Browsing by Author "Senger, R. T."

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Open Access
Ab-initio electron transport calculations of carbon based string structures
(American Physical Society, 2004) Tongay, S.; Senger, R. T.; Dag, S.; Çıracı, Salim
The new stable structures of carbon-based strings and their unusual electronic transport properties were discussed. Total energy and electronic structure calculations using first principles pseudopotential plane wave method within density functional theory (DFT) and supercell geometries were also carried out. It was found that carbon chains were suitable for structural and chemical functionalizations because of their flexibility. These carbon chains also form stable ring, helix, grid and network structures. The results show that the double covalent bonding of carbon atoms underlies their unusual chemical, mechanical and transport properties and carbon chains can form stable string structures with impressive physical properties.
Open Access
Atomic and electronic structure of carbon strings
(IOP Publishing Ltd., 2005) Tongay, S.; Dag, S.; Durgun, Engin; Senger, R. T.; Çıracı, Salim
This paper presents an extensive study of various string and tubular structures formed by carbon atomic chains. Our study is based on first-principles pseudopotential plane wave and finite-temperature ab initio molecular dynamics calculations. Infinite- and finite-length carbon chains exhibit unusual mechanical and electronic properties such as large cohesive energy, axial strength, high conductance, and overall structural stability even at high temperatures. They are suitable for structural and chemical functionalizations. Owing to their flexibility and reactivity they can form linear chain, ring, helix, two-dimensional rectangular and honeycomb grids, three-dimensional cubic networks, and tubular structures. Metal-semiconductor heterostructures and various quantum structures, such as multiple quantum wells and double-barrier resonant tunnelling structures, can be formed from the junctions of metallic carbon and semiconducting BN linear chains. Analysis of atomic and electronic structures of these periodic, finite, and doped structures reveals fundamentally and technologically interesting features, such as structural instabilities and chiral currents. The double covalent bonding of carbon atoms depicted through self-consistent charge density analysis underlies the chemical, mechanical, and electronic properties.
Open Access
Atomic chains of group-IV elements and III-V and II-VI binary compounds studied by a first-principles pseudopotential method
(American Physical Society, 2005) Senger, R. T.; Tongay, S.; Durgun, Engin; Çıracı, Salim
Using the first-principles plane wave pseudopotential method we have studied structural, electronic, and transport properties of atomic chains of group-IV elements and group III-V and group II-VI binary compounds. Several materials which are insulating or semiconducting in bulk are found to be metallic in nanowire structures. Our calculations reveal that monatomic chains of Si, Ge, and Sn elements, and of binary compounds such as InP, GaAs, and AlSb, are stable and metallic. On the other hand, compound wires of BN, SiC, GaN, ZnSe, and several others have semiconducting or insulating properties. Ideal mechanical strength calculations show that some of these atomic chains can sustain strains of up to ε=0.3. We have presented ab initio electron transport calculations for Si and AlP linear chain segments in between Al electrodes. Conductance of Si monatomic chains displays some nontrivial features as the number of atoms in the chain is varied or as the chain is strained. In addition to single atomic chain structures, junctions and grid structures of Si are investigated.
Open Access
Bilayer SnS2: tunable stacking sequence by charging and loading pressure
(American Physical Society, 2016-03) Bacaksiz, C.; Cahangirov, S.; Rubio, A.; Senger, R. T.; Peeters, F. M.; Sahin, H.
Employing density functional theory-based methods, we investigate monolayer and bilayer structures of hexagonal SnS2, which is a recently synthesized monolayer metal dichalcogenide. Comparison of the 1H and 1T phases of monolayer SnS2 confirms the ground state to be the 1T phase. In its bilayer structure we examine different stacking configurations of the two layers. It is found that the interlayer coupling in bilayer SnS2 is weaker than that of typical transition-metal dichalcogenides so that alternative stacking orders have similar structural parameters and they are separated with low energy barriers. A possible signature of the stacking order in the SnS2 bilayer has been sought in the calculated absorbance and reflectivity spectra. We also study the effects of the external electric field, charging, and loading pressure on the characteristic properties of bilayer SnS2. It is found that (i) the electric field increases the coupling between the layers at its preferred stacking order, so the barrier height increases, (ii) the bang gap value can be tuned by the external E field and under sufficient E field, the bilayer SnS2 can become a semimetal, (iii) the most favorable stacking order can be switched by charging, and (iv) a loading pressure exceeding 3 GPa changes the stacking order. The E-field tunable band gap and easily tunable stacking sequence of SnS2 layers make this 2D crystal structure a good candidate for field effect transistor and nanoscale lubricant applications.
Open Access
Binding energies of excitons in II-VI compound-semiconductor based quantum well structures
(Wiley - V C H Verlag GmbH & Co., 2004) Senger, R. T.; Bajaj, K. K.
