Nanomechanics of a Hydrogen Molecule Suspended between Two Equally Charged Tips

Geometric configuration and energy of a hydrogen molecule centered between two point-shaped tips of equal charge are calculated with the variational quantum Monte-Carlo (QMC) method without the restriction of the Born-Oppenheimer (BO) approximation. Ground state nuclear distribution, stability, and low vibrational excitation are investigated. Ground state results predict significant deviations from the BO treatment that is based on a potential energy surface (PES) obtained with the same QMC accuracy. The quantum mechanical distribution of molecular axis direction and bond length at a sub-nanometer level is fundamental for understanding nanomechanical dynamics with embedded hydrogen. Because of the tips' arrangement, cylindrical symmetry yields a uniform azimuthal distribution of the molecular axis vector relative to the tip-tip axis. With approaching tips towards each other, the QMC sampling shows an increasing loss of spherical symmetry with the molecular axis still uniformly distributed over the azimuthal angle but peaked at the tip-tip direction for negative tip charge while peaked at the equatorial plane for positive charge. This directional behavior can be switched between both stable configurations by changing the sign of the tip charge and by controlling the tip-tip distance. This suggests an application in the field of molecular machines.Comment: 20 pages, 10 figure

Effective calculation of LEED intensities using symmetry-adapted functions

The calculation of LEED intensities in a spherical-wave representation can be substantially simplified by symmetry relations. The wave field around each atom is expanded in symmetry-adapted functions where the local point symmetry of the atomic site applies. For overlayer systems with more than one atom per unit cell symmetry-adapted functions can be used when the division of the crystal into monoatomic subplanes is replaced by division into subplanes containing all symmetrically equivalent atomic positions

Influence Of Ch4/h2 Reactive Ion Etching On The Deep Levels Of Si-doped Alxga1-xas (x=0.25)

