Browsing by Author "Buldum, A."

Now showing 1 - 15 of 15

Open Access
Atomic scale study of friction and energy dissipation
(Elsevier, 2003-05) Çıracı, Salim; Buldum, A.
This paper presents an analysis of the interaction energy and various forces between two surfaces, and the microscopic study of friction. Atomic-scale simulations of dry sliding friction and boundary lubrication are based on the classical molecular dynamics (CMD) calculations using realistic empirical potentials. The dry sliding of a single metal asperity on an incommensurate substrate surface exhibits a quasi-periodic variation of the lateral force with two different stick-slip stage involving two structural transformation followed by a wear. The contact area of the asperity increases discontinuously with increasing normal force. Xe atoms placed between two atomically flat Ni surfaces screen the Ni-Ni interaction, decrease the corrugation of the potential energy as well as the friction force at submonolayer coverage. We present a phononic model of energy dissipation from an asperity to the substrates. (C) 2003 Elsevier Science B.V. All rights reserved.
Open Access
Atomic-scale study of dry sliding friction
(American Physical Society, 1997) Buldum, A.; Çıracı, Salim
We present a theoretical study of dry sliding friction, which has a close bearing on the experiments done by using the atomic and friction force microscope. By performing atomic-scale calculations for the friction between a single atom and monoatomic infinite chain, we examined the effect of various material parameters on the stick-slip motion. We found that the perpendicular elastic deformation of the substrate that is induced by the sliding object is crucial for the energy damping in friction. In this case, the average friction force strongly depends on the perpendicular force constant of the substrate and the friction constant varies with the normal force. In particular, soft materials that continue to be elastic for a wide range of perpendicular compression may exhibit a second state. As a result, the hysteresis curve in the stick-slip motion becomes anisotropic.
Open Access
Conductance through a single atom
(American Physical Society, 1997) Mehrez, H.; Çıracı, Salim; Buldum, A.; Batra, I. P.
In this paper we present an analysis of conduction through a single atom between two metal electrodes. Based on ab initio total-energy and electronic-structure calculations, and molecular-dynamics simulations using the embedded-atom model, we show that the conductance through an atom depends on the electronic structure of both the single atom and the metal electrodes, as well as the binding structure between the single atom and the surfaces of the metal electrodes. Our results enable us to interpret experimental results obtained by using a mechanical break junction on atomic-scale wires.
Open Access
Contact, nanoindentation, and sliding friction
(American Physical Society, 1998) Buldum, A.; Çıracı, Salim; Batra, I. P.
This paper presents an atomic-scale study of contact, indentation, and subsequent pulling and dry sliding of a sharp and blunt metal tip on a metal surface. The evolution of atomic structure and the variation of perpendicular and lateral forces are calculated by molecular-dynamics methods using an empirical potential based on the embedded-atom model. The sharp tip experiences multiple jumps to contact in the attractive force range. The contact interface grows discontinuously mainly due to disorder-order transformation leading to disappearance of a layer and hence abrupt changes in the normal-force variation. Atom exchange occurs in the repulsive range. During the pulling off, the connective neck is reduced discontinuously; however, not all the abrupt changes of the pulling force are associated with the creation of a new layer in the neck. The sliding of the sharp tip (or single asperity) induces two consecutive structural transformations that occur periodically, but end with the wear of a layer. The situation for a blunt tip is, however, quite different.
Open Access
Controlled lateral and perpendicular motion of atoms on metal surfaces
(American Physical Society, 1996) Buldum, A.; Çıracı, Salim
We present the theoretical study of the controlled lateral and perpendicular motion of Xe on the Pt(111) surface. The lateral translation of Xe is manipulated by a tungsten tip of a scanning tunneling microscope. Using molecular statics and dynamics the energetics and different modes of atom translation are revealed. In the controlled and reversible transfer of Xe between two flat Pt(111) surfaces, effective charge on Xe, and the dipole moment of the Xe-Pt bond, are calculated as functions of the Xe-surface separation. The contributions of various mechanisms to the transfer rate of Xe are investigated by using the calculated quantum states of Xe under the applied bias voltage. These are tunneling and ballistic transfer, dipole excitation and excitation due to resonant tunneling of electrons, and electron wind force. We found that a single power law for the transfer rate does not exist in the whole range of applied pulse voltage. At high pulse voltage the transfer rate is dominated by the inelastic electron tunneling. At low pulse voltage the rate due to thermally assisted tunneling and ballistic transfer becomes important.
