Pinning Potential Research Articles

Magnetic Nonreciprocity in a Hybrid Device of Asymmetric Artificial Spin-Ice-Superconductors

Controlling the size and distribution of potential barriers within a medium of interacting particles can unveil unique collective behaviors and innovative functionalities. We introduce a unique superconducting hybrid device using a novel artificial spin ice structure composed of asymmetric nanomagnets. This structure forms a distinctive superconducting pinning potential that steers unconventional motion of superconducting vortices, thereby inducing a magnetic nonreciprocal effect, in contrast to the electric nonreciprocal effect commonly observed in superconducting diodes. Furthermore, the polarity of the magnetic nonreciprocity is in situ reversible through the tunable magnetic patterns of artificial spin ice. Our findings demonstrate that artificial spin ice not only precisely modulates superconducting characteristics but also opens the door to novel functionalities, offering a groundbreaking paradigm for superconducting electronics.

The behavior of ultra-cold atoms in two-dimensional optical lattices with asymmetric periodic pinning potentials

We examine the disorder effect of the pinning potential acting on ultracold atoms in the optical lattice based on the expanded Bose–Hubbard model for soft-core bosons. Using Quantum Monte Carlo simulation with local optimal Worm algorithm, we obtain three additional crystal-like phase types at commensurate densities, ρ = 1/2, 1, and 3/2 besides two basic phases which are superfluid and Mott insulator phases as observed in the bare soft-core boson model. In the presence of disorder, the superfluid and the charge-density wave coexist in the sandwiched regions between the superfluid and the solid orders around the commensurate densities. This observation signifies our work because it has not been revealed previously in other models of the soft-core bosons.

Higher-order spin-hole correlations around a localized charge impurity

Analysis of higher-order correlation functions has become a powerful tool for investigating interacting many-body systems in quantum simulators, such as quantum gas microscopes. Experimental measurements of mixed spin-charge correlation functions in the 2D Hubbard have been used to study equilibrium properties of magnetic polarons and to identify coherent and incoherent regimes of their dynamics. In this paper we consider theoretically an extension of this technique to systems which use a pinning potential to reduce the mobility of a single dopant in the Mott insulating regime of the 2D Hubbard model. We find that localization of the dopant has a dramatic effect on its magnetic dressing. The connected third-order spin correlations are weakened in the case of a mobile hole but strengthened near an immobile hole. In the case of the fifth-order correlation function, we find that its bare value has opposite signs in cases of the mobile and of fully pinned dopant, whereas the connected part is similar for both cases. We study suppression of higher-order correlators by thermal fluctuations and demonstrate that they can be observed up to temperatures comparable to the spin-exchange energy $J$. We discuss implications of our results for understanding the interplay of spin and charge in doped Mott insulators.

Depinning behavior of the vortex domain wall at the asymmetric triangular notch in permalloy wires

The depinning field (H D) of vortex domain walls in a permalloy wire with an asymmetric triangle notch was investigated through magneto-optic Kerr effect (MOKE) microscopy and micromagnetic simulations. Wires of various widths with notches fixed on the wall’s incoming side angle were studied for various outgoing side angles (ϕ). The curves of H D of wall versus ϕ were measured by MOKE microscopy. Micromagnetic simulations were used to obtain curves of the H D of the wall versus ϕ. The maximum of such a curve of tail-to-tail -clockwise wall is known as the transition angle (ϕ T). The shape-anisotropic energy (E A) of the notch outgoing side wire and the exchange energy (E Ex) of the wall–notch interaction competed to influence the ϕ T value. Pinning potential was increased by the E Ex when ϕ was smaller than the ϕ T. Pinning potential was considerably reduced by the small E A when ϕ was larger than the ϕ T. Furthermore, the ϕ T value changed with the decrease in the depth of the notch because E A was influenced by notch depth.

Transport Properties of the Classical Toda Chain: Effect of a Pinning Potential

We consider energy transport in the classical Toda chain in the presence of an additional pinning potential. The pinning potential is expected to destroy the integrability of the system and an interesting question is to see the signatures of this breaking of integrability on energy transport. We investigate this by a study of the non-equilibrium steady state of the system connected to heat baths as well as the study of equilibrium correlations. Typical signatures of integrable systems are a size-independent energy current, a flat bulk temperature profile and ballistic scaling of equilibrium dynamical correlations, these results being valid in the thermodynamic limit. We find that, as expected, these properties change drastically on introducing the pinning potential in the Toda model. In particular, we find that the effect of a harmonic pinning potential is drastically smaller at low temperatures, compared to a quartic pinning potential. We explain this by noting that at low temperatures the Toda potential can be approximated by a harmonic inter-particle potential for which the addition of harmonic pinning does not destroy integrability.

