arXiv subjects

M. S. Vasconcelos

Publications and source records attributed to M. S. Vasconcelos.

7 recordsLinked to original sources

arXiv2019

Phononic Topological States in 1D Quasicrystals

We theoretically analyze the spectrum of phonons of a one-dimensional quasiperiodic lattice. We simulate the quasicrystal from the classic system of spring-bound atoms with a force constant modulated by the Aubry-André model, so that its value is slightly different in each site of the lattice. From the equations of motion, we obtained the equivalent phonon spectrum of the Hofstadter butterfly, characterizing a multifractal. In this spectrum, we obtained the extended, critical and localized regimes, and we observed that the multifractal characteristic is sensitive to the number of atoms and the $λ$ parameter of our model. We also verified the presence of border states for phonons, where some modes in the system boundaries present vibrations. Through the measurement of localization of the individual displacements in each site, we verify the presence of a phase transition through the Inverse Participation Rate (IPR) for $λ= 1.0 $, where the system changes from extended to localized.

cond-mat.dis-nn↗

arXiv2018

Logarithm corrections in the critical behavior of the Ising model on a triangular lattice modulated with the Fibonacci sequence

We investigated the critical behavior of the Ising model in a triangular lattice with ferro and anti-ferromagnetic interactions modulated by the Fibonacci sequence, by using finite-size numerical simulations. Specifically, we used a replica exchange Monte Carlo method, known as Parallel Tempering, to calculate the thermodynamic quantities of the system. We have obtained the staggered magnetization $q$, the associated magnetic susceptibility ($χ$) and the specific heat $c$, to characterize the universality class of the system. At the low-temperature limit, we have obtained a continuous phase transition with a critical temperature around $T_{c} \approx 1.4116$ for a particular modulation of the lattice according to the Fibonacci letter sequence. In addition, we have used finite-size scaling relations with logarithmic corrections to estimate the critical exponents $β$, $γ$ and $ν$, and the correction exponents $\hatβ$, $\hatγ$, $\hatα$ and $\hatλ$. Our results show that the system obeys the Ising model universality class and that the critical behavior has logarithmic corrections.

cond-mat.dis-nn↗

arXiv2017

Critical behavior of the 2D Ising model modulated by the Octonacci sequence

We investigated the Ising model on a square lattice with ferro and antiferromagnetic interactions modulated by the quasiperiodic Octonacci sequence in both directions of the lattice. We have applied the Replica Exchange Monte Carlo (Parallel Tempering) technique to calculate the thermodynamic quantities of the system. We obtained the order parameter, the associated magnetic susceptibility ($χ$) and the specific heat $(c)$ in order to characterize the universality class of the phase transition. Also, we use the finite size scaling method to obtain the critical temperature of the system and the critical exponents $β$, $γ$ and $ν$. In the low temperature limit we have obtained a continuous transition with critical temperature around $T_{c} \approx 1.413$. The system obeys the Ising universality class with logarithmic corrections. We found estimatives for the correction exponents $\hatβ$, $\hatγ$ and $\hatλ$ by using the finite size scaling technique.

cond-mat.stat-mech↗

arXiv2017

Influence of the Terbium film size for the magnetocaloric effect at low temperatures

Significative enhance in magnetocaloric effect due to finite size and surface effects is reported in Terbium(Tb) thin films in the helimagnetic phase (corresponding to a temperature range from $T_C$=219 K to $T_N$=231 K), for external fields of the order of kOe. For a Tb thin film of 6 monolayers submitted to an applied field ($Δ$H= 30 kOe, $Δ$H= 50 kOe and $Δ$H= 70 kOe) we report a significative change in adiabatic temperature, $Δ$T/$Δ$H, near the Néel temperature, of the order ten times higher than that observed for Tb bulk. On the other hand, for small values of the magnetic field, large thickness effects are found. For external field strength around few kOe, we have found that the thermal caloric efficiency increases remarkably for ultrathin films. For an ultrathin film with 6 monolayers, we have found $Δ$T/$Δ$H=43 K/T while for thicker films, with 20 monolayers, $Δ$T/$Δ$H=22 K/T. Our results suggest that thin films of Tb are a promising material for magnetocaloric effect devices for applications at intermediate temperatures.

cond-mat.mtrl-sci↗

arXiv2016

Octonacci Photonic Crystals with Negative Refraction Index Materials

We investigate the optical transmission spectra for $s$-polarized (TE) and $p$-polarized (TM) waves in one-dimensional photonic quasicrystals on a quasiperiodic multilayer structure made up by alternate layers of SiO$_{2}$ and \textit{metamaterials}, organized by following the Octonacci sequence. Maxwell's equations and the transfer-matrix technique are used to derive the transmission spectra for the propagation of normaly and obliquely incident optical fields. We assume Drude-Lorentz-type dispersive response for the dielectric permittivity and magnetic permeability of the metamaterials. For normally incident waves, we observe that the spectra does not have self-similar behavior or mirror symmetry and it also features the absence of optical band gap. Also for normally incident waves, we show regions of full transmittance when the incident angle $θ_{C} = 0^{\circ}$ in a particular frequency range.

cond-mat.mtrl-sci↗

arXiv2015

Octonacci Photonic Quasicrystals

We study theoretically the transmission spectra in one-dimensional photonic quasicrystals, made up of SiO$_2$($A$) and TiO$_2$($B$) materials, organized following the Octonacci sequence, where the $n$th-stage of the multilayer $S_{n}$ is given by the rule $S_{n}=S_{n-1}S_{n-2}S_{n-1}$, for $n\geq 3$ and with $S_{1}=A$ and $S_{2}=B$. The expression for transmittance was obtained by employing a theoretical calculation based in the transfer-matrix method. To normally incident waves, we observe that, for a same generation, the transmission spectra for TE and TM waves are equal, at least qualitatively, and they present a scaling property where a self-similar behavior is obtained, as an evidence that these spectra are fractals. The spectra show regions where the omnidirectional band gaps emerges for specific generations of Octonacci photonic structure, except to TM waves. For TE waves, we note that all of them have the almost same width, for different generations. We also report the localization of modes as a consequence of the quasiperiodicity of the heterostructure

cond-mat.mtrl-sci↗

arXiv2014

Magnetic Phases and Specific Heat of Ultra-Thin Holmium Films

We report model calculations of the magnetic phases of very thin Ho films in the temperature interval between 20K and 132K, and show that slab size, surface effects and magnetic field due to spin ordering may impact significantly the magnetic phase diagram. There is a relevant reduction of the external field strength required to saturate the magnetization and for ultra-thin films the helical state does not form. We explore the heat capacity and the susceptibility as auxiliary tools to discuss the nature of the phase transitions.

cond-mat.mtrl-sci↗