arXiv subjects

R. C. Silva

Publications and source records attributed to R. C. Silva.

18 recordsLinked to original sources

arXiv2026

Scale-dependent universality class crossover in magnetic skyrmion polymers

Dipolar magnetic skyrmions can assemble into chains with alternating helicity that act as one-dimensional polymers, yet their statistical mechanics violates the universal harmonic scaling observed in actin, DNA, and microtubules. From first principles, we compute the inter-skyrmion pair potential and find a bi-exponential form of competing interactions with two characteristic decay lengths that encode the distinct microscopic mechanisms of repulsion and attraction. Multiscale simulations reveal a power-law temperature dependence with exponent $1$ in the worm-chain limit of a single bond, and exponent $1/2$ in the three-bond limit. We find that the power-law behavior is remarkably independent of magnetic field strength, and the crossover is due to competing radial interactions responsible for the bonds, resulting in a quartic transverse confinement. We show that the precise form of the competing interactions (e.g., Morse or double-Yukawa) does not affect the temperature dependence.

cond-mat.soft↗

arXiv2026

Static vacancies as parametrized conformal defects in the critical $J_1$--$J_2$ transverse-field Ising chain

We revisit the problem of two static nonmagnetic vacancies in the transverse-field Ising chain with first- and second-neighbor couplings $J_1$ and $J_2$, now on the critical line, using density-matrix renormalization-group (DMRG) calculations in open chains of up to $N=300$ sites. In contrast to the gapped regime studied previously, where the vacancy-vacancy interaction decays exponentially, along the entire quantum critical line the interaction becomes algebraic, $|Δ_b(r)|\sim r^{-α}$, with $α$ close to the universal Casimir value of unity and a weak but systematic dependence on the second-neighbor coupling, $α_\infty \simeq 1.070 + 0.091\, J_2/J_1)$ across $J_2/J_1\in[0.1,1.0]$. The transmission ratio of the spin correlator across a vacancy approaches a $J_2$-dependent plateau $T_\infty(J_2)$ that grows from $0.11$ to $0.33$ over the same range, and the Affleck-Ludwig boundary entropy is small and approximately constant, $\log g_\infty \approx -0.073$, well above the Ising fixed-BC value $-\ln\sqrt{2}$ and close to the free-boundary value. The three observables vary smoothly and monotonically with $J_2$, consistent with a one-parameter family of partially transmissive conformal defects controlled by $J_2$. Throughout, the critical line is located using the bulk spin-correlator exponent $η=1/4$, the order-parameter exponent of the Ising universality class, which provides a robust criterion in this open geometry.

cond-mat.stat-mech↗

arXiv2025

Periodic transitions of topological charge in skyrmions confined within FeGe and Co/Pt nanodisks

The dynamic control of skyrmion properties such as polarity, vorticity, and topological charge is crucial for their implementation in spintronic applications. In this work, we investigate the periodic inversion of the topological charge in two distinct systems: FeGe, a bulk chiral magnet, and Co/Pt, an interfacial system with strong Dzyaloshinskii-Moriya interaction. By applying an oscillating magnetic field perpendicular to the film plane, we induce cyclic transitions in the spin texture. In FeGe, the skyrmion evolves through a $Q=1 \rightarrow 0 \rightarrow -1$ sequence via an intermediate skyrmionium state. In Co/Pt, the process involves skyrmion annihilation and re-nucleation, resulting in alternating topological charges. These results reveal distinct dynamic mechanisms for topological charge modulation, offering potential pathways for the energy-efficient control of skyrmion-based devices.

cond-mat.mes-hall↗

arXiv2024

A proposal for skyrmion-based diode-like device in antiferromagnetic nanostripe

Micromagnetic simulations were employed to investigate the dynamics of a single skyrmion within an antiferromagnetic nanostripe with spatially engineered magnetic properties. This study investigates skyrmion motion within an antiferromagnetic nanostripe engineered with trapezoidal regions of enhanced magnetic anisotropy, enabling diode-like functionality by selectively directing skyrmion movement. Our findings demonstrate that skyrmions can cross these barriers in one direction while being obstructed in the reverse direction, mimicking diode behavior. A detailed analysis is presented on how geometric parameters, such as the inclination angle of the trapezoidal barriers, impact skyrmion motion and device efficacy. Additionally, we reveal that an optimal combination of current density and anisotropy is essential to facilitate efficient skyrmion transport through the nanostripe without reverse movement or annihilation. This work advances the development of skyrmion-based devices for spintronic applications. It provides valuable insights into designing structures that harness controlled topological dynamics

cond-mat.mes-hall↗

arXiv2024

Channeling Skyrmions: suppressing the skyrmion Hall effect in ferrimagnetic nanostripes

