Critical Anisotropy Research Articles

Thickness effect on the evolution of superconducting properties in FeSe0.4Te0.6 thin films

Abstract A series of high-quality FeSe0.4Te0.6 epitaxial thin films with different thicknesses were fabricated on CaF2 substrates by pulsed laser deposition. With increasing the film thickness, the superconducting transition temperature is enhanced rapidly but deteriorates gradually in over thick films. The superconducting transition occurs in all films including the 5 nm one, and Tc above 21 K persists in the films with thicknesses from 45 to 180 nm. We find that Tc and the c-axis lattice constant show a nearly linear correlation, suggesting a strong interdependence between Tc and the lattice strain characteristics. The upper critical field, anisotropy and effective pinning energy are studied in detail for the 15, 60, 90 and 180 nm films. These observations provide key insights in understanding the thickness effect on the superconductivity of the FeSe0.4Te0.6 films.

Anisotropy-driven topological quantum phase transition in magnetic impurities

Abstract A few years ago, a topological quantum phase transition (TQPT) has been found in Anderson and Kondo 2-channel spin-1 impurity models that include a hard-axis anisotropy term DSz^2 with D > 0. The most remarkable manifestation of the TQPT is a jump in the spectral density of localized electrons, at the Fermi level, from very high to very low values as D is increased. If the two conduction channels are equivalent, the transition takes place at the critical anisotropy Dc ∼ 2.5 TK, where TKis the Kondo temperature for D = 0. This jump might be important to develop a molecular transistor. The jump is due to a corresponding one in the Luttinger integral, which has a topological non-trivial value π/2 for D > Dc. Here, we review the main results for the spectral density and highlight the significance of the theory for the interpretation of measurements conducted on magnetic atoms or molecules on metallic surfaces. In these experiments, where D is held constant, the energy scale TK is manipulated by some parameters. The resulting variation gives rise to a differential conductance dI/dV , measured by scanning-tunneling spectroscopy, which is consistent with a TQPT at an intermediate value of TK. We also show that the theory can be extended to integer spin S > 1 and two-impurity systems. This is also probably true for half-integer spin and non-equivalent channels in some cases.

Iron-Based Superconductors for High-Field Applications: Realization of High Engineering Critical Current Density.

Iron-based superconductors have strong potential for magnet applications through their very high upper critical field, low anisotropy and manufacturability through the powder-in-tube (PIT) route. The engineering critical current density (Je) is a key parameter for measuring the maximum current density that superconducting materials can withstand in practical applications. It serves as a bridge between theoretical research and practical applications of superconductors and has great significance in promoting the development and application of superconducting technology. In this study, Ag sheathed Ba0.6K0.4Fe2As2 (Ba-122) iron-based superconducting tapes were prepared by using the process of drawing, flat rolling and heat treatment by hot pressing (HP). For the first time, the filling factor of the tapes increased to about 40%, leading to a reduction in the volume fraction of Ag, consequently lowering the overall cost. The optimal parameters for achieving high transport Je were obtained by comparing the effects of different HP pressures on the properties and micro-morphology of the tapes. The prepared mono-filament tapes are capable of carrying the transport Je of 4.1 × 104 A/cm2 (Ic = 350 A) at 4.2 K, 10 T, marking the highest Je reported for Ba-122 wires and tapes to date. Our results show that high transport Je can be obtained in Ba-122 superconducting tapes, and iron-based superconductors have a promising future in practical applications.

Enhanced mechanical strength and texture of (Ba,K)Fe2As2 Cu/Ag composite sheathed tapes with Nb barrier layer

