Weak Pinning Research Articles

Kelvin–Helmholtz Instability in 3\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^3$$\\end{document}He Superfluids in Zero-Temperature Limit

In rotating 3\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^3$$\\end{document}He superfluids, the Kelvin–Helmholtz (KH) instability of the AB interface has been found to follow the theoretical model above 0.4Tc\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$0.4 \\, T_\ extrm{c}$$\\end{document}. A deviation from this dependence has been assumed possible at the lowest temperatures. Our NMR and thermal bolometer measurements down to 0.2Tc\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$0.2 \\, T_\ extrm{c}$$\\end{document} show that the critical KH rotation velocity follows the extrapolation from higher temperatures. We interpret this to mean that the KH instability is a bulk phenomenon and is not compromised by interactions with the wall of the rotating container, although weak pinning of the interface to the wall is observed during slow sweeping of the magnetic field. The KH measurement provides the only so far existing determination of the interfacial surface tension at temperatures down to 0.2Tc\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$0.2 \\, T_\ extrm{c}$$\\end{document} as a function of pressure.

Peak effect and dynamics of stripe- and pattern-forming systems on a periodic one-dimensional substrate.

We examine the ordering, pinning, and dynamics of two-dimensional pattern-forming systems interacting with a periodic one-dimensional substrate. In the absence of the substrate, particles with competing long-range repulsion and short-range attraction form anisotropic crystal, stripe, and bubble states. When the system is tuned across the stripe transition in the presence of a substrate, we find that there is a peak effect in the critical depinning force when the stripes align and become commensurate with the substrate. Under an applied drive, the anisotropic crystal and stripe states can exhibit soliton depinning and plastic flow. When the stripes depin plastically, they dynamically reorder into a moving stripe state that is perpendicular to the substrate trough direction. We also find that when the substrate spacing is smaller than the widths of the bubbles or stripes, the system forms pinned stripe states that are perpendicular to the substrate trough direction. The system exhibits multiple reentrant pinning effects as a function of increasing attraction, with the anisotropic crystal and large bubble states experiencing weak pinning but the stripe and smaller bubble states showing stronger pinning. We map out the different dynamic phases as a function of filling, the strength of the attractive interaction term, the substrate strength, and the drive, and demonstrate that the different phases produce identifiable features in the transport curves and particle orderings.

Weak Pinning and Long-Range Anticorrelated Motion of Phase Boundaries in Driven Diffusive Systems.

We show that domain walls separating coexisting extremal current phases in driven diffusive systems exhibit complex stochastic dynamics with a subdiffusive temporal growth of position fluctuations due to long-range anticorrelated current fluctuations and a weak pinning at long times. This weak pinning manifests itself in a saturated width of the domain wall position fluctuations that increases sublinearly with the system size. As a function of time t and system size L, the width w(t,L) has a scaling behavior w(t,L)=L^{3/4}f(t/L^{9/4}), with f(u) constant for u≫1 and f(u)∼u^{1/3} for u≪1. An Orstein-Uhlenbeck process with long-range anticorrelated noise is shown to capture this scaling behavior. The exponent 9/4 is a new dynamical exponent for relaxation processes in driven diffusive systems.

Critical current of a layered high-temperature superconductor with artificial pinning centers: the Monte Carlo simulation

Current–voltage characteristics (EJ curves) and magnetic field dependences of the critical current have been calculated for a superconductor with artificial pinning in the form of submicron-sized holes and tilted radiation defects. Calculations have been performed within the framework of the three-dimensional model of a layered HTS by means of the Monte Carlo method. S-shaped features of the EJ curves have been observed for a sample with a rectangular lattice of holes. Such features have not occurred in calculations for HTSs with non-magnetic pinning centers before, but they have been observed in experimental studies. In this paper, the features occurred in magnetic fields close to 290 Gs (which is the lower critical field for the Bi2Sr2CaCu2O8-δ superconductor at 1 K) and they were sensitive to the magnitude of the external magnetic field. In addition, the features were more prominent at temperatures below 30 K and in samples with weak intrinsic pinning, and they were connected with matching-like effects in the vortex system (i. e. a certain number of vortices being pinned on each hole, screening new vortices from entering the sample). For samples with tilted radiation defects, decreasing field dependences of the critical current have been obtained, showing weak maxima near the lower critical field of the superconductor. Calculations have shown that, at a fixed value of the external field, the critical current decreases with the increasing tilt angle of the defects.

