arXiv subjects

T. Santos

Publications and source records attributed to T. Santos.

8 recordsLinked to original sources

arXiv2026

Strain-Induced Curvature in Monolayer Graphene: Effects on Electronic Structure, Phonon Dynamics, and Lattice Thermal Conductivity

We present a comprehensive set of calculations to investigate the effect of strain-induced x-y topological perturbation in the monolayer graphene sheet. We show that the induced curvature with the defined strain constraint, energetically stabilizes the systems. The electronic properties are modified when the amplitude of the curvature of the sheet increases, which induces Van Hove singularities of the electronic Density of States to approach the Fermi energy. The highly curved system exhibits coexisting flat and linear dispersions close to the Fermi level, which is a promising feature for thermoelectric applications. We also demonstrate, through the phonon dispersion curves, that respective systems are dynamically stable within the studied range of strains/curvatures. Moreover, the flexural acoustic mode transitions from quadratic to linear dispersion under strain, mimicking the 3D behavior and enhancing phonon scattering. The increase of phonon scattering will therefore decrease the value of the lattice thermal conductivity, $\kappa_L$. Such results allows us to conclude that it is possible to tune $\kappa_L$ by applying x-y strain to the monolayer sheet, and inducing different topological curvatures.

cond-mat.mes-hall↗

arXiv2020

Thermal stability for domain wall mediated magnetization reversal in perpendicular STT MRAM cells with W insertion layers

We present an analytical model for calculating energy barrier for the magnetic field-driven domain wall-mediated magnetization reversal of a magneto-resistive random access memory (MRAM) cell and apply it to study thermal stability factor $Δ$ for various thicknesses of W layers inserted into the free layer (FL) as a function of the cell size and temperature. We find that, by increasing W thickness, the effective perpendicular magnetic anisotropy (PMA) energy density of the FL film monotonically increases, but at the same time, $Δ$ of the cell mainly decreases. Our analysis shows that, in addition to saturation magnetization $M_s$ and exchange stiffness constant $A_\mathrm{ex}$ of the FL film, the parameter that quantifies the $Δ$ of the cell is its coercive field $H_c$, rather than the net PMA field $H_k$ of the FL film comprising the cell.

cond-mat.mes-hall↗

arXiv2020

Impact of Distinct Ca$^{2+}$ Sources on the Physical Properties of Alumina-based Macroporous Refractories for Thermal Insulation at High Temperature

Calcium aluminate cement (CAC), calcium carbonate (CaCO$_3$), calcium hydroxide [Ca(OH)$_2$] and calcium oxide (CaO) were investigated in alumina-based macroporous compositions for $\textit{in situ}$ formation of hibonite (also known as CaO.6Al$_2$O$_3$ or CA$_6$). Due to its volumetric expansion, this phase could counteract the linear shrinkage observed when macroporous ceramics are fired. In order to evaluate the impact of each Ca$^{2+}$ source on the physical properties, different formulations were processed and characterized for their total porosity, crushing strength and linear shrinkage. CaCO$_3$-containing samples presented high porosity and small dimensional changes after thermal treatment. Thus, a novel composition was formulated with $CaCO_3$ in order to result in 100 % CA$_6$ on thermodynamic equilibrium. Besides the usual properties, its $\textit{in situ}$ changes in the hot elastic modulus ($E_{\rm in\ situ}$) and thermal conductivity ($k_{\rm eff}$), were evaluated. This composition presented promising results, as a constant and low $k_{\rm eff}$, low linear expansion and Young modulus increase at low temperatures. These properties make it possible to produce more efficient insulators with better performance in service.

physics.app-ph↗

arXiv2018

Electronic and Phonon Instabilities in Bilayer Graphene under Applied External Bias

We have performed electronic-structure and lattice-dynamics calculations on the AB and AA structures of bilayer graphene. We study the effect of external electric fields and compare results obtained with different levels of theory to existing theoretical and experimental results. Application of an external field to the AB bilayer alters the electronic spectrum, with the bands changing under bias from a parabolic to a "Mexican hat" double-well structure. This results in a semi-metal-to-semiconductor phase transition, with the size of the induced electronic band-gap being tuneable through the field strength. A reduction of continuous symmetry from a hexagonal to a triangular lattice is also evidenced through in-plane electronic charge inhomogeneities between the sublattices. When spin-orbit coupling is turned on for the AB system, we find that the bulk gap decreases, gradually increasing for larger intensities of the bias. Under large bias the energy dispersion recovers the Mexican hat structure, since the energy interaction between the layers balances the coupling interaction. We find that external bias perturbs the harmonic phonon spectra and leads to anomalous behaviour of the out-of-plane flexural ZA and layer-breathing ZO' modes. For the AA system, the electronic and phonon dispersions both remain stable under bias, but the phonon spectrum exhibits zone-center imaginary modes due to layer-sliding dynamical instabilities.

