Microscopic Impurities Research Articles

Microscopic theory of current-induced skyrmion transport and its application in disordered spin textures

Introduction: Magnetic skyrmions hold great promise for realizing compact and stable memory devices that can be manipulated at very low energy costs via electronic current densities.Methods: In this work, we extend a recently introduced method to describe classical skyrmion textures coupled to dynamical itinerant electrons. In this scheme, the electron dynamics is described via nonequilibrium Green’s function (NEGF) within the generalized Kadanoff–Baym ansatz, and the classical spins are treated via the Landau–Lifshitz–Gilbert equation. Here, the framework is extended to open systems by the introduction of a non-interacting approximation to the collision integral of NEGFs. This, in turn, allows us to perform computations of the real-time response of skyrmions to electronic currents in large quantum systems coupled to electronic reservoirs, which exhibit linear scaling in the number of time steps. We use this approach to investigate how electronic spin currents and dilute spin disorder affect skyrmion transport and the skyrmion Hall drift.Results: Our results show that the skyrmion dynamics is sensitive to a specific form of the spin disorder, such that different disorder configurations lead to qualitatively different skyrmion trajectories for the same applied bias.Discussion: This sensitivity arises from the local spin dynamics around the magnetic impurities, a feature that is expected not to be well-captured by phenomenological or spin-only descriptions. At the same time, our findings illustrate the potential of engineering microscopic impurity patterns to steer skyrmion trajectories.

Universal scaling of tunable Yu-Shiba-Rusinov states across the quantum phase transition

Quantum magnetic impurities give rise to a wealth of phenomena attracting tremendous research interest in recent years. On a normal metal, magnetic impurities generate the correlation-driven Kondo effect. On a superconductor, bound states emerge inside the superconducting gap called the Yu-Shiba-Rusinov (YSR) states. Theoretically, quantum impurity problems have been successfully tackled by numerical renormalization group (NRG) theory, where the Kondo and YSR physics are shown to be unified and the normalized YSR energy scales universally with the Kondo temperature divided by the superconducting gap. However, experimentally the Kondo temperature is usually extracted from phenomenological approaches, which gives rise to significant uncertainties and cannot account for magnetic fields properly. Using scanning tunneling microscopy at 10 mK, we apply a magnetic field to several YSR impurities on a vanadium tip to reveal the Kondo effect and employ the microscopic single impurity Anderson model with NRG to fit the Kondo spectra in magnetic fields accurately and extract the corresponding Kondo temperature unambiguously. Some YSR states move across the quantum phase transition (QPT) due to the changes in atomic forces during tip approach, yielding a continuous universal scaling with quantitative precision for quantum spin-frac{1}{2} impurities.

Development of a Temperature Distribution Measurement System for Transmission Oil for Transportation Equipment

In this study, an optical sensor using thermo-sensitive phosphor and its measurement system for visualizing and measuring the temperature distribution in an arbitrary cross-section of transmission oil using one type of phosphor, whose peak wavelength changes with temperature, is proposed. Because the intensity of the excitation light is gradually attenuated by the scattering of the laser light owing to microscopic impurities in the oil, we attempted to reduce the scattering effect by increasing the excitation light wavelength. Therefore, Pyrromethene 597 was selected as the optical sensor using thermo-sensitive phosphor, and a DPSS (Diode Pumped Solid State) laser with a wavelength of 532 nm was used as the excitation light. Using this measurement system, we measured the temperature distribution of a vertical buoyant jet of transmission oil and validated the measurement method. In addition, it was shown that this measurement system could be applied to the measurement of the temperature distribution in transmission oil with cavitation foaming.

Characterization of microscopic ferromagnetic defects in thin films using magnetic microscope based on Nitrogen-Vacancy centres

In this work we present results acquired by applying magnetic field imaging technique based on Nitrogen-Vacancy centres in diamond crystal for characterization of magnetic thin films defects. We used the constructed wide-field magnetic microscope for measurements of two kinds of magnetic defects in thin films. One family of defects under study was a result of non-optimal thin film growth conditions. The magnetic field maps of several regions of the thin films created under very similar conditions to previously published research revealed microscopic impurity islands of ferromagnetic defects, that potentially could disturb the magnetic properties of the surface. The second part of the measurements was dedicated to defects created post deposition - mechanical defects introduced in ferromagnetic thin films. In both cases, the measurements identify the magnetic field amplitude and distribution of the magnetic defects. In addition, the magnetic field maps were correlated with the corresponding optical images. As this method has great potential for quality control of different stages of magnetic thin film manufacturing process and it can rival other widely used measurement techniques, we also propose solutions for the optimization of the device in the perspective of high throughput.