Critical Current in a Long Josephson Contact with Weak Pinning in an External Magnetic Field

Critical Current in a Long Josephson Contact with Weak Pinning in an External Magnetic Field

Bias-polarity-dependent asymmetric domain switching in fatigued epitaxial BiFeO3 (001) capacitors

We investigated domain switching dynamics in pristine and fatigued epitaxial BiFeO3 (001) capacitors using piezoresponse force microscopy and switching current measurements. In contrast to domain growth-dominant switching in the pristine capacitors, we observed two different bias-polarity-dependent switching dynamics in the fatigued capacitors. Under a positive bias on the top electrode of the fatigued capacitors, the domain walls were strongly pinned, indicating that nucleation played a critical role in domain switching. However, when a negative bias was applied, the domain wall velocity decreased owing to weak pinning of the domain walls, but domain growth remained the governing process of the domain switching behavior. The intriguing asymmetric domain switching and related domain wall pinning behavior in the fatigued BiFeO3 capacitors may be attributed to the formation of a defect layer near the interface between the ferroelectric film and electrode.

Defect-induced weak collective pinning in superconducting YB6 crystals

In a previous study (2017 Phys. Rev. B 96 144501), a strong variation in the superconducting transition temperature T c of YB6 differing by a factor of two has been explained by a change in the density of yttrium and boron vacancies tuning the electron–phonon interaction. Here, by using an array of miniature Hall probes, we address the penetration of the magnetic field, pinning, and critical current density on a series of YB6 single crystals with T c variation between 4.25 and 7.35 K. The analysis of the superconducting and normal-state specific heat characteristics allowed us to determine T c and the stoichiometry of our samples. We observed almost no pinning in the most stoichiometric YB6 crystal with the lowest T c. Upon increasing the number of vacancies weak pinning appears, and the critical current density is enhanced following the increased transition temperature in a linear variation. We argue that such an increase is, within weak collective pinning theory, consistent with the increasing number of vacancies that serve as pinning centers.

Instability of Magnetic Skyrmion Strings Induced by Longitudinal Spin Currents.

It is well established that spin-transfer torques exerted by in-plane spin currents give rise to a motion of magnetic skyrmions resulting in a skyrmion Hall effect. In films of finite thickness or in three-dimensional bulk samples the skyrmions extend in the third direction forming a string. We demonstrate that a spin current flowing longitudinally along the skyrmion string instead induces a Goldstone spin wave instability. Our analytical results are confirmed by micromagnetic simulations of both a single string as well as string lattices, suggesting that the instability eventually breaks the strings. A longitudinal current is thus able to melt the skyrmion string lattice via a nonequilibrium phase transition. For films of finite thickness or in the presence of disorder a threshold current will be required, and we estimate the latter assuming weak collective pinning.

Effects of He ion irradiation on the structural stability and deformation behavior in Zr63.5Cu23Al9Fe4.5 and Fe80B13Si7 metallic glasses

The effects of He2+ irradiation on the structural stability and deformation behavior in Zr63.5Cu23Al9Fe4.5 and Fe80B13Si7 metallic glasses (MGs) were investigated by TEM and nano-indenter. He bubbles in Fe80B13Si7 MG distributed more narrowly along the depth direction, packed more densely and exhibited larger size. At 4 × 1017/cm2 dose, Fe3B nanocrystals generated in Fe80B13Si7 MG caused by enhanced atomic diffusion due to increased excess free volume and larger compressive stress on atoms of bubbles gaps, while Zr63.5Cu23Al9Fe4.5 MG could maintain the amorphous structure. According to serrated flow phenomenon and bubbles distribution, the underlying mechanism of the transformation in deformation behavior of MGs with increasing He dose was analyzed as follows: localized inhomogeneous deformation in as-cast MGs, homogeneous deformation in the rejuvenated stage, localized inhomogeneous deformation in the stage where small bubbles act as weak pinning, homogeneous deformation in the stage where large bubbles act as strong pinning, localized fracture in the stage where bubbles collapse.

