Evolution Of Domain Walls Research Articles

Revisiting evolution of domain walls and their gravitational radiation with CosmoLattice

Employing the publicly available 𝒞osmoℒattice code, we conduct numerical simulations of a domain wall network and the resulting gravitational waves (GWs) in a radiation-dominated Universe in the Z2-symmetric scalar field model. In particular, the domain wall evolution is investigated in detail both before and after reaching the scaling regime, using the combination of numerical and theoretical methods. We demonstrate that the total area of closed walls is negligible compared to that of a single long wall stretching throughout the simulation box. Therefore, the closed walls are unlikely to have a significant impact on the overall network evolution. This is in contrast with the case of cosmic strings, where formation of loops is crucial for maintaining the system in the scaling regime. To obtain the GW spectrum, we develop a technique that separates physical effects from numerical artefacts arising due to finite box size and non-zero lattice spacing. Our results on the GW spectrum agree well with refs. [29,30], which use different codes. Notably, we observe a peak at the Hubble scale, an exponential falloff at scales shorter than the wall width, and a plateau/bump at intermediate scales. We also study sensitivity of obtained results on the choice of initial conditions. We find that different types of initial conditions lead to qualitatively similar domain wall evolution in the scaling regime, but with important variations translating into different intensities of GWs.

Magnetic-field driven domain wall evolution in rhombohedral magnetostrictive single crystals: a phase-field simulation

Magnetic-field driven domain wall evolution in rhombohedral magnetostrictive single crystals: a phase-field simulation

Stability of domain wall network with initial inflationary fluctuations and its implications for cosmic birefringence

We study the formation and evolution of domain walls with initial inflationary fluctuations by numerical lattice calculations that, for the first time, correctly take into account correlations on superhorizon scales. We find that, contrary to the widely-held claim over the past few tens of years, the domain wall network exhibits remarkable stability even when the initial distribution is largely biased toward one of the minima. This is due to the fact that the domain wall network retains information about initial conditions on superhorizon scales, and that the scaling solution is not a local attractor in this sense. Our finding immediately implies that such domain walls will have a significant impact on cosmology, including the production of gravitational waves, baryogenesis, and dark matter from domain walls. Applying this result to the axion-like particle domain wall, we show that it not only explains the isotropic cosmic birefringence suggested by the recent analysis, but also predicts anisotropic cosmic birefringence that is nearly scale-invariant on large scales and can be probed by future CMB observations.

Implementation of habituation on single ferroelectric memristor

As a basic form of behavioral plasticity, habituation enables organisms to adjust their behaviors in response to external stimulation and is a fundamental ability of organisms. The emulation of habituation in hardware becomes critical in highly efficient artificial neuromorphic computing. However, few devices can mimic habituation because it includes two opposite responses to the same repeated stimulus. Herein, we fabricate a device of Au/LiTaO3/Pt, which not only shows synaptic plasticity, such as long-term potentiation/depression and paired-pulse facilitation, but also demonstrates habituation behavior. In addition, the habituation characteristics, which is dependent on the intensity and frequency of stimulus, are realized on the device. The analysis of electrical transport behavior indicates that the variation in resistance states correlates with the injection/extraction of free carriers during the resistance switching and suggests that the habituation behavior originates from the evolution of conductive domain walls in LiTaO3 ferroelectric.

Domain Wall Evolution in Hf 0.5 Zr 0.5 O 2 Ferroelectrics under Field-Cycling Behavior

HfO 2 -based ferroelectrics have evoked considerable interest owing to the complementary metal–oxide semiconductor compatibility and robust ferroelectricity down to a few unit cells. However, the unique wake-up effect of HfO 2 -based ferroelectric films severely restricts the improvement of their performance. In particular, the domain structure is an important characteristic of ferroelectric materials, which still has not been well understood in HfO 2 -based ferroelectrics. In this work, a Hf 0.5 Zr 0.5 O 2 ferroelectric thin film is grown on a typical Si substrate buffered with TiN electrode. The 90° domains of the Pca 2 1 ferroelectric phase with head-to-tail and tail-to-tail structures can be observed by C s-corrected scanning transmission electron microscope under their pristine condition. After waking up, the 180° domain is displayed in the ferroelectric phase. The remarkable differences in domain walls for 90° and 180° domains are characterized by qualitatively mapping the polarization distributions at the atomic scale. The domain wall changes from the [ 1 0 1 ¯ ] of the Hf 0.5 Zr 0.5 O 2 film to the [001] of the Hf 0.5 Zr 0.5 O 2 film. This result provides fundamental information for understanding the domain structure of HfO 2 -based ferroelectrics.

