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U Landman

Publications and source records attributed to U Landman.

At least 19 recordsLinked to original sources
PubMed2001

Charge migration in DNA: ion-gated transport.

Electron hole (radical cation) migration in DNA, where the quantum transport of an injected charge is gated in a correlated manner by the thermal motions of the hydrated counterions, is described here. Classical molecular dynamics simulations in conjunction with large-scale first-principles electronic structure calculations reveal that different counterion configurations lead to formation of states characterized by varying spatial distributions and degrees of charge localization. Stochastic dynamic fluctuations between such ionic configurations can induce correlated changes in the spatial distribution of the hole, with concomitant transport along the DNA double helix. Comparative ultraviolet light-induced cleavage experiments on native B DNA oligomers and on ones modified to contain counterion (Na(+))-starved bridges between damage-susceptible hole-trapping sites called GG steps show in the latter a reduction in damage at the distal step. This reduction indicates a reduced mobility of the hole across the modified bridge as predicted theoretically.

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PubMed2001

Photoabsorption spectra of Na(+)(n) clusters: thermal line-broadening mechanisms.

Photoabsorption cross sections of small sodium cluster cations ( Na(+)(n), n = 3, 5, 7, and 9) were calculated at various temperatures with the time-dependent local-density approximation in conjunction with ab initio molecular dynamics simulations, yielding spectra that agree with measured ones without ad hoc line broadening or renormalization. Three thermal line-broadening mechanisms are revealed: (I) lifting of level degeneracies caused by symmetry-breaking ionic motions, (II) oscillatory shifts of the entire spectrum caused by breathing vibrations, and (III) cluster structural isomerizations.

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PubMed2001

COI oxidation on a single Pd atom supported on magnesia.

The oxidation of CO on single Pd atoms anchored to MgO(100) surface oxygen vacancies is studied with temperature-programmed-reaction mass spectrometry and infrared spectroscopy. In one-heating-cycle experiments, CO(2), formed from O(2) and CO preadsorbed at 90 K, is detected at 260 and 500 K. Ab-initio simulations suggest two reaction routes, with Pd(CO)(2)O(2) and PdCO(3)CO found as precursors for the low and high temperature channels, respectively. Both reactions result in annealing of the vacancy and induce migration and coalescence of the remaining Pd-CO to form larger clusters.

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PubMed2001

Multiply charged metal cluster anions.

Formation and stability patterns of silver dianionic and gold trianionic clusters are investigated with Penning-trap experiments and a shell-correction method including shape deformations. The theoretical predictions pertaining to the appearance sizes and electronic shell effects are in remarkable agreement with the experiments. Decay of the multiply anionic clusters occurs predominantly by electron tunneling through a Coulomb barrier rather than via fission, leading to appearance sizes unrelated to those of multiply cationic clusters.

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PubMed2001

Structure and magnetism of neutral and anionic palladium clusters.

The properties of neutral and anionic Pd(N) clusters were investigated with spin-density-functional calculations. The ground-state structures are three dimensional for N>3 and they are magnetic with a spin triplet for 2 < or = N < or = 7 and a spin nonet for N = 13 neutral clusters. Structural and spin isomers were determined and an anomalous increase of the magnetic moment with temperature is predicted for a Pd7 ensemble. Vertical electron detachment and ionization energies were calculated and the former agrees well with measured values for Pd(-)(N).

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PubMed2000

Metal-semiconductor nanocontacts: silicon nanowires

Silicon nanowires assembled from clusters or etched from the bulk, connected to aluminum electrodes and passivated, are studied with large-scale local-density-functional simulations. Short ( approximately 0.6 nm) wires are fully metallized by metal-induced gap states resulting in finite conductance ( approximately e(2)/h). For longer wires ( approximately 2.5 nm) nanoscale Schottky barriers develop with heights larger than the corresponding bulk value by 40% to 90%. Electric transport requires doping dependent gate voltages with the conductance spectra exhibiting interference resonances due to scattering of ballistic channels by the contacts.

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PubMed2000

Collective and independent-particle motion in two-electron artificial atoms

Investigations of the exactly solvable excitation spectra of two-electron quantum dots with a parabolic confinement, for different values of the parameter R(W) expressing the relative magnitudes of the interelectron repulsion and the zero-point kinetic energy, reveal for large R(W) a rovibrational spectrum associated with a linear trimeric rigid molecule composed of the two electrons and the infinitely heavy confining dot. This spectrum transforms to that of a "floppy" molecule for smaller R(W). The conditional probability distribution calculated for the exact two-electron wave functions allows identification of the rovibrational excitations as rotations and stretching/bending vibrations.

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PubMed2000

Formation, stability, and breakup of nanojets

Atomistic molecular dynamics simulations reveal the formation of nanojets with velocities up to 400 meters per second, created by pressurized injection of fluid propane through nanoscale convergent gold nozzles with heating or coating of the nozzle exterior surface to prevent formation of thick blocking films. The atomistic description is related to continuum hydrodynamic modeling through the derivation of a stochastic lubrication equation that includes thermally triggered fluctuations whose influence on the dynamical evolution increases as the jet dimensions become smaller. Emergence of double-cone neck shapes is predicted when the jet approaches nanoscale molecular dimensions, deviating from the long-thread universal similarity solution obtained in the absence of such fluctuations.

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