Abstract Iron-based superconductors with ultra-high upper critical fields and low anisotropy have attracted much attention for superconducting mechanisms and high-field applications. In practical applications, improving the mechanical strength and heat treatment temperature of superconducting tapes is of great significance for the improvement of transport current as well as stability. In this paper, (Ba, K)Fe2As2(Ba-122) superconducting tapes with Cu/Nb/Ag composite sheaths were successfully fabricated using a pre-composite process, which provides a feasible method for the fabrication of high-strength superconducting wires and tapes. It is shown that Cu/Nb/Ag composite sheathed tapes can be sintered at 880 °C, and tapes sintered at 880 °C have the highest transport properties as well as excellent superconductivity of the superconducting cores, as demonstrated by a series of characterizations. In addition, other superconducting properties of the tapes sintered at 880 °C, including grain orientation, flux pinning, upper critical field and irreversible field, were also studied. It was found that none of the three sheaths fractured after sintering and the superconducting core had a high c-axis texture and densities. The high mechanical strength of the Cu/Nb/Ag composite sheathed tape was also demonstrated by comparative tensile experiments. The results indicate that the low-cost Ba-122 tapes with Cu/Nb/Ag composite sheaths hold great promise for future practical applications.

Influence of magnetic anisotropy on the vortex stability in circular Permalloy nanodots using energy analysis

We thoroughly investigated the stability of vortex magnetization in Permalloy (Py) circular nanodots with and without magnetocrystalline anisotropy using micromagnetic simulations. This study explores a wide-ranging understanding of influence of anisotropy on hysteresis loops, and spin-configurations, and provides a detailed analysis of the energy profile. The ground state vortex configuration considered of size 64 × 20-nm2 Py-nanodot at zero out-of-plane anisotropy had been modulated till the anisotropy 250 kJ/m3. We analyzed different energy terms at nucleation field, annihilation field and remanent states to gain deeper insights into the magnetization reversal mechanism. The energy analysis revealed an interplay between exchange and demagnetization energies, resulting in the stability of vortex up to a critical anisotropy (CK) 170 kJ/m3. Beyond this CK, the vortex destabilizes, switching the magnetization to a single-domain state. Additionally, we estimated the energy barrier involved in single domain to vortex state transformation. These findings provide valuable insights for designing vortex-based magnetic data storage and memory devices.

Properties and Applications of Iron-Chalcogenide Superconductors.

Iron-chalcogenide superconductors continue to captivate researchers due to their diverse crystalline structures and intriguing superconducting properties, positioning them as both a valuable platform for theoretical investigations and promising candidates for practical applications. This review begins with a comprehensive overview of the fabrication techniques employed for various iron-chalcogenide superconductors, accompanied by a summary of their phase diagrams. Subsequently, it delves into the upper critical field, anisotropy, and critical current density. Furthermore, it discusses the successful fabrication of meters-long coated conductors and explores their applications in superconducting radio-frequency cavities and coils. Finally, several prospective avenues for future research are proposed.

Static universality of the Ising and Blume–Capel models on two-dimensional Penrose tiles

We study the phase transitions and the critical behavior of the Ising and Blume–Capel models on two kinds of two-dimensional Penrose tiles employing Monte Carlo methods and detailed finite-size scaling analysis. The Wolff algorithm is used for the Ising model whereas the Wang–Landau and Metropolis algorithms are used for the Blume–Capel model to obtain critical temperatures and exponents. The phase diagram of the Blume–Capel model reveals the presence of the double transition, the reentrant behavior, and the tricriticality in the vicinity of the critical single-ion anisotropy. Our results indicate that continuous phase transitions of the Ising and Blume–Capel models on the Penrose tiles belong to the two-dimensional Ising universality class. However, the scaling functions depend on the spin magnitude, anisotropy, and graph shape. Lastly, we verify that the critical Binder cumulant is also universal but depends on the shape of the graph.

Topological features of the Haldane model on a dice lattice: Flat-band effect on transport properties

