arXiv subjects

Clécio C. de Souza Silva

Publications and source records attributed to Clécio C. de Souza Silva.

18 recordsLinked to original sources

arXiv2025

Many-body skyrmion interactions in helimagnets

Many-body interactions strongly influence the structure, stability, and dynamics of condensed-matter systems, from atomic lattices to interacting quasi-particles such as superconducting vortices. Here, we investigate theoretically the pairwise and many-body interaction terms among skyrmions in helimagnets, considering both the ferromagnetic and conical spin backgrounds. Using micromagnetic simulations, we separate the exchange, Dzyaloshinskii-Moriya, and Zeeman contributions to the skyrmion-skyrmion pair potential, and show that the binding energy of skyrmions within the conical phase depends strongly on the film thickness. For small skyrmion clusters in the conical phase, three-body interactions make a substantial contribution to the cohesive energy, comparable to that of pairwise terms, while four-body terms become relevant only at small magnetic fields. As the system approaches the ferromagnetic phase, these higher-order contributions vanish, and the interactions become essentially pairwise. Our results indicate that realistic models of skyrmion interactions in helimagnets in the conical phase must incorporate many-body terms to accurately capture the behavior of skyrmion crystals and guide strategies for controlling skyrmion phases and dynamics.

cond-mat.mes-hall↗

arXiv2024

Polarization and dynamic phases of aligning active matter in periodic obstacle arrays

We numerically examine a system of monodisperse self-propelled particles interacting with each other via simple steric forces and aligning torques moving through a periodic array of obstacles. Without obstacles, this system shows a transition to a polarized or aligned state for critical alignment parameters. In the presence of obstacles, there is still a polarization transition, but for dense enough arrays, the polarization is locked to the symmetry directions of the substrate. When the obstacle array is made anisotropic, at low densities the particles can form a quasi-isotropic state where the system can be polarized in any of the dominant symmetry directions. For intermediate anisotropy, the particles self-organize into a coherent lane state with one-dimensional polarization. In this phase, a small number of highly packed lanes are adjacent to less dense lanes that have the same polarization, but lanes further away can have the opposite polarization, so that global polarization is lost. For the highest anisotropy, hopping between lanes is suppressed, and the system forms uniformly dense uncoupled but polarized lanes.

cond-mat.soft↗

arXiv2023

Polar order, shear banding, and clustering in confined active matter

We investigate the collective behavior of sterically interacting self-propelled particles confined in a harmonic potential. Our theoretical and numerical study unveils the emergence of distinctive collective polar organizations, revealing how different levels of interparticle torques and noise influence the system. The observed phases include the shear-banded vortex, where the system self organizes in two concentric bands rotating in opposite directions around the potential center; the uniform vortex, where the two bands merge into a close packed configurations rotating uniformly as a quasi-rigid body; and the orbiting polar state, characterized by parallel orientation vectors and the cluster revolving around the potential center, without rotation, as a rigid body. Intriguingly, at lower filling fractions, the vortex and polar phases merge into a single phase where the trapped cluster breaks into smaller polarized clusters, each one orbiting the potential center as a rigid body.

cond-mat.soft↗

arXiv2023

Towards Magnonic Logic and Neuromorphic Computing: Controlling Spin-Waves by Spin-Polarized Current

Spin-waves (magnons) are among the prime candidates for building fast yet energy-efficient platforms for information transport and computing. We here demonstrate theoretically and in state-of-the-art micromagnetic simulation the effects that strategically-injected spin-polarized current can have on controlling magnonic transport. We reveal analytically that the Zhang-Li spin-transfer-torque induced by applied current is analogous to the Dzyaloshinskii-Moriya interaction for scattering the magnons in the linear regime, to then provide a generalized Snell's law that describes the spin-wave propagation across regions with different current densities. We validate the latter in numerical simulations of realistic systems, and exemplify how these findings may help advance the design of spin-wave logic and neuromorphic computing devices.

cond-mat.mtrl-sci↗

arXiv2021

Mesoscale phase separation of skyrmion-vortex matter in chiral magnet-superconductor heterostructures

