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S. Ahmad Ketabi

Dean of School of Physics and Professor of Condensed Matter Physics

Education

  • Ph.D. 1997-2002

    Theoretical Condensed Matter Physics

    Thesis: The Properties of Electron Transport in π - Conjugated Polymers

    Supervisor: Prof. N. Shahtahmasebi

    Ferdowsi University of Mashhad, Mashhad, Iran

  • M.Sc. 1992-1995

    Solid States Physics

    Thesis: Vibrational and thermal properties of 3-dimentional disordered systems

    Supervisor:  Prof. N. Shahtahmasebi

    Teacher Training University, Tehran, Iran

  • B.Sc. 1988-1992

    Physics

    Project:  Determination of Miller Indices: Rotational Crystal Method
    Supervisor: Prof. N. Tajabor

    Subject Class: Solid States Physics

    Ferdowsi University of Mashhad, Mashhad, Iran

Teaching

  • Condensed Matter Physics (Enz)
  • Classical Mechanics I and II (Symon and Goldstein)
  • Electromagnetism (Reitz and Milford)
  • Electrodynamics (Jackson)
  • Advanced Quantum Physics (Sakurai)
  • Advanced Solid State Physics (Ashcraft and Mermin)
  • Advanced Statistical Mechanics and thermodynamics (Pathria)
  • Mathematical Methods for Physicists (Arfken)
  • Programming in Fortran 90 and MatLab

Selected Publications

Nakhaee, M., Ketabi, S.A., Peeters, F.M. Dirac nodal line in bilayer borophene: Tight-binding model and low-energy effective Hamiltonian (2018) 98 (11), art. no. 115413, .

DOI: 10.1103/PhysRevB.98.115413

Bilayer hexagonal borophene, which is bound together through pillars, is a novel topological semimetal. Using density functional theory, we investigate its electronic band structure and show that it is a Dirac material which exhibits a nodal line. A tight-binding model was constructed based on the Slater-Koster approach, which accurately models the electronic spectrum. We constructed an effective four-band model Hamiltonian to describe the spectrum near the nodal line. This Hamiltonian can be used as a new platform to study the new properties of nodal line semimetals. We found that the nodal line is created by edge states and is very robust against perturbations and impurities. Breaking symmetries can split the nodal line, but cannot open a gap. © 2018 American Physical Society.

PUBLISHER: American Physical Society

Pourhassan, B., Faizal, M., Ketabi, S.A. Logarithmic correction of the BTZ black hole and adaptive model of graphene (2018) 27 (12), art. no. 1850118, .

DOI: 10.1142/S0218271818501183

It is known that almost all approaches to quantum gravity produce a logarithmic correction term to the entropy of a black hole, but the exact coefficient of such a term varies between the different approach to quantum gravity. Such logarithmic terms can also occur due to thermal fluctuations in both analogous and real black holes so that we will analyze the effects of logarithmic corrections term with variable coefficient on properties of analogous black hole. As these properties can be experimentally tested, they can be used to obtain the correct coefficient for such terms for an analogous black hole. We will argue that as even the real black holes can be considered as thermodynamical objects in Jacobson formalism, so such analogous black holes can be used to obtain the correct coefficient for the real black holes, and this in turn can be used to select the correct approach to quantum gravity. In that case, we use an adaptive model of graphene, which is still far from real graphene, to investigate some thermodynamics quantities of BTZ black hole. © 2018 World Scientific Publishing Company.

AUTHOR KEYWORDS: Black hole; quantum gravity; thermal fluctuation
PUBLISHER: World Scientific Publishing Co. Pte Ltd

Nakhaee, M., Ketabi, S.A., Peeters, F.M. Tight-binding model for borophene and borophane (2018) 97 (12), art. no. 125424, .

DOI: 10.1103/PhysRevB.97.125424

Starting from the simplified linear combination of atomic orbitals method in combination with first-principles calculations, we construct a tight-binding (TB) model in the two-centre approximation for borophene and hydrogenated borophene (borophane). The Slater and Koster approach is applied to calculate the TB Hamiltonian of these systems. We obtain expressions for the Hamiltonian and overlap matrix elements between different orbitals for the different atoms and present the SK coefficients in a nonorthogonal basis set. An anisotropic Dirac cone is found in the band structure of borophane. We derive a Dirac low-energy Hamiltonian and compare the Fermi velocities with that of graphene. © 2018 American Physical Society.

PUBLISHER: American Physical Society

Mirzakhani, M., Zarenia, M., Vasilopoulos, P., Ketabi, S.A., Peeters, F.M. Landau levels in biased graphene structures with monolayer-bilayer interfaces (2017) 96 (12), art. no. 125430, .