We study the passivation and recovery of shallow and deep levels in Si-doped AlGaAs exposed to CH4/H2 and H2 reactive ion etching (RIE). The carrier concentration depth profile is determined by capacitance-voltage measurements. The activation energy to recover the silicon donors is found to be 1.1 eV for samples exposed to CH4/H2 RIE and 1.3 eV for samples exposed to H2 RIE. We study the behavior of DX centers in Si-doped AlGaAs layers after RIE exposure and subsequent thermal annealing by using deep level transient spectroscopy. For CH4TH2 RIE a new emission is detected at the high temperature side. We identify this emission as the DX3 center, which is assigned to a DX center with three aluminum atoms surrounding the Si donor. This DX center is only detected on the samples exposed to CH4/H2 RIE. We explain the formation of this deep level to the highly selective removal of Ga atoms in favor of Al atoms. Consequently Al-rich regions are created near the surface. © 1996 American Vacuum Society.14317731779Chevallier, J., Clerjaud, B., Pajot, B., (1991) Semiconductors and Semimetals, 34, p. 449. , Chap. 13Dautremont-Smith, W.C., (1988) Mater. Res. Soc. Symp. Proc., 104, p. 313Pearton, S.J., Dautremont-Smith, W.C., Chevallier, J., Tu, C.W., Cummings, K.D., (1986) J. Appl. Phys., 59, p. 2821Jalil, A., Chevallier, J., Pesant, J.C., Mostefaoui, R., Pajot, B., Murawala, P., Azoulay, R., (1987) Appl. Phys. Lett., 50, p. 439Chevallier, J., Pajot, B., Jalil, A., Mostefaoui, R., Rahbi, R., Boissy, M.C., (1988) Mater. Res. Soc. Symp. Proc., 104, p. 337Pavesi, L., Giannozzi, P., Reinhart, F.K., (1990) Phys. Rev. B, 42, p. 1864Pavesi, L., Giannozzi, P., (1991) Phys. Rev. B, 43, p. 2446Tavendale, A.J., Pearton, S.J., Williams, A.A., Alexiev, D., (1990) Appl. Phys. Lett., 56, p. 1457Yuan, M.H., Wang, L.P., Jin, S.X., Chen, J.J., Qin, G.G., (1991) Appl. Phys. Lett., 58, p. 925Srivastava, P.C., Chandra, S., Singh, U.P., (1991) Semicond. Sci. Technol., 6, p. 1126Cho, H.Y., Kim, E.K., Min, S., Chang, K.J., Lee, C., (1990) J. Appl. Phys., 68, p. 5077Zundel, T., Weber, J., (1989) Phys. Rev. B, 39, p. 13549Roos, G., Johnson, N.M., Herring, C., Harris, J.S., (1991) Appl. Phys. Lett., 56, p. 461Pearton, S.J., Abernathy, C.R., Lopata, J., (1991) Appl. Phys. Lett., 59, p. 3571Morrow, R.A., (1989) J. Appl. Phys., 66, p. 2973Cameron, N.I., Beaumont, S.P., Wilkinson, C.D.W., Johnson, N.P., Kean, A.H., Stanley, C.R., (1990) J. Vac. Sci. Technol. B, 8, p. 1966Cameron, N.I., Beaumont, S.P., Wilkinson, C.D.W., Johnson, N.P., Kean, A.H., Stanley, C.R., (1990) Microelectron. Eng., 11, p. 607Cheung, R., Thorns, S., McIntyre, I., Wilkinson, C.D.W., Beaumont, S.P., (1988) J. Vac. Sci. Technol. B, 6, p. 1911Jackson, G.S., Beberman, J., Feng, M.S., Hsieh, K.C., Holonyak Jr., N., Verdeyen, J., (1988) J. Appl. Phys., 64, p. 5175Dautremont-Smith, W.C., Nabity, J.C., Swaminathan, V., Stavola, M., Chevallier, J., Tu, C.W., Pearton, S.J., (1986) Appl. Phys. Lett., 49, p. 1098Jalil, A., Heurtel, A., Marfaing, Y., Chevallier, J., (1989) J. Appl. Phys., 66, p. 5854Nabity, J.C., Stavola, M., Lopata, J., Dautremont-Smith, W.C., Tu, C.W., Pearton, S.J., (1987) Appl. Phys. Lett., 50, p. 921Morrow, R.A., (1991) J. Appl. Phys., 69, p. 4306Chadi, D.J., Chang, K.J., (1988) Phys. Rev. Lett., 61, p. 873Chadi, D.J., Chang, K.J., (1988) Phys. Rev. B, 39, p. 10063Mooney, P.M., (1990) J. Appl. Phys., 67, pp. R1Chang, K.J., (1990) Proc. 20th Conference on Physics Semiconductor, 1, p. 787. , edited by E. M. Anastassakis and J. D. JoannopoulosCollot, P., Gaonach, C., (1990) Semicond. Sci. Technol., 5, p. 237Constantine, C., Johnson, D., Pearton, S.J., Chakrabarti, U.K., Emerson, A.B., Hobson, W.S., Kinsella, A.P., (1990) J. Vac. Sci. Technol. B, 8, p. 596Pearton, S.J., Chakrabarti, U.K., Hobson, W.S., (1989) J. Appl. Phys., 66, p. 2061Pearton, S.J., Abernathy, C.R., (1989) Appl. Phys. Lett., 55, p. 678Pearton, S.J., Hobson, W.S., Jones, K.S., (1989) J. Appl. Phys., 66, p. 5009Pereira, R., Van Hove, M., De Potter, M., Van Rossum, M., (1990) Electron. Lett., 26, p. 462Pereira, R., Van Hove, M., De Raedt, W., Jansen, Ph., Borghs, G., Jonckheere, R., Van Rossum, M., (1991) J. Vac. Sci. Technol. B, 9, p. 1978Chevallier, J., Dautremont-Smith, W.C., Tu, C.W., Pearton, S.J., (1985) Appl. Phys. Lett., 47, p. 108Hansen, W.L., Haler, E.E., Luke, P.N., (1982) IEEE Trans. Nucl. Sci., NS-29, p. 738Hall, R.N., (1984) IEEE Trans. Nucl. Sci., NS-31, p. 320Mooney, P.M., Theis, T.N., Wright, S.L., (1988) Appl. Phys. Lett., 53, p. 2546Mooney, P.M., Theis, T.N., Calleja, E., (1991) J. Electron. Mater., 20, p. 23Baba, T., Mizuta, M., Fujizawa, T., Yoshino, J., Kukimoto, H., (1989) Jpn. J. Appl. Phys., 28, pp. L891Morgan, T.N., (1991) J. Electron. Mater., 20, p. 6