We study the topological properties of a Haldane model on a band deformed dice lattice, which has three atoms per unit cell (call them A, B, and C) and the spectrum comprises of three bands, including a flat band. The bands are systematically deformed with the aim to study the evolution of topology and transport properties. The deformations are induced through hopping anisotropies and are achieved in two distinct ways. In one of them, the hopping amplitudes between the sites of B and C sublattices and those between A and B sublattices are varied along a particular direction, and in the other, the hopping between the sites of A and B sublattices are varied (keeping B-C hopping unaltered) along the same direction. The first case retains some of the spectral features of the familiar dice lattice and yields Chern insulating lobes in the phase diagram with $C=\ifmmode\pm\else\textpm\fi{}2$ till a certain critical deformation. The topological features are supported by the presence of a pair of chiral edge modes at each edge of a ribbon and the plateaus observed in the anomalous Hall conductivity support the above scenario. Whereas, a selective tuning of only the A-B hopping amplitudes distorts the flat band and has important ramifications on the topological properties of the system. The insulating lobes in the phase diagram have distinct features compared to the case above, and there are dips observed in the Hall conductivity near the zero bias. The dip widens as the hopping anisotropy is made larger, and thus the scenario registers significant deviation from the familiar plateau structure observed in the anomalous Hall conductivity. However, a phase transition from a topological to a trivial insulating region demonstrated by the Chern number changing discontinuously from $\ifmmode\pm\else\textpm\fi{}2$ to zero beyond a certain critical hopping anisotropy remains a common feature in the two cases.

Skyrmion and meron ordering in quasi-two-dimensional chiral magnets

We revisit the well-known theoretical phase diagram for a two-dimensional chiral magnet with an easy-axis uniaxial anisotropy and an applied magnetic field. A particular emphasis is given to merons---quanta of the phase transition between the cycloid and the hexagonal skyrmion lattice, which possess fractional topological charges $\ifmmode\pm\else\textpm\fi{}1/2$. A localized state of two coupled merons with opposite topological charges exhibits the nonreciprocal current-driven dynamics: it experiences no skyrmion Hall effect, but due to the asymmetry of the attracting intermeron potential gives rise to asymmetric current-velocity characteristics. We also address the problem of skyrmion ordering into hexagonal and square skyrmion crystals. In an entire field range, the square lattice turns out a saddle point state between differently oriented hexagonal lattices. Merons separating elongated skyrmions were also shown to form a local energy minimum by forming a hexagonal lattice shadowing the ordinary lattice of skyrmions. In addition, we focus on the anisotropy-driven transformations of modulated phases and their isolated counterparts, which exist as detached branches of solutions on both sides from the critical anisotropy value. Moreover, a family of isolated skyrmions with both rotational senses can be found near this anisotropy value. Our results are useful from both the fundamental point of view, since they complement previous findings, and from the point of view of applications, since they provide guidelines for developing meron-based spintronic devices.

The Einasto model for dark matter haloes

Context. The Einasto model has become one of the most popular models for describing the density profile of dark matter haloes. There have been relatively few comprehensive studies on the dynamical structure of the Einasto model, mainly because only a limited number of properties can be calculated analytically. Aims. We want to systematically investigate the photometric and dynamical structure of the family of Einasto models over the entire model parameter space. Methods. We used the SpheCow code to explore the properties of the Einasto model. We systematically investigated how the most important properties change as a function of the Einasto index n. We considered both isotropic models and radially anisotropic models with an Osipkov-Merritt orbital structure. Results. We find that all Einasto models with n < 1/2 have a formal isotropic or Osipkov-Merritt distribution function that is negative in parts of phase space, and hence cannot be supported by such orbital structures. On the other hand, all models with larger values of n can be supported by an isotropic orbital structure, or by an Osipkov-Merritt anisotropy, as long as the anisotropy radius is larger than a critical value. This critical anisotropy radius is a decreasing function of n, indicating that less centrally concentrated models allow for a larger degree of radial anisotropy. Conclusions. Studies of the structure and dynamics of models for galaxies and dark matter haloes should not be restricted to completely analytical models. Numerical codes such as SpheCow can help open up the range of models that are systematically investigated. This applies to the Einasto model discussed here, but also to other proposed models for dark matter haloes, including different extensions to the Einasto model.