We investigate theoretically the equilibrium configurations of many magnetic skyrmions interacting with many superconducting vortices in a superconductor-chiral magnet bilayer. We show that miscible mixtures of vortices and skyrmions in this system breaks down at a particular wave number for sufficiently strong coupling, giving place to remarkably diverse mesoscale patterns: gel, stripes, clusters, intercalated stripes and composite gel-cluster structures. We also demonstrate that, by appropriate choice of parameters, one can thermally tune between the homogeneous and density-modulated phases.

cond-mat.supr-con↗

arXiv2021

Tailoring High-Frequency Magnonics in Monolayer Chromium Trihalides

Monolayer chromium trihalides, the archetypal two dimensional (2D) magnetic materials, are readily suggested as a promising platform for high frequency magnonics. Here we detail the spin wave properties of monolayer CrBr$_3$ and CrI$_3$, using spin dynamics simulations parametrized from the first principles. We reveal that spin wave dispersion can be tuned in a broad range of frequencies by strain, paving the way towards flexo magnonic applications. We further show that ever present halide vacancies in these monolayers host sufficiently strong Dzyaloshinskii Moriya interaction to scatter spin waves, which promotes design of spin-wave guides by defect engineering. Finally we discuss the spectra of spin-waves propagating across a moiré periodic modulation of magnetic parameters in a van der Waals heterobilayer, and show that the nanoscale moiré periodicities in such samples are ideal for realization of a magnonic crystal in the terahertz frequency range. Recalling the additional tunability of magnetic 2D materials by electronic gating, our results situate these systems among the front-runners for prospective high frequency magnonic applications.

physics.comp-ph↗

arXiv2020

Spin textures in chiral magnetic monolayers with suppressed nearest-neighbor exchange

High tunability of two dimensional magnetic materials (by strain, gating, heterostructuring or otherwise) provides unique conditions for studying versatile magnetic properties and controlling emergent magnetic phases. Expanding the scope of achievable magnetic phenomena in such materials is important for both fundamental and technological advances. Here we perform atomistic spin-dynamics simulations to explore the (chiral) magnetic phases of atomic monolayers in the limit of suppressed first-neighbors exchange interaction. We report the rich phase diagram of exotic magnetic configurations, obtained for both square and honeycomb lattice symmetries, comprising coexistence of ferromagnetic and antiferromagnetic spin-cycloids, as well as multiple types of magnetic skyrmions. We perform a minimum-energy path analysis for the skyrmion collapse to evaluate the stability of such topological objects, and reveal that magnetic monolayers could be good candidates to host the antiferromagnetic skyrmions that are experimentally evasive to date.

physics.comp-ph↗

arXiv2019

Manipulation of Magnetic Skyrmions by Superconducting Vortices in Ferromagnet-Superconductor Heterostructures

Dynamics of magnetic skyrmions in hybrid ferromagnetic films harbors novel physical phenomena and holds promise for technological applications. In this work, we discuss the behavior of magnetic skyrmions when coupled to superconducting vortices in a ferromagnet-superconductor heterostructure. We use numerical simulations and analytic arguments to reveal broader possibilities for manipulating the skyrmion-vortex dynamic correlations in the hybrid system, that are not possible in its separated constituents. We explore the thresholds of particular dynamic phases, and quantify the phase diagram as a function of the relevant material parameters, applied current and induced magnetic torques. Finally, we demonstrate the broad and precise tunability of the skyrmion Hall-angle in presence of vortices, with respect to currents applied to either or both the superconductor and the ferromagnet within the heterostructure.

cond-mat.mtrl-sci↗

arXiv2018

Deflection of (anti)ferromagnetic skyrmions at heterochiral interfaces

Devising magnetic nanostructures with spatially heterogeneous Dzyaloshinskii-Moriya interaction (DMI) is a promising pathway towards advanced confinement and control of magnetic skyrmions in potential devices. Here we discuss theoretically how a skyrmion interacts with a heterochiral interface using micromagnetic simulations and analytic arguments. We show that a heterochiral interface deflects the trajectory of ferromagnetic (FM) skyrmions, and that the extent of such deflection is tuned by the applied spin-polarized current and the difference in DMI across the interface. Further, we show that this deflection is characteristic for the FM skyrmion, and is completely absent in the antiferromagnetic (AFM) case. In turn, we reveal that the AFM skyrmion achieves much higher velocities than its FM counterpart, yet experiences far stronger confinement in nanoengineered heterochiral tracks, which reinforces AFM skyrmions as a favorable choice for skyrmion-based devices.