Modulation of Contact Resistance of Dual-Gated MoS2 FETs Using Fermi-Level Pinning-Free Antimony Semi-Metal Contacts.

Achieving low contact resistance (RC ) is one of the major challenges in producing 2D FETs for future CMOS technology applications. In this work, the electrical characteristics for semimetal (Sb) and normal metal (Ti) contacted MoS2 devices are systematically analyzed as a function of top and bottom gate-voltages (VTG and VBG ). The semimetal contacts not only significantly reduce RC but also induce a strong dependence of RC on VTG , in sharp contrast to Ti contacts thatonly modulate RC by varying VBG . The anomalous behavior is attributedto the strongly modulated pseudo-junction resistance (Rjun ) by VTG , resulting from weak Fermi level pinning (FLP) of Sb contacts. In contrast, the resistances under both metallic contacts remain unchanged by VTG as metal screens the electric field from the applied VTG . Technology computer aided design simulations further confirm the contribution of VTG to Rjun , which improves overall RC of Sb-contacted MoS2 devices. Consequently, the Sb contact has a distinctive merit in dual-gated (DG) device structure, as it greatly reduces RC and enables effective gate control by both VBG and VTG . The results offer new insight into the development of DG 2D FETs with enhanced contact properties realized by using semimetals.

Microwave response of type-II superconductors at weak pinning

Theory of linear microwave response of thin films of type-II superconductors in the mixed state is developed taking into account random spatial fluctuations of the parameters of the system, such as the order parameter, diffusion coefficient, or film thickness. In the regime of collective pinning the microwave response of the system exhibits strong frequency dispersion, arising from nonequilibrium vortex core quasiparticles. The corresponding contribution to the ac conductivity is controlled by the inelastic relaxation time, and may exceed the usual Bardeen-Stephen conductivity. It is caused by the Debye-type inelastic relaxation. Debye mechanism of microwave losses may be responsible for strong effects of electromagnetic noise upon dc conductivity in the mixed state at low temperatures.

Extremely fast vortex dynamics in Bi2Sr2Ca2Cu3O10+δ crystalline nanostrip

The maximum velocity of a mobile vortex in movement is generally limited by the phenomenon of flux-flow instability (FFI), which necessitates weak vortex pinning and fast heat removal from non-equilibrium electrons. We here demonstrate exfoliations and nano-fabrications of Bi2Sr2Ca2Cu3O10+δ crystalline nanostrips, which possess a rather weak pinning volume of vortices, relatively low resistivity, and large normal electron diffusion coefficient. The deduced vortex velocity in Bi2Sr2Ca2Cu3O10+δ crystalline nanostrips can be up to 300 km/s near the superconducting transition temperature, well above the speed of sound. The observed vortex velocity is an order of magnitude faster than that of conventional superconducting systems, representing a perfect platform for exploration of ultra-fast vortex matter and a good candidate for fabrications of superconducting nanowire single photon detectors or superconducting THz modulator.

Strong-coupling superconductivity and weak vortex pinning in Ta-doped CsV3Sb5 single crystals

By measuring magnetizations of pristine and Ta-doped ${\mathrm{CsV}}_{3}{\mathrm{Sb}}_{5}$ single crystals, we have carried out systematic studies on the lower critical field, critical current density, and equilibrium magnetization of this kagome system. The lower critical field has been investigated in the two typical samples, and the temperature-dependent lower critical field obtained in Ta-doped sample can be fitted by using the model with two $s$-wave superconducting gaps yielding the larger gap of $2{\mathrm{\ensuremath{\Delta}}}_{s1}/{k}_{\mathrm{B}}{T}_{\mathrm{c}}=7.9\phantom{\rule{0.28em}{0ex}}(\ifmmode\pm\else\textpm\fi{}1.8)$. This indicates a strong-coupling feature of the V-based superconductors. The measured magnetization hysteresis loops allow us to calculate the critical current density, which shows a very weak bulk vortex pinning. The magnetization hysteresis loops measured in these two kinds of samples can be well described by a recently proposed generalized phenomenological model, which leads to the determination of many fundamental parameters for these superconductors. Our systematic results and detailed analysis conclude that this V-based kagome system has features of strong-coupling superconductivity, relatively large Ginzburg-Landau parameter, and weak vortex coupling.

