A theoretical study on the dynamics of an antiferromagnetic (AFM) skyrmion is indispensable for revealing the underlying physics and understanding the numerical and experimental observations. In this work, we present a reliable theoretical treatment of the spin-current induced motion of an AFM skyrmion in the absence and presence of pinning defect. For an ideal AFM system free of defect, the skyrmion motion velocity as a function of the intrinsic parameters can be derived, based on the concept that the skyrmion profile agrees well with the ${360}^{\circ }$ domain-wall formula, leading to an explicit description of the skyrmion dynamics. However, for an AFM lattice containing a defect, the skyrmion can be pinned and the depinning field as a function of damping constant and pinning strength can be described by Thiele's approach. It is revealed that the depinning behavior can be remarkably influenced by the time-dependent oscillation of the skyrmion trajectory. The present theory provides a comprehensive scenario for manipulating the dynamics of AFM skyrmion, informative for future spintronic applications based on antiferromagnets.

We report the discovery of a GeV-associated phenomenon which is strong (up to an order) stochastic reversible enhancements of photoluminescence intensity in a single GeV diamond synthesized with the high-pressure, high-temperature technique. We were lucky to observe this effect with only one crystal among dozens of similar microdiamonds. Each rise and fall of the intensity above its stable moderate level may be referred to as a superflare with smooth dynamics of the transients which develop on the timescale of seconds. These flares tend to recur infinitely at ambient conditions under cw-laser excitation above a certain input power threshold. To explain this phenomenon we propose a theory of intrinsic optical instabilities which develop in a dense ensemble of quantum emitters.

We study the scaling of logarithmic negativity between adjacent subsystems in critical fermion chains with various inhomogeneous modulations through numerically calculating its recently established lower and upper bounds. For random couplings, as well as for a relevant aperiodic modulation of the couplings, which induces an aperiodic singlet state, the bounds are found to increase logarithmically with the subsystem size, and both prefactors agree with the predicted values characterizing the corresponding asymptotic singlet state. For the marginal Fibonacci modulation, the prefactors in front of the logarithm are different for the lower and the upper bound and vary smoothly with the strength of the modulation. In the delocalized phase of the quasiperiodic Harper model, the scaling of the bounds of the logarithmic negativity and that of the entanglement entropy are compatible with the logarithmic scaling of the homogeneous chain. At the localization transition, the scaling of the above entanglement characteristics holds to be logarithmic, but the prefactors are significantly reduced compared to those of the translationally invariant case, roughly by the same factor.

We investigate the factorization, coherence, and asymmetry properties of the one-dimensional Heisenberg spin-$\frac{1}{2}$ XXZ chain with Dzyaloshinskii-Moriya interaction (DMI) and a transverse magnetic field using quantum information measures. Both longitudinal and transverse DM vectors are considered. Using numerical DMRG methods, we compute bipartite entanglement estimators like the one-tangle, two-spin concurrence and quantum coherence estimators like the Wigner-Yanase-skew information. We show that a longitudinal DMI destroys the factorizability property while a transverse DMI preserves it. We relate the absence of factorizability to the breaking of the $U\left(1\right)$ rotation symmetry about the local magnetization axis at each lattice site. Physically, the breaking of the symmetry manifests in the existence of a chiral current. Furthermore, we show that although the longitudinal DMI destroys the factorization property, there is a “pseudofactorizing” field at which the entanglement and hence violation of the $U\left(1\right)$ symmetry is minimal. Our calculations indicate a phase coherent ground state at $h_{\text{pf}}$. An entanglement transition (ET) occurs across this field which is characterized by an enhanced but finite range of two-spin concurrence in its vicinity in contrast with the diverging range of the concurrence for the ET across the factorizing field. We relate the asymmetry to the “frameness” or the ability for the state to act as a reference frame for some measurement. In the absence of the longitudinal DMI (or in the presence of a transverse DMI), at the factorizing field, the single site magnetization axis serves to specify the common $z$ axis for the full system but not the full Cartesian reference frame due to a lack of phase reference. On the other hand, in the presence of a longitudinal DMI, our results indicate that at the pseudofactorizing field, the local magnetization and the chiral current are sufficient to specify the full Cartesian reference frame with the chiral current serving as the macroscopic quantity to determine the phase reference.

