Centro de Ciências Exatas e da Terra

URI Permanente desta comunidadehttps://repositorio.ufrn.br/handle/1/135

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Low - temperature interface specific heat due to localized modes
(ELSEVIER, 1982) Carriço, Artur da Silva; Albuquerque, E. L.
Green functions are derived for the displacement associated with acoustic vibrations in isotropic elastic media, and used to evaluate the surface specific heat in the harmonic approximation. The contribution of surface elastic waves of the Rayleigh and love types are pointed out and their features discussed. The nature of the results and their relations to previous works in this field is also presented and discussed.
Modelling the raman spectrum of the amorphous- crystal Si system
(Institute of Physics (the “Institute”) and IOPscience, 1986) Carriço, Artur da Silva; Barrio, R. A.; Elliot, R. J.
A model to calculate the Raman response of small Si crystallites immersed in an amorphous Si matrix is developed. The structure of the system is modelled by an infinite crystalline slab of varying width, in which the atoms on the (111) surfaces on both sides of the slab are attached to Bethe lattices. The Raman response is obtained from a simple polarisability model using the Green function technique. The results are compared with recent experiments. The model can be used for Si crystallites with free and hydrogenated (111) surfaces.
Effects of electron-electron correlations on defect and interface states in amorphous Si and Si02 systems
(American Physical Society, 1986-07-15) Carriço, Artur da Silva; Barrio, R. A.; Elliot, R. J.
The effects of electron-electron correlations are included in the nearest-neighbor tight-binding model of tetrahedrally coordinated covalent solids in the amorphous phase. The amorphous systems are represented by Bethe lattices and the electron-electron interactions are parametrized for a short- range interaction in a Hubbard-like Hamiltonian. Self-consistent solutions are obtained in the Hartree-Fock approximation. If defects are included the self-consistent energy parameters vary with distance away from the defect but the solution is still possible if this variation falls away rapid- ly over a few shells. The cases of dangling bonds in Si and Si02 are considered by terminating the Bethe lattice. The former shows localized states in the gap for two charge configurations, but Si02 shows only one. The interface between Si and Si02 is represented by joining two Bethe lattices. The self-consistent solution shows appreciable charge transfer on to the first 0 atom and consequential changes in the local density of states.
Model of electronic states at the Si-Si02 interface
(American Physical Society, 1986-07-15) Carriço, Artur da Silva; Barrio, R. A.; Elliot, R. J.
The electronic properties of the interface between crystalline Si and its amorphous oxide SiO 2 have been studied within the tight-binding approximation by saturating the dangling bonds in three different surfaces (111), (110), and (100) of a semi-infinite Si crystal with SiO2 Bethe lattices. The localized states due to isolated dangling bonds in the interfaces (Pb centers) are found to be roughly at the same energy, in the Si gap for the (111) and (110) cases, but the (100) interface differs substantially from the other two. It is also found that the value of the site-energy interaction parameter on the Si defect site is important in determining the energy of the defect state.
Transfer matrix approach to metal surfaces vibrations
(Elsevier B.V., 1989) Carriço, Artur da Silva
Surface phonons propagating in arbitrary directions in the surface of FCC metals are studied via vibrational Green's functions obtained through the definition of transfer matrices. The dynamical matrix is described in the central force model. The method is applicable to any direction in the surface and is specially efficient for high symmetry directions. Ni (111) surface phonons are obtained with excellent agreement with previous experimental results.
Vibrational properties of amorphous Si-N and Ge- N alloys
(IOP Publishing Ltda, 1989) Carriço, Artur da Silva; Barrio, R. A.; Marques, F. C.; Sanjurjo, J.; Chambouleyron, I.
A theoretical study of the vibrational density of states of amorphous alloys was developed by applying a simple effective-medium theory with a Born Hamiltonian. This theory is equivalent to the coherent-potential approximation, with the advantage that it is set in real space in a very simple way. With this theory, one can observe the differences in the spectra of amorphous Si and Ge when several concentrations of N atoms are added. The predictions of the theory are compared with data from Raman scattering experiments.
Influence of nonlinearities on the pass band structure of magnetic superlattices
(Elsevier, 1989) Carriço, Artur da Silva; Almeida, N. S.
We consider superlattices constructed with alternate layers of antiferromagnetic and nonmagnetic materials to study the effect produced by the first nonlinear correction of the susceptibility of the antiferromagnetic material, on the electromagnetic pass bands of these artificial structures. We focus specially on the modifications produced on the pass band edges, as compared to the behavior exhibited under very low magnetic fields. Applications are made for FeF2/ZnF2 superlattices with different thicknesses.
A model for disorder in vicinal surfaces
(IOP Publishing Ltda, 1991) Carriço, Artur da Silva
The average spectrum of a disordered f.c.c. vicinal surface is obtained from effective medium Green's function. The disorder is restricted to the surface plane and the scattering in the surface is treated in the Coherent Potential Approximation. The method applies to site-diagonal disorder and is valid for any crystallographic orientation. An application is made for an s-like Tight-Binding Hamiltonian with substitutional disorder. Depending on the strength of the perturbation potential, resonances or local bands are found in the surface spectrum.