Open Access
First-principles investigation of structural and electronic properties of solid cubane and its doped derivatives
(American Physical Society, 2000) Yildirim, T.; Çıracı, Salim; Kılıç, Ç.; Buldum, A.
The electronic and structural properties of molecular and solid cubane have been studied by first-principles, self-consistent field total energy calculations. Calculated molecular properties such as equilibrium geometry and electronic and vibrational spectra are found to be in good agreement with experimental data. Structural parameters and the energetics of both the low-temperature, orientationally ordered and high-temperature, orientationally disordered or plastic phases of solid cubane are determined. The valence band of solid cubane is derived from the molecular states; the energy gap between the lowest unoccupied and highest occupied molecular orbital bands is rather large due to the saturated carbon atoms. The effect of alkali-metal-atom doping on the electronic energy bands is investigated. It is found that the metallic band of doped cubane is derived from the undoped solid cubane’s lowest conduction band with a significant contribution from the alkali-metal atom.
Open Access
Interplay between stick-slip motion and structural phase transitions in dry sliding friction
(American Physical Society, 1997) Buldum, A.; Çıracı, Salim
Simulations of dry sliding friction between a metal asperity and an incommensurate metal surface reveal unusual atomic processes. The lateral force exhibits a quasiperiodic variation with the displacement of an asperity; each period consists of two different stick-slip processes involving structural transitions. While one layer of asperity changes and matches the substrate lattice in the first slip, two asperity layers merge into a new one through a structural transition during the second slip. This leads to wear. The lateral force decreases abruptly during these slip stages, but it increases between two consecutive slips and resists the relative motion. The analysis of the order suggests that each structural transition is associated with a first-order phase transition. Nonadiabatic atomic rearrangements during these phase transitions involve a new kind of mechanism of energy dissipation in the dry sliding friction.
Open Access
Interpretation of long-range interatomic force
(American Physical Society, 1999) Buldum, A.; Çıracı, Salim; Fong, C. Y.; Nelson J. S.
Recent direct mechanical measurements of atomic force microscopy showed that the force between the silicon tip and the silicon sample is long range in the attractive region and its magnitude at maximum is relatively smaller. These features disagree with previous theoretical predictions based on the ab initio calculations. We investigated the nature of forces between a silicon tip and the silicon (111)-(2×1) surface by performing first-principles pseudopotential and classical molecular dynamics calculations and by calculating the van der Waals interaction. The first two methods give forces that are short range in nature. Fair agreement between the experiment and theory is obtained when the van der Waals interaction is included. The effect of the tip induced deformation is analyzed.
Open Access
Lateral translation of an Xe atom on metal surfaces
(Institute of Physics Publishing, 1995) Buldum, A.; Çıracı, Salim; Erkoç, Ş.
This work presents the theoretical study of the controlled lateral translation of an Xe atom physisorbed on the Ni(110) surface. The motion of the Xe is manipulated by the tip of the scanning tunnelling microscope. The interaction of the physisorbed atom with the tip and sample surface is described by empirical potentials. Using molecular statics and dynamics, the energetics and different modes of the translation are revealed. Important effects of electrode relaxation, tip geometry and material parameters are briefly discussed.
