Crystalline Cluster Model For Ionic Solids: Nacl

A crystalline cluster model for ionic crystals is presented, in the large context of the self-consistent-field multiple-scattering method, by imposing that the cluster potential has the same features as the crystal potential. The model is particularly investigated with respect to NaCl, and the results are compared with those of a conventional cluster calculation. It is seen that the main features of the electronic structure emerge from the occupied cluster states and the obtained results of our model provide a good interpretation of experimentally observed properties, e.g., the experimental one-electron spectrum. © 1979 The American Physical Society.2083415342

Monte Carlo Simulations Applied To Alx Gay In1-x-y X Quaternary Alloys (x=as,p,n): A Comparative Study

We develop a different Monte Carlo approach applied to the Ax By C1-x-y D quaternary alloys. Combined with first-principles total-energy calculations, the thermodynamic properties of the (Al,Ga,In) X (X=As, P, or N) systems are obtained and a comparative study is developed in order to understand the roles of As, P, and N atoms as the anion X in the system Alx Gay In1-x-y X. Also, we study the thermodynamics of specific compositions in which AlGaInN, AlGaInP, and AlGaInAs are lattice matched, respectively, to the GaN, GaAs, and InP substrates. We verify that the tendency for phase separation is always towards the formation of an In-rich phase. For arsenides and phosphides this occurs in general for lower temperatures than for their usual growth temperatures. This makes these alloys very stable against phase separation. However, for nitrides the In and/or Al concentrations have to be limited in order to avoid the formation of In-rich clusters and, even for low concentrations of In and/or Al, we observe a tendency of composition fluctuations towards the clustering of the ternary GaInN. We suggest that this latter behavior can explain the formation of the InGaN-like nanoclusters recently observed in the AlGaInN quaternary alloys. © 2005 The American Physical Society.7120Stringfellow, G.B., (1983) J. Appl. Phys., 54, p. 404. , JAPIAU 0021-8979 10.1063/1.331719Olego, D., Chang, T.Y., Silberg, E., Caridi, E.A., Pinczuk, A., (1982) Appl. Phys. Lett., 41, p. 476. , APPLAB 0003-6951 10.1063/1.93537Fujii, T., Nakata, Y., Sigiyama, Y., Hiyiamizu, S., (1986) Jpn. J. Appl. Phys., Part 1, 25, p. 254. , JAPNDE 0021-4922Mowbray, D.J., Kowalski, O.P., Hopkinson, M., Skolnick, M.S., David, J.P.R., (1994) Appl. Phys. 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Theoretical Study Of Strain-induced Ordering In Cubic Inxga 1-xn Epitaxial Layers

Chemical ordering in cubic epitaxial InxGa1-xN layers is investigated by combining first-principles pseudo-potential plane-wave total-energy calculations, a local concentration-dependent cluster-based method, and Monte Carlo simulations. It is found that for the unstrained or fully relaxed layers there are no stable ordered structures, indicating the tendency of the alloy to undergo phase separation, in agreement with previous calculations and experiment. The energetics of the InxGal 1-xN layers pseudomorphycally grown on fully relaxed GaN (001) buffers shows that biaxial strain acts as the driving force for chemical ordering in the alloys. 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Electronic Excitation Of H2 By Electron Impact Using Soft Norm-conserving Pseudopotentials