Spin freezing of the spin-glass Fe0.25Zn0,75F2: a Mossbauer study
(Brazil Journal Physics, 1991-01) Araújo, José Humberto de; Cunha, Joao Batista Marimon da; Vasquez, Adalberto; Montenegro, F. C.; Rezende, S. M.; Amara, Lovelyn; Moro, J. T.; Coutinho-Filho, Mauricio D.
We report 57~e Mõssbauer measurements in the diluted Ising antiferromagnet Fe0.25Zn0,75F2 at temperatures between 4.2 and 28 K. DC susceptibility measurements in the same sample show a spin-glass phase at a freezing temperature Tg = 10K. We found that for this concentration there is a competitive coexistence of spin-glass behaviour and antiferromagnetic order. The freezing temperature is frequency-dependent and follows a power law
Vibrational spectrum of vicinal surfaces
(Elsevier B.V., 1991-07) Carriço, Artur da Silva; Nogueira Junior, Edvaldo
The vibrational spectrum of FCC metal vicinal surfaces is obtained from displacement-displacement Green's functions. Results are presented for the Pt(332) surface. Surface spectral densities are calculated, in the neighbourhood of the surface, for selected directions in the Brillouin zone. The coherent surface vibrations contain features that reproduce the available experimental results and predict others that might be observable by He scattering. The dispersion relation of a high frequency surface mode is calculated. The surface mode frequency is shown to depend weakly on the wave-vector along high symmetry lines in the surface Brillouin zone. The polarization and local character of this mode are closely related. Near the Brillouin zone centre and also near the zone boundary it is strongly polarized perpendicularly to the surface and a sudden polarization change is seen when its frequency approaches the surface projection of the bulk spectrum.
Mössbauer effect study of a pleistocenic Brazilian fossil
(Springer, 1991-11) Araújo, José Humberto de; Kunrath, J. I.; Costa Jr, M. I. da; Vasquez, A.; Alves Jr, C.; Oliveira, L. D. Dantas de
Some preliminary Mössbauer effect results for a pleistocenic bone fossilized by mineralization process are presented. The presence of some iron-sulphur compounds permits to infer a correlation between the oxidation state of iron and the environmental conditions of fossilization
Mössbauer study of spin-glass Fe x Zn1−x F2 system
(Hyperfine Interaction, 1991-11) Araújo, Jose Humberto de; Cunha, Joao Batista Marimon da; Vasquez, A.; Amaral, L.; Moro, J. T.; Montenegro, F. C.; Rezende, S. M.; Coutinho-Filho, Mauricio D.
57Fe Mössbauer effect measurements in the diluted Ising antiferromagnet Fe x Zn1−x F2 withx=0.25 andx=0.57 at temperatures between 4.2 and 55 K, are reported. DC suceptibility measurements show a spin-glass (SG) phase at low temperatures forx≤0.31. Our Mössbauer spectra show a phase transition to a SG state with antiferromagnetic order (AFSG) forx=0.25 and only antiferromagnetic order forx=0.57.
Size and interface effects in the phase transitions of antiferromagnetic superlattices
(Elsevier B.V., 1992-04) Carriço, Artur da Silva; Camley, R. E.
A self-consistent mean field theory is used to study composition effects on the phase transitions of FeF2/CoF2 and FeF2/MnF2 superlattices under weak applied fields. The temperature dependence of the magnetization is shown to depend strongly on the superlattice layering pattern and also on the interface coupling. Thin films of low anisotropy compounds display stronger shifts of transition temperature. Comparison with bulk results shows that size effects persist even for very thick layers for weakly coupled systems.
Phase transitions in antiferromagnetic superlattices
(American Physical Society, 1992-06-01) Carriço, Artur da Silva; Camley, A. S.
We investigate the temperature dependence of the magnetization of films and superlattices of uniaxial antiferromagnets using a self-consistent, mean-field approach and apply the theory to FeF2/CoF2 superlattices. The number of magnetic phase transitions in this system (one or two) depends on the thicknesses of the films in the superlattice and on the interface exchange constant. Weak interface coupling leads to two transitions as do thick films. Our findings are in good agreement with recent experimental results and indicate that the FeF2/CoF2 system displays strong interface coupling.