We present a brief description of the calculation of the variation of the binding energy of the heavy-hole exciton as a function of well width in quantum well structures composed of II-VI compound semiconductors including the effects of exciton-optical phonon interaction as formulated by Pollmann and Büttner [J. Pollmann and H. Büttner, Phys. Rev. B 16, 4480 (1977)], and of particle masses and dielectric mismatches between the well and the barrier layers. We compare the results of our calculations with the available experimental data in ZnSe/MgS, ZnSe/Mg0.15Zn0.85Se, and ZnS/Mg0.19Z0.81S quantum well structures and find a good agreement
Open Access
Carbon string structures: First-principles calculations of quantum conductance
(American Physical Society, 2005) Senger, R. T.; Tongay, S.; Dag, S.; Durgun, Engin; Çıracı, Salim
Carbon forms various nanostructures based on the monatomic chains or strings which show transport properties of fundamental and technological interest. We have carried out first-principles quantum conductance calculations using optimized structures within density functional theory. We treated finite segments of carbon monatomic chain, metal-semiconductor heterostructure, and resonant tunneling double barrier formed of C-BN chains, as well as symmetric and antisymmetric loop devices between two electrodes. We examined the effects of electrode, contact geometry, size of the device, strain, and foreign atoms adsorbed on the chain. Calculated quantum ballistic conductance of carbon chains showing even-odd disparity depending on the number of atoms and strain are of particular interest. Notably, chains consisting of an even number of carbon atoms contacted to metal electrodes display a resonant tunneling-like behavior under axial strain. The double covalent bonding of carbon atoms depicted through self-consistent charge density analysis underlies unusual transport properties.
Open Access
Chiral single-wall gold nanotubes
(American Physical Society, 2004) Senger, R. T.; Dag, S.; Çıracı, Salim
The formation of freestanding and tip-suspended chiral-wall (n,m) nanotubes, which were composed of helical atomic strands, from gold atoms was investigated using first-principles calculations, where (n,m) notation defines the structure of the tube. The tubes with 3≤n≤5 were found to be stable and exhibited electronic and transport properties investigated. The (5,3) gold tube was energetically the most favourable. It was observed from the quantum ballistic conductance, band structure and charge density analysis that the current on these wires was less chiral, and no direct correlation between the numbers of conduction channels and helical strands was found.
Open Access
Determination of energy-band offsets between GaN and AlN using excitonic luminescence transition in AlGaN alloys
(American Institute of Physics, 2006) Westmeyer, A. N.; Mahajan, S.; Bajaj, K. K.; Lin J. Y.; Jiang, H. X.; Koleske, D. D.; Senger, R. T.
We report the determination of the energy-band offsets between GaN and AlN using the linewidth (full width at half maximum) of an extremely sharp excitonic luminescence transition in Alx Ga1-x N alloy with x=0.18 at 10 K. Our sample was grown on C -plane sapphire substrate by metal-organic chemical-vapor deposition at 1050 °C. The observed value of the excitonic linewidth of 17 meV is the smallest ever reported in literature. On subtracting a typical value of the excitonic linewidth in high-quality GaN, namely, 4.0 meV, we obtain a value of 13.0 meV, which we attribute to compositional disorder. This value is considerably smaller than that calculated using a delocalized exciton model [S. M. Lee and K. K. Bajaj, J. Appl. Phys. 73, 1788 (1993)]. The excitons are known to be strongly localized by defects and/or the potential fluctuations in this alloy system. We have simulated this localization assuming that the hole, being much more massive than the electron, is completely immobile, i.e., the hole mass is treated as infinite. Assuming that the excitonic line broadening is caused entirely by the potential fluctuations experienced by the conduction electron, the value of the conduction-band offset between GaN and AlN is determined to be about 57% of the total-band-gap discontinuity. Using our model we have calculated the variation of the excitonic linewidth as a function of Al composition in our samples with higher Al content larger than 18% and have compared it with the experimental data. We also compare our value of the conduction-band offset with those recently proposed by several other groups using different techniques.
Open Access
Dynamics of phononic dissipation at the atomic scale: Dependence on internal degrees of freedom
(American Physical Society, 2007) Sevinçli, H.; Mukhopadhyay, S.; Senger, R. T.; Çıracı, Salim
Dynamics of dissipation local vibrations to the surrounding substrate is a key issue in friction between sliding surfaces as well as in boundary lubrication. We consider a model system consisting of an excited nano-particle which is weakly coupled with a substrate. Using three different methods, we solve the dynamics of energy dissipation for different types of coupling between the nanoparticle and the substrate, where different types of dimensionality and phonon densities of states were also considered for the substrate. In this paper, we present a microscopic analysis of transient properties of energy dissipation via phonon discharge toward the substrate. Finally, important conclusions of our theoretical analysis are verified by a realistic study, where the phonon modes and interaction parameters involved in the energy dissipation from an excited benzene molecule to the graphene are calculated by using first-principles methods. The methods used are applicable also to dissipative processes in the contexts of infrared Raman spectroscopy and atomic force microscopy of molecules on surfaces.
