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We compare the driven dynamics of skyrmions, antiskyrmions, and skyrmionium interacting with random disorder, circular defects, and asymmetric potentials. When interacting with a line defect at a constant drive, skyrmions and antiskyrmions show an acceleration effect for motion along the wall and a drop in velocity when they can cross the barrier. In contrast, skyrmionium travels at a reduced velocity when moving along a wall, and exhibits an increase in velocity once it can cross the barrier. For point defects, skyrmionium can be pinned for a finite fixed period of time, while for skyrmions and antiskyrmions, the Magnus force creates a deflection from the defect and an acceleration effect. For a given drive, skyrmionium moves twice as fast as skyrmions; however, skyrmionium is more susceptible to pinning effects than skyrmions and antiskyrmions. Additionally, there is a critical threshold where the skyrmionium transforms to a skyrmion that is associated with a drop in the velocity of the texture. We show that all three textures exhibit diode and ratchet effects when interacting with an asymmetric substrate, but skyrmions and antiskyrmions show a stronger ratcheting effect than skyrmionium due to the Magnus force., Comment: 21 pages, 13 figures

This paper addresses Yukawa theory, focusing on the scattering between two identical fermions mediated by an intermediate scalar boson, considering the effects of thermal contributions and Lorentz symmetry breaking. Temperature is introduced into the theory through the TFD formalism, while Lorentz violation arises from a background tensor coupled to the kinetic part of the Klein-Gordon Lagrangian. Two important quantities are calculated: the cross-section for the scattering process and the modified Yukawa potential. The main results obtained in this work demonstrate that considering Lorentz symmetry breaking has several implications for changes in symmetries and physical states, while the presence of temperature is strongly related to the strength of the interaction. This interplay between symmetry breaking and temperature effects provides deeper insights into the behavior of the Yukawa theory under different conditions., Comment: 20 pages, 4 figures

Polymer mixtures fractionate between phases depending on their molecular weight. Consequently, by varying solvent conditions, a polydisperse polymer sample can be separated between phases so as to achieve a particular molecular weight distribution in each phase. In principle, predictive physics-based theories can help guide separation design and interpret experimental fractionation measurements. Even so, applying the standard Flory-Huggins model can present a computational challenge for mixtures with many polymeric components of different length, particularly for scarce components at the tails of a distribution. Here, we apply our recently-derived exact analytical solution of multi-component Flory-Huggins theory for polydisperse polymers to understand the principles of polymer fractionation for common molecular weight distributions. Our method reveals that polymer fractionation is highly sensitive to the shape, and in particular the tails, of this distribution. Our results highlight the need for considering the full molecular weight distribution in phase coexistence calculations.

We compare the dynamical behavior of magnetic skyrmions interacting with square and triangular defect arrays just above commensuration using both an atomistic model and a particle-based model. Under an applied drive, the initial motion is a kink traveling through the pinned skyrmion lattice. For the square defect array, both models agree well and show a regime in which the soliton motion is locked along 45$^\circ$. The atomistic model also produces locking of a soliton along 30$^\circ$, while the particle-based model does not. For the triangular defect array, the atomistic model exhibits soliton motion locked to 30$^\circ$ over a wide region of external driving force values. In contrast, the particle-based model gives soliton motion locked to 45$^\circ$ over only a small range of external driving force values. The difference arises because the nondeforming particle model facilitates meandering skyrmion orbits while the deformable atomistic model enables stronger skyrmion-skyrmion interactions that reduce the meandering. Our results indicate that soliton motion through pinned skyrmion lattices on a periodic substrate is a robust effect and could open the possibility of using solitons as information carriers. Our results also provide a better understanding of the regimes for which particle-based models of skyrmions are best suited., Comment: 7 pages, 5 figures

Using atomistic simulations, we investigate the dynamical behavior of magnetic skyrmions in dimer and trimer molecular crystal arrangements, as well as bipartite lattices at 3/2 and 5/2 fillings, under ac driving over a square array of anisotropy defects. For low ac amplitudes, at all fillings we find reversible motion where the skyrmions return to their original positions at the end of each ac drive cycle and the diffusion is zero. We also identify two distinct irreversible regimes. The first is a translating regime in which the skyrmions form channels of flow in opposing directions and translate by one substrate lattice constant per ac drive cycle. The translating state appears in the dimer and trimer states, and produces pronounced peaks in the diffusivity in the direction perpendicular to the external drive. For larger ac amplitudes, we find chaotic irreversible motion in which the skyrmions can randomly exchange places with each other over time, producing long-time diffusive behavior both parallel and perpendicular to the ac driving direction., Comment: 16 pages, 5 figures

