arXiv subjects

Charles Reichhardt

Publications and source records attributed to Charles Reichhardt.

At least 19 recordsLinked to original sources

arXiv2026

Planar Interfaces for Transmission of Chiral Spin Textures

Lateral magnetic interfaces provide a direct way to test whether skyrmions remain robust when driven across abrupt changes in material parameters and magnetic order. Here we study skyrmion transmission across planar ferromagnet-ferromagnet (FM-FM), antiferromagnet-antiferromagnet (AFM-AFM), ferromagnet-antiferromagnet (FM-AFM), and antiferromagnet-ferromagnet (AFM-FM) interfaces using micromagnetic simulations and analytic reduced-coordinate criteria. The outcomes are organized into phase diagrams according to the morphology formed in the receiving region, distinguishing compact transmission from deformed skyrmions, stripe-domain states, amorphous textures, and relaxation into the background. Same-order FM-FM and AFM-AFM skyrmion transmission is captured by an analytically defined range of the reduced Dzyaloshinskii-Moriya interaction, identifying the wall-softening regime that supports compact transmission without stripe formation. Mixed-order FM-AFM and AFM-FM interfaces are directionally distinct, requiring conversion between ferromagnetic magnetization and antiferromagnetic N\'eel textures. These results show that planar interfaces act as active transport elements and provide reduced design criteria for heterogeneous skyrmion tracks.

cond-mat.mes-hall↗

arXiv2025

Janus skyrmion: Interfacial quasiparticle with two-faced helicity

Janus particles are functional particles with at least two surfaces showing asymmetric properties. We show at the interface between two magnetic regions with different antisymmetric exchange interactions, an alternative species of two-dimensional topological quasiparticles can emerge, in which different helicity structures can coexist. We name such an interfacial quasiparticle a "Janus skyrmion," in analogy to the Janus particle. As the Janus skyrmion shows helicity asymmetry, its size could vary with both the in-plane and out-of-plane magnetic fields. A vertical spin current could drive the Janus skyrmion into one-dimensional motion along the interface without showing the skyrmion Hall effect, at a speed which depends on both the in-plane spin-polarization direction and current density. Thermal fluctuations could also lead to one-dimensional random walk of a Brownian Janus skyrmion. This work uncovers unique dynamics intrinsic to interfacial quasiparticles with exotic helicity, which may be realized in interface-engineered magnetic layers.

cond-mat.mes-hall↗

arXiv2025

Diffusion asymmetry of repulsive skyrmions in structured environment

The diffusion of matter in structured environments can give rise to emergent phenomena that do not occur in unstructured environments. Here, we report the asymmetric diffusion of magnetic skyrmions in structured chambers, where repulsive skyrmion-skyrmion and skyrmion-environment interactions play vital roles in their diffusive behavior. By fabricating an off-center asymmetric gate separating two chambers, skyrmions could demonstrate asymmetric diffusion through the gate, which depends on the gate symmetry, the gate opening width, and the skyrmion density. Although the diffusion is affected by the skyrmion density, the simulation outcomes are generally in line with rate equation solutions assuming time-independent diffusion rates. Diffusive skyrmions within the chamber can transiently form bonds through repulsive skyrmion-skyrmion interactions, leading to emergent rotational dynamics that is unique to interacting skyrmion systems. Our results uncover asymmetric diffusive behavior of skyrmions, offering insights that will appeal to the wide audience interested in magnetism, active matter, and statistical physics.

cond-mat.mes-hall↗

arXiv2023

Resistance to Critiques in the Academic Literature: An Example from Physics Education Research

Research framed around issues of diversity and representation is often controversial. The question of what constitutes a valid critique of such research, or the appropriate manner of airing such a critique, thus has a heavy ideological and political subtext. Here we outline an attempt to comment on a paper recently published in the research journal $\mathit{Physical}$ $\mathit{Review}$ - $\mathit{Physics}$ $\mathit{Education}$ $\mathit{Research}$ (PRPER). The article in question claimed to find evidence of 'whiteness' in introductory physics from analysis of a six-minute video. We argue that even if one accepts the rather tenuous proposition that 'whiteness' is sufficiently well defined to observe, the study lacks the proper controls, checks, and methodology to allow for confirmation or disconfirmation of the authors' interpretation of the data. The authors of the whiteness study, however, make the stunning claim that their study cannot be judged by standards common in science. We summarize our written critique and its fate, along with a brief description of its genesis as a response to an article in which senior officers of the American Physical Society (which publishes PRPER) explained that the appropriate venue for addressing issues with the paper at hand is via normal editorial processes.