Open Access
Energy and mass of 3D and 2D polarons in the overall range of the electron-phonon coupling strength
(Institute of Physics Publishing Ltd., 1994) Ercelebi, A.; Senger, R. T.
The ground-state characterization of the polaron problem is retrieved within the framework of a variational scheme proposed previously by Devreese et al for the bound polaron. The formulation is based on the standard canonical transformation of the strong coupling ansatz and consists of a variationally determined perturbative extension serving for the theory to interpolate in the overall range of the coupling constant. Specializing our considerations to the bulk and strict two-dimensional polaron models we see that the theory yields significantly improved energy upper bounds in the strong coupling regime and, moreover, extrapolates itself successfully towards the well-established weak coupling limits for all polaron quantities of general interest.
Open Access
Energy-transfer rate in a double-quantum-well system due to Coulomb coupling
(Elsevier, 2002) Senger, R. T.; Tanatar, Bilal
We study the energy-transfer rate for electrons in a double-quantum-well structure, where the layers are coupled through screened Coulomb interactions. The energy-transfer rate between the layers (similar to the Coulomb drag effect in which the momentum-transfer rate is considered) is calculated as functions of electron densities, interlayer spacing, the temperature difference of the 2DEGs, and the electron drift velocity in the drive layer. We employ the full wave vector and frequency-dependent random-phase approximation at finite temperature to describe the effective interlayer Coulomb interaction. We find that the collective modes (plasmons) of the system play a dominant role in the energy-transfer rates.
Open Access
First-principles calculations of spin-dependent conductance of graphene flakes
(The American Physical Society, 2008) Şahin, H.; Senger, R. T.
Using ab initio density-functional theory and quantum transport calculations based on nonequilibrium Green's function formalism we study structural, electronic, and transport properties of hydrogen-terminated short graphene nanoribbons (graphene flakes) and their functionalization with vanadium atoms. Rectangular graphene flakes are stable, having geometric and electronic structures quite similar to that of extended graphene nanoribbons. We show that a spin-polarized current can be produced by pure hydrogenated rectangular graphene flakes by exploiting the spatially separated edge states of the flake using asymmetric nonmagnetic contacts. Functionalization of the graphene flake with magnetic adatoms such as vanadium also leads to spin-polarized currents even with symmetric contacts. We observe and discuss sharp discontinuities in the transmission spectra which arise from Fano resonances of localized states in the flake.
Open Access
Ground-state description of quasi-one-dimensional polarons with arbitrary electron-phonon coupling strength
(American Physical Society, 1996) Erçelebi, A.; Senger, R. T.
We consider the interaction of a confined electron with bulk polar-optical phonons in a cylindrical quantum well wire with infinite boundary potential. Expressions for the polaron self-energy and mass are derived within a variational scheme over reasonably broad ranges of the wire radius and the phonon-coupling strength. The formulation is based on the standard canonical transformation of the strong-coupling ansatz and consists of a variationally determined perturbative extension serving for the theory to interpolate in the overall range of the coupling constant. Contrary to the general trend that the electron-phonon interaction is inherently stronger in systems of lower dimensionality, our results indicate that, at weak coupling, the binding energy of the polaron can be smaller and its mass less inertial compared with the bulk case when the wire is made narrow.
Open Access
Half-metallic properties of atomic chains of carbon-transition-metal compounds
(American Physical Society, 2005) Dag, S.; Tongay, S.; Yildirim, T.; Durgun, Engin; Senger, R. T.; Fong, C. Y.; Çıracı, Salim
We found that magnetic ground state of one-dimensional atomic chains of carbon-transition-metal compounds exhibit half-metallic properties. They are semiconductors for one spin direction, but show metallic properties for the opposite direction. The spins are fully polarized at the Fermi level and net magnetic moment per unit cell is an integer multiple of Bohr magneton. The spin-dependent electronic structure can be engineered by changing the number of carbon atoms and type of transition metal atoms. These chains, which are stable even at high temperatures and some of which keep their spin-dependent electronic properties even under moderate axial strain, hold the promise of potential applications in nanospintronics.
Open Access
Hartree-Fock approximation of bipolaron state in quantum dots and wires
(Springer, 2010) Senger, R. T.; Kozal, B.; Chatterjee, A.; Erçelebi, A.