We use the James Webb Space Telescope (JWST) Mid-Infrared Instrument (MIRI) medium-resolution spectrometer (MRS) observations of the radio loud AGN host UGC 8782 to map the warm molecular and ionized gas kinematics. The data reveal spatially resolved outflows in the inner 2 kpc, seen in low ionization (traced by the [Ar ii] 6.99um emission) and in warm molecular gas (traced by the H2 rotational transitions). We find a maximum mass-outflow rate of 4.90 (2.04) M/yr at ~900 pc from the nucleus for the warm outflow (198 K< T< 1000 K) and estimate and outflow rate of up to 1.22 (0.51) M/yr for the hotter gas phase (T > 1000 K). These outflows can clear the entire nuclear reservoir of warm molecular gas in about 1 Myr. The derived kinetic power of the molecular outflows lead to coupling efficiencies of 2-5 percent of the AGN luminosity, way above the minimum expected to the AGN feedback be effective quenching the star formation., Comment: Accepted for publication at APJ

Using atomistic simulations, we show that new types of skyrmion states called skyrmion molecular crystals and skyrmion superlattices can be realized on triangular substrates when there are two or three skyrmions per substrate minimum. We find that as a function of the magnetic field and substrate periodicity, a remarkably wide variety of ordered phases appear similar to those found in colloidal or Wigner molecular crystals, including ferromagnetic and herringbone states. The ability of the skyrmions to annihilate, deform, and change size gives rise to a variety of superlattice states in which a mixture of different skyrmion sizes and shapes produces bipartate or more complex lattice structures. Our results are relevant for skyrmions on structured triangular substrates, in magnetic arrays, or skyrmions in moire materials., Comment: 6 pages, 4 figures

In this paper, our objective is to explore a time-machine space-time formulated in general relativity, as introduced by Li (Phys. Rev. D {\bf 59}, 084016 (1999)), within the context of modified gravity theories. We consider Ricci-inverse gravity of all Classes of models, {\it i.e.}, (i) Class-{\bf I}: $f(\mathcal{R}, \mathcal{A})=(\mathcal{R}+{\kappa\,\mathcal{R}^2}+\beta\,\mathcal{A})$, (ii) Class-{\bf II}: $f(\mathcal{R}, A^{\mu\nu}\,A_{\mu\nu})=(\mathcal{R}+{\kappa\,\mathcal{R}^2}+\gamma\,A^{\mu\nu}\,A_{\mu\nu})$ model, and (iii) Class-{\bf III}: $f(\mathcal{R}, \mathcal{A}, A^{\mu\nu}\,A_{\mu\nu})=(\mathcal{R}{\kappa\,\mathcal{R}^2}+\beta\,\mathcal{A}+\delta\,\mathcal{A}^2+\gamma\,A^{\mu\nu}\,A_{\mu\nu})$ model, where $A^{\mu\nu}$ is the anti-curvature tensor, the reciprocal of the Ricci tensor, $R_{\mu\nu}$, $\mathcal{A}=g_{\mu\nu}\,A^{\mu\nu}$ is its scalar, and $\beta, {\kappa}, \gamma, \delta$ are the coupling constants. Moreover, we consider $f(\mathcal{R})$ modified gravity theory and investigate the same time-machine space-time. In fact, we show that Li time-machine space-time serve as valid solutions both in Ricci-inverse and $f(\mathcal{R})$ modified gravity theories. Thus, both theory allows the formation of closed time-like curves analogue to general relativity, thereby representing a possible time-machine model in these gravity theories theoretically., Comment: 23 pages, 4 figures

The Flory-Huggins theory describes the phase separation of solutions containing polymers. Although it finds widespread application from polymer physics to materials science to biology, the concentrations that coexist in separate phases at equilibrium have not been determined analytically, and numerical techniques are required that restrict the theory's ease of application. In this work, we derive an implicit analytical solution to the Flory-Huggins theory of one polymer in a solvent by applying a procedure that we call the implicit substitution method. While the solutions are implicit and in the form of composite variables, they can be mapped explicitly to a phase diagram in composition space. We apply the same formalism to multicomponent polymeric systems, where we find analytical solutions for polydisperse mixtures of polymers of one type. Finally, while complete analytical solutions are not possible for arbitrary mixtures, we propose computationally efficient strategies to map out coexistence curves for systems with many components of different polymer types.

We use atomistic simulations to examine the sliding dynamics of a skyrmion in a two-dimensional system containing a periodic one-dimensional stripe pattern of variations between low and high values of the perpendicular magnetic anisotropy. The skyrmion changes in size as it crosses the interface between two anisotropy regions. Upon applying combined dc and ac driving in either parallel or perpendicular directions, we observe a wide variety of Shapiro steps, Shapiro spikes, and phase-locking phenomena. The phase-locked orbits have two-dimensional dynamics due to the gyrotropic or Magnus dynamics of the skyrmions, and are distinct from the phase-locked orbits found for strictly overdamped systems. Along a given Shapiro step when the ac drive is perpendicular to the dc drive, the velocity parallel to the ac drive is locked while the velocity in the perpendicular direction increases with increasing drive to form Shapiro spikes. At the transition between adjacent Shapiro steps, the parallel velocity jumps up to the next step value, and the perpendicular velocity drops. The skyrmion Hall angle shows a series of spikes as a function of increasing dc drive, where the jumps correspond to the transition between different phase-locked steps. At high drives, the Shapiro steps and Shapiro spikes are lost. When both the ac and dc drives are parallel to the stripe periodicity direction, Shapiro steps appear, while if the dc drive is parallel to the stripe periodicity direction and the ac drive is perpendicular to the stripe periodicity, there are only two locked phases, and the skyrmion motion consists of a combination of sliding along the interfaces between the two anisotropy values and jumping across the interfaces., Comment: 12 pages, 10 figures