Open Access
Model for phononic energy dissipation in friction
(American Physical Society, 1999) Buldum, A.; Leitner, D. M.; Çıracı, Salim
We have developed a microscopic model of phononic energy dissipation in friction that involves the generation of a local excess phonon distribution in a nanoparticle between two sliding objects, and its damping into the objects. The conversion of the energy stored in the nanoparticle into excess phonons and their decay rates are calculated. The model can be extended to include randomly distributed nanoparticles and phonon-phonon interaction through anharmonic couplings. By using this model we present a quantitative analysis of energy dissipation in sliding friction.
Open Access
Quantum effects in electrical and thermal transport through nanowires
(Institute of Physics Publishing, 2001) Çıracı, Salim; Buldum, A.; Batra, I. P.
Nanowires, point contacts and metallic single-wall carbon nanotubes are one-dimensional nanostructures which display important size-dependent quantum effects. Quantization due to the transverse confinement and resultant finite level spacing of electronic and phononic states are responsible for some novel effects. Many studies have revealed fundamental and technologically important properties, which are being explored for fabricating future nanodevices. Various simulation studies based on the classical molecular dynamics method and combined force and current measurements have shown the relationship between atomic structure and transport properties. The atomic, electronic and transport properties of these nanostructures have been an area of active research. This brief review presents some quantum effects in the electronic and phononic transport through nanowires.
Open Access
Quantum heat transfer through an atomic wire
(IOP Publishing Ltd., 2000) Buldum, A.; Çıracı, Salim; Fong, C. Y.
We studied the phononic heat transfer through an atomic dielectric wire with both infinite and finite lengths by using a model Hamiltonian approach. At low temperature under ballistic transport, the thermal conductance contributed by each phonon branch of a uniform and harmonic chain cannot exceed the well known value which depends linearly on temperature but is material independent. We predict that this ballistic thermal conductance will exhibit stepwise behaviour as a function of temperature. By performing numerical calculations on more realistic systems, where small atomic chains are placed between two reservoirs, we also found resonant modes, which should also lead to stepwise behaviour in the thermal conductance.
Open Access
Structure of aluminum atomic chains
(American Physical Society, 2001) Sen, P.; Çıracı, Salim; Buldum, A.; Batra, I. P.
First-principles density-functional calculations reveal that aluminum can form planar chains in zigzag and ladder structures. The most stable one has equilateral triangular geometry with four nearest neighbors; the other stable zigzag structure has wide bond angle and allows for two nearest neighbors. An intermediary structure has the ladder geometry and is formed by two strands. While all these planar geometries are more favored energetically than the linear chain, the binding becomes even stronger in nonplanar geometries. We found that by going from bulk to a chain the character of bonding changes and acquires directionality. The conductance of zigzag and linear chains is 4e2/h under ideal ballistic conditions.
Open Access
Theoretical study of boundary lubrication
(American Physical Society, 1999) Buldum, A.; Çıracı, Salim
We analyzed the dynamics of xenon atoms as lubricant between two Ni(110) slabs in relative motion. Atomic simulations are carried out by using classical molecular dynamics with realistic empirical potentials, where nickel as well as xenon atoms are relaxed. The resistance of the xenon layer against the loading force is examined and critical forces are determined to destroy the lubricant layer at different coverages. The relative motion of slabs in the lateral direction is investigated under constant normal force as a function of coverage ranging from zero to the monolayer xenon. Important lubrication properties of xenon atoms are analyzed by calculating the variation of potential energy, lateral force, and local hydrodynamic pressure. It is predicted that the corrugation of the potential energy associated with the sliding has a minimum value at submonolayer coverage. A phononic energy dissipation mechanism together with the theoretical analysis is proposed.
Open Access
Thermal conduction through a molecule
(E D P Sciences, 1999) Buldum, A.; Leitner, D. M.; Çıracı, Salim
The quantum features of phononic thermal conduction through a molecule between two reservoirs have been studied in the weak-coupling limit. As opposed to ballistic heat transfer through a uniform bridge or atomic chain strongly coupled to reservoirs investigated earlier, thermal conductance in the present case shows a nonlinear temperature dependence that is sensitive to the mode frequencies of the molecule.