We perform micromagnetic simulations to study the switching barriers in square artificial spin ice systems consisting of elongated single domain magnetic islands arranged on a square lattice. By considering a double vertex composed of one central island and six nearest neighbor islands, we calculate the energy barriers between two types of double vertices by applying the simplified and improved string method. We investigate by means of micromagnetic simulations the consequences of the neighboring islands, the inhomogeneities in the magnetization of the islands and the reversal mechanisms on the energy barrier by comparing three different approaches with increasing complexity. The micromagnetic models, where the string method is applied, are compared to a method commonly in use, the mean barrier approximation. Our investigations indicate that a proper micromagnetic modeling of the switching process leads to significantly lower energy barriers, by up to 35% compared to the mean-barrier approximation, so decreasing the expected average life time up to seven orders of magnitude. Hereby, we investigate the influence of parallel switching channels and the conceptional approach of using a mean-barrier to calculate the corresponding rates.

We develop the theory of hydrodynamic electron transport in a long-range disorder potential for conductors in which the underlying electron liquid lacks Galilean invariance. For weak disorder, we express the transport coefficients of the system in terms of the intrinsic kinetic coefficients of the electron liquid and the correlation function of the disorder potential. We apply these results to analyze the doping and temperature dependence of transport coefficients of graphene devices. We show that at charge neutrality, long-range disorder increases the conductivity of the system above the intrinsic value. The enhancement arises from the predominantly vortical hydrodynamic flow caused by local deviations from charge neutrality. Its magnitude is inversely proportional to the shear viscosity of the electron liquid and scales as the square of the disorder correlation radius. This is qualitatively different from the situation away from charge neutrality. In that case, the flow is predominantly potential and produces negative viscous contributions to the conductivity, which are proportional to the sum of shear and bulk viscosities and inversely proportional to the square of disorder correlation radius.

Gaussian deformation in graphene structures exhibits an interesting effect in which flower-shaped confinement states are observed in the deformed region [Carrillo-Bastos et al., Phys. Rev. B 90, 041411 (2014)]. To exploit such a deformation for various applications, tunable electronic features including a band-gap opening for semimetallic structures are expected. Besides, the effects of disorders and external excitations also need to be considered. In this work, we present a systematic study on quantum transport of graphene ribbons with Gaussian deformation. Different levels of deformation are explored to find a universal behavior of the electron transmission. Using a tight-binding model in combination with nonequilibrium Green's-functions formalism, we show that the first plateau of the transmission of semimetallic armchair ribbons is just weakly affected in the case of small Gaussian deformations. However, significant large Gaussian bumps can induce a strong drop of this plateau and a transport gap is formed. The transmission at zero energy is found to decrease exponentially with increasing the size of the Gaussian bump. Moreover, the gap of semiconducting ribbons is enlarged with large deformations. The opening or the widening of the transport gap in large deformed armchair structures is interpreted by a formation of a spatially three-zone behavior of the hopping profile. On the other hand, a transport gap is not observed in zigzag ribbons regardless of the size of Gaussian bumps. This behavior is due to the strong localization of edge states at the energy point E = 0. Furthermore, it unveils the opposite effect of vertical electric fields $+{\mathit{E}}_z$ and $-{\mathit{E}}_z$, stemming from the breaking of the mirror symmetry. Additionally, it is also pointed out that the electronic behavior of a Gaussian deformed ribbon including edge roughness is dominated by the characteristics of the edge-roughness effect with strong Anderson-type localized states reflected by sharp peaks in the transmission profile.

A physical understanding of the nature of spatially modulated phases (SMPs) in rare-earth-doped antiferrodistortive (AFD) multiferroics and how they behave close to surfaces and interfaces is lacking. Here the emergence of the antiferroelectric (AFE), ferroelectric (FE), or ferrielectric (AFE-FE) spatial modulation in the vicinity of the morphotropic phase transition in ${\mathrm{La}}_x{\mathrm{Bi}}_{1–x}\mathrm{Fe}{\mathrm{O}}_3$ ($x\sim 0.2$) is explored on the atomic level using high-resolution scanning transmission electron microscopy (HRSTEM). The suppression, or “melting,” of the AFE-type SMP in the vicinity of the AFD twin wall/surface junction is revealed by HRSTEM in ${\mathrm{La}}_{0.22}{\mathrm{Bi}}_{0.78}\mathrm{Fe}{\mathrm{O}}_3$ films and explained by the hybrid approach combining Landau-Ginzburg-Devonshire (LGD) phenomenology and the semimicroscopic four-sublattice model (FSM). The LGD-FSM approach reduces the problem of AFE (or AFE-FE) SMP emergence and stability to the thermodynamic analysis of the free-energy functional with AFE, FE, and AFD long-range order parameters and two master parameters: the FE-AFE coupling strength between four neighboring A sites and the nonstoichiometry factor, which are proportional to the variations of La concentration in ${\mathrm{La}}_x{\mathrm{Bi}}_{1–x}\mathrm{Fe}{\mathrm{O}}_3$ films. We establish that the surface-induced melting of SMPs and the associated broadening of AFE AFD domain walls minimize the film free energy under certain conditions imposed on the master parameters and gradient energy below the critical value. The observed behavior provides insight into the origin of SMPs in AFD multiferroics.

