XANES Study of Structural Disorder in Amorphous Silicon

An investigation of the structure of several amorphous silicon (a-Si) films is presented. Samples were prepared by using the ion beam sputtering technique at different substrate deposition temperatures. X-ray absorption spectroscopy and multiple scattering formalism have been used to detect structural variations of the a-Si films. The analysis of the XANES (X-ray absorption near-edge structure) spectra shows that increasing the substrate deposition temperature leads to a structural change toward a higher-level short-range order.

Indented Barrier Resonant Tunneling Rectifiers

This article concerns a novel negative-conductance device consisting of a series of N laterally indented barriers which exhibits resonant tunneling under one bias polarity and simple tunneling under the opposite one, thus acting as a rectifier. Electrons undergo resonant tunneling when the bias creates a band profile with N triangular wells which can each contain a resonant state. From 1 to N the addition of each indentation can be used to increase the current density and the rectification ratio, calculated at the current-peak bias at resonance, provided that at a given bias all the states in the triangular wells align each other with the emitter Fermi energy in order to form a resonance along the structure. © 1996 American Institute of Physics.80741744176Chang, L.L., Esaki, L., Tsu, R., (1974) Appl. Phys. Lett., 24, p. 593Sollner, T.C.L.G., Goodhue, W.D., Tannenwald, P.E., Parker, C.D., Peck, D.D., (1983) Appl. Phys. Lett., 43, p. 588Sollner, T.C.L.G., Tannenwald, P.E., Goodhue, W.D., Peck, D.D., (1984) Appl. Phys. Lett., 45, p. 1319Ricco, B., Azbel, M.Ya., (1984) Phys. Rev. B, 29, p. 1970Pötz, W., (1989) J. Appl. Phys., 66, p. 2458Frensley, W.R., (1990) Rev. Mod. Phys., 62, p. 745Chevoir, F., Vinter, B., (1993) Phys. Rev. B, 47, p. 7260Chang, L.L., Mendez, E.E., Tejedor, C., (1991) Resonant Tunneling in Semiconductors: Physics and Applications, , Plenum, New YorkSollner, T.C.L.G., Brown, E.R., Goodhue, W.C., Microwave and Milimeter-Wave Resonant Tunneling Diodes (1987) Picosecond Electronics and Optoelectronics Technical Digest, 87 (1), pp. 143-145. , Optical Society of America, Washington, D.CLiu, H.C., Coon, D.D., (1987) Appl. Phys. Lett., 50, p. 1669Schulz, P.A., Da Silva, G., (1988) Appl. Phys. Lett., 52, p. 960Papp, G., Di Ventra, M., Coluzza, C., Baldereschi, A., Margaritondo, G., (1995) Superlattices Microstruct., 17, p. 273Di Ventra, M., Papp, G., Coluzza, C., Baldereschi, A., (1994) Proceedings of the 22nd International Conference on the Physics of Semiconductors, p. 1600. , edited by D. J. Lockwood World Scientific, SingaporeVassel, M.O., Lee, J., Lockwood, H.F., (1983) J. Appl. Phys., 54, p. 5206Papp, C., Coluzza, C., Di Ventra, M., Baldereschi, A., Margaritondo, G., Gu, B.-Y., (1995) Superlattices Microstruct., 17, p. 117Adachi, S., (1985) J. Appl. Phys., 58, pp. R1Rossmanith, M., Syassen, K., Böckenhoff, E., Ploog, K., Von Klitzing, K., (1991) Phys. Rev. B, 44, p. 3168Ohno, H., Mendez, E.E., Wang, W.I., (1990) Appl. Phys. Lett., 56, p. 1793Guéret, P., Rossel, C., Marclay, E., Meier, H., (1989) J. Appl. Phys., 66, p. 278Guéret, P., Rossel, C., Schulp, W., Meier, H., (1989) J. Appl. Phys., 66, p. 431

Spectral sum rules for the Tomonaga-Luttinger model

In connection with recent publications we discuss spectral sum rules for the Tomonaga-Luttinger model without using the explicit result for the one-electron Green's function. They are usefull in the interpretation of recent high resolution photoemission spectra of quasi-one-dimensional conductors. It is shown that the limit of infinite frequency and band cut\-off do not commute. Our result for arbitrary shape of the interaction potential generalizes an earlier discussion by Suzumura. A general analytical expression for the spectral function for wave vectors far from the Fermi wave vector $k_{F}$ is presented. Numerical spectra are shown to illustrate the sum rules.Comment: 9 pages, REVTEX 3.0, 2 figures added as postscript file

Analytic Markovian Rates for Generalized Protein Structure Evolution

A general understanding of the complex phenomenon of protein evolution requires the accurate description of the constraints that define the sub-space of proteins with mutations that do not appreciably reduce the fitness of the organism. Such constraints can have multiple origins, in this work we present a model for constrained evolutionary trajectories represented by a Markovian process throughout a set of protein-like structures artificially constructed to be topological intermediates between the structure of two natural occurring proteins. The number and type of intermediate steps defines how constrained the total evolutionary process is. By using a coarse-grained representation for the protein structures, we derive an analytic formulation of the transition rates between each of the intermediate structures. The results indicate that compact structures with a high number of hydrogen bonds are more probable and have a higher likelihood to arise during evolution. Knowledge of the transition rates allows for the study of complex evolutionary pathways represented by trajectories through a set of intermediate structures

Critical Properties of Spectral Functions for the 1D Anisotropic t-J Models with an Energy Gap

We exactly calculate the momentum-dependent critical exponents for spectral functions in the one-dimensional anisotropic t-J models with a gap either in the spin or charge excitation spectrum. Our approach is based on the Bethe ansatz technique combined with finite-size scaling techniques in conformal field theory. It is found that the spectral functions show a power-law singularity, which occurs at frequencies determined by the dispersion of a massive spin (or charge) excitation.We discuss how the nontrivial contribution of a massive excitation controls the singular behavior in optical response functions.Comment: 4 pages, REVTeX, 2 figures(available upon request), accepted for publication in JPSJ 66 (1997) No.

Implementing efficient concerted rotations using Mathematica and C code

In this article we demonstrate a general and efficient metaprogramming implementation of concerted rotations using Mathematica. Concerted rotations allow the movement of a fixed portion of a polymer backbone with fixed bending angles, like a protein, while maintaining the correct geometry of the backbone and the initial and final points of the portion fixed. Our implementation uses Mathematica to generate a C code which is then wrapped in a library by a Python script. The user can modify the Mathematica notebook to generate a set of concerted rotations suited for a particular backbone geometry, without having to write the C code himself. The resulting code is highly optimized, performing on the order of thousands of operations per second

Finite temperature spectral-functions of strongly correlated one-dimensional electron systems

The spectral functions of tJ and tJ_{XY} models in the limit of J/t-> 0 and at finite temperatures T>>t are calculated using the spin-charge factorized wave function. We find that the Luttinger-liquid like scaling behavior for a finite system with L sites is restricted below temperatures of the order T = J/L. We also observe weight redistribution in the photoemission spectral function in the energy range t, which is much larger than the temperature.Comment: revtex, 4 pages, 3 eps figure