physics.ed-ph↗

arXiv2021

Visualizing the strongly reshaped skyrmion Hall effect in multilayer wire devices

Magnetic skyrmions are nanoscale spin textures touted as next-generation computing elements. When subjected to lateral currents, skyrmions move at considerable speeds. Their topological charge results in an additional transverse deflection known as the skyrmion Hall effect (SkHE). While promising, their dynamic phenomenology with current, skyrmion size, geometric effects and disorder remain to be established. Here we report on the ensemble dynamics of individual skyrmions forming dense arrays in Pt/Co/MgO wires by examining over 20,000 instances of motion across currents and fields. The skyrmion speed reaches 24 m/s in the plastic flow regime and is surprisingly robust to positional and size variations. Meanwhile, the SkHE saturates at $\sim 22^\circ$, is substantially reshaped by the wire edge, and crucially increases weakly with skyrmion size. Particle model simulations suggest that the SkHE size dependence - contrary to analytical predictions - arises from the interplay of intrinsic and pinning-driven effects. These results establish a robust framework to harness SkHE and achieve high-throughput skyrmion motion in wire devices.

cond-mat.mes-hall↗

arXiv2019

Ice Rule and Emergent Frustration in Particle Ice and Beyond

Geometric frustration and the ice rule are two concepts that are intimately connected and widespread across condensed matter. The first refers to the inability of a system to satisfy competing interactions in the presence of spatial constraints. The second, in its more general sense, represents a prescription for the minimization of the topological charges in a constrained system. Both can lead to manifolds of high susceptibility and non-trivial, constrained disorder where exotic behaviors can appear and even be designed deliberately. In this Colloquium, we describe the emergence of geometric frustration and the ice rule in soft condensed matter. This Review excludes the extensive developments of mathematical physics within the field of geometric frustration, but rather focuses on systems of confined micro- or mesoscopic particles that emerge as a novel paradigm exhibiting spin degrees of freedom. In such systems, geometric frustration can be engineered artificially by controlling the spatial topology and geometry of the lattice, the position of the individual particle units, or their relative filling fraction. These capabilities enable the creation of novel and exotic phases of matter, and also potentially lead towards technological applications related to memory and logic devices that are based on the motion of topological defects. We review the rapid progress in theory and experiments and discuss the intimate physical connections with other frustrated systems at different length scales.

cond-mat.soft↗

arXiv2018

Ice Rule Fragility via Topological Charge Transfer in Artificial Colloidal Ice

Artificial particle ices are model systems of constrained, interacting particles. They have been introduced theoretically to study ice-manifolds emergent from frustration, along with domain wall and grain boundary dynamics, doping, pinning-depinning, controlled transport of topological defects, avalanches, and memory effects. Recently such particle-based ices have been experimentally realized with vortices in nano-patterned superconductors or gravitationally trapped colloids. Here we demonstrate that, although these ices are generally considered equivalent to magnetic spin ices, they can access a novel spectrum of phenomenologies that are inaccessible to the latter. With experiments, theory and simulations we demonstrate that in mixed coordination geometries, entropy-driven negative monopoles spontaneously appear at a density determined by the vertex-mixture ratio. Unlike its spin-based analogue, the colloidal system displays a "fragile ice" manifold, where local energetics oppose the ice rule, which is instead enforced through conservation of the global topological charge. The fragile colloidal ice, stabilized by topology, can be spontaneously broken by topological charge transfer.

cond-mat.stat-mech↗

arXiv2018

Skyrmion Topological Hall Effect near Room Temperature

Magnetic skyrmions are stable nanosized spin structures that can be displaced at low electrical current densities. Because of these properties, they have been proposed as building blocks of future electronic devices with unprecedentedly high information density and low energy consumption. The electrical detection of skyrmions via the Topological Hall Effect (THE), has so far been demonstrated only at cryogenic temperatures. Here, we report the observation of a skyrmion Topological Hall Effect near room temperature (276 K) in a mesoscopic lamella of FeGe. This region unambiguously coincides with the skyrmion lattice location revealed by neutron scattering. We provide clear evidence of a reentrant helicoid magnetic phase adjacent to the skyrmion phase, and discuss the large THE amplitude (5 n$Ω$.cm) in view of the ordinary Hall Effect.