The fluid-fluid interface is a complex environment for a floating object where the statics and dynamics may be governed by capillarity, gravity, inertia, and other external body forces. Yet, the alignment of these forces in intricate ways might result in beautiful pattern formation and self-assembly of these objects, as in the case of bubble rafts or colloidal particles. While interfacial self-assembly has been explored widely, controlled manipulation of floating objects, e.g. drops, at the fluid-fluid interface still remains a challenge largely unexplored. In this work, we reveal the self-assembly and manipulation of water drops floating at an oil-air interface. We show that the assembly occurs due to electrostatic interactions between the drops and their environment. We highlight the role of the boundary surrounding the system by showing that even drops with a net zero electric charge can self-assemble under certain conditions. Using experiments and theory, we show that the depth of the oil bath plays an important role in setting the distance between the self-assembled drops. Furthermore, we demonstrate ways to manipulate the drops actively and passively at the interface.

We revisit the proposal that an energy transfer from dark energy into dark matter can be described in field theory by a first order phase transition. We analyze the model proposed in Ref. Abdalla et al. (2013), using updated constraints on the decay time of a metastable dark energy from the work of Ref. Shafieloo et al. (2018). The results of our analysis show no prospects for potentially observable signals that could distinguish this scenario. We also show that such model would not drive a complete transition to a dark matter dominated phase even in a distant future. Nevertheless, the model is not excluded by the latest data and we confirm that the mass of the dark matter particle that would result from such a process corresponds to an axion-like particle, which is currently one of the best motivated dark matter candidates. We argue that extensions to this model, possibly with additional couplings, still deserve further attention as it could provide an interesting and viable description for an interacting dark sector scenario based in a single scalar field., Comment: 14 pages, 2 figures

Ionic liquids (ILs) are an extremely exciting class of electrolytes for energy storage applications because of their unique combination of properties. Upon dissolving alkali metal salts, such as Li or Na based salts, with the same anion as the IL, an intrinsically asymmetric electrolyte can be created for use in batteries, known as a salt-in-ionic liquid (SiIL). These SiILs have been well studied in the bulk, where negative transference numbers of the alkali metal cation have been observed from the formation of small, negatively charged clusters. The properties of these SiILs at electrified interfaces, however, have received little to no attention. Here, we develop a theory for the electrical double layer (EDL) of SiILs where we consistently account for the thermoreversible association of ions into Cayley tree aggregates. The theory predicts that the IL cations first populate the EDL at negative voltages, as they are not strongly bound to the anions. However at large negative voltages which are strong enough to break the alkali metal cation-anion associations, these IL cations are exchanged for the alkali metal cation because of their higher charge density. At positive voltages, we find that the SiIL actually becomes $\textit{more aggregated while screening the electrode charge}$ from the formation of large, negatively charged aggregates. Therefore, in contrast to conventional intuition of associations in the EDL, SiILs appear to become more associated in certain electric fields. We present these theoretical predictions to be verified by molecular dynamics simulations and experimental measurements.

Our focus is on a specific type-N space-time that exhibits closed time-like curves in general relativity theory within the framework of Ricci-inverse gravity model. The matter-energy content is solely composed of a pure radiation field, and it adheres to the energy conditions while featuring a negative cosmological constant. One of the key findings in this investigation is the non-zero determinant of the Ricci tensor ($R_{\mu\nu}$), which implies the existence of an anti-curvature tensor ($A^{\mu\nu}$) and, as a consequence, an anti-curvature scalar ($A \neq R^{-1}$). Furthermore, we establish that this type-N space-time serves as a solution within modified gravity theories via the Ricci-inverse model, which involves adjustments to the cosmological constant ($\Lambda$) and the energy density ($\rho$) of the radiation field expressed in terms of a coupling constant. As a result, our findings suggest that causality violations remain possible within the framework of this Ricci-inverse gravity model, alongside the predictions of general relativity., Comment: 15 pages, accepted for publication in AOP

