Department of Physics

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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
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
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
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
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
Surface metallization of silicon by potassium adsorption on Si(001)-(2×1)
(American Physical Society, 1988) Çıracı, Salim; Batra, I. P.
We present the detailed results of self-consistent and geometry-optimized total-energy, band-structure, and charge-density calculations for a potassium-covered Si(001)-(2×1) surface, and for an unsupported potassium monolayer. We found that the (2×1) reconstruction and the dimer bonds of the Si surface continue to be stable after the adsorption of alkali-metal atoms. At the monolayer coverage the charge from the adsorbed potassium atoms is transferred into the empty, antibonding dangling-bond surface states, resulting in the metallization of the Si(001) substrate surface. The bonding between the overlayer and the substrate surface is ionic, and the Fermi level is pinned by the partially filled silicon surface states. Our theory for the metallization and the surface collective excitations is different from previous ones developed for an alkali-metal overlayer on a metal substrate which suggest that the system undergoes a Mott transition, and can successfully account for recent experimental observations. The presence of the active dangling-bond states prevents the alkali-metal monolayer from metallization, and thus provides the crucial difference between metal and semiconductor substrates. © 1988 The American Physical Society.
Open Access
Critical relaxation of the two-dimensional three-state Potts model with conserved dynamics
(Institute of Physics (Great Britain), 1988) Aydın, M.; Yalabık, M. C.
The dynamical critical exponent z is obtained using the finite-size scaling method for the two-dimensional three-state Potts model with conserved dynamics. It is observed that there is a change in the dynamical behaviour of the system at the size n = 15. For finite lattices of sizes n ≤ 15, the value of z is predicted as z = 2.0±0.1. For n ≥ 15, the asymptotic behaviour is observed and z is calculated as z = 2.78 ± 0.20. © 1987 IOP Publishing Ltd.
Open Access
Tip induced localized states in scanning tunneling microscopy
(Institute of Physics Publishing Ltd., 1988) Tekman, E.; Çıracı, Salim
We have investigated the Scanning Tunneling Microscopy (STM) of graphite with varying tip-to-surface distance. Using an LCAO type approach we showed that at small separations states are localized between the tip and the surface. The energies and the characters of these Tip Induced Localized States (TILS) depend on the height and the lateral position of the tip. These states play a significant role in the tunneling process and influence the STM corrugations predicted from the local density of states. We have developed a current expression, which includes these local interactions, but differes significantly from earlier theories.
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 strained Sin / Gen (001) superlattices
(Pergamon Press, 1988) Çıracı, Salim; Gülseren, O.; Ellialtioğlu, Ş.
Using the empirical tight binding method we have investigated the electronic properties of the Sin/Gen(001) strained superlattices as a function of the superlattice periodicity and the band misfit. For n ≥ 4 we have found that first and second conduction band states are localized in Si. The hole states localized in Ge appear for n ≥ 4. The difference between the direct and indirect band gaps is reduced from 2.01 eV for bulk Si to 0.01 eV for n=6 which can be considered to be quasi-direct. For the cases n=6 and n=8, the band gap might become direct for large values of band misfit. © 1988.
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
Atomic theory of scanning tunneling microscopy
(American Physical Society, 1989) Tekman, E.; Çıracı, Salim
We present a quantitative analysis of the modifications of the scanning-tunneling-microscopy images due to the local perturbations of the electronic states induced by the tip in close proximity to the sample surface. Using an empirical tight-binding method, we have calculated the electronic states of a prototype tip-sample system consisting of a single-atom tip and the graphite surface, as a function of the tip-sample distance. We find that as the tip approaches the sample, their states start to interact and become laterally confined in the vicinity of the tip at small tip-sample separation. These states influence the tunneling phenomenon by connecting the tip and sample surface electronically. The effect of the tip-induced localized states is discussed, and the expression for the tunneling current is reformulated by incorporating the tip-induced states. Calculations using this expression show that the corrugation amplitude obtained from scanning tunneling microscopy is enhanced and deviates from the proportionality to the local density of states of the bare sample at the Fermi level evaluated at the center of the tip. © 1989 The American Physical Society.
Open Access
Novel features of quantum conduction in a constriction
(American Physical Society, 1989) Tekman, E.; Çıracı, Salim
The effects of the geometry and temperature on the quantum conductance for one-dimensional (1D) ballistic transport through a constriction in a 2D electron gas are investigated by use of a refined formalism. As the length of the constriction increases, weak oscillations around the classical conductance evolve into a steplike structure and the resonances on the plateaus become pronounced. Quantization at integer multiples of 2e2/h occurs only for uniform constriction of finite length. At finite temperature of 0.6 K significantly long uniform constriction is necessary to observe plateaus devoid of resonance structure. © 1989 The American Physical Society.