cond-mat.mes-hall↗

arXiv2017

Conformal Vortex Crystals

We investigate theoretically globally nonuniform configurations of quantized-flux vortices in clean superconductors trapped by an external force field that induces a nonuniform vortex density profile. Using an extensive series of numerical simulations, we demonstrate that, for suitable choices of the force field, and bellow a certain transition temperature, the vortex system self-organizes into highly inhomogeneous conformal crystals in a way as to minimize the total energy. These nonuniform structures are topologically ordered and can be mathematically mapped into a triangular Abrikosov lattice via a conformal transformation. Above the crystallization temperature, the conformal vortex crystal becomes unstable and gives place to a nonuniform polycrystalline structure. We propose a simple method to engineer the potential energy profile necessary for the observation of conformal crystals of vortices, which can also be applied to other 2D particle systems, and suggest possible experiments in which conformal or quasi-conformal vortex crystals could be observed in bulk superconductors and in thin films.

cond-mat.supr-con↗

arXiv2017

Stick-slip Phenomena and Memory Effects in Moving Vortex Matter

Manipulating vortices in non-conventional superconductors is nowadays a challenging path toward controlling functionalities for superconducting nanodevices. Here, we directly observe and control single vortex core trajectories with unmatched resolution using a new scanning tunneling spectroscopy at very low temperature. Our data show the depinning threshold of a Bragg-glass in a weakly disordered superconductor, a clean 2H-NbSe2 crystal. We first experimentally capture the linear and collective response, the Campbell regime. Upon strong drives, the oscillating trajectories perform a series of stick-slip motions that mimics the lattice periodicity. We then theoretically elucidate this peculiar non-linear regime by solving the Langevin dynamics equations. We additionally explore the impact of initial conditions and reveal an enhancement of the long-range correlations with the cooling procedure. Finally, our work establishes a connection between theory of vortex pinning, memory effects and vortex lineshapes, thus offering a new platform to investigate the relationship between viscous media and individual controllable objects in any many-body systems.

cond-mat.supr-con↗

arXiv2011

Vortex density waves and negative absolute resistance in patterned superconductors

We study theoretically dynamical phases of vortices in superconducting films with arrays of obstacles. By performing a series of molecular dynamics simulations and analytical calculations, we demonstrate the existence of a phase of soliton-like vortex-density waves appearing in a wide range of parameters. These waves are formed by a self-assembled phase separation process induced by strongly nonlinear density fluctuations of the moving vortex matter above a certain critical driving current. At high vortex concentrations, the waves move at an approximately current-independent speed resulting in a wide plateau in the voltage-current characteristics. At stronger drives, the vortex system enters into a fully jammed (zero-voltage) phase. By combining ac and dc drives, the interplay between the vortex-density-wave and jammed phases leads to the observation of negative absolute mobility of vortices, which induces the superconducting film into a negative resistance state.

cond-mat.supr-con↗

arXiv2009

Vortex-Antivortex annihilation dynamics in a square mesoscopic superconducting cylinder

The dynamics of the annihilation of a vortex-antivortex pair is investigated. The pair is activated magnetically during the run of a simulated hysteresis loop on a square mesoscopic superconducting cylinder with an antidot inserted at its center. We study the nucleation of vortices and antivortices by first increasing the magnetic field, applied parallel to the axis of the sample, from zero until the first vortex is created. A further increase of the field pulls the vortex in, until it reaches the antidot. As the polarity of the field is reversed, an antivortex enters the scene, travels toward the center of the sample and eventually the pair is annihilated. Depending on the sample size, its temperature, and Ginzburg-Landau parameter, the vortex-antivortex encounter takes place at the antidot or at the superconducting sea around it. The position and velocity of the vortex and antivortex singularities were evaluated as a function of time. The current density, magnetization and order parameter topology were also calculated.

