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Electrons in $\ensuremath{\alpha}\text{\ensuremath{-}}{T}_{3}$ lattices behave as condensed-matter analogies of integer-spin Dirac fermions. The three atoms making up the unit cell bestow the energy spectrum with an additional energy band that is completely flat, providing unique electronic properties. The interatomic hopping term, $\ensuremath{\alpha}$, is known to strongly affect the electronic spectrum of the two-dimensional (2D) lattice, allowing it to continuously morph from graphenelike responses to the behavior of fermions in a dice lattice. For pristine lattice structures the energy bands are gapless, but small deviations in the atomic equivalence of the three sublattices will introduce gaps in the spectrum. It is unknown how these affect transport and electronic properties such as the energy spectrum of superlattice minibands. Here we investigate the dependency of these properties on the parameter $\ensuremath{\alpha}$ accounting for different symmetry-breaking terms, and we show how it affects band-gap formation. Furthermore, we find that superlattices can force band gaps to close and shift in energy. Our results demonstrate that $\ensuremath{\alpha}\text{\ensuremath{-}}{T}_{3}$ superlattices provide a versatile material for 2D band-gap engineering purposes.

The heat flux inverse analysis is significantly affected by the heat flux distribution in laser welding simulation. This work proposed a different approach for the heat distribution which predicts the temperature and the weld bead in laser welding. The influence of the heat flux distribution in an inverse heat conduction problem applied on laser welding simulation is studied by comparing different heat distribution models. An inverse heat conduction algorithm based on the three-dimensional (3D) heat diffusion equation and the enthalpy function to model the phase change problem are used to estimate the heat flux under different heat distributions. The Time Traveling Regularization is applied with the Golden Section method to estimate the heat flux in the studied cases. The proposed cubic root and square root of the volumetric heat distribution presented a good agreement between the weld profile and the experimental temperatures. The heat rate estimation proves to be dependent on the heat distribution. The proposed methodology is an alternative to predict the weld bead profile and the thermal efficiency in low penetration laser welding.

In this article, the tribological behavior and energy efficiency of surfaces coated with WC-CoCr/HVOF were evaluated after a laser remelting process, using low and high laser energy densities, respectively, 33.3 and 150 J/mm2. The purpose of laser remelting was to adequately modify the microstructure of the coatings and promote better performance during surface sliding. Therefore, the microstructure, phase composition and microhardness of the coatings were investigated, and heat effect on the substrate. During dry and lubricated tribological tests, friction coefficient (COF), wear, and surface roughness also were evaluated. In the dry tests, friction, wear and heat dissipation energies were obtained, which were then correlated with the energy consumed by the tribometer. A HEPR-type biodegradable oil was used in the lubricated tests. The proper formation of W2C and Co3W3C, obtained for the lower energy density, increased the hardness, without weakening the material, while the CrC phase acted as an anti-wear barrier. Whereas for the high energy density, thermal decomposition produced fragile phases, which were easily removed from the matrix during the wear test. The sample tested in as-sprayed condition had a high adhesion and friction compared to the remelted samples. On the remelted surfaces, greater friction stability was obtained. The lower dry friction was not linked to the lower power consumption, a result which was then attributed to the higher heat dissipation from the surface during the tests. Finally, this study proposes a methodology for quantifying the efficiency of sliding surfaces and points to a sustainable solution for tribology.