cond-mat.mes-hall↗

arXiv2016

Active Particles in Complex and Crowded Environments

Differently from passive Brownian particles, active particles, also known as self-propelled Brownian particles or microswimmers and nanoswimmers, are capable of taking up energy from their environment and converting it into directed motion. Because of this constant flow of energy, their behavior can only be explained and understood within the framework of nonequilibrium physics. In the biological realm, many cells perform directed motion, for example, as a way to browse for nutrients or to avoid toxins. Inspired by these motile microorganisms, researchers have been developing artificial particles that feature similar swimming behaviors based on different mechanisms; these manmade micro- and nanomachines hold a great potential as autonomous agents for healthcare, sustainability, and security applications. With a focus on the basic physical features of the interactions of self-propelled Brownian particles with a crowded and complex environment, this comprehensive review will put the reader at the very forefront of the field, providing a guided tour through its basic principles, the development of artificial self-propelling micro- and nanoparticles, and their application to the study of nonequilibrium phenomena, as well as the open challenges that the field is currently facing.

cond-mat.soft↗

arXiv2014

AC current generation in chiral magnetic insulators and skyrmion motion induced by the spin Seebeck effect

We show that a temperature gradient induces an ac electric current in multiferroic insulators when the sample is embedded in a circuit. We also show that a thermal gradient can be used to move magnetic skyrmions in insulating chiral magnets: the induced magnon flow from the hot to the cold region drives the skyrmions in the opposite direction via a magnonic spin transfer torque. Both results are combined to compute the effect of skyrmion motion on the ac current generation and demonstrate that skyrmions in insulators are a promising route for spin caloritronics applications.

cond-mat.mtrl-sci↗

arXiv2014

Dynamics of skyrmions in chiral magnets: dynamic phase transitions and equation of motion

We study the dynamics of skyrmions in a metallic chiral magnet. First we show that skyrmions can be created dynamically by destabilizing the ferromagnetic background state through a spin polarized current. We then treat skyrmions as rigid particles and derive the corresponding equation of motion. The dynamics of skyrmions is dominated by the Magnus force, which accounts for the weak pinning of skyrmions observed in experiments. Finally we discuss the quantum motion of skyrmions.

cond-mat.mtrl-sci↗

arXiv2013

Realizing three-dimensional artificial spin ice by stacking planar nanoarrays

Artificial spin ice is a frustrated magnetic two-dimensional nano-material, recently employed to study variety of tailor-designed unusual collective behaviours. Recently proposed extensions to three dimensions are based on self-assembly techniques and allow little control over geometry and disorder. We present a viable design for the realization of a three-dimensional artificial spin ice with the same level of precision and control allowed by lithographic nano-fabrication of the popular two-dimensional case. Our geometry is based on layering already available two-dimensional artificial spin ice and leads to an arrangement of ice-rule-frustrated units which is topologically equivalent to that of the tetrahedra in a pyrochlore lattice. Consequently, we show, it exhibits a genuine ice phase and its excitations are, as in natural spin ice materials, magnetic monopoles interacting via Coulomb law.

cond-mat.mes-hall↗

arXiv2013

Plastic response by dislocation glide in solid helium under dc strain rate loading

We develop a model for the gliding of dislocations and plasticity in solid He-4. This model takes into account the Peierls barrier, multiplication and interaction of dislocations, as well as classical thermally and mechanically activated processes leading to dislocation glide. We specifically examine the dc stress-strain curve and how it is affected by temperature, strain rate, and dislocation density. As a function of temperature and shear strain, we observe plastic deformation and discuss how this may be related to the experimental observation of elastic anomalies in solid hcp He-4 that have been discussed in connection with the possibility of supersolidity or giant plasticity. Our theory gives several predictions for the dc stress strain curves, for example, the yield point and the change in the work-hardening rate and plastic dissipation peak, that can be compared directly to constant strain rate experiments and thus provide bounds on model parameters.

cond-mat.mtrl-sci↗

arXiv2013

Particle model for skyrmions in metallic chiral magnets: dynamics, pinning and creep

