BaFe_{12}O_{19} substrate on the magnetoresistance of a superconducting Pb microbridge. The system was designed in such a way that the bridge is oriented perpendicular to the domain walls. It is demonstrated that depending on the ratio between the amplitude of the nonuniform magnetic field B_0, induced by the ferromagnet, and the upper critical field H_{c2} of the superconducting material, the regions of the reverse-domain superconductivity in the H-T plane can be isolated or can overlap (H is the external magnetic field, T is temperature). The latter case corresponds to the condition B_0/H_{c2}<1 and results in the formation of superconductivity above the magnetic domains of both polarities. We discovered the regime of edge-assisted reverse-domain superconductivity, corresponding to localized superconductivity near the edges of the bridge above the compensated magnetic domains. Direct verification of the formation of inhomogeneous superconducting states and external-field-controlled switching between normal state and inhomogeneous superconductivity were obtained by low-temperature scanning laser microscopy.
Ferromagnet/superconductor heterostructures allow for the combination of unique physical phenomena offered by the both fields of magnetism and superconductivity. It was shown recently that spin waves can be efficiently scattered in such structures by a lattice of static or moving magnetic flux quanta (Abrikosov vortices), resulting in bandgaps in the spin-wave spectra. Here, we realize a nonreciprocal motion of a vortex lattice in nanoengineered symmetric and asymmetric pinning landscapes and investigate the non-reciprocal scattering of magnons on fluxons. We demonstrate that the magnon bandgap frequencies can be tuned by the application of a low-dissipative transport current and by its polarity reversal. Furthermore, we exploit the rectifying (vortex diode or ratchet) effect by the application of a 100 MHz-frequency ac current to deliberately realize bandgap up- or downshifts during one ac halfwave while keeping the bandgap frequency constant during the other ac halfwave. The investigated phenomena allow for the realization of energy-efficient hybrid magnonic devices, such as microwave filters with an ultra-high bandgap tunability of 10 GHz/mA and a fast modulation of the transmission characteristics on the 10 ns time scale.
We demonstrate experimentally that the presence of a single domain wall in an underlying ferromagnetic BaFe_{12}O_{19} substrate can induce a considerable asymmetry in the current (I) - voltage (V) characteristics of a superconducting Al bridge. The observed diode-like effect, i.e. polarity-dependent critical current, is associated with the formation of a vortex-free channel inside the superconducting area which increases the total current flowing through the superconducting bridge without dissipation. The vortex-free region appears only for a certain sign of the injected current and for a limited range of the external magnetic field.
The theoretical and experimental results concerning the thermodynamical and low-frequency transport properties of hybrid structures, consisting of spatially-separated conventional low-temperature superconductor (S) and ferromagnet (F), is reviewed. Since the superconducting and ferromagnetic parts are assumed to be electrically insulated, no proximity effect is present and thus the interaction between both subsystems is through their respective magnetic stray fields. Depending on the temperature range and the value of the external field H_{ext}, different behavior of such S/F hybrids is anticipated. Rather close to the superconducting phase transition line, when the superconducting state is only weakly developed, the magnetization of the ferromagnet is solely determined by the magnetic history of the system and it is not influenced by the field generated by the supercurrents. In contrast to that, the nonuniform magnetic field pattern, induced by the ferromagnet, strongly affect the nucleation of superconductivity leading to an exotic dependence of the critical temperature T_{c} on H_{ext}. Deeper in the superconducting state the effect of the screening currents cannot be neglected anymore. In this region of the phase diagram various aspects of the interaction between vortices and magnetic inhomogeneities are discussed. In the last section we briefly summarize the physics of S/F hybrids when the magnetization of the ferromagnet is no longer fixed but can change under the influence of the superconducting currents. As a consequence, the superconductor and ferromagnet become truly coupled and the equilibrium configuration of this soft S/F hybrids requires rearrangements of both, superconducting and ferromagnetic characteristics, as compared with hard S/F structures.
Noncollinear magnetism opens exciting possibilities to generate topological superconductivity. Here, we focus on helical and cycloidal magnetic textures in magnet-superconductor hybrid structures in a background magnetic field. We demonstrate that this system can enter a topological phase which can be understood as a set of parallel topological wires. We explore and confirm this idea in depth with three different approaches: a continuum model, a tight-binding model based on the magnetic unit cell, and exact diagonalization on a finite two-dimensional lattice. The key signature of this topological state is the presence of Majorana bound states at certain disclination defects in the magnetic texture. Based on the $C_2$ symmetry imposed by the helical or cycloidal texture, we employ the theory of topological crystalline superconductors with rotation invariants to obtain the Majorana parity at disclinations. Furthermore, we consider a 90-degree helimagnet domain wall, which is formed by a string of alternating disclinations. We discuss how the resulting chain of disclination bound states hybridizes into two chiral modes with different velocities. We suggest that hybrid systems of chiral magnets and superconductors are capable of hosting Majorana modes in various spatial configurations with potentially far less nano-engineering than in, e.g., semiconductor wires.
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