Additive manufacturing applications in the aeronautic, automotive, and biomedical industries have increased significantly in recent years as a result of design flexibility. In this regard, several methods are available, notably Directed Energy Deposition with laser (DED-L). However, to achieve the optimum parameters for each situation, a high number of experiments is necessary. Numerical simulations are useful in this context, as they are able to predict trends without conducting exhaustive tests. Mathematical models based on geometrical functions, also known as multi-layer multi-bead (MLMB) models, have been recently introduced in this field and they present good results for the surface profile. Also, their implementation is relatively simple. The aim of this study was to develop and validate a model that can provide a good approximation of the geometrical profile of DED beads. The existing models for DED have not been validated for different materials or applied to different build-up strategies, such as alternated overlapping beads. Thus, a new system of equations had to be implemented for the alternate overlapping build-up strategy. When an existing model was used for iron and Inconel 625, it was noted that the bead width and area were not kept constant during the deposition. The powder catchment efficiency was estimated for both materials and in fact, changed depending on bead position. Therefore, the adjustment was required and a correction factor was applied. In addition, for Inconel, the type of function used for the individual beads was not the same as that for the central beads, which also contradicts the initial assumptions. After applying the new assumptions, errors of 4% for the iron deposits and 10% for the Inconel deposits were obtained. This demonstrates that the proposed mathematical model can be used for the prediction of the deposited layer in additive manufacturing.

We investigate the effects of a perpendicular electric field applied to multilayer phosphorene nanoribbons with zigzag and armchair edges. Within the context of the tight-binding model, we explore the electronic properties of these systems giving emphasis to the appearance of Dirac-like spectra, a transition that occurs when the gate density associated with the applied displacement field is greater than the critical value $n_c$. We show that the confinement properties and the screening effects in such systems play an important role on the determination of $n_c$, suggesting a scheme to determine the thickness, width and edge orientation of multilayered phosphorene nanoribbons. We also explore how this transition affects the electronic transport properties of such systems., Comment: 6 pages, 5 figures

We investigate the electronic properties of $N$-layer black phosphorus by means of an analytical method based on a recently proposed tight-binding Hamiltonian involving $14$ hopping parameters. The method provides simple and accurate general expressions for the Hamiltonian of $N$-layer phosphorene, which are suitable for the study of electronic transport and optical properties of such systems, and the results show the features that emerge as the number of layers increases. In addition, we show that the $N$-layer problem can be translated into $N$ effective monolayer problems in the long wavelength approximation and, within this analytical picture, we obtain expressions for the energy gap and the effective masses for electrons and holes along the $N$-layer black phosphorus plane directions as function of the number of layers, as well as for the Landau levels as function of perpendicular magnetic field., 12 pages, 7 figures

Banana is a typical fruit that grows in warm tropical regions of the world including everywhere in Brazil. The production of the banana plant takes place year round and its pseudo-stem, mostly made of lignocellulosic fibers, is discarded as waste. The banana fibers posses a relatively higher content of potassium, which is an alkaline element of interest to the production of common clayey ceramics, like red brick, owing to its fluxing action. Therefore, the present work investigated the effect of banana pseudo-stem fiber incorporation on the technological properties of a clayey ceramic fired at 700, 900 and 1050°C. In spite of a possible contribution to liquid phase formation, the incorporation of up to 10 wt% of banana fiber impaired the ceramic properties, especially when fired at 1050°C

We investigate the energy spectrum of single layer black phosphorene nanoribbons (BPN) by means of a low-energy expansion of a recently proposed tight-binding model that describes electron and hole bands close to the Fermi energy level. Using the continuum approach, we propose boundary conditions based on sublattice symmetries for BPN with zigzag and armchair edges and show that our results for the energy spectra exhibit good agreement with those obtained by using the five-parameter tight-binding model. We also explore the behaviour of the energy gap versus the nanoribbon width $W$. Our findings demonstrate that band gap of armchair BPNs scale as $1/W^2$, while zigzag BPNs exhibit a $1/W$ tendency. We analyse the different possible combinations of the zigzag edges that result two-fold degenerate and non-degenerate edge states. Furthermore, we obtain expressions for the wave functions and discuss the limit of validity of such analytical model., 11 pages, 4 figures

A Green's function method is used to obtain the spectrum of excitations of two neighboring impurities in a semi-infinite ferromagnet. The equations of motion for Green's functions are determined in the framework of the Heisenberg model. The energies of non-resonant localized modes are calculated as a function of the interaction parameters for the exchange coupling between impurity–spin pairs, host–spin pairs, and impurity–host neighbors, as well as the effective field parameter at the impurity sites. With two impurities the system is less symmetric and has more localized modes when compared with a single impurity case.