Recently spin textures called skyrmions have been discovered in certain chiral magnetic materials without spatial inversion symmetry, and have attracted enormous attention due to their promising application in spintronics since only a low applied current is necessary to drive their motion. When a conduction electron moves around the skyrmion, its spin is fully polarized by the spin texture and acquires a quantized phase; thus, the skyrmion yields an emergent electrodynamics that in turn determines skyrmion motion and gives rise to a finite Hall angle. While Skyrmions behave as particles, no particle level description of their dynamics exists to date. Such a model would have tremendous impact on understanding skyrmion dynamics by theoretical analysis and computational modeling. Here we derive the equation of motion from a microscopic continuum model and obtain the short-range interaction between skyrmions, and the interaction between skyrmions and defects. Skyrmions also experience a Magnus force perpendicular to their velocity due to the underlying emergent electromagnetic field. We validate the equation of motion by studying the depinning transition using both the particle and the continuum models. By using the particle description, we explain the recent experimental observations of the rotation of a skyrmion lattice in the presence of a temperature gradient. We also predict quantum and thermal creep motion of skyrmions in the pinning potential.

cond-mat.str-el↗

arXiv2013

Manipulation of skyrmions in nanodisks with a current pulse and skyrmion rectifier

A skyrmion in a nanosized disk of a chiral magnet can be used as a bit of information. To this end, it is desirable to control the creation and removal of a skyrmion only by currents without using external magnetic fields. Here we propose to create a skyrmion by applying a current pulse to a nanodisk. The skyrmion can be removed from the disk by applying a dc current. We show that the dynamics of the created skyrmion can lead to a rectification effect, in which a dc voltage is generated by the motion of skyrmion in the presence of an ac current.

cond-mat.mes-hall↗

arXiv2013

Driven skyrmions and dynamical transitions in chiral magnets

We study the dynamics of skyrmions in chiral magnets in the presence of a spin polarized current. The motion of skyrmions in the ferromagnetic background excites spin waves and contributes to additional damping. At a large current, the spin wave spectrum becomes gapless and skyrmions are created dynamically from the ferromagnetic state. At an even higher current, these skyrmions are strongly deformed due to the damping and become unstable at a threshold current, leading to a chiral liquid. We show how skyrmions can be created by increasing the current in the magnetic spiral state. We then construct a dynamic phase diagram for a chiral magnet with a current. The instability transitions between different states can be observed as experimentally clear signatures in the transport measurements, such as jumps and hysteresis.

cond-mat.str-el↗

arXiv2013

Stabilizing fractional vortices in multiband superconductors with periodic pinning arrays

Multiband superconductors support the excitation of vortices with fractional quantum flux, known as fractional vortices. In the ground state, the fractional vortices in different bands bond together to form a composite vortex with the standard flux quantum $Φ_0$; thus, it is difficult to stabilize the fractional vortices. Here we show that fractional vortex lattices can be stabilized in multiband superconductors with a periodic pinning array at the half matching field of the composite system, when full matching of the fractionalized vortices occurs. In the presence of a high current, the fractional vortices in different bands decouple and move at different velocities. When the current is turned off suddenly, the fractional vortices in different bands may be trapped at different pinning sites. This system also exhibits rich dynamic behavior, and for fractional vortices in two-band superconductors we find a phases where the vortices one band are pinned and the vortices in the other band are moving. These different phases can be observed in transport measurements, and produce a new type of self-induced Shapiro steps that can arise under the application of only a dc drive.

cond-mat.supr-con↗

arXiv2013

Onset of Irreversibility and Chaos in Amorphous Solids Under Periodic Shear

An important aspect of the physics of amorphous solids is the onset of irreversible behavior usually associated with yield. Here we study amorphous solids under periodic shear using quasi-static molecular dynamics simulations and observe a transition from reversible to irreversible deformation at a critical strain amplitude. We find that for small strain amplitudes the system exhibits a noisy but repetitive limit-cycle, similar to return point memory \cite{sethna1993hysteresis}. However, for large strain amplitudes the behavior becomes chaotic (shows sensitivity to initial conditions) and thus irreversible. We show that the chaotic behavior is a result of the shear band instabilities that arise for large strains and the convective displacement fields they create.

cond-mat.soft↗