DOI: 10.1103/PhysRevB.96.125430

The electron energy spectrum in monolayer-bilayer-monolayer and in bilayer-monolayer-bilayer graphene structures is investigated and the effects of a perpendicular magnetic field and electric bias are studied. Different types of monolayer-bilayer interfaces are considered as zigzag (ZZ) or armchair (AC) junctions which modify considerably the bulk Landau levels (LLs) when the spectra are plotted as a function of the center coordinate of the cyclotron orbit. Far away from the two interfaces, one obtains the well-known LLs for extended monolayer or bilayer graphene. The LL structure changes significantly at the two interfaces or junctions where the valley degeneracy is lifted for both types of junctions, especially when the distance between them is approximately equal to the magnetic length. Varying the nonuniform bias and the width of this junction-to-junction region in either structure strongly influence the resulting spectra. Significant differences exist between ZZ and AC junctions in both structures. The densities of states (DOSs) for unbiased structures are symmetric in energy whereas those for biased structures are asymmetric. An external bias creates interface LLs in the gaps between the LLs of the unbiased system in which the DOS can be quite small. Such a pattern of LLs can be probed by scanning tunneling microscopy. © 2017 American Physical Society.

PUBLISHER: American Physical Society

Derakhshan, V., Ketabi, S.A. Sizable band gap in organometallic topological insulator (2017) 85, pp. 253-258.

DOI: 10.1016/j.physe.2016.09.003

Based on first principle calculation when Ceperley–Alder and Perdew–Burke–Ernzerh type exchange-correlation energy functional were adopted to LSDA and GGA calculation, electronic properties of organometallic honeycomb lattice as a two-dimensional topological insulator was calculated. In the presence of spin–orbit interaction bulk band gap of organometallic lattice with heavy metals such as Au, Hg, Pt and Tl atoms were investigated. Our results show that the organometallic topological insulator which is made of Mercury atom shows the wide bulk band gap of about ∼120 meV. Moreover, by fitting the conduction and valence bands to the band-structure which are produced by Density Functional Theory, spin–orbit interaction parameters were extracted. Based on calculated parameters, gapless edge states within bulk insulating gap are indeed found for finite width strip of two-dimensional organometallic topological insulators. © 2016 Elsevier B.V.

AUTHOR KEYWORDS: Density functional theory; Edge states; Organometallic lattice; Spin–orbit interaction; Topological insulator
INDEX KEYWORDS: Electric insulators; Electronic properties; Energy gap; Heavy metals; Honeycomb structures; Lattice theory; Organometallics, Edge state; Exchange-correlation energy functionals; First principle calculations; Honeycomb lattices; Insulating gap; Interaction parameters; Mercury atoms; Topological insulators, Density functional theory
PUBLISHER: Elsevier B.V.

Derakhshan, V., Ketabi, S.A., Moghaddam, A.G. Tunable band gap, magnetoresistance and pseudo-magnetoresistance in silicene-based nanodevices (2016) 100, pp. 214-227.

DOI: 10.1016/j.spmi.2016.09.036

Spin-dependent transport in two terminal zigzag silicene nanoribbon is investigated numerically in the presence of spin-orbit interactions, external spin splittings or exchange fields, and perpendicular electric field. We show by applying an exchange field, a tunable band gap emerges which depends on the exchange field vector angle with the plane of nanoribbon. Such behavior is interpreted using the low-energy Hamiltonian of the silicene which indicates qualitative agreement with results obtained from lattice model. Moreover, it is found that by decreasing the width of nanoribbon larger band gaps are achievable which can be promising for nanoelectronic applications. On the other hand, by imposing exchange fields inside the electrodes, the magnetoresistance of the junction is investigated. As a main result we observe that when the Rashba interaction becomes stronger the magnetoresistance decreases but it is not fully suppressed. Finally, in the presence of perpendicular electric fields applied to the electrodes, depending on their relative configuration, the so-called pseudomagnetoresistance is calculated. Our results indicates that the conductance of the silicene nanoribbon significantly differs between the parallel and anti-parallel configurations of electric fields in the electrodes. © 2016 Elsevier Ltd

AUTHOR KEYWORDS: Magentoresistance; Pseudo-magnetoresistance; Quantum transport; Silicene; Spintronics
INDEX KEYWORDS: Electric fields; Electrodes; Galvanomagnetic effects; Magnetoelectronics; Magnetoresistance; Nanoribbons; Quantum chemistry; Quantum electronics; Terminals (electric), Magentoresistance; Nanoelectronic applications; Parallel configuration; Quantum transport; Silicene; Spin dependent transport; Spin orbit interactions; Tunable Band-gap, Energy gap
PUBLISHER: Academic Press

Mirzakhani, M., Zarenia, M., Da Costa, D.R., Ketabi, S.A., Peeters, F.M. Energy levels of ABC-stacked trilayer graphene quantum dots with infinite-mass boundary conditions (2016) 94 (16), art. no. 165423, .

