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The concept of topological quantum number, or topological charge, has been used extensively to describe topological defects or solitons. Nematic liquid crystals contain both integer and half-integer topological defects, making them useful models for testing the rules that govern topological defects. Here, we investigated topological defects in nematic liquid crystals using the phase-field method. If there are no defects along a loop path, the total charge number is described by an encircled loop integral. We found that the total charge number is conserved, and the conservation of defects number is determined by a boundary during the generation and annihilation of positive–negative topological defects when the loop integral is confined. These rules can be extended to other two-dimensional systems with topological defects. [ABSTRACT FROM AUTHOR]

In this paper, the effect of heat treatment on the structure and magnetic properties of the alloy is investigated by heat treating the melted Fe2Cr1-xMnxGa samples at different temperatures. It is found that the structure and magnetic properties of Fe2Cr1-xMnxGa are very sensitive to heat treatment and there is a certain degree of disorder. The structure of the prepared samples changes with different conditions of heat treatment, and the change in structure will in turn affect the magnetic behavior of the alloy. The magnetization intensity increases with increasing annealing temperature, indicating that the orderliness of the samples is improved by heat treatment. The magnetization intensity of the samples decreases with the doping of Mn content, which is due to the anti-parallel arrangement of the Fe-Mn spin magnetic moments by the doped Mn atoms. [ABSTRACT FROM AUTHOR]

Fully compensated ferrimagnets do not create any magnetic stray field and allow for a completely polarized current of charges. As a result, these alloys show promising prospects for applications as spintronic devices. In this paper, we investigated the phase stability, the site preference, the tetragonal distortion and the influence of symmetry from the crystal structure and chemical environments of magnetic ions on the magnetic properties of Cr2YZ and Mn2YZ (Y = void, Ni, Cu, and Zn; Z = Ga, Ge, and As) full Heusler alloys by first-principles calculations. We found that the selected Cr2-based alloys, except for Cr2NiGa and Cr2NiGe, prefer to crystallize in the centrosymmetric L21-type structure, while the selected Mn2-based alloys, except for Mn2CuAs, Mn2ZnGe and Mn2ZnAs, tend to crystallize in the non-centrosymmetric XA-type structure. Due to the symmetry, the antiferromagnetism of the selected L21-type alloys is very stable, and no spin-polarized density of states could be generated. In contrast, the magnetic moment of the selected XA-type alloys depends heavily on the number of valence electrons and tetragonal distortion, and spin-polarized density of states is generated. Therefore, the selected alloys with L21-type structures and their tetragonal-distorted structure are potential candidates for conventional antiferromagnets, while those with XA-type structure and their tetragonal-distorted structure are promising candidates for (fully) compensated ferrimagnets. [ABSTRACT FROM AUTHOR]

Nematic liquid crystals (LCs) normally contain both integer and half-integer topological charges. In this work, we investigate topological charge evolution dynamics in nematic LCs by the phase-field model. We observe the annihilation of the topological charge, and the total number of charges decreases to two cores with time evolution in a disk. A significant decrease in charges is observed during the initial evolution. The positive topological charge and negative topological charge annihilate depending on the distance between two charges, which can be treated as the interaction force. The form of interaction force is similar to Coulomb's law, but the force is inversely the distance of defects instead of the square of the distance in Coulomb's law. Therefore, the present study contributes to the understanding of topological charge evolution dynamics and provides guidance for experiments to design in nematic LCs. [ABSTRACT FROM AUTHOR]

Twinning is a common phenomenon in crystal growth and mechanical deformation. It plays a key role in tuning the mechanical and transport properties of metals. In transition metal oxides, the twinning may give additional changes related to magnetism. Herein, the atomic configuration and the electronic structure of the twin boundaries (TBs) in rock‐salt‐type NiO are studied, combining aberration‐corrected transmission electron microscopy and first‐principles calculations. For NiO, the twinning occurs on the (111) plane, with oxygen atoms located at the TB and antiferromagnetic (AFM) coupling between the mirroring Ni atomic planes near the TB. The configuration gives the TB energy of 0.26 J m−2. Atomic and magnetic structures of fivefold twins in NiO are also investigated. In the structure of fivefold twins with the lowest energy, ferromagnetic coupling is formed in the AFM TBs due to the competing exchange interaction at the core of fivefold twins. [ABSTRACT FROM AUTHOR]

In this work, we conducted a high-throughput atomistic simulation of the interstitial solid solutions of hydrogen in α-Fe. The elastic constants and moduli were calculated. Through statistical analysis of structures and results, the influences of the microscopic distribution of hydrogen on the elastic moduli, as well as hydrogen-induced hardening and softening, are discussed. We found that even though the uniformly distributed hydrogen caused slight softening in α-Fe, the distribution of hydrogen at different adjacent positions significantly affected the elastic moduli. For example, hydrogen increased the Young's modulus and shear modulus at the 5th and 10th nearest neighbors, resulting in hardening, but decreased the bulk modulus at the 7th nearest neighbor, making the material easier to compress. These phenomena are related to the distribution densities of the positions that hydrogen atoms can occupy on the two major slip families, {110} and {112}, at different nearest neighbors distinguished by distances. [ABSTRACT FROM AUTHOR]

