Showing papers by "Fernando Flores published in 1988"

Contact resistance in the scanning tunneling microscope at very small distances

Abstract: A theoretical analysis of the contact resistance in the scanning tunneling microscope at very small distances is presented. For a single atom, we find that the resistance saturates at close contact, its value being $\frac{h}{{2e}^{2}}$. Our analysis of the recent experimental results of Gimzewski and M\"oller shows that the mechanical instability predicted by Pethica and co-workers appears for a distance of 1.5 \AA{} between the tip and the sample.

Molecular orbital theory and tunnelling currents

Abstract: The parameters appearing in an effective tight-binding Hamiltonian are shown to be simply related to the tunnelling currents and the overlap between the different orbitals of the constituent atoms. Different systems are considered and the results show that our approach applies both to molecular and condensed-matter systems.

K/Si(100) 2 × 1: A Case Study for the Transfer of Charge between Alkali Metals and Semiconductor Surfaces

Abstract: Experimental data and theoretical calculations for K/Si(100) 2 × 1 provide a picture of the interface interaction essentially similar to the one established for adsorption of alkali metals on metals in contradiction with recent calculations. In particular, the transfer of charge from K to Si decreases with coverage and it is far from being complete at saturation coverage. The K-Si bond length and the charge transfer indicate that the bonding has a substantial covalent character.

Tight-binding theory of tunneling current with chemisorbed species.

Abstract: A tight-binding calculation of the tunneling current for the scanning tunneling microscope with chemisorbed species is presented. The calculation employs no adjustable parameters: some contact can be made with (a) the experiments of Bar\'o et al. [Phys. Rev. Lett. 52, 1304 (1984)] for chemisorbed O on Ni(110) and (b) local-density calculations; in both cases, quite good agreement is obtained.

Dynamic screening: Capture and loss processes of protons moving in solids

Abstract: The many-body self-energy approach combined with ordinary atomic physics has been used to calculate the charge states and the capture and loss cross sections of protons moving in solids. The results show that for high velocities the processes are atomic like, while at intermediate velocities solid-state effects are responsible for the proton charges.

Inelastic cross sections and charge states for B, C, N, and O ions moving in metals

Abstract: We calculate the cross sections associated with the electronic capture and loss of B, C, N, and O ions moving in a solid target with velocities vapprox.Zv/sub 0/ (where v/sub 0/ is the Bohr velocity). Three mechanisms are considered: Auger processes, whereby an electron is captured or lost creating an electronic excitation in the solid; resonant processes, coherently induced by the target crystal potential; and direct capture of electrons from the target inner shells. Atomiclike processes control the charge transfer for the velocities vapprox. >Zv/sub 0/. The distribution of charge states has been calculated as a function of the ion velocity. Good agreement is found with the available experimental data.

Theoretical aspects of scanning tunneling microscopy

Abstract: Two of the most interesting fields to which the recently discovered Scanning Tunneling Microscopy (STM) has been applied, are surface topography and surface spectroscopy. In this paper theoretical work carried out to interpret STM images and spectroscopic data will be reviewed. Special emphasis is made on models introduced to study tunneling through localised states (image or surface states). The close relationship between surface spectroscopy and surface topography, as viewed by STM, is also highlighted. Besides, theoretical analyses of the mechanisms involved in energy dissipation in STM will be also discussed; both metallic and insulating samples will be considered.

Contact resistance and saturation effects in the scanning tunnelling microscope: The resistance quantum unit

Abstract: SUMMARY We present a theoretical analysis of the contact resistance in the Scanning Tunnelling Microscope. Under the assumption of a single contact atom on the tip's apex, we find that the resistance saturates at close contact, its value being h/2e2. Our analysis of the recent experimental results of Gimzewski & Moller (1987), shows that the mechanical instability pradicted by Pethica & Sutton (1988), appears for a distance 1·5 A larger than the close contact distance between the tip and the sample.

Theory of tunneling in metal-superconductor devices: Supercurrents in the superconductor gap at zero temperature

Abstract: Tunneling experiments in metal‐oxide superconductor have shown the existence of ‘‘leakage’’ currents for applied voltages V smaller than one‐half of the superconductor gap Δ. These currents are independent of temperature T. Recently experiments with scanning tunneling microscopy (STM) and squeezable tunnel junctions have shown that the observation of the superconductor gap depends strongly on the resistance in the junction. In fact only for resistances larger than ∼106 Ω the gap is clearly observable. These experiments have been explained in terms of the perturbative Hamiltonian formalism of Bardeen. However, it may happen that this theory while applicable for very large resistances may not be so for small tunnel resistances. We present here a nonperturbative theory in all orders of the transmitivity ‖To‖2 and show the existence of supercurrents for values of V<Δ at T=0. This is done by performing a matching in the junction between the superconductor Bogolubov wave functions for Copper pairs and the elect...

Reply to "Comment on 'Heterojunction valence-band-discontinuity dependence on face orientation' "

Abstract: We respond to the criticisms of Van de Walle and Martin in the preceding Comment and present new calculations of the valence-band offsets.

Energy barriers and interface states at heterojunctions

Abstract: In this paper homopolar and heteropolar hcterojunctions have been analysed. The energy band discontinuities at heteropolar heterojunctions have been obtained self-consistently by analysing the charge induced at the interface. The crucial point of the analysis is the flow of charge between both semiconductors as a function of the difference in energy between their charge neutrality levels. For homopolar heterojunctions interface relaxation is discussed and it appears to be a function of the state of occupation of the interface. The analysis gives a relaxation of 4% for the 111-GeGaAs interface.

Raman scattering from atomic adsorbates

Abstract: We investigate the simplest system which can be anticipated to show Raman Scattering, for adsorbates on a substrate; the case of a single atom. Incident light provides the necessary energy to promote an electron from a state primarily located in the metal to a state which has the main weight on the adsorbed atom or vice versa. This charge transfer takes the atom from a neutral state bound weakly to the metal by van der Waals forces to a state more strongly bound of "image" type. This will change the vibrational properties both with respect to equilibrium separation as well as vibrational frequency and compared to SERS there is no enhancement since the vibration of the atom against the surface is a feature which is not present for the separated systems. From the scattered light that is vibrationally shifted from the incoming light we can then get out information about the atoms binding to the surface which ultimately can be used as a tool for investigating surface phenomena like physi- and chemisorption. We calculate and compare the differential cross-sections for the Ap interaction and the A2-mechanism. Special attention is devoted to adsorbate induced resonances in the vicinity of the Fermi level. Our results yield cross-sections of the order 10−31−10−30cm2, which are clearly observable even for a smooth surface without any field enhancements.