The minimum driving force strategy is applied to promote the exciton dissociation in organic solar cells (OSCs) without significant loss of open-circuit voltage. However, this strategy tends to promote F\"orster resonance energy transfer (FRET) from the donor to the acceptor (D-A), a consequence generally ignored until recently. In spite of the advances reported on this topic, the correlation between charge-transfer (CT) state binding energy and driving force remains unclear, especially in the presence of D-A FRET. To address this question, we employ a kinetic approach to model the charge separation in ten different D/A blends using non-fullerene acceptors. The model considers the influence of FRET on photoluminescence (PL) quenching efficiency. It successfully predicts the measured PL quenching efficiency for D or A photoexcitation in those blends, including the ones for which the D-A FRET process is relevant. Furthermore, the application of the model allows to quantifying the fractions of quenching loss associated with charge transfer and energy transfer. Fundamental relationships that controls the exciton dissociation was derived evidencing the key roles played by the Marcus inverted regime, exciton lifetime and mainly by the correlation between the driving force and binding energy of CT state. Based on those findings, we propose some strategies to maximize the quenching efficiency and minimize energy loss of OSCs in the presence of D-A FRET., Comment: 44 pages, 10 figures, 3 tables

One strategy to improve the photovoltaic properties of non-fullerene acceptors (NFAs) is the rational fluorination or chlorination of those molecules. Although this modification improves important acceptor properties, little is known about the effects on the triplet states. Here, we combine the polarizable continuum model with optimally tuned range-separated hybrid functional to investigate this issue. We find that fluorination or chlorination of NFAs decreases the degree of HOMO-LUMO overlap along these molecules. Consequently, the energy gap between $T_{1}$ and $S_{1}$ states, $\Delta E_{ST} = E_{S_{1}} - E_{T_{1}}$, also decreases. This effect simultaneously enhances the generation of triplet excitons and reduce the binding energy of the triplet excitons ($E_{b,T}$) which favor their dissociation into free charges. Interestingly, although Cl has a lower electronegativity than F, the chlorination is more effective to reduce $\Delta E_{ST}$. Since chlorination of NFAs is easier than fluorination, Cl substitution can be a useful approach to enhance solar energy harvesting using triplet excitons., Comment: 26 pages, 6 figures, 1 table

Half-Heusler systems host a plethora of different ground states, especially with non-trivial topology. However, there is still a lack of spectroscopic insight into the corresponding band inversion in this family. In this work, we locally explore the half-Heuslers Y$T$Bi ($T =$ Pt and Pd) by means of scanning tunneling microscopy/spectroscopy. From our analysis of the (120) surface plane, we infer that the increase of the spin--orbit coupling upon going from Pd to Pt is the main player in tuning the surface states from trivial to topologically non-trivial. Our measurements unveil a ($2 \times 1$) reconstruction of the (120) surface of both systems. Using density functional theory calculations, we show that the observed different behavior of the local density of states near the Fermi level in these two materials is directly related to the presence of metallic surface states. Our work sheds new light on a well known tunable family of materials and opens new routes to explore the presence of topological states of matter in half-Heusler systems and its microscopic observation., Comment: 16 pages, 12 figures. To appear in Phys. Rev. B

Using atomistic simulations, we have investigated the transport and annihilation of skyrmions interacting with a funnel array under a current applied perpendicular to the funnel axis. We find that transport without annihilation is possible at low currents, when the motion is dominated by skyrmion-skyrmion interactions and skyrmions push each other through the funnel opening. Skyrmion annihilation occurs for higher currents when skyrmions in the upper half of the sample exert pressure on skyrmions in the bottom half of the sample due to the external current. Upon interacting with the funnel wall, the skyrmions undergo a size reduction that makes it easier for them to pass through the funnel opening. We find five phases as a function of the applied current and the size of the funnel opening: (i) pinned, (ii) transport without annihilation, (iii) transport with annihilation, (iv) complete annihilation, and (v) a reentrant pinning phase that only occurs for very narrow openings. Our findings provide insight into how to control skyrmion transport using funnel arrays by delineating regimes in which transport of skyrmions is possible as well as the conditions under which annihilation occurs., Comment: 17 pages, 10 figures

In this paper, Ricci-inverse gravity is investigated. It is an alternative theory of gravity that introduces into the Einstein-Hilbert action an anti-curvature scalar that is obtained from the anti-curvature tensor which is the inverse of the Ricci tensor. An axially symmetric spacetime with causality violation is studied. Two classes of the model are discussed. Different sources of matter are considered. Then a direct relation between the content of matter and causality violation is shown. Our results confirm that Ricci-inverse gravity allows the existence of Closed Time-like Curves (CTCs) that lead to the violation of causality. Furthermore, a comparison is made between the results of general relativity and Ricci-inverse gravity. Other spacetimes, such as G\"{o}del and G\"{o}del-type universes, which are exact solutions of general relativity and allow for causality violations, are also explored in Ricci-inverse gravity framework., Comment: 11 pages, accepted for publication in EPJC

Using atomistic magnetic simulations we investigate the soliton motion along a pinned skyrmion chain containing an interstitial skyrmion. We find that the soliton can exhibit stable motion along the chain without a skyrmion Hall effect for an extended range of drives. Under a constant drive the solitons have a constant velocity. We also measure the skyrmion velocity-current curves and identify the signatures of different phases including a pinned phase, stable soliton motion, and quasi-free motion at higher drives where all of the skyrmions depin from the pinning centers and move along the rigid wall. In the quasi-free motion regime, the velocity is oscillatory due to the motion of the skyrmions over the pinning sites. For increasing pinning strength, the onset of soliton motion shifts to higher values of current density. We also find that for stronger pinning, the characteristic velocity-current shape is affected by the annihilation of single or multiple skyrmions in the drive interval over which the soliton motion occurs. Our results indicate that stable skyrmion soliton motion is possible and could be useful for technological applications., Comment: 19 pages, 7 figures