High precision studies of an intraplate earthquake sequence in northeast Brazil
(1992-09) Takeya, Mario Koechi; Pearce, Robert G.; ; http://lattes.cnpq.br/8658746289220629; Douglas, Ross Allan; Main, Ian
From May 1987 to April 1988, a 9 station, seismic network was operated near the town of João Câmara (533'S, 3551'W) in Rio Grande do Norte state, Brazil. The network was installed 6 months after the occurrence of a mb =5.1 earthquake. This thesis is concerned with the study of microearthquakes recorded by this network, which is in the Precambrian Borborema Province of northeast Brazilian shield. The study revealed a remarkably well-defined distribution of seismicity associated with a simple fault structure. Hypocentre location was done using the HYPO71 program. The results of the analysis show that the fault is divided into two main segments with the same N37E strike. The north segment is dipping 76NW. The south segment is more nearly vertical, dipping about 82NW and composed of three or more closely spaced almost parallel faults. A small gap and a low seismicity zone were found to separate the north and south segments. No events deeper than 9 km were found in the entire region studied. Examination of the epicentral map and corresponding vertical cross sections also revealed that events are not random but are clustered. Right lateral strike slip with a small normal component was inferred as the fault mechanism from composite fault plane solutions. Off-fault events distant from the main fault were also observed, their pattern showing a classical example explained by the model of Das & Scholz as a consequence of off-fault shear stress increase after the occurrence of the main earthquake. Magnitude determinations for the events recorded by the telemetric network have been performed by developing a specific earthquake magnitude scale based on the duration of the observed seismic signal. A maximum likelihood estimate of b-value shows no significant variation during the recording period. Shear wave splitting was observed in the J.Câmara region in all the selected events recorded by the three component station JCAZ. The polarization direction of the first split shear-wave arrival for those events lay in the north-south direction which does not agree with the premise of extensive dilatancy anisotropy for a maximum compressive stress in the east-west direction as inferred by the fault plane solutions.
Finite-size effects on spin configurations in antiferromagnetically coupled multilayers
(American Physical Society, 1992-11-01) Carriço, Artur da Silva; Camley, R. E.; Nörtemann, F. C.; Stamps, R. L.
Nonuniform canting states are found for multilayers of a finite size that are constructed from ferromagnetic films which are antiferromagnetically coupled. Our results are applicable to a wide variety of experimentally realizable systems such as Fe/Cr and Co/Ru. We find that at low fields, a twist in the ground-state configuration of the spins reduces the net Zeeman energy and is energetically favorable to the uniform canted state which has previously been assumed by most authors. At higher fields, the character of this twist changes and eventually leads to a state that is fully aligned along the direction of the applied magnetic field. A numerical self-consistent mean-field model is used to examine the properties of these states and a variational method is developed in order to obtain analytic expressions for the lengths and magnitudes of these twists. The deviations from the uniform spin-flop state at both high and low fields can be quite large and involve the entire sample.
Polaritons in modified antiferromagnets
(American Physical Society, 1992-11-01) Carriço, Artur da Silva; Fonseca, T. L.; Almeida, N. S.
A theoretical study of mixed modes of uniaxial Heisenberg antiferromagnets by a dc field is presented. The static field is applied in an arbitrary direction relative to the easy axis and special attention is given to the magnetostatic limit where previous calculations show intriguing behavior of surface modes. Retarded polaritons are also considered and the spectrum that should be observed through the attenuated-total-reflection technique is calculated. Numerical results are presented for MnF2 and FeF2 semi-infinite specimens.
Mössbauer effect characterization of Brazilian iron ores for mining industrial purposes
(Elsevier, 1993-04-04) Araujo, Jose Humberto; Costa Jr., M. I. da; Kunrath, J. I.; Cunha, J. B. M. da; Moro, J. T.
Iron ore is one of the important export products of Brazil. As only ore bodies with over 50% iron contents are mined, continuous efforts in prospecting are made. The use of Mössbauer spectroscopy as an aid to these efforts is illustrated by an example given in this report.
Nonlinear response of antiferromagnetic films to radiation at oblique incidence
(American Physical Society, 1993-10-01) Carriço, Artur da Silva; Almeida, N. S.
The response of an antiferromagnetic film to incident radiation at oblique incidence is studied in the nonlinear regime. For given values of the frequency and incidence angle, the reflectivity of a film of arbitrary thickness is obtained as an implicit function of the incident field amplitude. The compound effect of the incidence angle and field strength on the reflected radiation is studied by means of the analysis of the reflection coefficient calculated for different values of these external variables. We use the physical parameters of the uniaxial Heisenberg antiferromagnet FeF2 to obtain numerical results. It is found that the threshold for nonlinear optical behavior is strongly dependent on the incidence angle.
Phase diagram of thin antiferromagnetic films in strong magnetic fields.
(American Physical Society, 1994-11-01) Carriço, Artur da Silva; Camley, R. E.; Stamps, R. L.
A theoretical study of the phase diagram of antiferromagnetic thin films is presented. The theory is based on a numerical self-consistent local-field calculation that allows for size and surface effects. The properties of magnetic structures as functions of the temperature and external field are calculated. Spatially nonuniform canted states are shown to intermediate the transition from the antiferromagnetic to the spin-flop phases. The model is applied to thin films of FeF2 and MnF2 as examples in order to clarify the role of anisotropy.

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