cond-mat.supr-con↗

arXiv2008

Reversible Transport of Interacting Brownian Ratchets

The transport of interacting Brownian particles in a periodic asymmetric (ratchet) substrate is studied numerically. In a zero-temperature regime, the system behaves as a reversible step motor, undergoing multiple sign reversals of the particle current as any of the following parameters are varied: the pinning potential parameters, the particle occupation number, and the excitation amplitude. The reversals are induced by successive changes in the symmetry of the effective ratchet potential produced by the substrate and the fraction of particles which are effectively pinned. At high temperatures and low frequencies, thermal noise assists delocalization of the pinned particles, rendering the system to recover net motion along the gentler direction of the substrate potential. The joint effect of high temperature and high frequency, on the other hand, induces an additional current inversion, this time favoring motion along the direction where the ratchet potential is steeper. The dependence of these properties on the ratchet parameters and particle density is analyzed in detail.

cond-mat.stat-mech↗

arXiv2008

Dynamics of driven vortex-antivortex matter in superconducting films with a magnetic dipole array

We investigate theoretically vortex-antivortex (v-av) matter moving in thin superconducting films with a regular array of in-plane magnetic dipoles. Our model considers v-av pair creation induced by the local current density generated by the magnetic texture and the transport current and simulates the dynamics of vortices and antivortices by numerical integration of the Langevin equation of motion. Calculations of the transport properties at zero applied field show a strong dependence of the v-av dynamics on the current intensity and direction. The dynamics of the v-av matter is characterized by a series of creation and annihilation processes, which reflect on the time dependence of the electrical field, and by guided motion, resulting in a zero-field transverse resistance.

cond-mat.supr-con↗

arXiv2004

Vortex configurations and metastability in mesoscopic superconductors

The vortex dynamics in mesoscopic superconducting cylinders with rectangular cross section under an axially applied magnetic field is investigated in the multivortex London regime. The rectangles considered range from a square up to an infinite slab. The flux distribution and total flux carried by a vortex placed in an arbitrary position of the sample is calculated analytically by assuming Clem's solution for the vortex core. The Bean-Livingston energy barrier is also analytically calculated in this framework. A Langevin algorithm simulates the flux penetration and dynamical evolution of the vortices as the external field is slowly cycled. The simulated magnetization process is governed by metastable states. The magnetization curves are hysteretic, with paramagnetic response in part of the downward branch, and present a series of peaks corresponding to the entry or expulsion of a single vortex. For elongated rectangles, the vortices arrange themselves into parallel vortex chains and an additional modulation of the magnetization, corresponding to creation or destruction of a vortex chain, comes out.

cond-mat.supr-con↗

arXiv2002

A simple model for dynamical melting of moving vortex lattices interacting with periodic pinning

Melting of moving vortex lattices in clean superconducting films with periodic pinning is studied by a dynamical "cage" model, based on a mean-field treatment of Langevin's equations for the whole vortex lattice, assuming elastic flow. In the frame moving with the velocity of the vortex lattice center of mass, the model describes forced vibrations of a single vortex tied to a spring. The vortex displacements due to thermal fluctuations and to the periodic pinning force, and the relationship between the driving force and the vortex velocity (V-I curves) are obtained by a perturbation method, valid for high velocities and for both weak and strong periodic pinning. The dynamical melting temperature is calculated as a function of the vortex velocity using Lindemann's criterion. Application to a square defect lattice gives, for strong pinning, dynamical melting lines and anisotropic V-I curves that agree qualitatively with numerical results.

cond-mat.supr-con↗

arXiv2001

Vortices in superconducting strips: interplay between surface effects and the pinning landscape

Vortices in a narrow superconducting strip with a square array of pinning sites are studied. The interactions of vortices with other vortices and with external sources (applied magnetic field and transport current) are calculated via a screened Coulomb model. The edge barrier is taken into account and shown to have an important role on the system dynamics. Numerical simulations in this approach show that the field dependent magnetic moment presents peaks corresponding to history dependent configurations of the vortex lattice. Some effects of the edge barrier on the I-V characteristics are also reported.

cond-mat.supr-con↗