The Skyrmion Hall Effect (SkHE) observed in ferromagnetic (FM) and ferrimagnetic (FI) skyrmions traveling due to a spin-polarized current can be a problematic issue when it comes to technological applications. By investigating the properties of FI skyrmions in racetracks through computational simulations, we have described the nature of their movement based on the relative values of the exchange, Dzyaloshinskii-Moriya, and anisotropy coupling constants. Beyond that, using a design strategy, a magnetic channel-like nano-device is proposed in which a spin-polarized current protocol is created to successfully control the channel on which the skyrmion will travel without the adverse SkHE. Additionally, a simple adjustment in the current strength can modify the skyrmion position sideways between different parallel channels in the nanostripe.

cond-mat.mes-hall↗

arXiv2023

Toward a search for axion-like particles at the LNLS

Axion-Like Particles (ALPs) appear in several dark sector studies. They have gained increasing attention from the theoretical and experimental community. In this work, we propose the first search for ALPs to be conducted at the Brazilian Synchrotron Light Laboratory (LNLS). In this work, we derive the projected sensitivity of a proposed experiment for the production of ALPs via the channel $e^+ e^- \to a γ$. We show that such an experiment could probe ALP masses between $1-55\,\mbox{MeV}$, and ALP-electron couplings down to $g_{aee}=2-6\times10^{-4} \,\mbox{GeV}^{-1}$ depending on the energy beam, thickness of the target, and background assumptions. Therefore, this quest would cover an unexplored region of parameter space for experiments of this kind, constitute a promising probe for dark sectors, and potentially become the first Latin-American dark sector detector.

hep-ph↗

arXiv2021

Skyrmion bound state and dynamics in an antiferromagnetic bilayer racetrack

We investigate the dynamics of two skyrmions lying in distinct layers of an antiferromagnetic bilayer system, consisting of nanostripes with the shape of racetracks. The top and bottom nanostripes are separated by a height offset and they are coupled through a ferromagnetic exchange, allowing the interaction between the skyrmions from both layers. Depending on the distance between the skyrmions they attract each other sufficiently to achieve a bound state. We also analyze their dynamics when an electric current is applied in a unique layer and we determine how the bound-state nucleation depends on the current density and vertical distance between the skyrmions. Finally, we analyzed the robustness of the bound states by considering two situations: 1) a system constituted by clean or homogeneous antiferromagnetic racetracks; 2) a system in which randomly distributed magnetic impurities in both layers are included in the system.

cond-mat.mes-hall↗

arXiv2020

Robust dynamics of antiferromagnetic skyrmion driven by spin-polarized current in small thin disks

We investigate skyrmion configuration and dynamics in antiferromagnetic thin disks. It is shown that the skyrmion acquires oscillatory dynamics with well-defined amplitude and frequency which may be controlled on demand by the spin-polarized current. Such dynamics are robust in the sense that an interface between two half-disks cannot change the dynamics appreciably. Indeed, the skyrmion keeps its oscillatory despite crossing this interface. The way skyrmion found to do that is by modifying its core region shape so that its total energy is unaltered for several cycles.

cond-mat.mes-hall↗

arXiv2020

Typical skyrmions versus bimerons: a long-distance competition in ferromagnetic racetracks

During the last years, topologically protected collective modes of the magnetization have called much attention. Among these, skyrmions and merons have been the object of intense study. In particular, topological skyrmions are objects with an integer skyrmion number $Q$ while merons have a half-integer skyrmion charge $q$. In this work, we consider a $Q=1$ skyrmion, composed by a meron and an antimeron (bimeron), displacing in a ferromagnetic racetrack, disputing a long-distance competition with its more famous counterpart, the typical $Q=1$ cylindrically symmetrical skyrmion. Both types of topological structures induce a Magnus force and then are subject to the Hall effect. The influence of the Dzyaloshinskii-Moriya interaction ($DMI$) present in certain materials and able to induces $DMI$-skyrmions is also analyzed. Our main aim is to compare the motions (induced by a spin-polarized current) of these objects along with their own specific racetracks. We also investigate some favorable factors which are able to give breath to the competitors, impelling them to remain in the race for longer distances before their annihilation at the racetrack lateral border. An interesting result is that the $DMI$-skyrmion loses this hypothetical race due to its larger rigidity.

