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Browsing by Author "Di Ventra, M."

Now showing 1 - 13 of 13
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    Can we make the SiC-SiO2 interface as good as the Si-SiO2 interface?
    Di Ventra, M. (AIP Publishing, 2001-10)
    A simple analysis based on the bulk valence and conduction densities of states was employed to estimate the interface-state densities for interfaces between the three most common SiC polytypes (3C, 4H, and 6H) and SiO2. We found that all polytypes had comparable conduction-band interface-state density with silicon dioxide as Si, being higher for the valence band. The conduction-band interface-state density should be higher for 4H-SiC than for 6H-SiC for both the C- or Si-terminated interfaces. On the contrary, the valence-band interface-state density can be either higher or lower for 4H-SiC compared to 6H-SiC according to which atom, C or Si, terminates the interface. The trends suggested by the above model are in agreement with recent mobility measurements in SiC-based field-effect transistors. (C) 2001 American Institute of Physics.
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    Chaotic transport in low-dimensional superlattices
    Zwolak, M.; Ferguson, D.; Di Ventra, M. (American Physical Society, 2003-02)
    We predict that in arrays of quantum dots (0D superlattice) and arrays of one-dimensional quantum wires (1D superlattice) chaotic transport should be observed in the presence of an ac field and for a wide range of physical parameters, like the external dc bias, contact charge, doping levels, and disorder in the array. Time-dependent current oscillations set in the array due to the formation of electric domain walls when sequential resonant tunneling is the main transport mechanism between adjacent units. Such oscillations can then be forced into spatiotemporal chaos. A similar phenomenon has been predicted and demonstrated for solid-state superlattices. However, contrary to the latter case, the domain walls move across a larger number of units in the superlattice the lower the dimensionality, due to the different spatial distribution of the electric-field across the array in the three cases.
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    Comment on "First-principles treatments of electron transport properties for nanoscale junctions"
    Lang, N. D.; Di Ventra, M. (American Physical Society, 2003-10)
    The use of a jellium model for the electrodes gives a good account of the conductance of a gold nanowire linking two metallic electrodes. The statement to the contrary in the recent paper of Fujimoto and Hirose [Phys. Rev. B 67, 195315 (2003)] is based on an incorrect positioning of the edge of the jellium relative to the outermost lattice plane of the electrode it represents.
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    DNA spintronics
    Zwolak, M.; Di Ventra, M. (AIP Publishing, 2002-07)
    We predict, using a tight-binding model, that spin-dependent transport can be observed in short DNA molecules sandwiched between ferromagnetic contacts. In particular, we show that a DNA spin valve can be realized with magnetoresistance values of as much as 26% for Ni and 16% for Fe contacts. Spin-dependent transport can broaden the possible applications of DNA as a component in molecular electronics and shed new light into the transport properties of this important biological molecule. (C) 2002 American Institute of Physics.
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    Effects of geometry and doping on the operation of molecular transistors
    Yang, Z. Q.; Lang, N. D.; Di Ventra, M. (AIP Publishing, 2003-03)
    We report first-principles calculations of current versus gate voltage characteristics of a molecular transistor with a phenyldithiolate molecule as active element. We show that (i) when the molecule is placed in proximity to the gate electrode, current modulation and resonant tunneling can occur at very small gate voltages. This is due to the first-order perturbation of the electronic states induced by the electrostatic potential of the gate in the molecular region. Such perturbation is present even if the molecule does not have an intrinsic dipole moment. (ii) The molecular transistor can be converted from n-type to p-type by the simple co-adsorption of a single oxygen atom placed near the molecule. While the latter finding suggests that the character of molecular transistors can be easily changed by doping the electrode surfaces, it also puts severe constraints on the experimental control of such structures for molecular electronics applications. (C) 2003 American Institute of Physics.
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    Koster-Slater model for the interface-state problem
    Di Ventra, M.; Berthod, C.; Binggeli, N. (American Physical Society, 2000-10)
    A Koster-Slater approach to the problem of localized states at semiconductor interfaces has been developed. It allows us to relate the existence and/or the energy position of interface states to some essential bulk features of the constituent materials and some interface-bonding parameters. The condition for the existence of localized states and the relevance of the model will be discussed comparing the predictions entailed by the latter with the results of ab initio calculations on the Ge/GaAs (110) interface.
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    Nonlinear current-induced forces in Si atomic wires
    Yang, Z. Q.; Di Ventra, M. (American Physical Society, 2003-04)
    We report first-principles calculations of current-induced forces in Si atomic wires as a function of bias and wire length. We find that these forces are strongly nonlinear as a function of bias due to the competition between the force originating from the scattering states and the force due to bound states. We also find that the shorter the wire, the larger the average force in the wire, suggesting that the wires are more difficult to break under current flow with increasing length. The last finding is in agreement with recent experimental data.