The Weyl semimetal CeAlGe is a promising material to study nontrivial topologies in real and momentum space due to the presence of a topological magnetic phase. Our results at ambient pressure show that the electronic properties of CeAlGe are extremely sensitive to small stoichiometric variations. In particular, the topological Hall effect (THE) present in CeAlGe is absent in some samples of almost identical chemical composition. The application of external pressure favors the antiferromagnetic ground state. It also induces a THE where it was not visible at ambient pressure. Furthermore, a small pressure is sufficient to drive the single region of the THE in magnetic fields into two different ones. Our results reveal an extreme sensitivity of the electronic properties of CeAlGe to tiny changes in its chemical composition, leading to a high tunability by external stimuli. We can relate this sensitivity to a shift in the Fermi level and to domain walls., Comment: Accepted in Physical Review Materials

We use atomistic-based simulations to investigate the behavior of ferromagnetic skyrmions being continuously compressed against a rigid wall under dc and ac drives. The compressed skyrmions can be annihilated close to the wall and form a conformal crystal with both a size and a density gradient, making it distinct from conformal crystals observed previously for superconducting vortices and colloidal particles. For both dc and ac driving, the skyrmions can move transverse to the compression direction due to a combination of density and size gradients. Forces in the compression direction are converted by the Magnus force into transverse motion. Under ac driving, the amount of skyrmion annihilation is reduced and we find a skyrmion Magnus ratchet pump. We also observe shear banding in which skyrmions near the wall move up to twice as fast as skyrmions further from the wall. When we vary the magnitude of the applied drive, we find a critical current above which the skyrmions are completely annihilated during a time scale that depends on the magnitude of the drive. By varying the magnetic parameters, we find that the transverse motion is strongly dependent on the skyrmion size. Smaller skyrmions are more rigid, which interferes with the size gradient and destroys the transverse motion. We also confirm the role of the size gradient by comparing our atomistic simulations with a particle-based model, where we find that the transverse motion is only transient. Our results are relevant for applications where skyrmions encounter repulsive magnetic walls, domain walls, or interfaces., Comment: 19 pages, 13 figures

In the ferromagnetic (FM) Weyl semimetal CeAlSi both space-inversion and time-reversal symmetries are broken. Our quantum oscillation (QO) data indicate that the FM ordering modifies the Fermi surface topology and also leads to an unusual drop in the QO amplitude. In the FM phase, we find a pressure-induced suppression of the anomalous and the loop Hall effects. This cannot be explained based on the electronic band structure or magnetic structure, both of which are nearly pressure independent. Instead, we show that a simplified model describing the scattering of Weyl fermions off FM domain walls can potentially explain the observed topological features. Our study highlights the importance of domain walls for understanding transport in FM Weyl semimetals., Comment: This article supersedes arXiv:2111.05742 including additional theoretical results. Published in Physical Review Research

The gap opening mechanism of a topological insulator, the quantum anomalous Hall effect and the axion physics are still pressing open questions and a microscopic viewpoint to further understand the role of magnetism in topology is highly desirable. In this work we have performed a microscopic investigation, by means of electron spin resonance (ESR) along with complementary bulk measurements, on the chalcogenide (Bi$_{1-x}$Gd$_{x}$)$_{2}$Se$_{3}$ ($x$ = 0, 0.001, 0.002 and 0.006). Our analysis of the Gd$^{3+}$ spin dynamics reveal no significant change of the Fermi surface as a function of Gd$^{3+}$ concentration, which indicates that the 4$f$ magnetism is different from the non-local effects induced by transition metals ($d$ electrons) substitutions. Additionally, we observe an unusual evolution of the Gd$^{3+}$ ESR spectra as a function of the applied magnetic field, which we discuss considering the magnetic interaction between Gd$^{3+}$ 4$f$ electrons and impurity centers such as Se vacancies. This interaction would give rise to a local weak antilocalization effect surrounding the Gd$^{3+}$ ions. Such mechanism is observable due to particular details of the Gd$^{3+}$ 4$f$ electrons magnetism in this system compared to $d$ electrons. Our work points out that rare earth substitutions in this model topological insulator is a promising path to explore the axion insulating systems., Comment: 10 pages, 3 figures. To appear in Phys. Rev. B