We calculate electronic excitation cross sections for the b 3∑u + a 3∑g + c 3∏u, and d 3∏u states of H2 by electron impact. Our results were obtained with the Schwinger multichannel method with pseudopotentials and real potentials at the two-channel level of approximation. Pseudo-H atoms are used to generate H2 molecules with almost the same low-energy spectrum as the real molecules. We show that the dynamics of the electronic excitation process of the pseudomolecules by electron impact is very similar to the real case. Our results support the idea that pseudopotentials can be used to obtain reliable molecular electronic excitation cross sections by low-energy electron impact, confirming the expectations of previous studies with CH2O and HBr.54654355437Takatsuka, K., McKoy, V., (1981) Phys. Rev. A, 24, p. 2473(1984) Phys. Rev. A, 30, p. 1734Bettega, M.H.F., Ferreira, L.G., Lima, M.A.P., (1993) Phys. Rev. A, 47, p. 1111Natalense, A.P.P., Bettega, M.H.F., Ferreira, L.G., Lima, M.A.P., (1995) Phys. Rev. A, 52, pp. R1Bettega, M.H.F., Natalense, A.P.P., Lima, M.A.P., Ferreira, L.G., (1995) J. Chem. Phys., 103, p. 10566Rescigno, T.N., McCurdy, C.W., (1996) J. Chem. Phys., 104, p. 120Rescigno, T.N., (1996) J. Chem. Phys., 104, p. 125Hamann, D.R., Schlüter, M., Chiang, C., (1979) Phys. Rev. Lett., 43, p. 1494Christiansen, P.A., Lee, Y.S., Pitzer, K.S., (1979) J. Chem. Phys., 71, p. 4445Topp, W.C., Hopfield, J.J., (1973) Phys. Rev. B, 7, p. 1295Bachelet, G.B., Hamann, D.R., Schlüter, M., (1982) Phys. Rev. B, 26, p. 4199Bettega, M.H.F., Natalense, A.P.P., Lima, M.A.P., Ferreira, L.G., (1996) Int. J. Quantum Chem., 60, p. 821Goddard III, W.A., Hunt, W.J., (1974) Chem. Phys. Lett., 24, p. 464Kolos, W., Wolniewicz, L., (1965) J. Chem. Phys., 43, p. 2429Kolos, W., Wolniewicz, L., (1968) J. Chem. Phys., 48, p. 3672Browne, J.C., (1964) J. Chem. Phys., 40, p. 43Lee, M.T., Machado, L.E., Brescansin, L.M., Meneses, G.D., (1991) J. Phys. B, 24, p. 509Lima, M.A.P., Gibson, T.L., McKoy, V., Huo, W.M., (1988) Phys. Rev. A, 38, p. 4527. , and references cited therei

Electron Scattering From Molecules: Applications Of The Schwinger Multichannel Method To E--co And E--c2h 4 Collisions

To illustrate our recent efforts to obtain electronic excitation cross sections of molecules by electron impact, we present in this paper results for the X 1Σ a 3Π and A 1Π transitions of CO obtained with the Schwinger multichannel method. Our results are in good agreement with other theoretical calculations, although not so good when compared with experiments. We also discuss the importance of inclusion of polarization effects to obtain electronic excitation cross sections of some molecules through an example using the C2H4 molecule, which has a triplet state with a low-energy threshold. Finally, we present a very simple rule to estimate integral electronic excitation cross sections using the differential cross section (DCS) at 900, which can be useful to experimentalists using apparatus with difficulties to measure the DCS's at angles around 0 and 180 degrees. We show its efficiency for the present electronic excitation of the C2H4 molecule by electron impact. © 2007 IOP Publishing Ltd.881Garscadden, A., (1992) Z. Phys., 24 (2), pp. 97-99Boudaïffa, B., Cloutier, P., Hunting, D., Huels, M.A., Sanche, L., (2000) Science, 287 (5458), pp. 1658-1660Da Costa, R.F., Da Paixão, F.J., Map, L., (2004) J. Phys. B: At. Mol. Phys., 37 (6), pp. 129-L135Da Costa, R.F., Da Paixão, F.J., Map, L., (2005) J. Phys. B: At. Mol. Phys., 38 (24), pp. 4363-4378Da Costa, R.F., Map, L., (2006) Int. J. Quantum Chem., 106 (13), pp. 2664-2676Nonum Da Costa, R.F., Map, L., (2007) Phys. Rev., 75, p. 022705Sun, Q., Winstead, C., McKoy, V., Lima, M.A.P., (1992) J. Chem. Phys., 96 (5), pp. 3531-3535Rescigno, T.N., Schneider, B.I., (1992) Phys. Rev., 45 (5), pp. 2894-2902Takatsuka, K., McKoy, V., (1981) Phys. Rev., 24 (5), pp. 2473-2480Takatsuka, K., McKoy, V., (1984) Phys. Rev., 30 (4), pp. 1734-1740Chaudhuri, P., Varella Do, T.M.N., Carvalho, C.R.C., Map, L., (2004) Nucl. Instrum. Methods Phys. Res., 221, pp. 69-75Chaudhuri, P., Varella Do, T.M.N., Carvalho, C.R.C., Map, L., (2004) Phys. Rev., 69, p. 042703Lane, N.F., (1980) Rev. Mod. Phys., 52 (1), pp. 29-119Sun, Q.Y., Winstead, C., McKoy, V., (1992) Phys. Rev., 46 (11), pp. 6987-6994Morgan, L.A., Tennyson, J., (1993) J. Phys. B: At. Mol. Opt. Phys., 26 (15), pp. 2429-2441Lee, M.-T., MacHado, A.M., Fujimoto, M.M., MacHado, L.E., Brescansin, L.M., (1996) J. Phys. B: At. Mol. Opt. Phys., 29 (18), pp. 4285-4301Furlong, J.M., Newell, W.R., (1996) J. Phys. B: At. Mol. Opt. Phys., 29 (2), pp. 331-338Leclair, L.R., Trajmar, S., (1996) J. Phys. B: At. Mol. Opt. Phys., 29 (22), pp. 5543-5566Zetner, P.W., Kanik, I., Trajmar, S., (1998) J. Phys. B: At. Mol. Opt. Phys., 31 (10), pp. 2395-2413Trajmar, S., Szabo, A., Ostlund, N.S., (1989) Modern Quantum Chemistry: Introduction to Advanced Electronic Structure Theory, pp. 194-197Da Costa, R.F., Bettega, M.H.F., Lima, M.A.P., Van Veen, E.H., (1976) Chem. Phys. Lett., 41 (3), p. 540Asmis, K.R., Allan, M., (1997) J. Chem. Phys., 106 (17), pp. 7044-7046Da Costa, R.F., Bettega, M.H.F., Lima, M.A.P., Da Costa, R.F., Bettega, M.H.F., Lima, M.A.P.

