Browsing by Author "Batra, I. P."

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Open Access
Absence of metallicity in Cs-GaAs(110): A Hubbard-model study
(American Physical Society, 1993) Gedik, Z.; Çıracı, Salim; Batra, I. P.
Using an approximate solution of the Hubbard-model Hamiltonian, we are able to establish that the Cs-GaAs(110) system becomes a Mott insulator at submonolayer Cs coverages. We also provide a consistent interpretation of electron-energy-loss and scanning-tunneling-spectroscopies data. The correlation effects are important for this system with an estimated correlation energy of 0.4 eV. © 1993 The American Physical Society.
Open Access
Adhesive energy, force and barrier height between simple metal surfaces
(1992) Çıracı, Salim; Tekman, E.; Gökçedag, M.; Batra, I. P.; Baratoff, A.
Using the self-consistent field pseudopotential method we calculated the adhesive energy, perpendicular and lateral forces and barrier height between two rigid A1(001) slabs modeling the sample and a blunt tip. We found that the adhesive energy and forces are site specific, and can lead to a significant corrugation in the constant force mode with negative force gradient. Lateral forces, which determine friction on the atomic scale are not simply proportional to the perpendicular force, and are typically one order of magnitude smaller. Our results confirm that perpendicular tip force and barrier height are interrelated for separations where the force gradient is positive. © 1992.
Open Access
Adsorption site of alkali metal overlayers on Si(001) 2 × 1
(1992) Batra, I. P.; Çıracı, Salim
The alkali metal semiconductor interfaces are currently being investigated by a variety of tools. Most studies to date at half a monolayer coverage have shown preference for either a quasi-hexagonal (H) site or a long-bridge (B) site. At this coverage one-dimensional chain structure for K on Si(001) 2 × 1 have now been confirmed by scanning tunneling microscopy (STM). The data, however, is consistent with either of the two sites. STM investigations at low coverages suggested that alkali metals like K and Cs occupy a novel site, Y, which is a bridge site between two Si atoms belonging to different dimers along the dimer row [110] direction. The total energy calculations for this new Y site, discovered by STM, have shown that it is indeed a site of (local) energy minimum. The ability of the surface silicon atoms, which are not adjacent to the alkali metal atom, to buckle makes the Y site a competitive adsorption site. We deduce the nature of bonding between alkali metals and Si using the STM data. It is concluded that the bond is substantially ionic in nature. © 1992.
Open Access
Calculations of STM linescans-general formalism
(Pergamon Press, 1988) Ellialtioğlu, Ş.; Çıracı, Salim; Batra, I. P.
We have developed a formalism for calculating the line scans of the scanning-tunneling microscopy from the realistic substrate and tip wave functions. The tip wave functions are calculated self-consistently by using a spherical jellium corresponding to a particular metal with various radii. This formalism provides a framework to analyze the experimental line scans, and to deduce information about the clean and adatom covered surfaces, and the radius and height of the tip, as well. We have found that the contribution of a tip wave function in tunneling current is strongly dependent on its symmetry. © 1988.
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
Conductance through atomic contacts created by scanning tunneling microscopy
(Elsevier, 1999) Kiliç, Ç.; Mehrez, H.; Çıracı, Salim; Batra, I. P.
We investigate conductance through contacts created by pressing a hard tip, as used in scanning tunneling microscopy, against substrates. Two different substrates are considered, one a normal metal (Cu) and another a semi-metal (graphite). Our study involves the molecular dynamics simulations for the atomic structure during the growth of the contact, and selfconsistent field electronic structure calculations of deformed bodies. We develop a theory predicting the conductance variations as the tip approaches the surface. We offer an explanation for a quasiperiodic variation of conductance of the contact on the graphite surface, a behavior which is dramatically different from contacts on normal metals.
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
Delta-Doping in strained (Si) / (Ge) superlattices
(American Physical Society, 1988) Çıracı, Salim; Batra, I. P.; Tekman, E.
We present a comparative study of the pseudomorphic (Si)6/(Ge)6 and -doped (Si)3(Sb)(Si)2/(Ge)6 superlattices using the self-consistent pseudopotential method. The strained (Si)6/(Ge)6 superlattice has the lowest conduction-band states of extended character, and the difference of energy between the direct and indirect band gap is 70 meV. Upon doping by Sb in the Si sublattice, a quasi-two-dimensional band confined to the Sb layer dips into the band gap. Furthermore, the average potential in the Ge sublattice rises relative to that of the Si side, which increases the band offset, and enhances the localization of the quantum well states. These results indicate that doping provides new means for controlling the electronic properties of strained superlattices. © 1988 The American Physical Society.