Power spectrum of domain-wall network, and its implications for isotropic and anisotropic cosmic birefringence

Recently, based on a novel analysis of the Planck satellite data, a hint of a uniform rotation of the polarization of cosmic microwave background photons, called isotropic cosmic birefringence, has been reported. The suggested rotation angle of polarization of about 0.2–0.4 degrees is close to the fine-structure constant, α ≃ 1/137 rad ≃ 0.42 deg. Interestingly, this coincidence can be naturally explained over a very wide parameter range by the domain walls of axion-like particles. Furthermore, the axion-like particle domain walls predict not only isotropic cosmic birefringence but also anisotropic one that reflects the spatial distribution of the axion-like particle field on the last scattering surface. In this paper, we perform lattice simulations of the formation and evolution of domain walls in the expanding universe and obtain for the first time the two-point correlation function and power spectrum of the scalar field that constitutes the domain walls. We find that for initial fluctuations at subhorizon scales, the power spectrum is roughly consistent with analytical predictions based on random wall distributions. However, there is some excess at scales corresponding to the Hubble radius. Applying our results to the anisotropic cosmic birefringence, we predict the power spectrum of the rotation angles induced by the axion-like particle domain walls for the similar initial condition, and show that it is within reach of future observations of the cosmic microwave background.

Ultrasensitive Sub-monolayer Palladium Induced Chirality Switching and Topological Evolution of Skyrmions.

Chiral spin textures are fundamentally interesting, with promise for device applications. Stabilizing chirality is conventionally achieved by introducing Dzyaloshinskii-Moriya interaction (DMI) in asymmetric multilayers, where the thickness of each layer is at least a few monolayers. Here we report an ultrasensitive chirality switching in (Ni/Co)n multilayer induced by capping with only 0.22 monolayer of Pd. Using spin-polarized low-energy electron microscopy, we monitor the gradual evolution of domain walls from left-handed to right-handed Néel walls and quantify the DMI induced by the Pd capping layer. We also observe the chiral evolution of a skyrmion during the DMI switching, where no significant topological protection is found as the skyrmion winding number varies. This corresponds to a minimum energy cost of <1 attojoule during the skyrmion chirality switching. Our results demonstrate the detailed chirality evolution within skyrmions during the DMI sign switching, which is relevant to practical applications of skyrmionic devices.

Alternating Current Field Effects in Atomically Ferroelectric Ultrathin Films.

In this work, atomically K1−xNaxNbO3 thin films are taken as examples to investigate the reversible and irreversible effects in a horizon plane, i.e., the changes of domain structures, phase states, free energies, etc., under a z-axis alternating current field via a phase-field method. The simulation results show the driving forces during the charging and discharging process, where there is a variation for the angles of the domain walls from 180° to 90° (and then an increase to 135°), which are the external electric field and domain wall evolution, respectively. As for the phase states, there is a transformation between the orthorhombic and rhombohedral phases which can’t be explained by the traditional polarization switching theory. This work provides a reasonable understanding of the alternating current field effect, which is essential in information and energy storage.

Stability of domain walls in models with asymmetric potentials

We study the evolution of cosmological domain walls in models with asymmetric potentials. Our research goes beyond the standard case of spontaneous breaking of an approximate symmetry. When the symmetry is explicitly broken the potential exhibits nearly degenerate minima which can lead to creation of a metastable network of domain walls. The time after which the network will decay depends on the difference of values of the potential in minima, its asymmetry around the maximum separating minima and the bias of the initial distribution of the field. Effect of asymmetry around the maximum separating minima is novel one that we study with a new type of potential. Using numerical lattice simulations we determine relative importance of these factors on decay time of networks for generic potentials. We find that even very small departures from the symmetric initial distribution case lead to rapid decay of the domain wall network. As a result creation of a long lasting network capable of producing observable gravitational wave signals is much more difficult than previously thought. On the other hand details of the shape of the potential turn out to be much less important than was expected and the evolution of network from symmetric distribution is controlled by the difference of values of the potential in the minima.