Magnetically actuated functional gradient nanocomposites have widely been used for ultra-durable biomimetic interfaces and surfaces. However, the mechanical and thermal mismatches in integrated systems containing dissimilar materials or structures usually cause failures. By modulating the concentration of magnetic particles, a suitable mechanical gradient morphology can be generated to match different integrated systems. In this work, a new model is developed to describe magnetic particle motion under the magnetic field. Hybrid nano-reinforcements with two different magnetic particle sizes and concentrations were employed to optimize the magnetic particle concentration gradient. It was observed that the diversification of concentration distribution can be achieved by tuning the sizes and concentrations of nanoparticles. The present study, therefore, contributes toward the understanding of the transport mechanism of magnetic-field-actuated functional gradient nanocomposites and provides guidance for experiments to design ultra-durable biomimetic interfaces and surfaces. [ABSTRACT FROM AUTHOR]

All magnetometers using alkali atoms rely on the monochromatic light driving the optical pumping process in the D1 or D2 line to create spin-polarized atoms. In the study of a sodium (Na) magnetometer, we find that 23Na exhibits the same hyperfine structure as exhibited by the 87Rb atom but differs from Rb in terms of level splitting. The narrowly split hyperfine levels of the 23Na 3P3/2 excited state are comparable to its natural broadening (9.8 MHz). We have modeled the nearly unresolved hyperfine structure as a partially resolved multilevel system in which the absorption at each photon detuning will induce adjacent allowed transition pathways simultaneously. Thus, the corresponding optical pumping processes of 23Na and 87Rb are governed by similar rate equations but result in different redistributed populations. By numerically solving the rate equations, we demonstrate that optically pumped 23Na has a much smaller spin polarization than that of 87Rb because the population imbalances between the ground state Zeeman levels of 23Na are very small. The inefficient optical pumping can explain why Na magnetometers are not studied extensively. The investigation into the optical pumping process of 23Na is helpful in preparing a highly spin-polarized atomic medium and optimizing the sensitivity of Na magnetometers. [ABSTRACT FROM AUTHOR]

Nanosecond laser pulses from a 1064 nm Nd:YAG laser were focused on the atomized spray of RbCl aqueous solution whose threshold energy of plasma ignition is around 550 mJ in atmosphere. A narrow-linewidth (100 MHz) vertical-cavity surface-emitting laser (VCSEL) has been used to probe the transient density of ground-state atomic Rb. The frequency of VCSEL probe beam was locked to the 87Rb transition |52S1/2, F = 2 → |52P1/2, F′ = 1 which corresponds to an absorption cross-section of up to 10−10 cm2. The atomic Rb number density was inferred from the extinction ratio of transmitted light intensity. The simple mathematical model developed to describe the density evolution n(t) conforms with the detected increase-peak-decline-vanish process well. It is a sensitive and real-time method used to diagnose the evolution process of generated plasma in air. The detection limit of RbCl concentration is lower than 1 gram per litre. Rb atoms have a mean lifetime of 250 µs in each pulsed plasma. [ABSTRACT FROM AUTHOR]

Cubic superconducting sample was simulated using time-dependent Ginzburg-Landau model under oscillating magnetic field with and without additional background static magnetic field. Vortex dynamics including entrance and exit from the sample was simulated. Magnetization and carrier concentration densities of the sample were studied as a function of external magnetic field variations. Anomalies in carrier concentration density were observed at certain values of the magnetic field which were correlated with the entrance and exit processes of vortices. Area swept by superconductor magnetization with magnetic field was observed to have a hysteresis-like behavior where area representing energy dissipated per cycle. This energy accumulation was suggested to cause instability in superconductor over the number of cycles and may result in thermal quenching. Temporal distribution of energy components showed consistency with the pattern observed for carrier concentration and magnetization under oscillating magnetic field. Rapid phase changes with magnetic oscillations resulted in oscillations in energy components, and irregular peaks and ripples in superconducting energy represent the situation of exit and entry of vortices. While the rise in interaction energy with cycles is referred to vortex relaxation time in a cycle, this energy is expected to accumulate and take other forms (e.g., heat) and is predicted to cause thermal quenching. In the presence of background static magnetic field, this energy dissipation was calculated to increase significantly while superconductor is subjected to oscillating magnetic field. [ABSTRACT FROM AUTHOR]