Dynamic Control of the Vortex Pinning Potential in a Superconductor Using Current Injection through Nanoscale Patterns.

Control over the vortex potential at the nanoscale in a superconductor is a subject of great interest for both fundamental and technological reasons. Many methods for achieving artificial pinning centers have been demonstrated, for example, with magnetic nanostructures or engineered imperfections, yielding many intriguing effects. However, these pinning mechanisms do not offer dynamic control over the strength of the patterned vortex potential because they involve static nanostructures created in or near the superconductor. Dynamic control has been achieved with scanning probe methods on the single vortex level but these are difficult so scale up. Here, we show that by applying controllable nanopatterned current injection, the superconductor can be locally driven out of equilibrium, creating an artificial vortex potential that can be tuned by the magnitude of the injected current, yielding a unique vortex channeling effect.

Thickness-modulated tungsten-carbon superconducting nanostructures grown by focused ion beam induced deposition for vortex pinning up to high magnetic fields.

We report efficient vortex pinning in thickness-modulated tungsten–carbon-based (W–C) nanostructures grown by focused ion beam induced deposition (FIBID). By using FIBID, W–C superconducting films have been created with thickness modulation properties exhibiting periodicity from 60 to 140 nm, leading to a strong pinning potential for the vortex lattice. This produces local minima in the resistivity up to high magnetic fields (2.2 T) in a broad temperature range due to commensurability effects between the pinning potential and the vortex lattice. The results show that the combination of single-step FIBID fabrication of superconducting nanostructures with built-in artificial pinning landscapes and the small intrinsic random pinning potential of this material produces strong periodic pinning potentials, maximizing the opportunities for the investigation of fundamental aspects in vortex science under changing external stimuli (e.g., temperature, magnetic field, electrical current).

Flux pinning distribution and E-J characteristics of 2G YBCO Tapes

E-J characteristics of SuperPower YBCO 2G tapes have been measured in the temperature range 15K-80K. It was found that the E-J characteristics deviate significantly from the standard power-law behaviour with apparent power exponent decreasing continuously at high voltage with increasing current. The deviation of E-J characteristic from the standard power-law was found to be consistent with a Weibull distribution of critical current. The identical scaling of E-J characteristics above 40K suggests a common critical current distribution. At lower temperature the critical current distribution becomes narrower to give higher apparent power exponent at lower temperature. With the critical current distribution model it is shown that the dissipation can be correlated directly to the flux flow resistance of Bardeen. The distribution of the critical current can be associated with a distribution of pinning potential of collective pinning and the headline pinning range is obtained as function of temperature.

Magnetic hysteresis behavior and magnetic pinning in a d ferromagnet/superconductor nanostructure

We investigate the interaction between superconductivity and defect-induced d0 ferromagnetism using a composite consisting of MgB2 and MgO nanocrystals. The composite exhibits a ferromagnetic hysteresis behavior in the temperature region from 40 to 300 K. Defective MgO nanocrystals (∼20 nm) embedded in the composite are considered to be responsible for the observed ferromagnetism. The zero field cool and field cool magnetization curves show that the superconducting transition occurs at Tc = 38.6 K, in agreement with Tc of pure MgB2. In the temperature region from Tc to 0.9Tc (∼35 K), the magnetization hysteresis curves show a superposition of ferromagnetic (F) and superconducting (S) signals. When the temperature of the system is decreased below 0.65Tc (∼25 K), the S signals dominate over the F signals. The resulting magnetic hysteresis loops are highly asymmetric and the descending filed branch is nearly flat, as predicted in the case of surface pinning. At temperatures below 0.5Tc (∼20 K), a sharp peak is developed near zero field in the magnetization hysteresis curves, implying an enhancement of superconducting vortex pinning. The observed pinning enhancement most likely results from magnetic pinning due to randomly distributed magnetic MgO grains, which yield the magnetic inhomogeneity and the related pinning potential in a length scale of ∼100 nm. Thus, the present ferromagnetic/superconducting composite provides an ideal model system that demonstrates the availability of d0 ferromagnetism as a source of magnetic potential for effective vortex pinning.

Determination of coordinate dependence of a pinning potential from a microwave experiment with vortices

The measurement of the complex impedance response accompanied by power absorption P(ω) in the radiofrequency and microwave ranges represents a most popular experimental method for the investigation of pinning mechanisms and vortex dynamics in type-II superconductors. In the theory, the pinning potential (PP) well for a vortex must be a priori specified in order to subsequently analyze the measured data. We have theoretically solved the inverse problem at T = 0 K and exemplify how the coordinate dependence of a PP can be determined from a set of experimental curves P(ω|j0) measured at subcritical dc currents 0 < j0 < jc under a small microwave excitation j1 ≪ jc with frequency ω. We furthermore elucidate how and why the depinning frequency ωp, which separates the non-dissipative (quasi-adiabatic) and the dissipative (high-frequency) regimes of small vortex oscillations in the PP, is reduced with increasing j0. The results can be directly applied to a wide range of conventional superconductors with a PP subjected to superimposed dc and small microwave ac currents at T ≪ Tc.