Phase field study on the performance of artificial synapse device based on the motion of domain wall in ferroelectric thin films

Artificial neural networks have gained intensive attention in recent years because of their potential in effectively reducing energy consumption and improving computation performance. Ferroelectric materials are considered to be promising candidates for artificial synapses because of their multiple and nonvolatile polarization states under external stimuli. Despite artificial ferroelectric synapses with multilevel states, long retention and fast switching speed have been reported, and some key fundamental issues, e.g., the influence of domain wall configuration and evolution on the performance of synapse behaviors, also remain unclear. In this work, we study the performance of artificial synapses based on the motion of 180° ferroelectric domain walls of stripe domain and cylinder domain in ferroelectric thin films via a dynamical phase field model. The results demonstrate that artificial synapses based on the stripe domain exhibit high linearity and symmetry in weight update under a weak electric field, compared with the cylinder domain. Based on such artificial synapses, the accuracy of an artificial neural network for the Modified National Institute of Standards and Technology handwritten digit recognition is over 92%. This work provides a domain-wall-based strategy to improve the weight updating linearity and symmetry of artificial synapse devices and the recognition accuracy of artificial neural networks.

Domain wall evolution at nanowires in terms of 3D LLG equation initial-boundary problem

Domain wall evolution at nanowires in terms of 3D LLG equation initial-boundary problem

Evolution of ferroelastic domain walls during phase transitions in barium titanate nanoparticles

In this work, ferroelastic domain walls inside ${\mathrm{BaTiO}}_{3}$ (BTO) tetragonal nanocrystals are distinguished by Bragg peak position and studied with Bragg coherent x-ray diffraction imaging (BCDI). Convergence-related features of the BCDI method for strongly phased objects are reported. A ferroelastic domain wall inside a BTO crystal has been tracked and imaged across the tetragonal-cubic phase transition and proves to be reversible. The linear relationship of relative displacement between two twin domains with temperature is measured and shows a different slope for heating and cooling, while the tetragonality reproduces well over temperature changes in both directions. An edge dislocation is also observed and found to annihilate when heating the crystal close to the phase transition temperature.

Gravitational waves from cosmic domain walls: a mini-review

Domain walls are sheet-like topological defects produced when a discrete symmetry is spontaneously broken in the early universe. Although the existence of stable domain walls is disfavored by cosmological considerations, it is possible to consider unstable domain walls which disappear early enough not to lead cosmological disasters. In this contribution, we discuss the possibility that a significant amount of gravitational waves is produced by annihilation of such unstable domain walls in the early universe. After reviewing cosmological evolution of domain walls, we give an estimate of the expected gravitational wave signal based on the results of numerical simulations. In addition, we briefly review some well-motivated particle physics models that predict the formation of unstable domain walls. The detectability of predicted signals is also discussed in prospect of planned gravitational wave observatories.

The atomic-scale domain wall structure and motion in HfO2-based ferroelectrics: A first-principle study

The HfO2-based ferroelectrics have aroused considerable attention due to their potential application in silicon process-compatible memory devices. However, the ferroelectricity origin and the domain evolution have not yet been well understood. It is now generally accepted the orthorhombic Pca21 phase is one of ferroelectric phases for HfO2. In this work, by performing density functional theory calculations, we systematically studied the domain wall structures and evolution based on the Pca21 ferroelectric phase. More specifically, the atomic structures of ten types of possible 180° and 90° ferroelectric domain walls are predicted and explored. And the motion of certain domain walls is expected as the microscopic mechanisms of the polarization switching and ferroelectricity activation of Pca21 HfO2 under external electric field. Our results are in good agreement with the recent experimental results on ferroelectric domain walls in HfO2-based epitaxial thin-film and are helpful to understand the ferroelectricity origin of the HfO2-based ferroelectrics.

Comparing parametric and non-parametric velocity-dependent one-scale models for domain wall evolution

We perform a detailed comparison between a recently proposed parameter-free velocity-dependent one-scale model and the standard parametric model for the cosmological evolution of domain wall networks. We find that the latter overestimates the damping of the wall motion due to the Hubble expansion and neglects the direct impact of wall decay on the evolution of the root-mean-square velocity of the network. We show that these effects are significant but may be absorbed into a redefinition of the momentum parameter. We also discuss the implications of these findings for cosmic strings. We compute the energy loss and momentum parameters of the standard parametric model for cosmological domain wall evolution using our {non-parametric} velocity-dependent one-scale model in the context of cosmological models having a power law evolution of the scale factor a with the cosmic time t (a ∝ tλ, 0 < λ < 1), and compare with the results obtained from numerical field theory simulations. We further provide simple linear functions which roughly approximate the dependence of the energy loss and momentum parameters on λ.