Self-induced decoherence approach: Strong limitations on its validity in a simple spin bath model and on its general physical relevance

The "self-induced decoherence" (SID) approach suggests that (1) the expectation value of any observable becomes diagonal in the eigenstates of the total Hamiltonian for systems endowed with a continuous energy spectrum, and (2), that this process can be interpreted as decoherence. We evaluate the first claim in the context of a simple spin bath model. We find that even for large environments, corresponding to an approximately continuous energy spectrum, diagonalization of the expectation value of random observables does in general not occur. We explain this result and conjecture that SID is likely to fail also in other systems composed of discrete subsystems. Regarding the second claim, we emphasize that SID does not describe a physically meaningful decoherence process for individual measurements, but only involves destructive interference that occurs collectively within an ensemble of presupposed "values" of measurements. This leads us to question the relevance of SID for treating observed decoherence effects.Comment: 11 pages, 4 figures. Final published versio

Effect of bonding of a CO molecule on the conductance of atomic metal wires

We have measured the effect of bonding of a CO molecule on the conductance of Au, Cu, Pt, and Ni atomic contacts at 4.2 K. When CO gas is admitted to the metal nano contacts, a conductance feature appears in the conductance histogram near 0.5 of the quantum unit of conductance, for all metals. For Au, the intensity of this fractional conductance feature can be tuned with the bias voltage, and it disappears at high bias voltage (above $\sim$ 200 mV). The bonding of CO to Au appears to be weakest, and associated with monotomic Au wire formation.Comment: 6 figure

Raman, infrared and optical spectra of the spin-Peierls compound NaV_2O_5

We have measured polarized spectra of Raman scattering, infrared and optical transmission of NaV_2O_5 single crystals above the temperature of the spin-Peierls transition Tsp=35 K. Some of the far-infrared (FIR) phonon lines are strongly asymmetric, due to the spin-phonon interaction. In addition to the phonon lines, a broad band was observed in the c(aa)c Raman spectrum and in the E||a FIR transmission spectrum. A possible origin of these bands is discussed. The absorption band at 10000 cm-1 1.25 eV is attributed to vanadium d-d electronic transitions while the absorption edge above 3 eV is supposed to correspond to the onset of charge-transfer transitions.Comment: 7 figures, 8 page

Laboratory Constraints on a 33.9 MeV/c^2 Isosinglet Neutrino: Status and Perspectives

An anomaly in the time behaviour of the signals observed by the KARMEN Collaboration may be interpreted as the possible decay signature of a 33.9 MeV/c$^2$ mainly sterile neutrino. This note discusses the parameter space still open for the mixing of this hypothetical particle with the neutrinos of known leptonic flavour, considering the experimental results which became available recently, as well as those to be expected from forthcoming measurements. It is concluded that if no positive signature is observed, the envisaged laboratory experiments are not expected to close enterily the parameter space of mixing amplitudes. However, a proper reassessment of the ALEPH upper bound on the $\nu_\tau$ neutrino mass including the possibility of $\tau$ flavour mixing, would certainly help in reducing the parameter space left open.Comment: LaTeX file, 11 pages, one figure available on reques