Purification of crude selenium by vacuum distillation and analysis

In this study, metallurgical grade crude selenium (99.4%), comprehensively recovered from copper electrolysis anode slime, was purified to 99.992% through triple consecutive vacuum distillation (TCVD) on a laboratory scale. The actual evaporation rate and accommodation coefficient α of crude selenium were found to be 0.0231 g cm−2 min and 0.3236, respectively, at 523 K and approximately 4 Pa by a vacuum thermogravimetric furnace. Internal heat vacuum distillation equipment was employed in the purification experiments with a dynamic vacuum level of approximately 4 Pa. Detailed analysis of the initial and purified selenium was performed using inductively coupled plasma mass spectrometry (ICP-MS) to identify the 15 most common impurity elements. The analysis confirmed the reduction in the total impurity content from 2997.38 ppmw to <79.08 ppmw. The microscopic impurity elemental distribution was analyzed using an electron probe microanalyzer (EPMA), and it shows that the impurities were relatively concentrated and gathered in crude selenium. The results enrich the database of selenium evaporation kinetics and lay a foundation for the industrial purification of selenium.

Extraction of Iron(III) from Chloride Nickel Solutions with Aliphatic Ketones

Extractive recovery of iron(III) from chloride nickel solutions with macromolecular aliphatic ketones was studied. It was found that the extraction of Fe(III) grows with increasing concentration of chloride ions in solution. For the hydrochloric acid solution, the maximum degree of extraction (99%) is observed at an HCl concentration < 6 mol dm−3. For the nickel chloride solution the highest extraction (95%) is reached at a Cl− concentration ≥ 8 mol dm−3. A high selectivity of ketones with respect to iron(III) was demonstrated, they hardly extract such microscopic impurities as Cu2+ and Co2+, as well as the macroscopic component Ni2+. The optimal ratio between the organic and aqueous phases for washing the extract to remove the carried-away nickel was determined. Enlarged laboratory tests on deep separation of iron and nickel were performed. As a result, a solution of ferric chloride with Fe(III) concentration of 76.8 g dm−3 with total content of nickel(II), cobalt(II), and copper(II) impurities less than 0.022 g dm−3.

Correlative Microscopy-Based Approach for Analyzing Microscopic Impurities in Carbon Black for Lithium-Ion Battery Applications

A novel method based on correlative microscopy for analyzing metal-rich particulate impurities randomly distributed in large volumes of carbon black powder is described here. Approximately 5 g of carbon black of tap density of 0.075 g cm−3 was used. The powder was encapsulated in epoxy and a combination of 3D X-ray computed tomography (X-CT), laser/ion beam milling and energy dispersive X-ray spectroscopy (EDS) was used to locate and analyze particles of interest. The locations of impurities were identified using X-CT and a focused ion beam (FIB) system with ion and laser beam milling capabilities was then used to analyze regions of interest in the encapsulated sample. The use of a nanosecond pulsed laser for milling allowed for enhanced material removal rates of up to 3 × 105 μm3 s−1. Finally, scanning electron microscopy (SEM) and EDS were used to image and analyze the composition of the exposed particles. This correlative workflow is shown to be capable of providing detailed spatial, morphological and compositional information about large metal-rich impurities in a carbon black product or a device with carbon black as additives, which may be used for identifying the source of impurities or predicting failure modes in the device.

Development of reliability assessment technique forwafer transfer robot

ABSTRACTSince the semiconductor manufacturing process requires high precision, performance deterioration is largely confirmed even by microscopic impact or impurities. However, it is difficult to m...

Competitive heterogeneous nucleation onto a microscopic impurity in a Potts model

Many metastable systems can nucleate to multiple competing stable or intermediate metastable states. In this work, a Potts model, subject to external fields, is used to study the competitive nucleation of two phases attempting to grow on a microscopic impurity. Monte Carlo simulations are used to calculate the free energy surfaces for the system under different conditions, where the relative stability of the phases is adjusted by changing the interaction parameters, and the nucleation rates obtained using multicomponent transition state theory are compared with the rates measured using the survival probability method. We find that the two methods predict similar nucleation rates when the free energy barrier used in the transition state theory is defined as the work required to form a critical embryo from the metastable phase. An analysis of the free energy surfaces also reveals that the competition between the nucleating phases leads to an effective drying of the impurity which slows down the nucleation rate compared to the single phase case.

