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Navegação Instituto de Física (FIS) por título

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Acoustic-plasmon branches in photoexcited semiconductors

Esperidião, Antônio Sérgio Cavalcante; Vasconcellos, Aurea R.; Luzzi, Roberto (Solid State Communications, 1990)

We analyze the spectra of electronic elementary excitations in a double component plasma of photoinjected electrons and holes in semiconductors under continuous laser illumination. We show that in the resulting nonequilibrium ...
Addendum to “Behavior of a bipartite system in a cavity”

Linhares, C. A.; Malbouisson, Adolfo Pedro Carvalho; Malbouisson, Jorge Mário Carvalho (2010)

This note is an Addendum to our previous article [ Phys. Rev. A 81 053820 (2010)]. We show that under the assumption of a Bose-Einstein distribution for the thermal reservoir, zero-temperature properties of the entangled ...
Adsorption of monovalent metal atoms on graphene: a theoretical approach

Medeiros, Paulo V. C.; Mota, F. de Brito; Mascarenhas, Artur J. S.; Castilho, Caio Mário Castro de (2010)

This work investigates, using first-principles calculations, electronic and structural properties of hydrogen, lithium, sodium, potassium and rubidium that are adsorbed, in a regular pattern, on a graphene surface. The ...
Advances on surface structural determination by LEED

Castilho, Caio Mário Castro de; Soares, Edmar A.; Carvalho, Vagner E. de (2011)

In the last 40 years, low energy electron diffraction (LEED) has proved to be the most reliable quantitative technique for surface structural determination. In this review, recent developments related to the theory that ...
Algebraic criteria for entanglement in multipartite systems

Vianna, J. D. M.; Trindade, M. A. S.; Fernandes, M. C. B. (2008)

Quantum computing depends heavily on quantum entanglement. It has been known that geometric models for correlated two-state quantum systems (qubits) can be developed using geometric algebra. This suggests that entanglement ...
Alternative hyperspherical adiabatic decoupling scheme for tetratomic molecules: quantum two-dimensional study of the ammonia umbrella motion

Guimarães, Marcilio N.; Ragni, Mirco; Bitencourt, Ana Carla Peixoto; Prudente, Frederico Vasconcellos (2013)

The prototypical example of the inversion of ammonia is studied as an approach to hyperspheric decoupling schemes for tetratomic molecule. We perform two-dimensional calculations including totally symmetric stretching and ...
An ab initio study of electric properties of linear (HCN)N and (HNC)N aggregates in gas phase

Brandão, I.; Rivelino, Roberto; Fonseca, T. L.; Castro, M. A. (Chemical Physics Letters, 2013)

We have accurately estimated the dipole moment (μ), the static linear polarizability View the MathML source and first hyperpolarizability (βtot and βHRS) per unit of long linear chains of HCN and HNC molecules, using the ...
An ab initio study of electric properties of linear (HCN)N and (HNC)N aggregates in gas phase

Brandão, I.; Rivelino, Roberto; Fonseca, T. L.; Castro, M. A. (2013)

We have accurately estimated the dipole moment (μμ), the static linear polarizability View the MathML sourceα¯ and first hyperpolarizability (βtotβtot and βHRSβHRS) per unit of long linear chains of HCN and HNC molecules, ...
An Abelian model for rainfall

Pinho, Suani Tavares Rubim de; Andrade, Roberto Fernandes Silva (1998)

Statistical analyses of long-term records of daily rain suggest that rain phenomena might be a manifestation of self-organized criticality. In this work the essential mechanisms of rain phenomena, the growth of droplets ...
An alternative theoretical approach to describe planetary systems through a Schrödinger-type diffusion equation

Carneiro, Saulo; Oliveira Neto, M. de; Maia, L.A. (2004)

In the present work we show that planetary mean distances can be calculated with the help of a Schrödinger-type diffusion equation. The obtained results are shown to agree with the observed orbits of all the planets and ...
An exact approach to the oscillator radiation process in an arbitrarily large cavity