Nonlinear dynamics, avalanches, and noise for driven Wigner crystals

We consider the driven dynamics of Wigner crystals interacting with random disorder. Using numerical simulations, we find a rich variety of transport phenomena as a function of charge density, drive, and pinning strength. For weak pinning, the system forms a defect free crystal that depins elastically. When the pinning is stronger, a pinned glass phase appears that depins into a filamentary flow state, transitions at higher drives into a disordered flow phase, and finally forms a moving smectic. Within the filamentary flow phase, the conduction curves can show switching dynamics as well as negative differential conductivity in which switching events cause some flow channels to be blocked. The velocity noise in the filamentary flow regime exhibits narrow band characteristics due to the one-dimensional nature of the motion, while the moving smectic has narrow band velocity noise with a washboard frequency. In the disordered flow state, the noise power reaches a peak value and the noise has a $1/f$ character. Our transport results are consistent with recent experimental transport studies in systems where Wigner crystal states are believed to be occurring. Below the conduction threshold, we find that avalanches with a power law size distribution appear when there are sudden local rearrangements of charges from one pinned configuration to another.

Effect of K-doping on the superconducting properties of FeSe0.5Te0.5 single crystals

Fe1−xKxSe0.5Te0.5 ( x = 0, 0.025, 0.05, 0.075 and 0.1) single crystals were grown by self-flux method. The effects of K-doping on the length of c-axis, critical temperature, anisotropy and critical current density were investigated. With the increase of K-doping content x, the length of c-axis increases, the critical temperature rises at first and then falls, and the anisotropy decreases. The critical current density displays a significant increase at x = 0.075. Subsequently, the superconducting properties of Fe0.925K0.075Se0.5Te0.5 were thoroughly studied. The field-dependent pinning energy U(H) and the temperature-dependent upper critical field Hc2(T) were given. Additionally, the Dew-Hughes model was used to scale the flux pinning force, and the result suggests that the superconductor had a temperature-induced pinning mechanism from normal point pinning to normal surface pinning, in which twins play an important role. We also discussed the temperature-dependent critical current density Jc(T)=Jc(0)(1-T/T1)n and found that the n-value depends on different flux pinning regimes, including single flux pinning, strong collective pinning associated with twins and weak collective pinning associated with point defects.

Ultrafast interfacial carrier dynamics and persistent topological surface states of Bi2Se3 in heterojunctions with VSe2

Vanadium diselenide (VSe2) has recently been highlighted as an efficient 2D electrode owing to its extra-high conductivity, thickness controllability, and van der Waals contact. However, as the electrode, applications of VSe2 to various materials are still lacking. Here, by employing ultrafast time-resolved spectroscopy, we study VSe2-thickness-dependent interfacial effects in heterostructures with topological insulator Bi2Se3 that is severely affected by contact with conventional 3D electrodes. Our results show unaltered Dirac surface state of Bi2Se3 against forming junctions with VSe2, efficient ultrafast hot electron transfer from VSe2 to Bi2Se3 across the interface, shortened metastable carrier lifetimes in Bi2Se3 due to dipole interactions enabling efficient current flow, and the electronic level shift (~tens meV) of bulk states of Bi2Se3 by interfacial interactions, which is ~10 times lower compared to conventional electrodes, implying weak Fermi level pinning. Our observations confirm VSe2 as an ideal electrode for efficient Bi2Se3-based-applications with full utilization of topological insulator characteristics.

