Browsing by Author "Mehrez, H."

Now showing 1 - 11 of 11

Open Access
An atomistic study on the stretching of nanowires
(1997) Mehrez, H.; Çıracı, Salim; Fong, C. Y.; Erkoç, Ş.
In this work we present an atomic-scale investigation of elastic and plastic deformation, and force variations in metal nanowires that are pulled from their ends. The atomic simulations are performed by using a molecular dynamics method with an empirical two-body potential; the effect of the initial size, shape, temperature and rate of stretching on the necking and fracture are investigated. We find that the necking occurs mainly due to the formation of a new layer with a smaller cross-section after every structural yield, and concurrently the tensile force falls abruptly. The relationship between the atomic structure and the conductance of the wire is analysed by constructing a realistic potential for the neck in terms of a linear combination of atomic pseudopotentials and by calculating the conductance using the transfer matrix method. Our results show that the variation of the conductance is strongly correlated with the sudden structural changes in the neck, and reflects the quantization of electronic states in the neck, but not the quantization of the conductance.
Open Access
Conductance in nanowires
(Kluwer Academic Publishers, 1997) Mehrez, H.; Çıracı, Salim; Abstreiter, G.; Aydınlı, Atilla; Leburton, J. -P.
This paper presents a detailed analysis of conductance and atomic structure in metal nanowires under tensile stress. We calculate the variation of conductance with the crossection of the constriction between two reservoirs, that is represented by three-dimensional circularly symmetric potentials. The absence of several observed features in the calculated conductance variations, in particular sudden jumps, suggests that the discontinuous rearrangements of atoms under stretch dominate the electron transport. To analyze the variations of atomic structure, we performed simulations based on the state of the art molecular dynamics simulations and revealed novel structural transformations. It is found that yielding and fracture mechanisms depend on the geometry, size, atomic arrangement and temperature. The elongation under uniaxial stress is realized by consecutive quasi elastic and yielding stages; the neck develops mainly by the implementation of a layer with a smaller crossection at certain stages of elongation. This causes to an abrupt decrease of the tensile force. Owing to the excessive strain at the neck, the original structure and atomic registry are modified; atoms show tendency to rearrange in closed-packed structures. In certain circumstances, a bundle of atomic chains or single atomic chain forms as a result of transition from the hallow site to the top site registry shortly before the break. The origin of the observed "giant" yield strength is explained by using results of present simulations and ab initio calculations of total energy and Young's modulus for an infinite atomic chain.
Open Access
Conductance of ferromagnetic nanowires
(American Physical Society, 1998) Mehrez, H.; Çıracı, Salim
The conductance distribution obtained from an ensemble of stretching ferromagnetic nanowires or point contacts do not exhibit peaks near the integer multiples of 2e2/h. This observation has been interpreted as the absence of conductance quantization. In this report, we examine various features of electron transport through the Ni nanowires and clarify the behavior of conductance distribution that is different from gold and copper nanowires. Our study concludes that the tunneling through the closely spaced states near EF that originate from crystal field and spin split 3d states of Ni prevents the plateaus of conductance from forming in the course of stretch.
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
A First-principles Study of the Structure and Dynamics of C8H8, Si8H8, and Ge8H8 Molecules
(American Chemical Society, 2000) Kiliç, Ç.; Yildirim, T.; Mehrez, H.; Çıracı, Salim
We present a first-principles study to elucidate the nature of the bonding, stability, energetics, and dynamics of individual X8H8 molecules (X = C, Si, Ge). The results obtained from both "local basis" and "pseudopotential" ab initio methods are in good agreement with the experimental data that exists for cubane (C8H8). The trends among these molecules are reminiscent of those prevailing in the bulk solids of C, Si, and Ge. High-temperature dynamics and fragmentation of X8H8 were studied by the quantum molecular dynamics method which shows that at high temperatures cubane is transformed to the 8-fold ring structure of cyclooctotetraene.
Open Access
Nanospintronic properties of carbon-cobalt atomic chains
(EDP Sciences, 2006) Durgun, Engin; Senger, R. T.; Mehrez, H.; Dag, S.; Çıracı, Salim
Periodic atom chains of carbon-cobalt compounds, (CnCo) ∞, comprise both conducting and insulating electronic properties simultaneously depending on the spin type of electrons, and hence are half-metals. Their band gap and the net magnetic moment oscillate with the number of carbon atoms in a unit cell. Finite segments of these chains also show interesting magnetic and transport properties. When connected to appropriate metallic electrodes the antiferromagnetic CoCnCo segments behave like molecular spin-valves, which can be conveniently manipulated. © EDP Sciences.