Standard self-confinement and extrinsic turbulence models for cosmic ray transport are fundamentally incompatible with observations

ABSTRACT Models for cosmic ray (CR) dynamics fundamentally depend on the rate of CR scattering from magnetic fluctuations. In the ISM, for CRs with energies ∼MeV-TeV, these fluctuations are usually attributed either to ‘extrinsic turbulence’ (ET) – a cascade from larger scales – or ‘self-confinement’ (SC) – self-generated fluctuations from CR streaming. Using simple analytic arguments and detailed ‘live’ numerical CR transport calculations in galaxy simulations, we show that both of these, in standard form, cannot explain even basic qualitative features of observed CR spectra. For ET, any spectrum that obeys critical balance or features realistic anisotropy, or any spectrum that accounts for finite damping below the dissipation scale, predicts qualitatively incorrect spectral shapes and scalings of B/C and other species. Even if somehow one ignored both anisotropy and damping, observationally required scattering rates disagree with ET predictions by orders of magnitude. For SC, the dependence of driving on CR energy density means that it is nearly impossible to recover observed CR spectral shapes and scalings, and again there is an orders-of-magnitude normalization problem. But more severely, SC solutions with super-Alfvénic streaming are unstable. In live simulations, they revert to either arbitrarily rapid CR escape with zero secondary production, or to bottleneck solutions with far-too-strong CR confinement and secondary production. Resolving these fundamental issues without discarding basic plasma processes requires invoking different drivers for scattering fluctuations. These must act on a broad range of scales with a power spectrum obeying several specific (but plausible) constraints.

Significant enhancement of transport Jc in Cu/Ag-sheathed (Ba,K)Fe2As2 superconducting tapes by pre-composite technique

Iron-based superconductors (IBSs) have received extensive attention in superconductivity mechanisms and high-field applications based on their ultra-high upper critical fields and low anisotropy. A major goal in the application research of IBSs is the fabrication of superconducting wires and tapes with high critical current density (Jc) and low cost. The use of Cu/Ag composite sheath can reduce the use of expensive silver, thereby reducing the cost of the tapes. In this paper, (Ba,K)Fe2As2 (Ba-122) tapes with Cu/Ag composite sheath were fabricated by a pre-composite process, which provides a feasible method for fabricating high-performance iron-based superconducting wires and tapes. Pre-composite tapes sintered at 750°C for 3 h achieved a high Jc of 6.2 × 104 A cm−2 at 10 T and 4.2 K. It is found that the high-density superconducting cores, large-size grains and uniform element distribution are the major causes of the high Jc. The results suggest that the low-cost Ba-122 tapes with pre-composite Cu/Ag sheath have great practical application prospect in the future.

Line-length-dependent dislocation glide in refractory multi-principal element alloys

Plastic deformation of refractory multi-principal element alloys (RMPEAs) is known to differ greatly from those of refractory pure metals. The fundamental cause is the different dislocation dynamics in the two types of metals. In this Letter, we use atomistic simulations to quantify dislocation glide in two RMPEAs: MoNbTi and NbTiZr. Edge and screw dislocations on the {110} and {112} slip planes are studied. A series of dislocation line lengths, ranging from 1 nm to 50 nm, are employed to elucidate the line-length-dependence. To serve as references, the same simulations are performed on pure metals. For the RMPEAs, the dependence of critical stresses on length becomes undetectable within the statistical dispersion for dislocations longer than 25 nm, as a result of the change in dislocation behavior. This length is in good agreement with those predicted by analytical models. Compared to the pure metals, the critical stress anisotropy among different slip planes and character angles is substantially reduced, providing an explanation for the homogeneous plasticity in RMPEAs observed in prior experiments.

Antiferromagnetic and ferromagnetic spintronics and the role of in-chain and inter-chain interaction on spin transport in the Heisenberg ferromagnet

Spin-transport and current-induced torques in ferromagnet heterostructures given by a ferromagnetic domain wall are investigated. Furthermore, the continuum spin conductivity is studied in a frustrated spin system given by the Heisenberg model with ferromagnetic in-chain interaction J_1<0 between nearest neighbors and antiferromagnetic next-nearest-neighbor in-chain interaction J_2>0 with aim to investigate the effect of the phase diagram of the critical ion single anisotropy D_c as a function of J_2 on conductivity. We consider the model with the moderate strength of the frustrating parameter such that in-chain spin-spin correlations that are predominantly ferromagnetic. In addition, we consider two inter-chain couplings J_{perp ,y} and J_{perp ,z}, corresponding to the two axes perpendicular to chain where ferromagnetic as well as antiferromagnetic interactions are taken into account.