Using a particle based model, we investigate the skyrmion dynamical behavior in a channel where the upper wall contains divots of one depth and the lower wall contains divots of a different depth. Under an applied driving force, skyrmions in the channels move with a finite skyrmion Hall angle that deflects them toward the upper wall for $-x$ direction driving and the lower wall for $+x$ direction driving. When the upper divots have zero height, the skyrmions are deflected against the flat upper wall for $-x$ direction driving and the skyrmion velocity depends linearly on the drive. For $+x$ direction driving, the skyrmions are pushed against the lower divots and become trapped, giving reduced velocities and a nonlinear velocity-force response. When there are shallow divots on the upper wall and deep divots on the lower wall, skyrmions get trapped for both driving directions; however, due to the divot depth difference, skyrmions move more easily under $-x$ direction driving, and become strongly trapped for $+x$ direction driving. The preferred $-x$ direction motion produces what we call a Magnus diode effect since it vanishes in the limit of zero Magnus force, unlike the diode effects observed for asymmetric sawtooth potentials. We show that the transport curves can exhibit a series of jumps or dips, negative differential conductivity, and reentrant pinning due to collective trapping events. We also discuss how our results relate to recent continuum modeling on a similar skyrmion diode system., Comment: 14 pages, 9 figures

Using a particle-based model, we examine the collective dynamics of skyrmions interacting with a funnel potential under dc driving as the skyrmion density and relative strength of the Magnus and damping terms are varied. For driving in the easy direction, we find that increasing the skyrmion density reduces the average skyrmion velocity due to jamming of skyrmions near the funnel opening, while the Magnus force causes skyrmions to accumulate on one side of the funnel array. For driving in the hard direction, there is a critical skyrmion density below which the skyrmions become trapped. Above this critical value, a clogging effect appears with multiple depinning and repinning states where the skyrmions can rearrange into different clogged configurations, while at higher drives, the velocity-force curves become continuous. When skyrmions pile up near the funnel opening, the effective size of the opening is reduced and the passage of other skyrmions is blocked by the repulsive skyrmion-skyrmion interactions. We observe a strong diode effect in which the critical depinning force is higher and the velocity response is smaller for hard direction driving. As the ratio of Magnus force to dissipative term is varied, the skyrmion velocity varies in a non-linear and non-monotonic way due to the pile up of skyrmions on one side of the funnels. At high Magnus forces, the clogging effect for hard direction driving is diminished., Comment: 16 pages, 14 figures

Understanding the bulk and interfacial properties of super-concentrated electrolytes, such as ionic liquids (ILs), has attracted significant attention lately for their promising applications in supercapacitors and batteries. Recently, McEldrew \textit{et al.} developed a theory for reversible ion associations in bulk ILs, which accounted for the formation of all possible Cayley tree clusters and a percolating ionic network (gel). Here we adopt and develop this approach to understand the associations of ILs in the electrical double layer at electrified interfaces. With increasing charge of the electrode, the theory predicts a transition from a regime dominated by a gelled or clustered state to a regime dominated by free, crowded ions. This transition from gelation to crowding is conceptually similar to the overscreening to crowding transition.

In this paper we present a subgradient method with non-monotone line search for the minimization of convex functions with simple convex constraints. Different from the standard subgradient method with prefixed step sizes, the new method selects the step sizes in an adaptive way. Under mild conditions asymptotic convergence results and iteration-complexity bounds are obtained. Preliminary numerical results illustrate the relative efficiency of the proposed method.

The structure of polar liquids and electrolytic solutions, such as water and aqueous electrolytes, at interfaces underlies numerous phenomena in physics, chemistry, biology, and engineering. In this work, we develop a continuum theory that captures the essential features of dielectric screening by polar liquids at charged interfaces, including oscillations in charge and mass, starting from the molecular properties of the solvent. The theory predicts an anisotropic dielectric tensor of interfacial polar liquids previously studied in molecular dynamics simulations. We explore the effect of the interfacial polar liquid properties on the capacitance of the electrode/electrolyte interface and on hydration forces between two plane-parallel polarized surfaces. In the linear response approximation, we obtain simple formulas for the characteristic decay lengths of molecular and ionic profiles at the interface.

Using a particle-based model we examine the depinning motion of solitons in skyrmion chains in quasi-one dimensional (1D) and two-dimensional (2D) systems containing embedded 1D interfaces. The solitons take the form of a particle or hole in a commensurate chain of skyrmions. Under an applied drive, just above a critical depinning threshold the soliton moves with a skyrmion Hall angle of zero. For higher drives, the entire chain depins, and in a 2D system we observe that both the solitons and chain move at zero skyrmion Hall angle and then transition to a finite skyrmion Hall angle as the drive increases. In a 2D system with a 1D interface that is at an angle to the driving direction, there can be a reversal of the sign of the skyrmion Hall angle from positive to negative. Our results suggest that solitons in skyrmion systems could be used as information carriers in racetrack geometries that would avoid the drawbacks of finite skyrmion Hall angles. The soliton states become mobile at significantly lower drives than the depinning transition of the skyrmion chains themselves., Comment: 13 pages, 14 figures