Andion, N. P.; Malbouisson, Adolfo Pedro Carvalho; Mattos Neto, A. (2001)

Starting from a solution of the problem of a mechanical oscillator coupled to a scalar field inside a reflecting sphere of radius R, we study the behaviour of the system in free space as the limit of an arbitrarily large ...
An upper limit to the photon fraction in cosmic rays above 1019 eV from the Pierre Auger Observatory

Abraham, J.; Aglietta, M.; Aguirre, C.; Allard, D.; Pepe, Iuri Muniz (2007)

An upper limit of 16% (at 95% c.l.) is derived for the photon fraction in cosmic rays with energies greater than 1019 eV, based on observations of the depth of shower maximum performed with the hybrid detector of the Pierre ...
Analogy between Landau theory of phase transitions and Lagrangian mechanics

Ribeiro Filho, A.; Tilley, D.R.; Žekš, B. (Physics Letters A, 1984)

It is shown that the definition of a stable point in landau theory is different from that used in mechanics. The implications for numerical work on phase transitions for systems that have a Lffshitz invariant are discussed.
Analysis of rainfall records: possible relation to self-organized criticality

Andrade, Roberto Fernandes Silva; Schellnhuber, H. J.; Claussen, M. (1998)

The hypothesis that rainfall might be a case of self-organized critical dynamics is tested using long-term data sets from weather stations around the world. It is found that the distribution of droughts in semi-arid regions ...
Analysis of vibrational modes of the P4 molecule through hyperspherical variants of the local orthogonal coordinates: the limit of dissociation in dimers

Ragni, Mirco; Prudente, Frederico Vasconcellos; Bitencourt, Ana Carla Peixoto; Barreto, Patrícia Regina Pereira (2011)

In this work, we discuss the representation in different orthogonal coordinates sets of highly symmetric P4 molecule whose equilibrium configuration is tetrahedral. For this purpose, the H (or diatom–diatom) and Wigner ...
Analytical approach to directed sandpile models on the Apollonian network

Vieira, André P.; Andrade Júnior, José Soares de; Herrmann, Hans J.; Andrade, Roberto Fernandes Silva (Brasil, 2007)

We investigate a set of directed sandpile models on the Apollonian network, which are inspired by the work of Dhar and Ramaswamy [Phys. Rev. Lett. 63, 1659 (1989)] on Euclidian lattices. They are characterized by a single ...
Analytical expression for K- and L-shell cross sections of neutral atoms near ionization threshold by electron impact

Campos, C. S.; Vasconcellos, M. A. Z.; Trincavelli, J. C.; Segui, S. (2007)

An analytical expression is proposed to describe the K- and L-shell ionization cross sections of neutral atoms by electron impact over a wide range of atomic numbers (4 ≤ Z ≤ 79) and over voltages U < 10. This study is ...
Analytical solutions for Yukawa potential applied to atomic systems embedded in debye plasmas.

Martins, M. Graças R.; Santos, Leandro Cerqueira; Vianna, J. David M. (2011)

A model to study atomic systems immersed in Debye plasmas is presented. Our model proposes a many-electron wave function built from analytical solutions for Yukawa potential. The variational principle is used considering ...
Análise multifractal de imagens médicas

Silva, Maria Caroline Santos da (Programa de Pós-Graduação em Física da UFBA, 2009)

Nos últimos anos, buscando melhorar os diagnósticos em imagens biomédicas, foram desenvolvidos diversos métodos de filtragem das imagens de forma a facilitar a detecção de padrões estruturais associados aos tumores. Por ...
Anisotropic cosmologies containing isotropic background radiation

Carneiro, Saulo; Marugán, Guillermo A. Mena (Brasil, 2001)

We present an anisotropic cosmological model based on a new exact solution of Einstein equations. The matter content consists of an anisotropic scalar field minimally coupled to gravity and of two isotropic perfect fluids ...

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