Abstract: Electrical current–induced deterministic magnetization switching in a magnetic multilayer structure without any external magnetic field is realized by utilizing interlayer exchange coupling. Two ferromagnetic Co layers, with in‐plane and out‐of‐plane anisotropy, respectively, are separated by a spacer Ta layer, which plays a dual role in inducing antiferromagnetic interlayer coupling, and contributing to the current‐induced effective magnetic field through the spin Hall effect. The current‐induced magnetization switching behavior can be tuned by premagnetizing the in‐plane Co layer. The antiferromagnetic exchange coupling field increases with decreasing thickness of the Ta layer, reaching 630 ± 5 Oe for a Ta thickness of 1.5 nm. A model is developed to separate the Joule heating and spin–orbit torques caused by the electrical current. The magnitude of the current‐induced perpendicular effective magnetic field from spin–orbit torque is 9.2 Oe/(107 A cm−2). The large spin Hall angle of Ta, opposite in sign to that of Pt, results in a low critical current density of 9 × 106 A cm−2. This approach is promising for the electrical switching of magnetic memory elements without any external magnetic field. [ABSTRACT FROM AUTHOR]

Abstract: Considering finite size effect, ferroelectric nanowires may show novel phenomena compared to ferroelectric thin film and bulk. Here we investigated the effect of width and surface compressive stress on electrocaloric cooling in ferroelectric nanowire by using thermodynamic calculations and phase‐field simulations. It was found that the isothermal entropy change and adiabatic temperature change in nanowire are 50% larger than that in thin film due to different mechanical boundary conditions of nanowire and thin film. The largest electrocaloric temperature changes were shown to increase significantly either with the decrease in nanowire width or the increase with the surface compressive stress. It was also revealed that the largest intrinsic entropy changes are almost the same for different nanowires with different widths and various stresses. The present study therefore contributes to the understanding of size effects of electrocaloric cooling and provides guidance for experiments to design high‐efficiency cooling devices using ferroelectric nanosystems. [ABSTRACT FROM AUTHOR]

Ferroelectric superlattice heterostructures have recently been explored for potential applications in electronic devices. In this letter, we employed the phase-field approach to simulate the domain structure and switching of a (BaTiO3)8/(SrTiO3)3 superlattice film constrained by a GdScO3 substrate. A constricted ferroelectric hysteresis loop was observed with a high saturation polarization but a small coercive field. The shape of the hysteresis loop is understood by analyzing the ferroelectric polarization distributions during switching. It is demonstrated that the multilayers stack behaves as dipole spring ferroelectric, named in analogy to exchange spring magnets in magnetic multilayers that show similar loops. [ABSTRACT FROM AUTHOR]

The local hydrogen distribution has significant influences on hydrogen embrittlement. In this work, mode-I fractures of (010)[100] pre-cracked α-Fe single crystal containing dissolved and absorbed hydrogen atoms are simulated by molecular dynamics and the time-stamped force-bias Monte Carlo methods. Statistics show that when located near the {112} plane, hydrogen atoms accelerate cleavage fracture and suppress the slip of {112}<111>; when located on the {110} plane, they promote martensite transformation and increase {110}<111> slip. Most adsorbed hydrogen atoms are concentrated near the inside of the crack surface and suppress fracture early by stress relaxation; therein concentrates stresses inside the matrix, and causes microvoid-coalescence fracture. • The hydrogen distribution and crack growth are linked by statistics. • Hydrogen promotes fracture by inhibiting {112}<111> slip in α-Fe. • The hydrogen on the {110} planes can enhance the martensitic transformation. • Adsorption of hydrogen can promote microvoid-coalescence fracture. • The voids in the matrix can be caused by hydrogen near the crack surface. [ABSTRACT FROM AUTHOR]

Time-dependent Ginzburg-Landau equations were solved by the finite difference scheme for a superconducting sample in steady and oscillating magnetic fields for 3D geometry. The dynamic behaviour of penetrating and leaving magnetic vortices in superconductor with the oscillating magnetic field was simulated. Carrier concentration density and the average magnetization of the sample were studied as a function of the external oscillating magnetic field. Anomalies in carrier concentrations at certain magnetic field values were observed and discussed. It was also observed that the area swept by magnetization versus external magnetic field is magnetic oscillation frequency dependent, which increases with increasing frequencies. It was suggested that this effect may cause instability in the superconducting characteristics of the sample over a number of cycles. Calculated energy patterns showed consistency with vortex patterns in the steady magnetic field. Magnetic oscillations initiated oscillations in energy components, ripples in superconducting energy are subjected to the entrance and leaving of vortices, while instability observed in interaction energy is referred to vortex relaxation time.

We report nanoscale bandgap engineering via a local strain across the inhomogeneous ferroelectric interface, which is controlled by the visible-light-excited probe voltage. Switchable photovoltaic effects and the spectral response of the photocurrent were explored to illustrate the reversible bandgap variation (∼0.3 eV). This local-strain-engineered bandgap has been further revealed by in situ probe-voltage-assisted valence electron energy-loss spectroscopy (EELS). Phase-field simulations and first-principle calculations were also employed for illustration of the large local strain and the bandgap variation in ferroelectric perovskite oxides. This reversible bandgap tuning in complex oxides demonstrates a framework for the understanding of the optically related behaviors (photovoltaic, photoemission, and photocatalyst effects) affected by order parameters such as charge, orbital, and lattice parameters.