cond-mat.mes-hall↗

arXiv2020

Topological Hall effect induced Skyrmion-Antiskyrmion coupling in inhomogeneous racetrack

In this paper we investigate a magnetic racetrack consisting of a junction of three materials with different properties. Indeed, this magnetic system is composed by two distinct regions (racetracks) connected by a thin interface: the first region (termed sector $1$) has isotropic in-plane magnetic chirality and supports skyrmion ($S$) excitations while the second (sector $3$) has anisotropic chirality and consequently supports antiskyrmions ($A$). The interface, which would be a third region (sector $2$, connecting sectors $1$ and $3$) located in the central part of the racetrack, is an easy-axis Heisenberg ferromagnetic material. The topological structures $S$ and $A$ are put in motion by applying a spin-polarized current. Under certain conditions, we show that the skyrmion and the antiskyrmion created in their respective sectors are simultaneously impelled to the interface (due to the Magnus force) to apparently become a unique object (a skyrmion-antiskyrmion pair or $SAP$). After glued by sector $2$, the skyrmion and the antiskyrmion move together (as a $SAP$) along the direction of the applied current. It is also shown that such an engineered racetrack can support a sequence of several $SAP$ structures in motion, forming a current.

cond-mat.mes-hall↗

arXiv2018

Predicted antiferromagnetic-vortex dynamics driven by spin polarized current in thin discs

We investigate vortex configuration in antiferromagnetic thin discs. It is shown that the vortex acquires oscillatory dynamics with well-defined amplitude and frequency which may be controlled on demand by an alternating spin polarized current. These findings may be useful for the emerging field of antiferromagnetic topological spintronics, once vortex dynamics may be controlled by purely electric means.

cond-mat.mes-hall↗

arXiv2014

Realization of magnetic monopoles current in an artificial spin ice device: A step towards magnetronics

Magnetricity- the magnetic equivalent of electricity- was recently verified experimentally for the first time. Indeed, just as the stream of electric charges produces electric current, emergent magnetic monopoles have been observed to roam freely (generating magnetic current) in geometrically frustrated magnets known as spin ice. However, this is realized only by considering extreme physical conditions as a single crystal of spin ice has to be cooled to a temperature of $0.36 K$. Candidates to overcome this difficulty are artificial analogues of spin ice crystals, the so-called artificial spin ices. Here we show that, by tuning geometrical frustration down, a peculiar type of these artificial systems is an excellent candidate. We produce this material and experimentally observe the emergent monopoles; then, we calculate the effects of external magnetic fields, illustrating how to generate controlled magnetic currents. This potential nano-device for use in magnetronics can be practical even at room temperature and the relevant parameters (such as magnetic charge strength etc) for developing this technology can be tuned at will.

cond-mat.mes-hall↗

arXiv2013

Vortex-crystal pattern in an anti-artificial spin ice system

We have proposed in this work an original system composed by anti-dots nanopatterned in a ferromagnetic thin film, mimicking negatively the structure of an articial spin ice. In the hysteresis loop we notice the emergency of an anisotropy in the magnetization saturation and in the micromagnetic simulations, in the beginning of the hysteresis loop (relaxation), the formation of a vortex crystal array with vortices in diferent positions possessing random polarization and chirality. The crystal of vortices in this electrically connected sample could be most eficient than those observed in non-connected nanodiscs for current-driven or magnetic vortices switching by electric currents.

cond-mat.mes-hall↗

arXiv2013

Phase growth control in low temperature PLD Co:TiO2 films by pressure

This paper reports on the structural and optical properties of Co-doped TiO2 thin films grown onto (0001) Al2O3 substrates by non-reactive pulsed laser deposition (PLD) using argon as buffer gas. It is shown that by keeping constant the substrate temperature at as low as 310 C and varying only the background gas pressure between 7 Pa and 70 Pa, it is possible to grow either epitaxial rutile or pure anatase thin films, as well as films with a mixture of both polymorphs. The optical band gaps of the films are red shifted in comparison to the values usually reported for undoped TiO2, which is consistent with n-type doping of the TiO2 matrix. Such band gap red shift brings the absorption edge of the Co-doped TiO2 films into the visible region, which might favour their photocatalytic activity. Furthermore, the band gap red shift depends on the films phase composition, increasing with the increase of the Urbach energy for increasing rutile content.