A Green's function method is used to obtain the spectrum of spin excitations associated with a linear array of magnetic impurities implanted in a ferromagnetic thin film. The equations of motion for the Green's functions of the anisotropic film are written in the framework of the Ising model in a transverse field. The frequencies of localized modes are calculated as a function of the interaction parameters for the exchange coupling between impurity-spin pairs, host-spin pairs, and impurity-host neighbors, as well as the effective field parameter at the impurity sites.

The present study intends to evaluate the measurement uncertainty for a scratching tester, which was developed in the Mechanical Engineering Department of the Federal University of Santa Catarina. This tester furnishes the scratching hardness and the energy required to displace an unitary volume from material surface. The test basically consists in scratching a sample surface, knowing the scratching conditions and morphology. The uncertainties considered are associated to the stylus geometry, the scratch width and the load application system. Results suggested a preferential use to the equipment in materials with high performance in sclerometric tests, where the uncertainties are less than 10% of the base value.

A microscopic model is employed to calculate the spectrum of spin waves in quasiperiodic magnetic multilayers in the dipole-exchange regime. Results are presented for structures in which thin ferromagnetic films are separated by non-magnetic spacers following a Fibonacci sequence and extend previous magnetostatic calculations. The results show the splitting of the frequency bands and the mode mixing caused by the dipolar interaction between the films as a function of spacer thickness, as well as the fractal aspect of the spectrum induced by the non-periodic aspect of the structure., Comment: 5 pages, 5 figures

A microscopic model is employed to calculate the spectrum of dipole-exchange spin waves in multilayers in which thin ferromagnetic films are separated by non-magnetic spacers. Two configurations are considered: in one the films have magnetizations parallel to each other, in the other the magnetizations are antiparallel. The calculations extend a previous microscopic formalism that allows the calculation of the dipole-exchange spin wave spectrum in thin films. The results show the splitting of the frequency bands and the mode mixing caused by the dipolar interaction between the films as a function of spacer thickness.

A method is presented for calculating the frequencies of non-retarded surface plasmons propagating on a semi-infinite medium with a surface profile described by a one-dimension quasiperiodic function. The profiles are generated, in analogy with previous work on quasiperiodic superlattices, by repeating unitary cells constructed according to an inflation rule. Dispersion relations are obtained for a semi-infinite free-electron metal as the active medium, with surface profiles obeying the Fibonacci and Thue-Morse sequences.

A finite-temperature perturbation formalism is employed to study the nonlinear processes for interactions between the discrete spin waves (SW) in ultrathin antiferromagnetic films, taking account of both dipole–dipole and exchange coupling between the spins. Within a microscopic Hamiltonian approach we obtain the dominant three- and four-magnon interactions at low temperatures, and their contributions to the frequency shift and damping (or reciprocal lifetime) of the discrete SW are evaluated. Numerical results applied to MnF2 are presented.

We evaluate the dispersion relation for massless fermions, described by the Dirac equation, and for zero-spin bosons, described by the Klein-Gordon equation, moving in two dimensions and in the presence of a one-dimensional periodic potential. For massless fermions the dispersion relation shows a zero gap for carriers with zero momentum in the direction parallel to the barriers in agreement with the well-known "Klein paradox". Numerical results for the energy spectrum and the density of states are presented. Those for fermions are appropriate to graphene in which carriers behave relativistically with the "light speed" replaced by the Fermi velocity. In addition, we evaluate the transmission through a finite number of barriers for fermions and zero-spin bosons and relate it with that through a superlattice., 9 pages, 12 figures

Within a minimal model, we present analytical expressions for the eigenstates and eigenvalues of carriers confined in quantum rings in monolayer and bilayer graphene. The calculations were performed in the context of the continuum model, by solving the Dirac equation for a zero width ring geometry, i.e. by freezing out the carrier radial motion. We include the effect of an external magnetic field and show the appearance of Aharonov-Bohm oscillations and of a non-zero gap in the spectrum. Our minimal model gives insight in the energy spectrum of graphene-based quantum rings and models different aspects of finite width rings., Comment: To appear in Phys. Rev. B

The energy spectrum and density of states of graphene bilayer superlattices (SLs) are evaluated. We take into account doping and/or gating of the layers as well as tunnel coupling between them. In addition, we evaluate the transmission through such SLs and through single or double barriers. The transmission exhibits a strong dependence on the direction of the incident wave vector.