The bipolaronic ground state of two electrons in a spherical quantum dot or a quantum wire with parabolic boundaries is studied in the strong electron-phonon coupling regime. We introduce a variational wave function that can conveniently conform to represent alternative ground state configurations of the two electrons, namely, the bipolaronic bound state, the state of two individual polarons, and two nearby interacting polarons confined by the external potential. In the bipolaron state the electrons are found to be separated by a finite distance about a polaron size. We present the formation and stability criteria of bipolaronic phase in confined media. It is shown that the quantum dot confinement extends the domain of stability of the bipolaronic bound state of two electrons as compared to the bulk geometry, whereas the quantum wire geometry aggravates the formation of stable bipolarons.
Open Access
Layer-and strain-dependent optoelectronic properties of hexagonal AIN
(American Physical Society, 2015) Kecik, D.; Bacaksiz, C.; Senger, R. T.; Durgun, Engin
Motivated by the recent synthesis of layered hexagonal aluminum nitride (h-AlN), we investigate its layer- and strain-dependent electronic and optical properties by using first-principles methods. Monolayer h-AlN is a wide-gap semiconductor, which makes it interesting especially for usage in optoelectronic applications. The optical spectra of 1-, 2-, 3-, and 4-layered h-AlN indicate that the prominent absorption takes place outside the visible-light regime. Within the ultraviolet range, absorption intensities increase with the number of layers, approaching the bulk case. On the other hand, the applied tensile strain gradually redshifts the optical spectra. The many-body effects lead to a blueshift of the optical spectra, while exciton binding is also observed for 2D h-AlN. The possibility of tuning the optoelectronic properties via thickness and/or strain opens doors to novel technological applications of this promising material.
Open Access
Magnetic field effects on an electron near an impenetrable dielectric surface
(1996) Saqqa, B.; Senger, R. T.; Erçelebi, A.
The interaction of an extrinsic electron with the surface optical modes of a semi-infinite medium is retrieved under the effect of a weak magnetic field. It is observed that for an electron in a bound state near the surface, the magnetic field enhances the effective phonon coupling rather prominently and thus leads to an increased degree of localisation of the electron towards the surface. This feature is seen to be more marked for larger coupling strengths. © Tübi̇tak.
Open Access
Monolayer honeycomb structures of group-IV elements and III-V binary compounds: First-principles calculations
(American Physical Society, 2009) Şahin, H.; Cahangirov, S.; Topsakal, M.; Bekaroglu, E.; Akturk, E.; Senger, R. T.; Çıracı, Salim
Using first-principles plane-wave calculations, we investigate two-dimensional (2D) honeycomb structure of group-IV elements and their binary compounds as well as the compounds of group III-V elements. Based on structure optimization and phonon-mode calculations, we determine that 22 different honeycomb materials are stable and correspond to local minima on the Born-Oppenheimer surface. We also find that all the binary compounds containing one of the first row elements, B, C, or N have planar stable structures. On the other hand, in the honeycomb structures of Si, Ge, and other binary compounds the alternating atoms of hexagons are buckled since the stability is maintained by puckering. For those honeycomb materials which were found stable, we calculated optimized structures, cohesive energies, phonon modes, electronic-band structures, effective cation and anion charges, and some elastic constants. The band gaps calculated within density functional theory using local density approximation are corrected by GW0 method. Si and Ge in honeycomb structure are semimetal and have linear band crossing at the Fermi level which attributes massless Fermion character to charge carriers as in graphene. However, all binary compounds are found to be semiconductor with band gaps depending on the constituent atoms. We present a method to reveal elastic constants of 2D honeycomb structures from the strain energy and calculate the Poisson’s ratio as well as in-plane stiffness values. Preliminary results show that the nearly lattice matched heterostructures of these compounds can offer alternatives for nanoscale electronic devices. Similar to those of the three-dimensional group-IV and group III-V compound semiconductors, one deduces interesting correlations among the calculated properties of present honeycomb structures.
Open Access
Monte Carlo simulation of localization dynamics of excitons in ZnO and CdZnO quantum well structures
(American Institute of Physics, 2006) Makino, T.; Saito, K.; Ohtomo, A.; Kawasaki, M.; Senger, R. T.; Bajaj, K. K.
Localization dynamics of excitons was studied for ZnOMgZnO and CdZnOMgZnO quantum wells (QWs). The experimental photoluminescence (PL) and absorption data were compared with the results of Monte Carlo simulation in which the excitonic hopping was modeled. The temperature-dependent PL linewidth and Stokes shift were found to be in a reasonable agreement with the hopping model, with accounting for an additional inhomogeneous broadening. The density of localized states used in the simulation for the CdZnO QW was consistent with the absorption spectrum.
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.