Anomalous Hall effect on the surface of topological Kondo insulators

We calculate the anomalous Hall conductivity $\sigma_{xy}$ of the surface states {in cubic topological Kondo insulators}. We consider a generic model for the surface states with three Dirac cones on the (001) surface. The Fermi velocity, the Fermi momentum and the Zeeman energy in different Dirac pockets may be unequal. The microscopic impurity potential mediates mixed intra and interband extrinsic scattering processes. Our calculation of $\sigma_{xy}$ is based on the Kubo-Streda diagrammatic approach. It includes diffractive skew scattering contributions originating from the rare two-impurity complexes. Remarkably, these contributions yield anomalous Hall conductivity that is independent of impurity concentration, and thus is of the same order as other known extrinsic side jump and skew scattering terms. We discuss various special cases of our results and the experimental relevance of our study in the context of the recent hysteretic magnetotransport data in SmB$_6$ samples.

Features of ZnS-powder doping with a Mn impurity during synthesis and subsequent annealing

Luminescence, electron spin resonance, and X-ray diffraction (XRD) methods were used to investigate the features of ZnS-powder doped by Mn impurity during self-propagating high-temperature synthesis and subsequent annealing. The obtained powder consists of ZnS microcrystals with mainly hexagonal phase (80 ± 5)%. It was found, that after synthesis Mn presents not only in the form of non-uniformly distributed microscopic impurities in ZnS, but also in the form of Mn metal nanocrystals. Thermal annealing at 800°C leads to the additional doping of ZnS from metallic Mn, to the redistribution of the embedded Mn in the volume of microcrystals, and to the ZnS oxidation. At the same time, the ratio between the cubic and hexagonal phases does not change. It was shown that annealing causes a decrease in the concentration of the defects responsible for the luminescence-excitation bands, which correspond to transitions from the ground to the excited states of the Mn2+ ion. As a result of annealing, there is also a change in XRD coherent domain size. Simultaneously, the intensity of peaks in the luminescence-excitation spectrum with wavelengths of 375 and 395 nm was changed. The causes of these changes and the nature of the corresponding bands are discussed.

Enhancement of the critical temperature in cuprate superconductors by inhomogeneous doping

We use a renormalized mean-field theory to investigate the superconducting properties of underdoped cuprates embedded with overdoped or metallic regions that carry excess dopants. The overdoped regions are considered, within two different models, first as stripes of mesoscopic size larger than the coherence length and then as point impurities. In the former case we compute the temperature-dependent superfluid stiffness by solving Bogoliubov-de Gennes equations within the slave boson mean-field theory. We average over stripes of different orientations to obtain an isotropic result. To compute the superfluid stiffness in the model with point impurities we resort to a diagrammatic expansion in the impurity concentration (to first order) and their strength (up to second order). We find analytic expressions for the disorder-averaged superfluid stiffness and the critical temperature. For both types of inhomogeneities we find increased superfluid stiffness. Remarkably, in the case of microscopic impurities we find that the maximal ${T}_{c}$ can be significantly increased compared to ${T}_{c}$ at optimal doping of a pure system.

Conduction type conversion in ion etching of Au- and Ag-doped narrow-gap HgCdTe single crystal

Fundamental aspects of the p-to-n conductivity type conversion induced by ion etching in HgCdTe single crystals diffusion-doped with Au and Ag have been studied. A conversion mechanism is suggested, according to which interstitial mercury atoms rapidly diffuse from the surface source with high concentration and “kick out” impurity atoms from the cation sublattice into interstitial positions and thereby convert impurity atoms from the acceptor state to the donor state. It is shown that the structure of defects in the converted layer is unstable and the electrical parameters of this layer vary when stored at room temperature. The most probable mechanism of this process at room temperature is decomposition of an oversaturated solution of the impurity. A re-conversion to the p-type, observed in isochronous annealing of samples, is due to impurity diffusion into the converted layer from the unconverted bulk of the sample or from microscopic impurity inclusions.

Finite element modeling of dislocation reduction in GaAs and InP single crystals grown from the VGF process

Dislocations in gallium arsenide (GaAs) and indium phosphide (InP) crystals are generated by thermal stresses induced during their solidification process of crystal growth. High dislocation density in these crystals will reduce the performance and reliability of the GaAs- and InP-based microelectronic and photonic devices/circuits. It has been known that doping impurity atoms into GaAs and InP crystals during their solidification process can significantly reduce dislocation densities generated in these crystals. A viscoplastic constitutive equation that couples a microscopic dislocation density and impurity atoms with a macroscopic plastic deformation is employed in a transient finite element model for predicting the dislocation density generated in the undoped and doped GaAs and InP crystals grown by the vertical gradient freeze (VGF) process. The effects of crystal growth parameters (i.e., imposed temperature gradient, crystal diameter, and crystal growth rate) on dislocation generation are also investigated. The numerical results show that doping impurity can significantly reduce the dislocation density generated in these crystals. It also shows that dislocation density reduces drastically as the crystal diameter and imposed temperature gradient decrease, but the crystal growth rate has almost no effect on dislocation generation in these crystals.