cond-mat.mtrl-sci↗

arXiv2016

Magnetization switching of FePt nanoparticle recording medium by femtosecond laser pulses

Manipulation of magnetization with ultrashort laser pulses is promising for information storage device applications. The dynamic of the magnetization response depends on the energy transfer from the photons to the spins during the initial laser excitation. A material of special interest for magnetic storage is FePt nanoparticles , on which optical writing with optical angular momentum was demonstrated recently by Lambert et al., although the mechanism remained unclear. Here we investigate experimentally and theoretically the all-optical switching of FePt nanoparticles. We show that the magnetization switching is a stochastic process. We develop a complete multiscale model which allows us to optimize the number of laser shots needed to write the magnetization of high anisotropy FePt nanoparticles in our experiments. We conclude that only angular momentum induced optically by the inverse Faraday effect will provide switching with one single femtosecond laser pulse.

cond-mat.mes-hall↗

arXiv2013

Speed limit of FePt spin dynamics on femtosecond timescales

Magnetization manipulation is becoming an indispensable tool for both basic and applied research. Theory predicts two types of ultrafast demagnetization dynamics classified as type I and type II. In type II materials, a second slower process takes place after the initial fast drop of magnetization. In this letter we investigate this behavior for FePt recording materials with perpendicular anisotropy. The magnetization dynamics have been simulated using a thermal micromagnetic model based on the Landau-Lifshitz-Bloch equation. We identify a transition to type II behavior and relate it to the electron temperatures reached by the laser heating. This slowing down is a fundamental limit to reconding speeds in heat assisted reversal.

cond-mat.mtrl-sci↗

arXiv2011

Resonant soft x-ray scattering from La(1-x)Sr(x)MnO(3) quantum wire arrays

We describe a strategy for using resonant soft x-ray scattering (RSXS) to study the electronic structure of transition metal oxide quantum wires. Using electron beam lithography and ion milling, we have produced periodic, patterned arrays of colossal magnetoresistance (CMR) phase La(1-x)Sr(x)MnO(3) consisting of ~ 5000 wires, each of which is 80 nm in width. The scattered intensity exhibits a series of peaks that can be interpreted as Bragg reflections from the periodic structure or, equivalently, diffraction orders from the grating-like structure. RSXS measurements at the Mn L(2,3) edge, which has a large magnetic cross section, show clear evidence for a magnetic superstructure with a commensurate period of five wires, which we interpret as commensurately modulated antiferromagnetism. This superstructure, which is accompanied by non-trivial reorganization of the magnetization within each wire, likely results from classical dipole interactions among the wires. We introduce a simple, exactly soluble, analytic model of the scattering that captures, semi-quantitatively, the primary features in the RSXS data; this model will act as a foundation for forthcoming, detailed studies of the magnetic structure in these systems.

cond-mat.str-el↗

arXiv2010

Conductance in Co/Al2O3/Si/Al2O3 permalloy with asymmetrically doped barrier

Permalloy magnetic tunnel junctions. Complementary low frequency noise measurements are used to understand the conductance results. The obtained data indicate the breakdown of the Coulomb blockade for thickness of the asymmetric silicon layer exceeding 1.2Å. The crossover in the conductance, the dependence of the tunnelling magnetoresistance with the bias voltage and the noise below 80K correspond to 1 monolayer coverage. Interestingly, the zero bias magnetoresistance remains nearly unaffected by the presence of the silicon layer. The proposed model uses Larkin-Matveev approximation of tunnelling through a single impurity layer generalized to 3D and takes into account the variation of the barrier shape with the bias voltage. The main difference is the localization of all the impurity levels within a single atomic layer. In the high thickness case, up to 1.8Å, we have introduced a phenomenological parameter, which reflects the number of single levels on the total density of silicon atoms.

cond-mat.other↗