Electronic Properties of Mn-Compounds Under Strain

We study the physical properties of MnAs under strain by using accurate first-principles pseudopotential calculations. Our results provide new insight on the physics of strained multilayer that are grown epitaxially on different lattice mismatched substrates and which are presently of interest for spintronic applications. We compute the strain dependence of the structural parameters, electronic bands, density of states and magnetization. In the region of strain/stress that is easily directly accessible to measurements, the effects on these physical quantities are linear. We also address the case of uniaxial stress inducing sizeable and strongly non linear effects on electronic and magnetic properties.Comment: 8 pages, 6 figure

Non-Nominal Value of the Dynamical Effective Charge in Alkaline-Earth Oxides

We calculate ab-initio the electronic states and the Born dynamical charge Z* of the alkaline-earth oxides in the local-density approximation. We investigate the trend of increasing Z* values through the series, using band-by-band decompositions and computational experiments performed on fake materials with artificially-modified covalence. The deviations of Z* from the nominal value 2 are due to the increasing interaction between O 2p orbitals and unoccupied cation d states. We also explain the variations, along the series, of the individual contributions to Z* arising from the occupied band manifolds.Comment: 12 pages Latex, plus 2 Postscript figure

Floating bonds and gap states in a-Si and a-Si:H from first principles calculations

We study in detail by means of ab-initio pseudopotential calculations the electronic structure of five-fold coordinated (T_5) defects in a-Si and a-Si:H, also during their formation and their evolution upon hydrogenation. The atom-projected densities of states (DOS) and an accurate analysis of the valence charge distribution clearly indicate the fundamental contribution of T_5 defects in originating gap states through their nearest neighbors. The interaction with hydrogen can reduce the DOS in the gap annihilating T_5 defects.Comment: To appear in Europhysics Let

Coordination defects in a-Si and a-Si:H : a characterization from first principles calculations

We study by means of first-principles pseudopotential method the coordination defects in a-Si and a-Si:H, also in their formation and their evolution upon hydrogen interaction. An accurate analysis of the valence charge distribution and of the ``electron localization function'' (ELF) allows to resolve possible ambiguities in the bonding configuration, and in particular to identify clearly three-fold (T_3) and five-fold (T_5) coordinated defects. We found that electronic states in the gap can be associated to both kind of defects, and that in both cases the interaction with hydrogen can reduce the density of states in the gap.Comment: To appear in Philos. Ma

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

Effective Hamiltonian for Ga_{1-x}Mn_xAs in the Dilute Limit

We derive an effective Hamiltonian for ${\rm Ga}_{1-x}{\rm Mn}_x {\rm As}$ in the dilute limit, where ${\rm Ga}_{1-x}{\rm Mn}_x {\rm As}$ can be described in terms of spin $F=3/2$ polarons hopping between the {\rm Mn} sites and coupled to the local {\rm Mn} spins. We determine the parameters of our model from microscopic calculations. Our approach treats the large Coulomb interaction in a non-perturbative way, captures the effects of spin-orbit coupling and disorder, and is appropriate for other p-doped magnetic semiconductors. Our model applies to uncompensated {\rm Mn} concentrations up to $x \sim 0.03$.Comment: 4 pages, 4 figures, Version to appear in Phys. Rev. Let