Pattern selection on axial-compressed bilayer systems with a non-zero Gauss curvature

When a soft bilayer system with curved surface is subject to uniaxial compression, surface wrinkles may appear when the compressive strain is beyond a critical value. Interestingly, diverse wrinkling patterns can occur by modulating the feature of the curved surface though the load is simple. Here we study the effect of the sign of Gauss curvature and curvature anisotropy on the wrinkling pattern selection and evolution with experiments, theoretical analysis and numerical simulations. For a system with negative Gauss curvature, our analysis reveals a critical curvature anisotropy below which the critical buckling pattern is a checkerboard mode and will evolve into a diamond-like pattern. Otherwise, the critical buckling mode is sinusoidal. In the post-buckling analysis, our analysis leads to an analytical solution to predict the critical value of the dimensionless curvature that dominates the wrinkling pattern transition. When the dimensionless curvature exceeds such a critical value, the system would branch into the diamond-like mode. Otherwise, the system would maintain the sinusoidal mode. Our results also show that the sign of the Gauss curvature has important influence on critical strain for the onset of wrinkling pattern transition. Negative Gauss curvature promotes the sinusoidal/ diamond-like mode transition, whereas positive Gauss curvature inhibits this wrinkling pattern evaluation. Phase diagrams have been proposed which facilitate the use of the key results achieved in this paper. This study shows an example that a curved system may evolve a surface with complex patterns under simple stimuli by tuning the surface curvature.

High-performance Ba1−xKxFe2As2 superconducting tapes with grain texture engineered via a scalable fabrication

Nowadays the development of high-field magnets strongly relies on the performance of superconducting materials. Iron-based superconductors exhibit high upper critical fields and low electromagnetic anisotropy, making them particularly attractive for high-field applications, especially in particle accelerator magnets, nuclear magnetic resonance spectrometers, medical magnetic resonance imaging systems and nuclear fusion reactors. Herein, through an industrially scalable and cost-effective manufacturing strategy, a practical level critical current density up to 1.1x10^5 A/cm2 at 4.2 K in an external magnetic field of 10 T was achieved in Cu/Ag composite sheathed Ba0.6K0.4Fe2As2 superconducting tapes. The preparation strategy combines flat rolling to induce grain texture and a subsequent hot-isostatic-pressing densification. By varying the parameters of rolling, the degree of grain texture was engineered. It is found that the transport properties of the Ba1-xKxFe2As2 tapes can be enhanced by applying a large amount of deformation during rolling, which can be attributed to the improved degree of c-axis texture. Microstructure characterizations on the highest-performance tape demonstrate that the Ba1-xKxFe2As2 phase has a uniform element distribution and small grains with good connectivity. Grain boundary pinning is consequently enhanced as proved by large currents circulating through the sample even at 25 K. Our work proves that Cu/Ag composite sheathed Ba1-xKxFe2As2 superconducting tapes can be a promising competitor for practical high-field applications in terms of the viable, scalable and cost-effective fabrication strategy applied and the high transport properties achieved in this work.

Finite element implementation of an isotach elastoplastic constitutive model for soft soils

An isotach elastoplastic constitutive model devised by Yang et al. (2016) and referred to as the Hunter Clay (HC) model attempts to capture a number of key behaviours of soft soils within a critical state framework, namely destructuration, fabric anisotropy and rate dependency, the latter often manifesting in creep settlement. Finite element implementation of the HC model is a useful means by which its application to practical problems can be facilitated. However, there are a number of significant challenges associated with the translation of isotach elastoplastic models into a finite element setting. In this paper, a detailed discussion of these challenges is undertaken and a new finite element implementation of the HC model is subsequently developed. This includes sophisticated numerical integration algorithms which employ automatic time substepping for solution of the governing finite element equations. The ability of the implemented HC model to predict the mechanical behaviour of soft soils under 1D compression is investigated via simulation of laboratory tests carried out on Ballina clay by Pineda et al. (2016) and Parkinson (2018).