Noise-Assisted Microwave Up-conversion by Vortices in Thin-Film Superconductors with a dc-Biased Washboard Pinning Potential

So far the main theoretical basis for understanding and optimization of the microwave properties of vortices in type II superconductors has been relying upon the Coffey–Clem (CC) approach for the linear impedance at nonzero temperature. However, the CC model does not account for the non-linear response and the possibility to control it by changing the value of the dc transport current in a superconducting sample. For this reason, we have exactly solved the Langevin equation for the two-dimensional non-linear vortex dynamics in a dc bias-tilted cosine pinning potential in the presence of an ac current of arbitrary amplitude and frequency ω and have, thereby, substantially generalized the CC results. In this work we analyze the behavior of the non-linear response on kω-frequency in a wide range of dc and ac current densities, ω, and temperature. The kω-response is found to depend strongly on all these parameters, as exemplified for the third-harmonic (k=3) transformation coefficient Z 3. The parametric window for the most enhanced up-conversion is presented. The predicted effects can be experimentally verified in thin-film superconductors with some pinning potential of the washboard type.

Current-controlled Filter on Superconducting Films with a Tilted Washboard Pinning Potential

Abstract The influence of an ac current of arbitrary amplitude and frequency on the mixed-state dc-voltage-ac-drive tiltingratchet response of a superconducting film with uniaxial cosine pinning potential at finite temperature is theoretically investigated. The results are obtained in the single-vortex approximation, within the frame of an exact solution of the Langevin equation for non-interacting vortices. Both experimentally achievable, the dc ratchet response and absorbed ac power are predicted to demonstrate a pronounced filter-like behavior at microwave frequencies. Based on our findings, we propose a cut-off filter and discuss its operating curves as functions of the driving parameters, i.e, ac amplitude, frequency, and dc bias. The predicted results can be examined, e.g, on superconducting films with a washboard pinning potential landscape.

Effect of element substitution on the superconducting properties of Gd-based melt-processed bulk-superconductors

We studied the effect of Co doping on the superconducting properties of melt-processed Gd-Ba-Cu-O bulk-superconductors and compared the results with Zn doping, the former substitutes for Cu of the Cu-O chain while the latter to the CuO2 plane. The critical current density (Jc) at 77 K of the region beneath the seed crystal increased with the Co3O4 content up to 0.23 wt.%, and that of the a-growth region about 5 mm away from the seed crystal up to 0.30 wt.%. The analysis of the pinning force showed that Co doping increases the amount of normal pins (δl pinning). The measurements of the time dependence of the trapped field of the samples doped with various amounts of Co3O4 indicated that the creep rate at 77 K increases with Co3O4 content. This result suggests that the pinning potential of the normal pin created by the addition of Co3O4 is relatively small.

Numerical studies on the collective vortex flow and RF responses of the intrinsic Josephson junction with periodic potentials

In-phase collective motion of Josephson vortices (JVs) over the junctions stack as well as the strong interaction between the vortices and the electromagnetic modes are needed to realize effective electromagnetic (EM) wave radiations from the intrinsic Josephson junction stacks. We have been studied the interaction from the RF responses of the intrinsic Josephson junctions in the flux-flow states. So far, the Shapiro-step-like responses were found in the flux-flow state with a cooperation of pancake vortices introduced by tilting the magnetic field direction out from the ab plane. The Shapiro-step-like responses suggest that the JVs collectively move keeping the rectangular lattice. We have proposed that the pancake vortices develop a periodic pinning potential that stabilizes the in-phase collective motion of JVs. We have carried out numerical simulations based on the coupled sine-Gordon equation considering the effect of the periodic pinning potential on the vortex dynamics. Current-voltage (I-V) properties of the inductively-coupled Josephson junction stack, which represent the intrinsic Josephson junction stack, were numerically obtained for both static and flux-flow states. It is shown that the periodic pinning potentials of appropriate strengths revealed Shapiro-step-like responses on the I-V properties of flux-flow states. It is also shown that the periodic pinning potentials stabilize the in-phase collective motion of the JVs over the junction stacks.

