Force Chain Evolution in a Two-Dimensional Granular Packing Compacted by Vertical Tappings

We experimentally study the statistics of force-chain evolution in a vertically-tapped two-dimensional granular packing by using photoelastic disks. In this experiment, the tapped granular packing is gradually compacted. During the compaction, the isotropy of grain configurations is quantified by measuring the deviator anisotropy derived from fabric tensor, and then the evolution of force-chain structure is quantified by measuring the interparticle forces and force-chain orientational order parameter. As packing fraction increases, the interparticle force increases and finally saturates to an asymptotic value. Moreover, the grain configurations and force-chain structures become isotropically random as the tapping-induced compaction proceeds. In contrast, the total length of force chains remains unchanged. From the correlations of those parameters, we find two relations: (i) a positive correlation between the isotropy of grain configurations and the disordering of force-chain orientations, and (ii) a negative correlation between the increasing of interparticle forces and the disordering of force-chain orientations. These relations are universally held regardless of the mode of particle motions with/without convection

X-ray method to study temperature-dependent stripe domains in MnAs/GaAs(001)

MnAs films grown on GaAs (001) exhibit a progressive transition between hexagonal (ferromagnetic) and orthorhombic (paramagnetic) phases at wide temperature range instead of abrupt transition during the first-order phase transition. The coexistence of two phases is favored by the anisotropic strain arising from the constraint on the MnAs films imposed by the substrate. This phase coexistence occurs in ordered arrangement alternating periodic terrace steps. We present here a method to study the surface morphology throughout this transition by means of specular and diffuse scattering of soft x-rays, tuning the photon energy at the Mn 2p resonance. The results show the long-range arrangement of the periodic stripe-like structure during the phase coexistence and its period remains constant, in agreement with previous results using other techniques.Comment: 4 pages, 4 figures, submitted to Applied Physics Letter

Magnetic reconfiguration of MnAs/GaAs(001) observed by Magnetic Force Microscopy and Resonant Soft X-ray Scattering

We investigated the thermal evolution of the magnetic properties of MnAs epitaxial films grown on GaAs(001) during the coexistence of hexagonal/orthorhombic phases using polarized resonant (magnetic) soft X-ray scattering and magnetic force microscopy. The results of the diffuse satellite X-ray peaks were compared to those obtained by magnetic force microscopy and suggest a reorientation of ferromagnetic terraces as temperature rises. By measuring hysteresis loops at these peaks we show that this reorientation is common to all ferromagnetic terraces. The reorientation is explained by a simple model based on the shape anisotropy energy. Demagnetizing factors were calculated for different configurations suggested by the magnetic images. We noted that the magnetic moments flip from an in-plane mono-domain orientation at lower temperatures to a three-domain out-of-plane configuration at higher temperatures. The transition was observed when the ferromagnetic stripe width L is equal to 2.9 times the film thickness d. This is in good agreement with the expected theoretical value of L = 2.6d.Comment: 16 pages in PD

Phonon Side Bands In The Optical Emission Of Zinc-blende-type Semiconductors

A model for the LO-phonon-related structure observed in the luminescence above the gap of InP is presented. The corresponding exciton-phonon quasiparticle spectrum is calculated for zinc-blende-type semiconductors using a Green's function formalism. It is shown that resonances may appear due to the interaction of the exciton continuum with excitations involving a 1s-exciton state plus an LO phonon. The corresponding electron-hole nonequilibrium distribution function is derived by solving the master equation, which depends on the rate of scattering by acoustic and optical phonons. These results enable the evaluation of the dependence of the luminescence intensity on light frequency and temperature. Explicit calculations are presented for InP, CdTe, and GaAs. In the case of InP they reproduce rather well the experimental luminescence profile observed above the gap and its dependence on temperature. The calculations explain why a similar structure has not been observed in the luminescence spectra of GaAs and CdTe. © 1993 The American Physical Society.4885187519

Aharonov-Bohm interference in quantum ring exciton: effects of built-in electric fields