Effects Of The Coulomb Correlation On The Calculated Results For Atoms With And Without Spin Polarization

In this paper we present the results of some atomic calculations as affected by the Coulomb correlation. We deal with atoms and ions with and without spin polarization. Thus we are able to compare calculated energy eigenvalues and hyperfine fields of contact with the experimental ionization energies and fields. Our approximation for the mass operator (exchange plus correlation) can thus be compared with the approximations proposed by Slater, Gaspar, Kohn and Sham, and Liberman. A broad evaluation of the results implies that our approximation for exchange and correlation is correct. Furthermore, when neglecting the correlation effects, we noticed that the results become poorer. Thus, this paper favors a single-particle description of many-body systems based on an effective-wave equation with the mass operator instead of the simpler exchange operator of the Hartree-Fock theory. © 1971 The American Physical Society.341224123

Wannier Functions And The Phases Of The Bloch Functions

We discuss the problem of what phases should be attributed to the Bloch functions in order to obtain Wannier functions with minimal widths. An exact solution to this problem is found by means of the kp perturbation formalism. We also consider localized crystal functions of a more general kind. These functions are found to obey a Schrödinger equation in reciprocal space, and may have some important assets when compared to the Wannier functions. © 1970 The American Physical Society.261614161

Microscopic Description Of The Phase Separation Process In Al Xgayin1-x-yn Quaternary Alloys