The recently proposed atomistic structure learning algorithm (ASLA) builds on neural network enabled image recognition and reinforcement learning. It enables fully autonomous structure determination when used in combination with a first-principles total energy calculator, e.g., a density functional theory (DFT) program. To save on the computational requirements, ASLA utilizes the DFT program in a single-point mode, i.e., without allowing for relaxation of the structural candidates according to the force information at the DFT level. In this work, we augment ASLA to establish a surrogate energy model concurrently with its structure search. This enables approximative but computationally cheap relaxation of the structural candidates before the single-point energy evaluation with the computationally expensive DFT program. We demonstrate a significantly increased performance of ASLA for building benzene while utilizing a surrogate energy landscape. Further, we apply this model-enhanced ASLA in a thorough investigation of the $c(4\times 8)$ phase of the Ag(111) surface oxide. ASLA successfully identifies a surface reconstruction which has previously only been guessed on the basis of scanning tunneling microscopy images.

The active control of small particles due to the Casimir force is of great interest for noncontact and low-friction micro- and nanoelectromechanical systems (MEMS and NEMS). In this work, we study theoretically the Casimir force between a nanoplate and a metal-oxide-semiconductor (MOS) substrate immersed in a liquid environment. The MOS structure consists of layers indium-tin-oxide (ITO)/Teflon/Au, where a gate electrode is connected between the ITO film and the gold substrate. Our results suggest that the Casimir equilibria are determined, not only by the layer thicknesses in MOS structures but also by the carrier densities of ITO (gate-controllable). Under proper conditions, a gold nanoplate can be stably trapped due to the Casimir forces. Moreover, we show that the trapping state can be released by decreasing the carrier density in the active layer of ITO, e.g., from ${10}^{21}$ to ${10}^{19}{\mathrm{cm}}^{-3}$. The switching between stable trapping and the release state of a suspended Teflon nanoplate is also demonstrated. Our findings can be useful in designing ultrafast switchable devices for applications in MEMS/NEMS.

We employ the jackknife algorithm to analyze the propagation of the statistical quantum Monte Carlo error through the Bethe-Salpeter equation. This allows us to estimate the error of dynamical mean-field theory calculations of the susceptibility and of dynamical vertex approximation calculations of the self-energy. We find that the different frequency components of the susceptibility are uncorrelated, whereas those of the self-energy are correlated. For improving the quality of the correlation matrix taking sufficiently many jackknife bins is key, while for reducing the standard error of the mean sufficiently many Monte Carlo measurements are necessary. We furthermore show that even in the case of the self-energy, the finite covariance does not have a sizable influence on the analytic continuation.

Trapping, levitating, and manipulating nanoscale objects with light forces shaped by patterned metamaterials continues to hold great interest for optical and condensed matter physics and engineering. Successful developments to date have concentrated on constraining movement only in one dimension, along the vertical axis to a material plane. Here we propose a realistic structure, consisting of alternating layers of graphene and dielectric, and periodically nanopatterned on the surface, that is capable of levitating and trapping nanoscale particles in two dimensions: one perpendicular and one parallel to the material plane. Repulsive forces arising from high- $k$ modes of the metamaterial provide particle levitation along the vertical axis. At the point where this repulsive force balances the downward gravitational force, the particle is trapped at stable equilibrium. Periodic nanopatterning in the metamaterial surface furnishes the second, horizontal axis constraining particle motion. We show that the equilibrium position above the surface can be controlled by adjusting both the Fermi level and the number of graphene layers. Furthermore, to explicate the role of the high-$k$ modes in generating the repulsive forces, we also propose a semianalytical method to calculate both the potential well and the forces generated by dipole radiation above the nanopatterned surface.

Multielectron bubbles provide a unique platform to study electrons in two dimensions and on curved surfaces, at densities which cannot be accessed using electrons on bulk helium or in semiconductor interfaces. Usually, MEBs are created by applying a large electric field and thereby inducing electrohydrodynamical instability on a charged surface of liquid helium. In the present study, we describe a method to create instability of the charged surface using ultrasound, in the presence of small electric fields. The ultrasound was applied close to the charged liquid-vapor interface, resulting in the formation of a liquid column, which breaks into liquid droplets. The mechanical impact of the droplets falling back into the bulk liquid resulted in the formation of highly charged multielectron bubbles. We estimated the initial charge density of the bubbles above the lambda point to be close to ${10}^{13}\mathrm{electrons}/{\mathrm{m}}^2$.