This work reports on the temperature dependence of the electron spin resonance (ESR) of Gd$^{3+}$-doped SmB$_{6}$ single crystals at X- and Q-band microwave frequencies in different crystallographic directions. We found an anomalous inhomogeneous broadening of the Gd$^{3+}$ ESR linewidth ($\Delta H$) within 5.3 K $\leq T \leq$ 12.0 K which is attributed to slow crystalline electric field (CEF) fluctuations, slower than the timescale of the ESR microwave frequencies used ($\sim$10 GHz). This linewidth inhomogeneity may be associated to the coupling of the Gd$^{3+}$ $S$-states to the breathing mode of the SmB$_{6}$ cage, and can be simulated by a random distribution of the 4$^{th}$ CEF parameter, $b_4$, that strikingly takes negative and positive values. The temperature at which this inhomogeneity sets in, is related to the onset of a continuous insulator-to-metal phase transition. In addition, based on the interconfigurational fluctuation relaxation model, the observed exponential $T$-dependence of $\Delta H$ above $T\simeq$ 10 K gives rise to an excitation energy notably close to the hybridization gap of SmB$_{6}$ ($\Delta\simeq$ 60 K). This charge fluctuation scenario provides important ingredients to the physical properties of SmB$_{6}$. We finally discuss the interplay between charge and valence fluctuations under the view of slow CEF fluctuations in SmB$_{6}$ by coupling the Gd$^{3+}$ ions to the breathing phonon mode via a dynamic Jahn-Teller-like mechanism.

We theoretically investigate the optical bistability phenomenon in an ensemble of $N$ non-interacting three-level atoms trapped inside an optical cavity. The atoms are in a $\Lambda$-level configuration, where one atomic transition is coupled by a cavity mode, while the other one is coupled by a classical field. In addition, we consider a pumping field driving the cavity mode. With this system, we are able to observe new kinds of hysteresis, while scanning either the frequency of the pumping field or the Rabi frequency (intensity) of the control field. We show that they can be highly controllable via external parameters of the system, achieving very narrow widths, thus being very useful for building new devices, such as small fluctuation detectors in either frequency or intensity of laser fields., Comment: 9 pages, 9 figures

Colossal magnetoresistance (CMR) emerges from intertwined spin and charge degrees of freedom in the form of ferromagnetic clusters also known as trapped magnetic polarons. As a result, CMR is rarely observed in antiferromagnetic materials. Here we use electron spin resonance (ESR) to reveal microscopic evidence for the formation of magnetic polarons in antiferromagnetic Eu$_{5}$In$_{2}$Sb$_{6}$. First, we observe a reduction of the Eu$^{2+}$ ESR linewidth as a function of the applied magnetic field consistent with ferromagnetic clusters that are antiferromagnetically coupled. Additionally, the Eu$^{2+}$ lineshape changes markedly below T' ~ 200 K, a temperature scale that coincides with the onset of CMR. The combination of these two effects provide strong evidence that magnetic polarons grow in size below T' and start influencing the macroscopic properties of the system., Comment: 7 pages, 4 figures. Accepted in Phys. Rev. B

The polarization dependence of the near edge x-ray absorption spectroscopy (XANES) is an element specific probe to the real-space distribution of the density of unoccupied states in solid-state materials. In this paper, we present Fe and As $K$-edge experiments of Ba(Fe$_{1-x}$$M_{x}$)$_{2}$As$_{2}$ ($M=$ Mn, Co and $x=0.0$ and $0.08$). The experiments reveal a strong polarization dependence of the probed XANES spectra, which concerns mainly an increase of the intensity of electronic transitions when the beam polarization is set out of the sample's $ab$ crystallographic plane. The results show that states with $p_{z}$-orbital character dominate the density of unoccupied states close to the Fermi level. Partial substitution of Fe by Co is shown to decrease the intensity anisotropy, suggesting that Co promotes electronic transfer preferentially to states with $p_{z}$-orbital character. On the other hand, Mn substitution causes the increase of the spectra $p_{z}$-orbital anisotropy, which is proposed to take place by means of an enhanced local Fe $3d4p$ mixing, unveiling the role of Fe $4p$ states in the localization of the Fe $3d$ orbitals. Moreover, by comparing our results to previous experiments, we identify the relative mixing between Fe and the pnictide $4p_{x,y,z}$ orbitals as a clear divide between the electronic properties of iron arsenides and selenides. Our conclusions are supported by multiple-scattering theory calculations of the XANES spectra and by quantum chemistry calculations of Fe coordination electronic structure., Comment: 9 pages, 6 figures, 1 table

In the ferromagnetic Weyl semimetal CeAlSi both space-inversion and time-reversal symmetries are broken. We use external pressure as an effective tuning parameter and relate three observations to the presence of a nontrivial topology in its ferromagnetic regime: an exceptional temperature response of the quantum oscillations amplitude, the presence of an anomalous Hall effect (AHE), and the existence of an unusual loop Hall effect (LHE). We find a suppression of the AHE and the LHE with increasing pressure, while the Curie temperature is enhanced. The magnetic structure and the electronic bands exhibit only a negligible pressure effect suggesting the importance of the domain wall landscape for the topological behavior in CeAlSi.