Open Access
Effect of tip profile on atomic-force microscope images: a model study
(American Physical Society, 1988) Abraham, F. F.; Batra, I. P.; Çıracı, Salim
Adopting the empirical silicon interatomic potential of Stillinger and Weber, we investigate the effect of the tip profile on the atomic-force microscope images for a prototype system, Si(001)-(2×1), and conclude that the tip profile has a profound effect on the observations. We also study relaxation of the surface under the influence of the tip using a many-body energy minimization procedure and find that the force exerted by the tip should be less than 10-9 N for the atomic-force microscope to be a nondestructive tool. © 1988 The American Physical Society.
Open Access
Electronic structure of Te-and As-covered Si(211)
(American Physical Society, 2003) Sen, P.; Batra, I. P.; Sivananthan, S.; Grein, C. H.; Dhar, N.; Çıracı, Salim
Electronic and atomic structures of the clean and As- and Te-covered Si(211) surface are studied using pseudopotential density-functional method. The clean surface is found to have (2 x 1) and rebonded (1 x 1) reconstructions as stable surface structures, but no π-bonded chain reconstruction. Binding energies of As and Te adatoms at a number of symmetry sites on the ideal and (2 x 1) reconstructed surfaces have been calculated because of their importance in the epitaxial growth of CdTe and other materials on the Si(211) surface. The special symmetry sites on these surfaces having the highest binding energies for isolated As and Te adatoms are identified. But more significantly, several sites are found to be nearly degenerate in binding-energy values. This has important consequences for epitaxial growth processes. Optimal structures calculated for 0.5 monolayer of As and Te coverage reveal that the As adatoms dimerize on the surface while the Te adatoms do not. However, both As- and Te-covered surfaces are found to be metallic in nature.
Open Access
Finite temperature studies of Te adsorption on Si(0 0 1)
(Elsevier, 2002) Sen, P.; Çıracı, Salim; Batra, I. P.; Grein, C. H.; Sivananthan, S.
We perform first principles density functional calculations to investigate the adsorption of Te on the Si(0 0 1) surface from low coverage up to a monolayer coverage. At low coverage, a Te atom is adsorbed on top of the Si surface dimer bond. At higher coverages, Te atoms adsorption causes the Si-Si dimer bond to break, lifting the (2 × 1) reconstruction. We find no evidence of the Te-Te dimer bond formation as a possible source of the (2 × 1) reconstruction at a monolayer coverage. Finite temperature ab initio molecular dynamics calculations show that Te covered Si(0 0 1) surfaces do not have any definitive reconstruction. Vibrations of the bridged Te atoms in the strongly anharmonic potentials prevent the reconstruction structure from attaining any permanent, two-dimensional periodic geometry. This explains why experiments attempting to find a definite model for the reconstruction reached conflicting conclusions. © 2002 Elsevier Science B.V. All rights reserved.
Open Access
Long-range order and segregation in semiconductor superlattices
(1987) Çıracı, Salim; Batra, I. P.
Results of self-consistent energy-minimization calculations provide strong evidence that the ordered phases in epitaxially grown Ga1-xAlxAs and strained Si1-xGex alloys are metastable, in the sense that segregation into constituents is favored. We show that the long-range order in intermediate metastable structures leads to significant changes in the electronic properties of semiconductor superlattices. Segregation gives rise to micro-quantum-wells with staggered band lineup and multiple confined states in the potential barrier. © 1987 The American Physical Society.
Open Access
Metallization of Silicon upon Potassium Adsorption
(1987) Çıracı, Salim; Batra, I. P.
We report novel features of potassium deposition on a Si(111)-(2×1) surface as a function of coverage. The binding is ionic even at the saturation coverage without any overlayer metallization. Up to a threshold coverage, the alkali-metal electrons are donated to the empty surface state resulting in a 1D metallic chain. Above this coverage, the conduction-band states are occupied, so that the surface electrons become itinerant leading to the metallization of the substrate and onset of enhanced conductivity. © 1987 The American Physical Society.