Open Access
Quantum point contact on graphite surface
(American Physical Society, 1998) Kiliç, Ç.; Mehrez, H.; Çıracı, Salim
The conductance through a quantum point contact created by a sharp and hard metal tip on the graphite surface has features which to our knowledge have not been encountered so far in metal contacts or in nanowires. In this paper we first investigate these features which emerge from the strongly directional bonding and electronic structure of graphite, and provide a theoretical understanding for the electronic conduction through quantum point contacts. Our study involves molecular-dynamics simulations to reveal the variation of interlayer distances and atomic structure at the proximity of the contact that evolves by the tip pressing toward the surface. The effects of the elastic deformation on the electronic structure, state density at the Fermi level, and crystal potential are analyzed by performing self-consistent-field pseudopotential calculations within the local-density approximation. It is found that the metallicity of graphite increases under the uniaxial compressive strain perpendicular to the basal plane. The quantum point contact is modeled by a constriction with a realistic potential. The conductance is calculated by representing the current transporting states in Laue representation, and the variation of conductance with the evolution of contact is explained by taking the characteristic features of graphite into account. It is shown that the sequential puncturing of the layers characterizes the conductance.
Open Access
Size-dependent alternation of magnetoresistive properties in atomic chains
(American Institute of Physics, 2006) Durgun, Engin; Senger, R. T.; Mehrez, H.; Sevinçli, H.; Çıracı, Salim
Spin-polarized electronic and transport properties of carbon atomic chains are investigated when they are capped with magnetic transition-metal (TM) atoms like Cr or Co. The magnetic ground state of the TM-C n-TM chains alternates between the ferromagnetic (F) and antiferromagnetic (AF) spin configurations as a function of n. In view of the nanoscale spintronic device applications the desirable AF state is obtained for only even-n chains with Cr; conversely only odd-n chains with Co have AF ground states. When connected to appropriate metallic electrodes these atomic chains display a strong spin-valve effect. Analysis of structural, electronic, and magnetic properties of these atomic chains, as well as the indirect exchange coupling of the TM atoms through non-magnetic carbon atoms are presented.
Open Access
Spintronic properties of carbon-based one-dimensional molecular structures
(American Physical Society, 2006) Durgun, Engin; Senger, R. T.; Sevinçli, H.; Mehrez, H.; Çıracı, Salim
In this paper we present an extensive study of the electronic, magnetic, and transport properties of finite and infinite periodic atomic chains composed of carbon atoms and 3d transition metal (TM) atoms using first-principles methods. Finite-size, linear molecules made of carbon atomic chains caped with TM atoms, i.e., TM- Cn -TM structures are stable and exhibit interesting magnetoresistive properties. The indirect exchange interaction of the two TM atoms through a spacer of n carbon atoms determines the type of the magnetic ground state of these structures. The n -dependent (n=1 to 7) variations of the ground state between ferromagnetic and antiferromagnetic spin configurations exhibit several distinct forms, including regular alternations for Ti, V, Mn, Cr, Fe, and Co, and irregular forms for Sc and Ni cases. We present a simple analytical model that can successfully simulate these variations, and the induced magnetic moments on the carbon atoms. Depending on the relative strengths of the carbon s, p and TM d orbital spin-dependent coupling and on the on-site energies of the TM atoms there induces long-range spin polarizations on the carbon atoms which mediate the exchange interaction. While periodically repeated TM- Cn atomic chains exhibit half-metallic properties with perfect spin polarization at the Fermi level, finite but asymmetric chains comprising single, double, and triple TM atoms display interesting spin-dependent features. These properties may be altered when these structures are coupled to electrodes. However, when connected to appropriate electrodes the TM- Cn -TM atomic chains act as molecular spin valves in their ferromagnetic states due to the large ratios of the conductance values for each spin type.
Open Access
Yielding and fracture mechanisms of nanowires
(American Physical Society, 1997) Mehrez, H.; Çıracı, Salim
This paper presents a detailed analysis of atomic structure and force variations in metal nanowires under tensile strain. Our work is based on state of the art molecular dynamics simulations and ab initio self-consistent field calculations within the local density approximation, and predicts structural transformations. It is found that yielding and fracture mechanisms depend on the size, atomic arrangement, and temperature. The elongation under uniaxial stress is realized by consecutive quasielastic and yielding stages; the neck develops by the migration of atoms, but mainly by the sequential implementation of a new layer with a smaller cross section at certain ranges of uniaxial strain. This causes an abrupt decrease of the tensile force. Owing to the excessive strain at the neck, the original structure and atomic registry are modified; atoms show a tendency to rearrange in closed-packed structures. In certain circumstances, a bundle of atomic chains or a single atomic chain forms as a result of transition from the hollow site to the top site registry shortly before the break. The wire is represented by a linear combination of atomic pseudopotentials and the current is calculated to investigate the correlation between conductance variations and atomic rearrangements of the wire during the stretch. The origin of the observed "giant" yield strength is explained by using results of the present simulations and ab initio calculations of the total energy and Young's modulus for an infinite atomic chain.