Room-temperature angular-dependent topological Hall effect in chiral antiferromagnetic Weyl semimetal Mn3Sn

Chiral antiferromagnetic (AFM) Weyl semimetal Mn3Sn shows a large anomalous Hall effect (AHE) around room temperature, due to the Berry curvature generated by Weyl nodes in electronic dispersions. Here, we study the temperature- and angular-dependent Hall effect and magnetic measurement in single-crystalline Mn3Sn. There are some intriguing phenomena: first, a large hysteretic-type AHE has been observed only above 270 K, while the coercivity is around 300 Oe and independent of temperature. Second, the temperature- and angular-dependent topological Hall effect is obtained, which may stem from the real space topological spin texture. Third, the coercivity extracted from the angular-dependent AHE is well fitted with the Stoner-Wohlfarth model, which reflects the evolution of domain walls and magnetic anisotropy. Thus, it shows that not only the topological structure in momentum space but also the real space topological spin texture plays an important role in anomalous transport properties of Mn3Sn. Our work pushes forward to the realization of room temperature AFM spintronics and paves the way toward the possible devices based on the unconventional Hall effect.

Transient magnetic-domain-wall ac dynamics by means of magneto-optical Kerr effect microscopy

The domain wall response under constant external magnetic fields reveals a complex behavior where sample disorder plays a key role. Furthermore, the response to alternating magnetic fields has only been explored in limited cases and analyzed in terms of the constant field solution. Here we unveil phenomena in the evolution of magnetic domain walls under the application of alternating magnetic fields within the creep regime, well beyond a small fuctuation limit of the domain wall position. Magnetic field pulses were applied in ultra-thin ferromagnetic films with perpendicular anisotropy, and the resulting domain wall evolution was characterized by polar magneto-optical Kerr effect microscopy. Whereas the DC characterization is well predicted by the elastic interface model, striking unexpected features are observed under the application of alternating square pulses: magneto-optical images show that after a transient number of cycles, domain walls evolve toward strongly distorted shapes concomitantly with a modification of domain area. The morphology of domain walls is characterized with a roughness exponent when possible and contrasted with alternative observables which result to be more suitable for the characterization of this transient evolution. The final stationary convergence as well as the underlying physics is discussed.

Evolution of thick domain walls in inflationary and p=wrho universe

We study the evolution of thick domain walls in the different models of cosmological inflation, in the matter-dominated and radiation-dominated universe, or more generally in the universe with the equation of state p=wrho . We have found that the domain wall evolution crucially depends on the time-dependent parameter C(t)=1/(H(t)delta _0)^2, where H(t) is the Hubble parameter and delta _0 is the thickness of the wall in flat space-time. For C(t)>2 the physical thickness of the wall, a(t)delta (t), tends with time to delta _0, which is microscopically small. Otherwise, when C(t) le 2, the wall steadily expands and can grow up to a cosmologically large size.

Controllable conductive readout in self-assembled, topologically confined ferroelectric domain walls.

Charged domain walls in ferroelectrics exhibit a quasi-two-dimensional conduction path coupled to the surrounding polarization. They have been proposed for use as non-volatile memory with non-destructive operation and ultralow energy consumption. Yet the evolution of domain walls during polarization switching makes it challenging to control their location and conductance precisely, a prerequisite for controlled read-write schemes and for integration in scalable memory devices. Here, we explore and reversibly switch the polarization of square BiFeO3 nanoislands in a self-assembled array. Each island confines cross-shaped, charged domain walls in a centre-type domain. Electrostatic and geometric boundary conditions induce two stable domain configurations: centre-convergent and centre-divergent. We switch the polarization deterministically back and forth between these two states, which alters the domain wall conductance by three orders of magnitude, while the position of the domain wall remains static because of its confinement within the BiFeO3 islands.

Phase-field simulation of domain walls in rhombohedral ferroelectric single crystals

Domain engineered ferroelectric relaxor-PbTiO3 single crystals exhibit ultrahigh piezoelectric coefficients. A 10th order Landau-Devonshire energy function for rhombohedral PIN-PMN-PT ferroelectric single crystals was implemented in a phase-field model to study the behavior of domain walls under external electric field. The domain formation and domain wall evolution were simulated. A new way to apply periodic boundary conditions was implemented to accommodate nonzero strain during the domain formation and evolution. It was found that 71 and 109 domain walls reacted differently to external electric field along the [110] direction. A domain wall broadening effect was observed in the 71 domain walls when the electric field was below the coercive field. When the electric field exceeded the coercive field, homogeneous polarization switching occurred with no motion of the 71 domain walls. Heterogeneous polarization switching occurred by the sweeping of the 109 domain walls. The evolution mechanism of the two types of domain walls helps explain the behavior of engineered domain structures subjected to external electric field.