Transition of vortex pinning behaviour induced by an artificial microstructure design in Ba(Fe0.94Co0.06)2As2 pnictide superconductor

Among various systems of iron-based superconductors, the doped Ba122 polycrystals have shown the largest application value because of their brilliant properties and chemical stability. Since there have been few reports on the vortex dynamics in Ba122 polycrystal, we firstly conduct a comparative investigation on the differences of vortex dynamics in Ba(Fe0.94Co0.06)2As2 single crystal, grain oriented polycrystal and random oriented powder in this paper. Herein, the polycrystal, which has c-axis grain texture, is prepared by the compaction of designed single crystal powder. The second magnetization peak (SMP) effects are observed in the hysteresis loops of single crystal and totally disappear in those of polycrystal and powder. Conventional magnetization relaxation and Maley's method are applied to analyse the vortex dynamics. In single crystal, the SMP effect is primarily related to the crossover of elastic-plastic vortex creep. The weak pinning in single vortex regime results in large magnetization relaxation rates (S) in low magnetic field regime. In polycrystal for H//AD (axial direction), the introduction of strong pins is effective to single vortex pinning, which brings about much lower S values in low magnetic field regime. Furthermore, the results of single crystal for H//ab, polycrystal for H//RD (radial direction) and random oriented powder are analyzed. Synergistic effects of strong pins and microscopic anisotropy on the vortex dynamics are discussed. These results will provide new knowledge for the design of high-performance Ba122 superconductors.

Critical current in a long Josephson contact with weak pinning in an external magnetic field

The analysis of possible current distributions when passing current through a periodically modulated long Josephson contact located in an external magnetic field is carried out. An approach based on the analysis of continuous configuration modification proceeding in the direction of Gibbs potential reduction is used for the calculation. The case when the pinning parameter is less than the critical value is considered. It is shown that at any value of the external magnetic field, there is a critical value of the transport current, when exceeded, the situation ceases to be stationary, as a result of which energy passes into radiation and heat, i.e. currents cease to be persistent. The value of the critical current is determined by the value of the magnetic field at which the vortices begin to fill the entire length of the contact. With an increase in the external magnetic field, the critical value of the current decreases. Keywords: high-temperature superconductors, pinning, Josephson contacts.

Development of a setup to characterize capillary liquid bridges between liquid infused surfaces

Capillary liquid bridges are ubiquitous in nature and are present in many industrial processes. In order to model their behavior, it is essential to develop suitable experimental tools that are able to characterize the bridges’ geometry and the associated capillary force they induce on the contacting surfaces. While many existing setups are capable of characterizing capillary bridges formed between conventional surfaces, quantitative measurements on smart surfaces such as liquid infused surfaces remain challenging. These surfaces typically exhibit weak contact line pinning and contact angle hysteresis, resulting in unusually small changes in the capillary force they exert upon extension or compression of the bridge. Although it is precisely these properties that drive the interest into liquid infused surfaces, they render experimental characterization challenging when compared to non-infused surfaces. Here, we tackle this issue by developing a relatively inexpensive setup capable of measuring capillary forces with sensitivity in the micronewton range while quantifying the bridge’s geometry. The setup is fully motorized and can vary the relative position of the contacting surfaces while maintaining synchronous force and geometry measurements. We also present a new analysis software developed to retrieve the relevant geometrical parameters of the bridge from optical observations while minimizing errors and noise. Using example surfaces, we demonstrate the setup’s capabilities, including for bridges between liquid infused surfaces.

Vickers micro-hardness variation during change in concentration of constituent elements in Ni50–xFexMn30Sn20–yIny, Heusler alloys

Present work is on Heusler alloys of the sequence Ni50–xFexMn30Sn20–yIny, were prepared in order to investigate the relationship between microstructure and mechanical property. The work represents the variations in the hardness of the alloy when the component elements are changed. Alloys show Vickers hardness HV = 3.5 GPa at x = 2 and y = 4. At x = 4 and y = 8, alloy exhibits an L10 tetragonal structure, whereas at x = 3 and y = 6 L21 austenite phase structure is observed. Interface piling up occurs which greatly reduces fracture propagation and dislocation at neighboring interfaces. Large piled-up interfaces available in the martensite phase due to the sub-strips significantly contribute this process resulting in large hardness value. In spite of thicker laminates in the austenite phase, the alloy exhibits higher hardness than martensite phase or even the composite. Hardness is particularly low in the martensitic phase (x = 4, y = 8), which is produced owing to interfacial motion. The hardness value falls as the Sn concentration increases due to weak pinning between the strips. A drastic increase in hardness of 3.5 GPa has been observed when x = 2 and y = 4.