New Hall Resistivity Scaling Relations in the Presence of Competition between Point-like and Anisotropic Planar Pinning Potential

Explicit current- and temperature-dependent expressions for anisotropic longitudinal and transverse nonlinear magnetoresistivities are represented and analyzed on the basis of a Fokker-Planck approach for two-dimensional vortex dynamics in a washboard pinning potential in the presence of point-like disorder. It is shown that new scaling relations in discussed below longitudinal and transverse geometries of experiment can be presented in the form which is: a) similar formally to the well-known scaling relations for the point-like disorder, but contains instead of the bare Hall conductivity a renormalized one which depends on the pinning anisotropy b) just the same as for purely anisotropic scaling relations in the absence of point-like pins. Physical meaning of these results is discussed.

Guiding of Vortices and Its Influence on the Hall Voltages in a Bianisotropic Planar Pinning Potential

The mediated by a small Hall effect odd resistive response of a superconductor in the mixed state is theoretically investigated within the framework of the two-dimensional stochastic model of bianisotropic pinning. It is shown that the odd magnetoresistivities ρ∥ , ⊥ arise due to the interaction of the nonlinear guiding- and Hall-effects, and their resistive properties are analyzed by means of the diagram of the dynamic states of the vortex system on the (j x , j y ) plane. Scaling relations for the Hall conductivity are found to be entirely different from the case of isotropic pinning.

Magnetic domains and flux pinning properties of a nanostructured ferromagnet/superconductor bilayer

We have investigated the magnetic and superconducting properties of a nanostructured ferromagnet/superconductor hybrid system, consisting of a Co layer with a square array of rectangular holes (antidots), covered with a thin continuous (nonperforated) superconducting Pb layer. The Co film with an antidot lattice shows a strong in-plane magnetic shape anisotropy determined by the array configuration and the rectangular shape of the antidots. Remanent domain structures in the uncovered Co antidot array are imaged at room temperature by magnetic force microscopy. The superconducting pinning properties of the hybrid ferromagnet/superconductor system are studied for different magnetic states of the Co antidot lattice. We demonstrate that the contribution to the superconducting pinning potential caused by the stray field from the domain walls is stronger than that arising from the periodic modulation due to the underlying antidot lattice.

Vortex structure and dynamics in kagomé and triangular pinning potentials

We study the dynamics of thermally driven superconducting vortices in two types of periodic pinning potentials: kagom\'e and triangular. For the first, second, and third matching fields, we obtain the corresponding ground-state vortex configurations and their phase diagrams. We analyze the system properties by looking at the vortex trajectories and the structure factor, as well as the linear and angular diffusion. The temperature versus pinning force phase diagram is analyzed in detail for each matching field. When the temperature is varied, we observe several stages of lattice pinning and melting. In most of the cases we find, for decreasing temperature, first a pinning of vortices and afterwards a freezing transition of the interstitial vortices. The intermediate regime corresponds to interstitial vortices in a confined liquidlike state and pinned vortices. The kagom\'e pinning potential shows interesting behavior at low temperatures: there is a phase with rotating vortex triangles caged by kagom\'e hexagons (``cooperative ring elementary excitations''), and there is geometric frustration for $\stackrel{\ensuremath{\rightarrow}}{T}0$ with a nearly degenerate ground state.

Dislocation kinetics in quasicrystals under small applied stress

The sliding of dislocations in quasicrystals under small applied stress is considered within phenomenological model. Implications of the quasi-periodic Peierls potential for the dislocation pinning is analyzed and associated activation mechanism is shown to be relevant for the plastic deformation of quasicrystals. The dependence of the dislocation velocity and strain rate on the applied stress is estimated. Kinetics of the dislocation movement in the quasi-periodic pinning potential is analyzed in exactly solvable one-dimensional model. The spectrum of the dislocation jump lengths is found to be a universal function of applied stress. The critical values of resolved stress leading to the drastic changes in the dislocation velocity are shown to form periodical sequence in a logarithm scale. Recent experiments on the mechanical properties of quasicrystals are discussed within proposed model.

Transition from thermally activated to regular flow of magnetic flux vortices in HTSC

The effect of structural inhomogeneities in a superconductor on a vortex medium flow in weak magnetic fields at temperatures varying from 78 to 83 K for various bias current densities is investigated by using transport measurements of Bi2Sr2CaCu2O8+x thin-film microbridges. The results obtained are analyzed on the basis of the theories of flux creep and the regular flow of vortices. It is shown that the current dependences of the effective potential for vortex pinning can be satisfactorily described in the framework of two statistical models, one of which was proposed earlier by the authors. Both models cover the regimes of thermally activated and regular flow of vortices as limiting cases. The wide transition region in which the creep and regular vortex flow processes simultaneously occur due to a large dispersion in the pinning energy distribution. It is found that when the magnetic field exceeds a certain value, the average value and dispersion of the pinning potential decrease sharply, so that the conditions of regular flow set in even for small values of the bias current. This fact is attributed to the destruction of vortex lines into two-dimensional segments.