We report a comprehensive discussion of quantum interference effects due to the finite structure of excitons in quantum rings and their first experimental corroboration observed in the optical recombinations. Anomalous features that appear in the experiments are analyzed according to theoretical models that describe the modulation of the interference pattern by temperature and built-in electric fields.Comment: 6 pages, 7 figure

E 1 Gap Of Wurtzite Inas Single Nanowires Measured By Means Of Resonant Raman Spectroscopy

Indium arsenide nanowires were synthesized with an intermixing of wurtzite and zincblende structure by chemical beam epitaxy with the vapor-liquid-solid mechanism. Resonant Raman spectroscopy of the transverse optical phonon mode at 215cm -1 reveals an E 1 gap of 2.47 eV which is assigned to the electronic band gap at the A point in the indium arsenide wurtzite phase. © 2011 American Institute of Physics.1399473474 Int. Union Pure Appl. Phys. (IUPAP C8 Comm.),Korean Ministry of Education, Science and Technology,Seoul Metropolitan Government,Office of Naval Research Global,Korea Tourism OrganizationDick, K.A., (2008) Prog. Cryst. Growth Charact. Mater., 54, pp. 138-173Milnes, A.G., Polyakov, A.Y., (1993) Mater. Sci. Eng. B, 18, pp. 237-259Dayeh, S.A., Susac, D., Kavanagh, K.L., Yu, E.T., Wang, D., (2009) Adv. Mater., 19, pp. 2102-2108Cardona, M., (1961) J. Appl. Phys., 32 (SUPPL.), pp. 2151-2155Antoci, S., Reguzzoni, E., Samoggia, G., (1970) Phys. Rev. Lett., 24, pp. 1304-1307Arguello, C.A., Rousseau, D.L., Porto, S.P.S., (1969) Phys. Rev., 181, pp. 1351-136

Defect spectroscopy of single ZnO microwires

The point defects of single ZnO microwires grown by carbothermal reduction were studied by microphotoluminescence, photoresistance excitation spectra, and resistance as a function of the temperature. We found the deep level defect density profile along the microwire showing that the concentration of defects decreases from the base to the tip of the microwires and this effect correlates with a band gap narrowing. The results show a characteristic deep defect levels inside the gap at 0.88 eV from the top of the VB. The resistance as a function of the temperature shows defect levels next to the bottom of the CB at 110 meV and a mean defect concentration of 4 1018 cm3 . This combination of techniques allows us to study the band gap values and defects states inside the gap in single ZnO microwires and opens the possibility to be used as a defect spectroscopy method.Fil: Villafuerte, Manuel Jose. Universidad Nacional de Tucumán. Facultad de Ciencias Exactas y Tecnología. Departamento de Física. Laboratorio de Física del Solido; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Ferreyra, J. M.. Universidad Nacional de Tucumán. Facultad de Ciencias Exactas y Tecnología. Departamento de Física. Laboratorio de Física del Solido; ArgentinaFil: Zapata, C.. Universidad Nacional de Tucumán. Facultad de Ciencias Exactas y Tecnología. Departamento de Física. Laboratorio de Física del Solido; ArgentinaFil: Barzola Quiquia, J.. University of Leipzig; AlemaniaFil: Iikawa, F.. Instituto de Física “Gleb Wataghin"; BrasilFil: Esquinazi, P.. University of Leipzig; AlemaniaFil: Huleani, S. P.. Universidad Nacional de Tucumán. Facultad de Ciencias Exactas y Tecnología. Departamento de Física. Laboratorio de Física del Solido; ArgentinaFil: de Lima, M. M.. Universidad de Valencia; EspañaFil: Cantarero, A.. Universidad de Valencia; Españ

Optical phonon modes of wurtzite InP

Optical vibration modes of InP nanowires in the wurtzite phase were investigated by Raman scattering spectroscopy. The wires were grown along the [0001] axis by the vapor-liquid-solid method. The A1(TO), E2h, and E1(TO) phonon modes of the wurtzite symmetry were identified by using light linearly polarized along different directions in backscattering configuration. Additionally, forbidden longitudinal optical modes have also been observed. Furthermore, by applying an extended 11-parameter rigid-ion model, the complete dispersion relations of InP in the wurtzite phase have been calculated, showing a good agreement with the Raman experimental data