The conjecture made recently by the group at Sherbrooke, that their observed anomalous thermal Hall effect in the pseudogap phase in the cuprates is due to phonons, is supported on the basis of an earlier result that the observed loop-current order in this phase must induce lattice distortions which are linear in the order parameter and an applied magnetic field. The lowered symmetry of the crystal depends on the direction of the field. A consequence is that the elastic constants change proportional to the field and are shown to induce axial thermal transport with the same symmetries as the Lorentz force enforces for the normal electronic Hall effect. Direct measurements of elastic constants in a magnetic field are suggested to verify the quantitative aspects of the results.

The phase diagram of the cuprate superconductors has posed a formidable scientific challenge for more than three decades. This challenge is perhaps best exemplified by the need to understand the normal-state charge transport as the system evolves from Mott insulator to Fermi-liquid metal with doping. Here we report a detailed analysis of the temperature $\left(T\right)$ and doping $\left(p\right)$ dependence of the planar resistivity of simple-tetragonal ${\mathrm{HgBa}}_2\mathrm{Cu}{\mathrm{O}}_{4+\delta }$ (Hg1201), the single-${\mathrm{CuO}}_2$-layer cuprate with the highest optimal superconducting transition temperature, $T_c$. The data allow us to test a recently proposed phenomenological model for the cuprate phase diagram that combines a universal transport scattering rate with spatially inhomogeneous (de)localization of the Mott-localized hole. We find that the model provides a good description of the data. We then extend this analysis to prior transport results for several other cuprates, including the Hall number in the overdoped part of the phase diagram, and find little compound-to-compound variation in the (de)localization gap scale. The results point to a robust, universal structural origin of the inherent gap inhomogeneity that is unrelated to doping-related disorder. They are inconsistent with the notion that much of the phase diagram is controlled by a quantum-critical point, and instead indicate that the unusual electronic properties exhibited by the cuprates are fundamentally related to strong nonlinearities associated with subtle nanoscale inhomogeneity.

The topological phases of graphene with spin-orbit coupling, an exchange field, and a staggered-sublattice potential determine the properties of the edge states of the zigzag nanoribbon. In the presence of the Hubbard interaction, the spontaneous magnetization at the zigzag terminations induces sizable magnetic moments at the lattice sites in the bulk region. Thus, the exchange field and staggered-sublattice potential in the bulk region are effectively changed, which in turn change the topological phase. Within a certain parameter regime, quasistable excited states of the zigzag nanoribbon exist, which have a different magnetism configuration at the zigzag terminations from the ground state. The quasistable excited states could effectively suppress the finite-size effect of the topological edge states. The investigation of the topological edge states in the presence of interaction helps the engineering of spintronic nanodevices based on realistic materials.

The calculation of interaction integrals is a bottleneck for the treatment of many-body quantum systems due to its high numerical cost. We conduct configuration interaction calculations of the few-electron states confined in III-V semiconductor two-dimensional structures using a shallow neural network to calculate the two-electron integrals, which can be used for general isotropic interaction potentials. This approach allows for a speedup of the evaluation of the energy levels and controllable accuracy.

We study the nanopatterning of silicon surfaces under near-normal 40-keV ${\mathrm{Ar}}^{+}$ sputtering with simultaneous Fe oblique codeposition. The ion-beam incidence was kept at 15°, for which no pattern is produced in the absence of metal incorporation. Morphological and compositional analyses were performed by atomic force microscopy, in its morphological and electrical modes, Rutherford backscattering spectrometry, x-ray photoelectron spectroscopy, scanning Auger, as well as transmission and scanning electron microscopy. Initially, nanodot structures randomly emerge, which, with increasing ion fluence, become progressively aligned along the perpendicular direction to the Fe flux. With increasing fluence, they coalesce, leading to a ripple pattern. The pattern dynamics and characteristics are faster and enhanced, respectively, as the distance to the metal source decreases (i.e., as the metal content increases). For the highest metal flux, the ripples can become rather large (up to 18 μm) and straighter, with few defects, and a pattern wavelength close to 500 nm, while keeping the surface roughness close to 15 nm. Furthermore, for a fixed ion fluence, the pattern order is improved for higher metal flux. In contrast, the pattern order enhancement rate with ion fluence does not depend on the metal flux. Our experimental observations agree with the predictions and assumptions of the model by Bradley [R. M. Bradley, Phys. Rev. B 87, 205408 (2013)] Several compositional and morphological studies reveal that the ripple pattern is also a compositional one, in which the ripple peaks have a higher iron silicide content, in agreement with the model. Likewise, the ripple structures develop along the perpendicular direction to the Fe flux, and the pattern wavelength increases as the metal flux decreases with a behavior qualitatively consistent with the model predictions.