Heterogeneous and Homogeneous Nucleation Compared: Rapid Nucleation on Microscopic Impurities

We use computer simulation to calculate the rates of both homogeneous nucleation and heterogeneous nucleation on microscopic impurities. We do so in perhaps the simplest model of fluids and magnets: the two-dimensional Ising model. We expect our results to be qualitatively applicable to many simple and complex fluids. We find that heterogeneous nucleation on an impurity that is not only microscopic but also as small as possible, that is, a single fixed spin, is more than four orders of magnitude faster than homogeneous nucleation. The rate of heterogeneous nucleation then increases by a factor of approximately five for each additional fixed spin in the impurity. These results suggest that impurities as small as single molecules can result in homogeneous nucleation being irrelevant due to heterogeneous nucleation on these microscopic impurities being much faster.

Correlations in the director-orientation fluctuations in the presence of microscopic impurities in a nematic liquid crystal

The model of a spatially restricted liquid-crystal system with microscopic impurities homogeneously distributed in the system is considered. The thermodynamic correlations of the director-orientation fluctuations and the correlations of the scalar order parameter characterizing impurities in the system are investigated for this system. It is shown that the impurity correlations are weaker than in a pure isotropic liquid. The correlations in the director deviations depend essentially on the wavevector and can be substantially weakened in the presence of impurities.

Imaging interedge-state scattering centers in the quantum Hall regime

We use an atomic force microscope tip as a local gate to study the scattering between edge channels in a two-dimensional electron gas in the quantum Hall regime. The scattering is dominated by individual, microscopic scattering centers, which we directly image here for the first time. The tip voltage dependence of the scattering indicates the presence of tunneling through weak links as well as scattering from microscopic impurities.

Impurity relaxation mechanism for dynamic magnetization reversal in a single domain grain

The interaction of coherent magnetization rotation with a system of two-level impurities is studied. Two different, but not contradictory mechanisms, the `slow-relaxing ion' and the `fast-relaxing ion' are utilized to derive a system of integro-differential equations for the magnetization. In the case that the impurity relaxation rate is much greater than the magnetization precession frequency, these equations can be written in the form of the Landau-Lifshitz equation with damping. Thus the damping parameter can be directly calculated from these microscopic impurity relaxation processes.

Localization of helium around microscopic impurities in liquid helium

The structure and properties of the environment of impurity atoms Im immersed in liquid helium are examined. It is demonstrated that there are two qualitatively different types of structure of the layer of helium atoms nearest to Im: structures characterized by attraction and repulsion. In the structures with attraction toward the center (for strong Im–He interaction) the Im–He distance is greater than the equilibrium distance for the Im–He pair potential, and the density and localization of helium atoms are greater than in the bulk. Here the number of helium atoms n in the layer is almost independent of the external pressure. In structures with repulsion, which exist for alkali metal atoms, the Im–He distances are shorter than the equilibrium ones, while the density is lower than in bulk helium. For T∼1 K several states with different n are populated, with energies differing only by ∼0.1 K, and n decreases substantially as the pressure is increased. On the basis of this analysis an interpretation is given for the optical and ESR spectra of atoms implanted in liquid and solid helium. A simple model is proposed for determining the characteristics of the helium environment from the experimental pressure dependence of the shifts of the atomic lines in the absorption and emission spectra. It is predicted that in He3-He4 mixtures the structure with attraction should be strongly enriched with He4 atoms, while the structures with repulsion should be enriched with He3. The possible existence of phase transitions in the helium shells surrounding impurity atoms is discussed.

Alignment of Microscopic Impurities in Incipient Ferroelectrics

Periodic boundary conditions were applied to compute polarization in solid solutions of KTaO3 : Li. Supercells of sizes 3 × 3 × 3, 4 × 4 × 4 and 5 × 5 × 5 lattice constants were considered. The polarization energy of the host lattice was found to be very sensitive to the wave vector which corresponds to alignment of the microscopic impurities. This alignment was shown to increase with increasing impurity concentration. This phenomenon can be explained by a strong enhancement of the interaction between the Li dipoles due to the lattice polarization of the incipient ferroelectric KTaO3. The results obtained are consistent with the experiment and consequences stemming from previous cluster computations of other authors.