Open Access
Molecular-dynamics study of self-interstitials in silicon
(American Physical Society, 1987) Batra, I. P.; Abraham, F. F.; Çıracı, Salim
Results of a molecular-dynamics computer simulation are presented for atomic relaxations and relaxation energies for self-interstitials in a silicon crystal. The Stillinger-Weber model potential containing two- and three-body terms is used and is expected to be more realistic than a simple Keating potential. The host crystal is represented by a cluster of 800 atoms, and the additional silicon atom was embedded in various interstitial sites near the center. The whole assembly was then periodically continued to fill the entire space. It is found that significant atomic relaxations occur in a shell of a radius 11 a.u. and decay exponentially. In fact the relaxation is oscillatory in nature and also nonuniform within some shells. The calculated formation energies of vacancy and self-interstitials at equilibrium show trends which are in agreement with the self-consistent field total-energy calculations. These energy values are also in agreement with the known self-diffusion activation energy. From calculated formation energy values, we are able to draw the conclusion that the tetrahedral-site interstitial can be most readily formed. The hexagonal-site interstitial, on the other hand, is most repulsive. The migration from tetrahedral to dumbbell interstitial site appears to be most favorable. © 1987 The American Physical Society.
Open Access
Pentagonal nanowires: a first-principles study of the atomic and electronic structure
(American Physical Society, 2002) Sen, P.; Gülseren, O.; Yildirim, T.; Batra, I. P.; Çıracı, Salim
We performed an extensive first-principles study of nanowires in various pentagonal structures by using pseudopotential plane wave method within the density functional theory. Our results show that nanowires of different types of elements, such as alkali, simple, transition, and noble metals and inert gas atoms, have a stable structure made from staggered pentagons with a linear chain perpendicular to the planes of the pentagons and passing through their centers. This structure exhibits bond angles close to those in the icosahedral structure. However, silicon is found to be energetically more favorable in the eclipsed pentagonal structure. These quasi-one-dimensional pentagonal nanowires have higher cohesive energies than many other one-dimensional structures and hence may be realized experimentally. The effects of magnetic state are examined by spin-polarized calculations. The origin of the stability is discussed by examining optimized structural parameters, charge density and electronic band structure, and by using analysis based on the empirical Lennard-Jones-type interaction. Electronic band structure of pentagonal wires of different elements are discussed and their effects on quantum ballistic conductance are mentioned. It is found that the pentagonal wire of silicon exhibits metallic band structure.
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 transport through one-dimensional aluminum wires
(American Vacuum Society, 2002) Batra, I. P.; Sen, P.; Çıracı, Salim
Quantum conductance in narrow channels has been well understood by using the two-dimensional electron gas, a model system which has been realized in semiconductor heterojunctions. An essential property of this electron gas is its ability to support a constriction of width comparable to the Fermi wavelength, a property not shared by even thin metal films. The advent of scanning tunneling microscope has made possible the fabrication of metallic wires of atomic widths. We investigate one-dimensional wires consisting of aluminum atoms, to be specific. Using the first-principles density functional calculations, we obtain the optimal structures and report the bonding as deduced from the charge density analysis. With the calculated electronic structure in hand, we discussed the quantum ballistic transport using channel capacity arguments motivated by the Heisenberg’s uncertainty principle. By comparing our results with the detailed pioneering calculations by Lang, we inferred an average value for channel transmitivity and touched upon material specific contact resistance. Finally, the validity of the Wiedemann–Franz law in the quantum domain is established by studying thermal conductance in nanowires.
Open Access
Scanning-tunneling microscopy at small tip-to-surface distances
(1987) Çıracı, Salim; Batra, I. P.
The scanning-tunneling microscopy (STM) of graphite at small tip-to-surface distances is investigated using the self-consistent-field pseudopotential method. We have calculated potential, charge density in the region between the tip and surface, and the force corrugation. Our results reveal that the tip at the close proximity to the surface disturbs the states near the Fermi level, and induces localized states. The STM images, which are usually related to the local density of states at the Fermi level of the clean surface, are affected by these localized states. The tunneling barrier is shown to collapse at small distances and a new mechanism for current is postulated. Some experimental evidence for this effect is presented. © 1987 The American Physical Society.
Open Access
Strain and dipole effects in covalent-polar semiconductor superlattices
(1991) Batra, I. P.; Çıracı, Salim; Özbay, Ekmel
The energetics and electronic structure of lattice-matched (Ge)4/(GaAs)2 and strained, pseudomorphic (Si)4/(GaAs)2 (001) semiconductor superlattices have been studied with use of a self-consistent-field pseudopotential method. The interfaces are assumed to be uniform, but the interlayer distances of the pseudomorphic lattice are optimized to achieve a minimum-total-energy configuration. The calculated enthalpy of formation is in the 100-meV/atom range for these two superlattices, which is almost an order of magnitude larger than the strain component in (Si)4/(GaAs)2. The superlattice dipole induces a metal-insulator transition by periodically tilting the potential. The electrostatic energy derived from this dipole field is the main cause of the instability relative to disproportionation. © 1991 The American Physical Society.
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.