The Role of Anisotropy in Distinguishing Domination of Néel or Brownian Relaxation Contribution to Magnetic Inductive Heating: Orientations for Biomedical Applications.

Magnetic inductive heating (MIH) has been a topic of great interest because of its potential applications, especially in biomedicine. In this paper, the parameters characteristic for magnetic inductive heating power including maximum specific loss power (SLPmax), optimal nanoparticle diameter (Dc) and its width (ΔDc) are considered as being dependent on magnetic nanoparticle anisotropy (K). The calculated results suggest 3 different Néel-domination (N), overlapped Néel/Brownian (NB), and Brownian-domination (B) regions. The transition from NB- to B-region changes abruptly around critical anisotropy Kc. For magnetic nanoparticles with low K (K < Kc), the feature of SLP peaks is determined by a high value of Dc and small ΔDc while those of the high K (K > Kc) are opposite. The decreases of the SLPmax when increasing polydispersity and viscosity are characterized by different rates of d(SLPmax)/dσ and d(SLPmax)/dη depending on each domination region. The critical anisotropy Kc varies with the frequency of an alternating magnetic field. A possibility to improve heating power via increasing anisotropy is analyzed and deduced for Fe3O4 magnetic nanoparticles. For MIH application, the monodispersity requirement for magnetic nanoparticles in the B-region is less stringent, while materials in the N- and/or NB-regions are much more favorable in high viscous media. Experimental results on viscosity dependence of SLP for CoFe2O4 and MnFe2O4 ferrofluids are in good agreement with the calculations. These results indicated that magnetic nanoparticles in the N- and/or NB-regions are in general better for application in elevated viscosity media.

Activation of non-basal &lt;c + a&gt; slip in multicomponent Mg alloys

Activating non-basal < c + a > slip is a key method to improve room temperature ductility and formability of Mg alloys. However, the detailed criterion for activation of the < c + a > slip in multicomponent Mg alloys, which can be utilized in commercial Mg alloys, requires further understanding. The present authors investigated the mechanism and criterion using a molecular statics simulation on dislocation behaviors in multicomponent Mg alloys. We found that if multicomponent Mg alloys have an equivalent dislocation binding intensity to associated binary Mg alloys that are optimized to minimize the critical resolved shear stress anisotropy and thus activate the < c + a > slip, then the critical resolved shear stress anisotropy between slip systems of the multicomponent Mg alloys can also be minimized, resulting in activation of the < c + a > slip. The activation is maximized in multicomponent Mg alloys when alloying a large amount of weak dislocation binding elements. It was confirmed through experiments that the multicomponent Mg alloys satisfying the above criterion show higher room-temperature tensile elongation and formability than other alloys.

The Upper Critical Field, Anisotropy, and Critical Current Density of Superconducting LaO1-xBiS2 with x = 0.07

The slice-shaped LaO1-xBiS2 (x = 0.07) superconducting single crystal was grown by a high-temperature flux method. The X-ray diffraction analysis and symmetric M(H) curves along the H-axis indicate no any other impurities are involved in current LaO1-xBiS2 sample. The M(T) curves show the suppression of the Tconset and Tirr with increasing magnetic field applied parallel to the c-axis which is more noteworthy than that under magnetic field perpendicular to the c-axis. The Hc2(T) curves show upward curvature near Tc, which cannot be described by the one-gap model, and similar Hc2(T) curves are observed in REO1-xFxBiS2 and captured by the two-gap model, suggesting LaO1-xBiS2 may be a two-gap superconductor. The superconducting anisotropy parameter (2.8~5.58) of LaO1-xBiS2 (x = 0.07) is estimated by different methods. The structural distortion plays an important role in the smaller superconducting anisotropy parameter of LaO1-xBiS2. The critical current density in the self-field is about 5.9 × 103 Acm−2 and 6.7 × 103 Acm−2 for H//c and H⊥c, respectively.