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F. S. Oliveira

Publications and source records attributed to F. S. Oliveira.

9 recordsLinked to original sources
arXiv2026

YNiSn$_2$: A candidate Dirac semimetal

We report the synthesis and physical properties of the new compound YNiSn$_2$, which crystallizes in the orthorhombic \textit{Cmcm} structure. The material exhibits semimetallic behavior and develops a giant positive magnetoresistance approaching 1200\% at $B = 16$ T. Pronounced de Haas-van Alphen and Shubnikov-de Haas oscillations reveal a dominant quasi-two-dimensional Fermi surface with an exceptionally small cyclotron effective mass of $m^{*} = 0.08 m{0}$, indicating light carriers and a tiny Fermi surface pocket. The strong anisotropy revealed by Shubnikov-de Haas quantum oscillation measurements highlights the low-dimensional electronic character of YNiSn$_2$, positioning it as a promising Dirac semimetal candidate.

cond-mat.str-el↗
arXiv2022

A Simple and Precise Way to Determine Electrical Resistivity of Isotropic Conductors: Simplifying the Four-Probe Method

COMSOL Multiphysics software is used to describe the behavior of the electrical resistivity of several samples with rectangular shape typically used in the Montgomery method. The simulation data obtained using four isotropic conductors allowed us to understand in detail the behavior of the electric potential and electric field of the samples. The results provide an analytical method which can substitute the four-probe method with much more simplicity and precision.

cond-mat.mtrl-sci↗
arXiv2022

On the Fermi gas, the Sommerfeld fine structure constant, and the electron-electron scattering in conductors

Electrical energy is considered as a fundamental parameter for inclusion in Fermi gas theory, in addition to thermal energy. It is argued that electrical energy can move some electrons to above the Fermi Level, providing free charges to carry the electrical current, even at absolute zero temperature. The Drude model, Ohm's law, quantum resistance, and the electrical resistivity due to electron-electron scattering appear naturally as a consequence of the theoretical description, which is based on the quantization of the angular momentum and the Fermi-Dirac distribution, considering total energy as $ε$ = k$_B$$T$ + $Φ_0$$I$. The electrical and magnetic forces acting on an electron are related to the ratio between the Fermi velocity and the speed of light and show that the electron motion is due to helical paths. Considering the center of mass description for the Bohr atom, it was possible to show that the magnetic force is related to the electrical force as $F_M$ = ($α$/$π$) $F_E$, which demonstrates that the electrons move in helical paths along the orbit. The helical motion naturally provides for quantization of the magnetic flux, the spin of the electron, and the first correction term of the anomalous magnetic moment. Applying the model to describe the electron-electron scattering allows prediction of the behavior of the electrical resistivity of many metals at low temperatures, which is in excellent agreement with empirical observations.

cond-mat.mtrl-sci↗
arXiv2021

Minimum Number of Bends of Paths of Trees in a Grid Embedding

We are interested in embedding trees T with maximum degree at most four in a rectangular grid, such that the vertices of T correspond to grid points, while edges of T correspond to non-intersecting straight segments of the grid lines. Such embeddings are called straight models. While each edge is represented by a straight segment, a path of T is represented in the model by the union of the segments corresponding to its edges, which may consist of a path in the model having several bends. The aim is to determine a straight model of a given tree T minimizing the maximum number of bends over all paths of T. We provide a quadratic-time algorithm for this problem. We also show how to construct straight models that have k as its minimum number of bends and with the least number of vertices possible. As an application of our algorithm, we provide an upper bound on the number of bends of EPG models of graphs that are both VPT and EPT.

math.CO↗
arXiv2021

B1-EPG representations using block-cutpoint trees

In this paper, we are interested in the edge intersection graphs of paths of a grid where each path has at most one bend, called B1-EPG graphs and first introduced by Golumbic et al (2009). We also consider a proper subclass of B1-EPG, the L-EPG graphs, which allows paths only in ``L'' shape. We show that two superclasses of trees are B1-EPG (one of them being the cactus graphs). On the other hand, we show that the block graphs are L-EPG and provide a linear time algorithm to produce L-EPG representations of generalization of trees. These proofs employed a new technique from previous results in the area based on block-cutpoint trees of the respective graphs.

math.CO↗
arXiv2021

On the Order Parameter of the Continuous Phase Transition in the Classical and Quantum Mechanical limits

The mean field theory is revisited in the classical and quantum mechanical limits. Taking into account the boundary conditions at the phase transition and the third law of the thermodynamics the physical properties of the ordered and disordered phases were reported. The equation for the order parameter predicts the occurrence of a saturation of $Ψ^2$ = 1 near $Θ_S$, the temperature below the quantum mechanical ground state is reached. The theoretical predictions are also compared with high resolution thermal expansion data of SrTiO$_{\text{3}}$ monocrystalline samples and other some previous results. An excellent agreement has been found suggesting a universal behavior of the theoretical model to describe continuous structural phase transitions.

cond-mat.other↗
arXiv2019

Structure and dielectric properties of Ba$_2$Cu$_x$Y$_{1-x}$TaO$_{6-y}$ double perovskite

In this paper, we reported the effect of Cu doping on the structural and dielectric properties of Ba$_2$Y$_{1-x}$Cu$_x$TaO$_{6-y}$ (0.00 $\leq$ x $\leq$ 0.50) ceramics at room temperature. The Copper for Yttrium substitution reduces the sintering temperature and leads to structural changes in the Ba$_2$YTaO$_6$ rock-salt crystalline structure. Dielectric permittivity and complex impedance spectroscopy measurements suggested enhancement of the dielectric constant and occurrence of interfacial Maxwell-Wagner polarization.

cond-mat.mtrl-sci↗
arXiv2014

Interplay between antiferrodistortive, ferroelectric and superconducting instabilities in Sr_{1-x}Ca_{x}$TiO_{3-δ}

SrTiO$_{3}$ undergoes a cubic-to-tetragonal phase transition at 105K. This antiferrodistortive transition is believed to be in competition with incipient ferroelectricity. Substituting strontium by isovalent calcium induces a ferroelectric order. Introducing mobile electrons to the system by chemical non-isovalent doping, on the other hand, leads to the emergence of a dilute metal with a superconducting ground state. The link between superconductivity and the other two instabilities is an open question, which gathers momentum in the context of the growing popularity of the paradigm linking unconventional superconductors and quantum critical points. We present a set of specific-heat, neutron-scattering and dielectric permittivity and polarization measurements on Sr$_{1-x}$Ca$_{x}$TiO$_{3}$ ($0<x<0.009$) and a low-temperature electric conductivity in Sr$_{0.9978}$Ca$_{0.0022}$TiO$_{3-δ}$. Calcium substitution was found to enhance the transition temperature for both anti-ferrodistortive and ferroelectric transitions. Moreover, we find that Sr$_{0.9978}$Ca$_{0.0022}$TiO$_{3-δ}$ has a superconducting ground state. The critical temperature in this rare case of a superconductor with a ferroelectric parent, is slightly lower than in SrTiO$_{3-δ}$ of comparable carrier concentration. A three-dimensional phase diagram for Sr$_{1-x}$Ca$_{x}$TiO$_{3-δ}$ tracking the three transition temperatures as a function of x and $δ$ results from this study, in which ferroelectric and superconducting ground states are not immediate neighbours.

cond-mat.str-el↗