Open Access
Quantum mechanical simulation of charge transport in very small semiconductor structures
(IEEE, 1989) Yalabik, M. C.; Diff, K.
A quantum mechanical simulation method of charge transport in very small semiconductor devices, based on the numerical solution of the time-dependent Schrödinger equation (coupled self-consistently to the Poisson equation to determine the electrostatic potential in the device), is presented. Carrier transport is considered within the effective mass approximation, while the effects of the electron-phonon interaction are included in an approximation that is consistent with the results of the perturbation theory and gives the correct two-point time correlation function. Numerical results for the transient behavior of a planar ultrasubmicrometer three-dimensional GaAs MESFET (gate length of 26 nm) are also presented. They indicate that extremely fast gate-step response times (switching times) characterize such short-channel GaAs devices. © 1989 IEEE
Open Access
Simulated annealing for texture segmentation with Markov models
(IEEE, 1989) Yalabık, M. Cemal; Yalabık, N.
Binary textured images are segmented into regions of different textures. The binary Markov model is used, and model parameters are assumed to be unknown prior to segmentation. The parameters are estimated using a weighted-least-squares method, while segmentation is performed iteratively using simulated annealing. To speed up the annealing process, an initial coarse segmentation algorithm that quickly determines the approximate region categories using k-means clustering algorithm is used. The results look promising, and the computational costs can be reduced further by optimization of the computations.
Open Access
Effects of the constriction geometry on quasi-one-dimensional transport: Adiabatic evolution and resonant tunneling
(American Physical Society, 1989) Tekman, E.; Çıracı, Salim
The geometry of a constriction, which plays a crucial role in quasi-one-dimensional (quasi-D) ballistic transport, is investigated by performing calculations of the conductance. If the constriction becomes smoothly narrower inside, the current-carrying states evolve adiabatically leading to quantized conductance without a resonance structure. In contrast, quasi-0D (confined) states can form in a local widening inside the constriction and give rise to resonant tunneling. The effects of an obstacle at the entrance and the roughening along the constriction are also studied. © 1989 The American Physical Society.
Open Access
A renormalisation group study of the BNNNI model
(IOP Publishing, 1989) Aydin, M.; Yalabik, M. C.
The phase diagram of the two-dimensional biaxially next-nearest-neighbour king (BNNNI) model is obtained using the real-space renormalisation group method with the mean-field approximation. The transitions between disordered and commensurate phases are observed to be single transitions for the ferromagnetic phase at intermediate and high temperatures and for the antiphase structure at high temperatures. However, within the approximation, successive transitions are observed between the antiphase structure and disordered phase at intermediate and low temperatures.
Open Access
A Monte Carlo study of the BNNNI model
(IOP Publishing, 1989) Aydin, M.; Yalabik, M. C.
The biaxially next-nearest-neighbour Ising (BNNNI) model is studied using the Monte Carlo procedure. The structure factor is used to identify the possible phases of the system. At intermediate temperatures, evidence for the presence of an additional incommensurate phase with a varying wavevector between the disordered and antiphase states is given. © 1989 IOP Publishing Ltd.
Open Access
Theory of transition from the tunneling regime to point contact in scanning tunneling microscopy
(American Physical Society, 1989) Çıracı, Salim; Tekman, E.
We analyze the transition from the tunneling regime to point contact in scanning tunneling microscopy. The variation of conductance as a function of tip-sample separation is sample and tip specific. Tunneling occurs through an effective barrier even if the potential barrier collapses. Subsequent to the collapse of the effective barrier the point contact is initiated leading to ballistic transport. The ballistic conductance through uniformly increasing contact area exhibits neither sharp quantized steps nor pronounced quantum oscillations. The observed oscillations are explained by the irregular enlargement of the contact area. © 1989 The American Physical Society.
Open Access
Phase transitions and renormalization-group theory
(1989-07) Berker, A. Nihat
The study of critical phenomena targets the qualitatively new phenomena that occur through the interactions of very large numbers of degrees of freedom. Such situations are typically realized near the onset of the transition between two thermodynamic phases. The special challenge of this field is thus due to the truly many-body nature of its subject. Another motivating challenge of the field has been the feasibility of immediate comparisons between the outcomes of sophisticated experiments and sophisticated theories, especially in condensed matter systems. Adding to this the wide variety of experimental and theoretical methods, it is seen that many cultures meet and exchange under the aegis of critical phenomena and phase transitions, contributing to the dynamism and intellectual enrichment of the field.