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    Shot noise in nanoscale conductors from first principles
    Chen, Y. C.; Di Ventra, M. (American Physical Society, 2003-04)
    We describe a field-theoretic approach to calculate quantum shot noise in nanoscale conductors from first principles. Our starting point is the second-quantization field operator to calculate shot noise in terms of single quasiparticle wave functions obtained self-consistently within the density-functional theory. The approach is valid in both linear and nonlinear response and is particularly suitable in studying shot noise in atomic-scale conductors. As an example, we study shot noise in Si atomic wires between metal electrodes. We find that shot noise is strongly nonlinear as a function of bias and it is enhanced for one- and two-Si wires due to the large contribution from the metal electrodes. For longer wires it shows an oscillatory behavior for even and odd number of atoms with opposite trend with respect to the conductance, indicating that current fluctuations persist with increasing wire length.
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    Switching behavior of semiconducting carbon nanotubes under an external electric field
    Rochefort, A.; Di Ventra, M.; Avouris, P. (AIP Publishing, 2001-04)
    We investigate theoretically the switching characteristics of semiconducting carbon nanotubes connected to gold electrodes under an external (gate) electric field. We find that the external introduction of holes is necessary to account for the experimental observations. We identify metal-induced gap states (MIGS) at the contacts and find that the MIGS of an undoped tube would not significantly affect the switching behavior, even for very short tube lengths. We also explore the miniaturization limits of nanotube transistors, and, on the basis of their switching ratio, we conclude that transistors with channels as short as 50 Angstrom would have adequate switching characteristics. (C) 2001 American Institute of Physics.
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    Targeting specific eigenvectors and eigenvalues of a given Hamiltonian using arbitrary selection criteria
    Tackett, A. R.; Di Ventra, M. (American Physical Society, 2002-12)
    We present a method for calculating some select eigenvalues and corresponding eigenvectors of a given Hamiltonian. We show that it is possible to target the eigenvalues and eigenvectors of interest without diagonalizing the full Hamiltonian, by using any arbitrary physical property of the eigenvectors. This allows us to target, for example, the eigenvectors based on their localization properties (e.g., states localized at a given surface or interface). We also show that the method scales linearly with system size.
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    Transport in molecular transistors: Symmetry effects and nonlinearities
    Rashkeev, S. N.; Di Ventra, M.; Pantelides, S. T. (American Physical Society, 2002-07)
    We report first-principles calculations of the current-voltage and current-gate-field characteristics of model molecular transistors to explore the factors that control current amplification and other properties. We show that both the position and amplitude of resonant peaks are modified by the use of substituents that affect the symmetry and dipole moments of the molecules, and allow a linear versus nonlinear Stark effect. In addition, strong nonlinearities arise at large source-drain currents.
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    Transport in nanoscale conductors from first principles
    Di Ventra, M.; Lang, N. D. (American Physical Society, 2002-01)
    We describe a first-principles atomistic approach to calculate the electronic and atomic dynamics of nanoscale conductors under steady-state current flow. The approach is based on a self-consistent solution of the Lippmann-Schwinger equation within the density-functional formalism for a sample connected to two bare metallic electrodes with a finite bias. Three-terminal device geometries can also be described easily using the present approach. The formalism provides the most fundamental quantities to describe the dynamics of the whole system: the self-consistent electronic wave functions. With these, the forces on the atoms are determined according to a Helmann-Feynman-like theorem that takes into account the contribution of the continuum of states as well as of the discrete part of the spectrum. Examples of applications will be given in the case of molecular devices with different anchoring groups at the interface between the molecule and the electrodes. In particular, we find that conductances close to the quantum unit (2e(2)/h) can be achieved with a given molecular structure simply by increasing the atomic number of the anchoring group..
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    Variational and nonvariational principles in quantum transport calculations
    Yang, Z. Q.; Tackett, A.; Di Ventra, M. (American Physical Society, 2002-07)
    A variational principle is not generally satisfied in steady-state quantum transport as opposed to the case of ground-state problems. We show that for a short-range potential, a functional for the scattering amplitude can be introduced that is stationary for arbitrary variations about the exact scattering wave function. However, except for the special case of spherically symmetric potentials, the functional does not satisfy any minimum principle even in linear response and for single-channel scattering. The absence of a minimum principle puts severe limitations on the choice of trial wave functions in transport calculations. Examples of electronic transport in selected quantum wires will be presented to illustrate the problem.