We evaluate the electronic transmission and conductance in bilayer graphene through a finite number of potential barriers. Further, we evaluate the dispersion relation in a bilayer graphene superlattice with a periodic potential applied to both layers. As a model we use the tight-binding Hamiltonian in the continuum approxi- mation. For zero bias the dispersion relation shows a finite gap for carriers with zero momentum in the direction parallel to the barriers. This is in contrast to single-layer graphene where no such gap was found. A gap also appears for a finite bias. Numerical results for the energy spectrum, conductance, and the density of states are presented and contrasted with those pertaining to single-layer graphene., Comment: 7 pages, 6 figures

A spin wave theory is developed for ultrathin antiferromagnetic films in the regime where the effects of the dipole–dipole and exchange interactions may be comparable. A microscopic model with a Hamiltonian representation of the above terms is employed, giving a description of linear spin waves in lowest order and the dominant spin wave interactions as higher-order effects. Applications are made to rutile-structure antiferromagnets (such as MnF2), perovskite-structure antiferromagnets (such as RbMnF3), and layered antiferromagnets (such as K2NiF4), which exhibit quite different behavior.

We present results for the resonant electronic transmission through graphene-based single and double barriers as a function of the incident wave vector, the widths and heights of the barriers, and the separation between them. Resonant features in the transmission result from resonant electron states in the wells or hole states in the barriers and strongly influence the ballistic conductance of the structures.

We obtain analytical expressions for the eigenstates and the Landau level spectrum of biased graphene bilayers in a magnetic field. The calculations are performed in the context of a four-band continuum model and generalize previous approximate results. Solutions are presented for the spectrum as a function of interlayer coupling, the potential difference between the layers, and the magnetic field. The explicit expressions allow us to calculate the oscillator strength and the selection rules for electric dipole transitions between the Landau states. Some transitions are significantly shifted in energy relative to those in an unbiased bilayer and exhibit a very different magnetic field dependence.

The electron states of a finite-width graphene sheet in the presence of an electrostatic confining potential and a perpendicular magnetic field are investigated. The confining potential shifts the Landau levels inside the well and creates current-carrying states at or close to the interface with the potential barriers in addition to the edge states caused by the finite width of the sheet. Detailed energy spectra are given as a function of the quantum wire parameters. The dependence of the density of states on the confinement potential is evaluated for finite and zero magnetic fields.

We demonstrate theoretically that quantum dots in bilayers of graphene can be realized. A position-dependent doping breaks the equivalence between the upper and lower layer and lifts the degeneracy of the positive and negative momentum states of the dot. Numerical results show the simultaneous presence of electron and hole confined states for certain doping profiles and a remarkable angular momentum dependence of the quantum dot spectrum, which is in sharp contrast with that for conventional semiconductor quantum dots. We predict that the optical spectrum will consist of a series of nonequidistant peaks.