cond-mat.mtrl-sci↗

arXiv2013

Nambu monopoles interacting with lattice defects in two-dimensional artificial square spin ice

The interactions between an excitation (similar to a pair of Nambu monopoles) and a lattice defect are studied in an artificial two-dimensional square spin ice. This is done by considering a square array of islands containing only one island different from all others. This difference is incorporated in the magnetic moment (spin) of the "imperfect" island and several cases are studied, including the special situation in which this distinct spin is zero (vacancy). We have shown that the two extreme points of a malformed island behave like two opposite magnetic charges. Then, the effective interaction between a pair of Nambu monopoles with the deformed island is a problem involving four magnetic charges (two pairs of opposite poles) and a string. We also sketch the configuration of the field lines of these four charges to confirm this picture. The influence of the string on this interaction decays rapidly with the string distance from the defect.

cond-mat.mes-hall↗

arXiv2012

Thermodynamics of elementary excitations in artificial magnetic square ice

We investigate the thermodynamics of artificial square spin ice systems assuming only dipolar interactions among the islands that compose the array. The emphasis is given on the effects of the temperature on the elementary excitations (magnetic monopoles and their Dirac strings). By using Monte Carlo techniques we calculate the specific heat, the density of poles and their average separation as functions of temperature. The specific heat and average separation between monopoles and antimonopoles exhibit a sharp peak and a local maximum, respectively, at the same temperature, $T_{p}\approx 7.2D/k_{B}$ (here, $D$ is the strength of the dipolar interaction and $k_{B}$ is the Boltzmann constant). As the lattice size is increased, the amplitude of these features also increases but very slowly. Really, the specific heat and the maximum in the average separation $d_{max}$ between oppositely charged monopoles increase logarithmically with the system size, indicating that completely isolated charges could be found only at the thermodynamic limit. In general, the results obtained here suggest that, for temperatures $T \geq T_{p}$, these systems may exhibit a phase with separated monopoles, although the quantity $d_{max}$ should not be larger than a few lattice spacings for viable artificial materials.

cond-mat.mes-hall↗

arXiv2009

Predicted defect induced vortex core switching in thin magnetic nanodisks

We investigate the influence of artificial defects (small holes) inserted into magnetic nanodisks on the vortex core dynamics. One and two holes (antidots) are considered. In general, the core falls into the hole but, in particular, we would like to remark an interesting phenomenon not yet observed, which is the vortex core switching induced by the vortex-hole interactions. It occurs for the case with only one hole and for very special conditions involving the hole size and position as well as the disk size. Any small deformation in the disk geometry such as the presence of a second antidot changes completely the vortex dynamics and the vortex core eventually falls into one of the defects. After trapped, the vortex center still oscillates with a very high frequency and small amplitude around the defect center.

cond-mat.mes-hall↗

arXiv2009

Magnetic monopole and string excitations in a two-dimensional spin ice

We study the magnetic excitations of a square lattice spin-ice recently produced in an artificial form, as an array of nanoscale magnets. Our analysis, based upon the dipolar interaction between the nanomagnetic islands, correctly reproduces the ground-state observed experimentally. In addition, we find magnetic monopole-like excitations effectively interacting by means of the usual Coulombic plus a linear confining potential, the latter being related to a string-like excitation binding the monopoles pairs, what indicates that the fractionalization of magnetic dipoles may not be so easy in two dimensions. These findings contrast this material with the three-dimensional analogue, where such monopoles experience only the Coulombic interaction. We discuss, however, two entropic effects that affect the monopole interactions: firstly, the string configurational entropy may loose the string tension and then, free magnetic monopoles should also be found in lower dimensional spin ices; secondly, in contrast to the string configurational entropy, an entropically driven Coulomb force, which increases with temperature, has the opposite effect of confining the magnetic defects.

cond-mat.mtrl-sci↗