Ab initio total energy electronic structure calculations are combined with Monte Carlo simulations to study the thermodynamic properties of Al xGayIn1-x-yN quaternary alloys. We provide a microscopic description of the phase separation process by analyzing the thermodynamic behavior of the different atoms with respect to the temperature and cation contents. We obtained, at growth temperatures, the range of compositions for the stable and unstable phases. The presence of Al in InGaN is proven to "catalyze" the phase separation process for the formation of the In-rich phase. Based on our results, we propose that the ultraviolet emission currently seen in samples containing AlInGaN quaternaries arises from the matrix of a random alloy, in which composition fluctuations toward InGaN- and AlGaN-like alloys formation may be present, and that a coexisting emission in the green-blue region results from the In-rich segregated clusters.707732021-073202-4Lemos, V., Silveira, E., Leite, J.R., Tabata, A., Trentin, R., Scolfaro, L.M.R., Frey, T., Lischka, K., (2000) Phys. Rev. Lett., 84, p. 3666. , and references thereinKung, P., Razegui, M., (2000) Opto-Electron. Rev., 8, p. 201Kneissl, M., Treat, D.W., Teepe, M., Miyashita, N., Johnson, N.M., (2003) Appl. Phys. Lett., 82, p. 2386Adivarahan, V., Chitnis, A., Zhang, J.P., Shatalov, M., Yang, J.W., Simin, G., Khan, M.A., Shur, M.S., (2001) Appl. Phys. Lett., 79, p. 4240Yasan, A., McClintock, R., Mayes, K., Darvish, S.R., Zhang, H., Kung, P., Razeghi, M., Han, J.Y., (2002) Appl. Phys. Lett., 81, p. 2151Nagahama, S., Yanamoto, T., Sano, M., Mukai, T., (2001) Jpn. J. Appl. Phys., Part 2, 40, pp. L778Teles, L.K., Furthmüller, J., Scolfaro, L.M.R., Leite, J.R., Bechstedt, F., (2000) Phys. Rev. B, 62, p. 2475Teles, L.K., Scolfaro, L.M.R., Leite, J.R., Furthmüller, J., Bechstedt, F., (2002) J. Appl. Phys., 92, p. 7109Tamulaitis, G., Kazlauskas, K., Jursenas, S., Zukauskas, A., Khan, M.A., Yang, J.W., Zhang, J., Gaska, R., (2000) Appl. Phys. Lett., 77, p. 2136Hirayama, H., Kinoshita, A., Yamabi, T., Enomoto, Y., Hirata, A., Araki, T., Nanishi, Y., Aoyagi, Y., (2002) Appl. Phys. Lett., 80, p. 207Chen, C.H., Chen, Y.F., Lan, Z.H., Chen, L.C., Chen, K.H., Jiang, H.X., Lin, J.Y., (2004) Appl. Phys. Lett., 84, p. 1480Feng, S.W., Cheng, Y.C., Chung, Y.Y., Yang, C.C., Ma, K.J., Yan, C.C., Hsu, C., Jiang, H.X., (2003) Appl. Phys. Lett., 82, p. 1377Yamaguchi, S., Kariya, M., Nitta, S., Kato, H., Takeuchi, T., Wetzel, C., Amano, H., Akasaki, I., (1998) J. Cryst. Growth, 195, p. 309Takayama, T., Yuri, M., Itoh, K., Harris Jr., J.S., (2001) J. Appl. Phys., 90, p. 2358Matsuoka, T., (1998) MRS Internet J. Nitride Semicond. Res., 3, p. 54Marques, M., Teles, L.K., Ferreira, L.G., Scolfaro, L.M.R., Leite, J.R., unpublishedKresse, G., Furthmüller, J., (1996) Comput. Mater. Sci., 6, p. 15Marques, M., Teles, L.K., Scolfaro, L.M.R., Leite, J.R., Furthmüller, J., Bechstedt, F., (2003) Appl. Phys. Lett., 83, p. 890Ferreira, L.G., Wei, S.-H., Zunger, A., (1991) Int. J. Opt. Sens., 5, p. 34Metropolis, N., Rosenbluth, A.W., Rosenbluth, M.N., Teller, A.H., Teller, E., (1953) J. Chem. Phys., 21, p. 1087Cowley, J.M., (1950) J. Appl. Phys., 21, p. 2

Statistical Model Applied To Ax By C1-x-y D Quaternary Alloys: Bond Lengths And Energy Gaps Of Alx Gay In1-x-y X (x=as, P, Or N) Systems

We extend the generalized quasichemical approach (GQCA) to describe the Ax By C1-x-y D quaternary alloys in the zinc-blende structure. Combining this model with ab initio ultrasoft pseudopotential calculations within density functional theory, the structural and electronic properties of Alx Gay In1-x-y X (X=As, P, or N) quaternary alloys are obtained, taking into account the disorder and composition effects. Results for the bond lengths show that the variation with the compositions is approximately linear and also does not deviate very much from the value of the corresponding binary compounds. The maximum variation observed amounts to 3.6% for the In-N bond length. For the variation of band gap, we obtain a bowing parameter b=0.26 eV for the (Ga0.47 In0.53 As)z (Al0.48 In0.52 As)1-z quaternary alloy lattice matched to InP, in very good agreement with experimental data. In the case of AlGaInN, we compare our results for the band gap to data for the wurtzite phase. 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Assessment of Cement Durability in Repository Environment

Portland cement paste is proposed as the material to filling in the annulus between the casing of a borehole and the geological formation in a deep repository for spent sealed radiation sources in Brazil. The cement paste is intended to function as structural material, an additional barrier against the migration of radionuclides outside the repository, and as a blockage against the transport of water between the different strata of the geological setting. The objective of this research is to investigate the behavior of the cement paste and to estimate its service life. In this paper we present the results of mechanical strength measurements and chemical and mineralogical analysis of samples to detect the changes caused by radiation, temperature and aggressive chemicals of groundwater to which the material will be exposed. Methods of analysis included Inductively Coupled Plasma Atomic Emission Spectroscopy, Ion Chromatography, XRay Diffraction, and Thermo Gravimetric Analysi