The time evolution of the magnetization of a magnetic molecular crystal is obtained in an external time-dependent magnetic field, with sweep rates in the kT/s range. We present the 'exact numerical' solution of the time dependent Schr\"{o}dinger equation, and show that the steps in the hysteresis curve can be described as a sequence of two-level transitions between adiabatic states. The multilevel nature of the problem causes the transition probabilities to deviate significantly from the predictions of the Landau-Zener-St\"{u}ckelberg model. These calculations allow the introduction of an efficient approximation method that accurately reproduces the exact results. When including phase relaxation by means of an appropriate master equation, we observe an interplay between coherent dynamics and decoherence. This decreases the size of the magnetization steps at the transitions, but does not modify qualitatively the physical picture obtained without relaxation., Comment: 8 pages, 7 figures

We investigate the properties of carriers in graphene-based quantum wires created by potential barriers, by means of analytical and numerical calculations. We obtain expressions for the energy spectrum as a function of barrier height, well width and linear momentum along the wire. The results demonstrate a direction-dependent resonant transmission across the potential well.

We report the existence of confined massless fermion states in a graphene quantum well (QW) by means of analytical and numerical calculations. These states show an unusual quasilinear dependence on the momentum parallel to the QW: their number depends on the wave vector and is constrained by electron-hole conversion in the barrier regions. An essential difference with nonrelativistic electron states is a mixing between free and confined states at the edges of the free-particle continua, demonstrated by the direction-dependent resonant transmission across a potential well.

In this work we calculate the spectrum of surface plasmon polaritons propagating on a medium with a surface described by a generalized quasiperiodic function. The profile function is defined in terms of unit cells containing binary elements, corresponding to elevations of different shapes, arranged according to an inflation rule that generates a quasiperiodic sequence. Our theoretical model, based upon a well-known formalism that has been applied to periodic surfaces, generalizes a previous calculation performed for the nonretarded limit. Numerical results show the effect of the quasiperiodicity on the dispersion relations and on the density of surface polaritons modes propagating on a semi-infinite free-electron metal.

A Green's function formalism is used to calculate the spectrum of excitations of two neighboring impurities implanted in a semi-infinite ferromagnetic. The equations of motion for the Green's functions are determined in the framework of the Ising model in a transverse field and results are given for the effect of the exchange coupling, position and orientation of the impurities on the spectra of localized spin wave modes., 8 pages, 7 figures

A Green's function formalism is used to calculate the spectrum of localized modes of an impurity layer implanted within a ferromagnetic thin film. The equations of motion for the Green's functions are determined in the framework of the Ising model in a transverse field. We show that depending on the thickness, exchange and effective field parameters, there is a ``crossover'' effect between the surface modes and impurity localized modes. For thicker films the results show that the degeneracy of the surface modes can be lifted by the presence of an impurity layer., Comment: 12 pages, 5 figures

The damping and frequency shift of spin wave modes due to magnon-magnon interactions in ultrathin antiferromagnetic films are studied in the dipole-exchange regime. We employ a Hamiltonian formalism that uses a transformation of the spin operators to boson operators in the film geometry and calculate the effects of the three- and four-magnon interactions on the spin wave spectrum by means of a diagrammatic perturbation technique. With this formalism we obtain expressions for the damping and energy shift of the discrete spin wave modes in bcc antiferromagnetic films as a function of wave vector and temperature. Numerical results are shown for ultrathin films of the antiferromagnet MnF 2 .

We present calculations for the dipole-exchange spin wave spectrum of two ferromagnetic thin films separated by a non-magnetic spacer. We employ a microscopic model that takes into account the position dependence of the spin sites in each media. The results show the frequency shift and mode mixing caused by the dipolar interaction between the films as a function of spacer thickness, for films with parallel magnetization.

A theory is presented for the effect of surface orientation on the spectra of localized surface spin waves in antiferromagnetic films. The calculations apply to the exchange-dominated regime and exploit properties of banded matrices to find analytic expressions for the spin-wave frequencies.Results are presented for films with a simple-cubic structure for (0 0 1), (0 1 1), and (1 1 1) orientations of the surface planes. The spin-wave characteristics are found to be quite distinct in the three cases.

Resonant electronic transmission through graphene-based double barriers (wells) is studied as a function of the incident wave vector, the widths and heights (depths) of the barriers (wells), and the separation between them. Resonant features in the transmission result from resonant electron states in the wells or hole states in the barriers and strongly influence the ballistic conductance of the structures.