The interplay between non-trivial topological states of matter and strong electronic correlations is one of the most compelling open questions in condensed matter physics. Due to experimental challenges, there is an increasing desire to find more microscopic techniques to complement the results of more traditional experiments. In this work, we locally explore the Kondo insulator Sm$_{1-x}$Gd$_{x}$B$_{6}$ by means of electron spin resonance (ESR) of Gd$^{3+}$ ions at low temperatures. Our analysis reveals that the Gd$^{3+}$ ESR line shape shows an anomalous evolution as a function of temperature, wherein for highly dilute samples (x $\approx$ 0.0002) the Gd$^{3+}$ ESR line shape changes from a localized ESR local moment character to a diffusive-like character. Upon manipulating the sample surface with a focused ion beam we demonstrate, in combination with electrical resistivity measurements, that the localized character of the Gd$^{3+}$ ESR line shape is recovered by increasing the penetration of the microwave in the sample. This provides compelling evidence for the contribution of surface or near-surface excitations to the relaxation mechanism in the Gd$^{3+}$ spin dynamics. Our work brings new insights into the importance of non-trivial surface excitations in ESR, opening new routes to be explored both theoretically and experimentally., Comment: 10 pages, 5 figures, to appear in Phys. Rev. Research

We show that the resistivity plateau of SmB$_6$ at low temperature, typically taken as a hallmark of its conducting surface state, can systematically be influenced by different surface treatments. We investigate the effect of inflicting an increasing number of hand-made scratches and microscopically defined focused ion beam-cut trenches on the surfaces of flux-grown Sm$_{1-x}$Gd$_x$B$_6$ with $x =$ 0, 0.0002. Both treatments increase the resistance of the low-temperature plateau, whereas the bulk resistance at higher temperature largely remains unaffected. Notably, the temperature at which the resistance deviates from the thermally activated behavior decreases with cumulative surface damage. These features are more pronounced for the focused ion beam treated samples, with the difference likely being related to the absence of microscopic defects like subsurface cracks. Therefore, our method presents a systematic way of controlling the surface conductance., Comment: 7 pages,5 figures

Using a particle-based model, we simulate the behavior of a skyrmion under the influence of asymmetric funnel geometries and ac driving at zero temperature. We specifically investigate possibilities for controlling the skyrmion motion by harnessing a ratchet effect. Our results show that as the amplitude of a unidirectional ac drive is increased, the skyrmion can be set into motion along either the easy or hard direction of the funnel depending on the ac driving direction. When the ac drive is parallel to the funnel axis, the skyrmion flows in the easy direction and its average velocity is quantized. In contrast, when the ac drive is perpendicular to the funnel axis, a Magnus-induced ratchet effect occurs, and the skyrmion moves along the hard direction with a constant average velocity. For biharmonic ac driving of equal amplitude along both the parallel and perpendicular directions, we observe a reentrant pinning phase where the skyrmion ratchet vanishes. For asymmetric biharmonic ac drives, the skyrmion exhibits a combination of effects and can move in either the easy or hard direction depending on the configuration of the ac drives. These results indicate that it is possible to achieve controlled skyrmion motion using funnel geometries, and we discuss ways in which this could be harnessed to perform data transfer operations., Comment: 11 pages, 19 figures

We numerically examine a single skyrmion dynamics under the influence of triangular and honeycomb obstacle arrays at zero temperature. The skyrmion Hall angle $\theta_{sk}$, that is the angle between the applied external drive and the direction of the skyrmion motion, increases in quantized steps or continuously as a function of the applied drive. For the obstacle arrays studied in this work, the skyrmion exhibits two main directional locking effects, where the skyrmion motion locks with $\theta_{sk}=-30^\circ$ and $-60^\circ$. We show that these directions are privileged due to the obstacle landscape symmetry, where there are channels that the skyrmion may move with less or no obstacle collisions. Besides that, the skyrmion Hall angles can be modified by changing the obstacle density in the sample, where some dynamic phases may appear, vanish or be stimulated. This interesting behavior can be useful to guide skyrmions using regions with different obstacle densities to set the skyrmion into designed trajectories. We have also investigated for fixed obstacle densities how the phases with $\theta_{sk}=-30^\circ$ and $-60^\circ$ evolve as a function of the Magnus force, where possibilities for switching between these phases and topological selection is discussed., Comment: 19 pages, 12 figures

The electrostatic screening of charge in one-dimensional confinement leads to long-range breakdown in electroneutrality within a nanopore. Through a series of continuum simulations, we demonstrate the principles of electroneutrality breakdown for electrolytes in one dimensional confinement. We show how interacting pores in a membrane can counteract the phenomenon of electroneutrality breakdown, eventually returning to electroneutrality. Emphasis is placed on applying simplifying formulas to reduce the multidimensional partial differential equations into a single ordinary differential equation for the electrostatic potential. Dielectric mismatch between the electrolyte and membrane, pore aspect ratio, and confinement dimensionality are studied independently, outlining the relevance of electroneutrality breakdown in nanoporous membranes for selective ion transport and separations.