DOI: 10.1103/PhysRevB.94.165423

Using the continuum model, we investigate the confined states and the corresponding wave functions of ABC-stacked trilayer graphene (TLG) quantum dots (QDs). First, a general infinite-mass boundary condition is derived and applied to calculate the electron and hole energy levels of a circular QD in both the absence and presence of a perpendicular magnetic field. Our analytical results for the energy spectra agree with those obtained by using the tight-binding model, where a TLG QD is surrounded by a staggered potential. Our findings show that (i) the energy spectrum exhibits intervalley symmetry EKe(m)=-EK′h(m) for the electron (e) and hole (h) states, where m is the angular momentum quantum number, (ii) the zero-energy Landau level (LL) is formed by the magnetic states with m≤0 for both Dirac valleys, that is different from monolayer and bilayer graphene QD with infinite-mass potential in which only one of the cones contributes, and (iii) groups of three quantum Hall edge states in the tight-binding magnetic spectrum approach the zero LL, which results from the layer symmetry in TLG QDs. © 2016 American Physical Society.

PUBLISHER: American Physical Society

Derakhshan, V., Ketabi, S.A., Moghaddam, A.G. Time-reversal breaking and spin transport induced by magnetic impurities in a 2D topological insulator (2016) 49 (35), art. no. 355001, .

DOI: 10.1088/0022-3727/49/35/355001

We employed the formalism of bond currents, expressed in terms of non-equilibrium Green's function to obtain the local currents and transport features of zigzag silicene ribbon in the presence of magnetic impurity. When only intrinsic and Rashba spin-orbit interactions are present, silicene behaves as a two-dimensional topological insulator with gapless edge states. But in the presence of finite intrinsic spin-orbit interaction, the edge states start to penetrate into the bulk of the sample by increasing Rashba interaction strength. The exchange interaction induced by local impurities breaks the time-reversal symmetry of the gapless edge states and influences the topological properties strongly. Subsequently, the singularity of partial Berry curvature disappears and the silicene nanoribbon becomes a trivial insulator. On the other hand, when the concentration of the magnetic impurities is low, the edge currents are not affected significantly. In this case, when the exchange field lies in the x-y plane, the spin mixing around magnetic impurity is more profound rather than the case in which the exchange field is directed along the z-axis. Nevertheless, when the exchange field of magnetic impurities is placed in the x-y plane, a spin-polarized conductance is observed. The resulting conductance polarization can be tuned by the concentration of the impurities and even completely polarized spin transport is achievable. © 2016 IOP Publishing Ltd.

AUTHOR KEYWORDS: berry curvature; bond current; conductance polarization; silicene; spintronics; time-reversal symmetry
INDEX KEYWORDS: Electric insulators; Fruits; Magnetism; Polarization; Silicene; Spintronics; Topology, berry curvature; Interaction strength; Magnetic impurity; Non-equilibrium Green's function; Rashba spin orbit interaction; Time reversal symmetries; Topological insulators; Topological properties, Impurities
PUBLISHER: Institute of Physics Publishing

Ketabi, S.A. Structural and electronic properties of trans-polyacetylene under local strain (2016) 80, pp. 95-100.

DOI: 10.1016/j.physe.2016.01.027

A theoretical study is presented to investigate the structural and electronic properties of trans-polyacetylene (trans-PA) molecule under local strain. The influence of a local bending or compression of the space between neighboring carbon atoms on the band gap of the molecule was studied. Making use of an effective difference equation based on tight-binding procedure the band structure of trans-PA has been calculated. Our results indicate that the energy gap of the strained molecule modified significantly which affects the electronic properties of the molecule. We found that the size of the molecular gap is proportional to the bending angle so that for the bending perpendicular to π-orbitals plane the band gap reduced drastically and for the parallel one the band gap gradually increased. Furthermore, the current-voltage characteristics of the strained trans-PA molecule are studied. We found that under the local strain the threshold voltage for the current flow through the bent molecule decreased (increased) depending on the bending is perpendicular (parallel) to the molecule plane. © 2016 Elsevier B.V. All rights reserved.

AUTHOR KEYWORDS: Electronic transport; Green's function method; Polyacetylene; Strain
INDEX KEYWORDS: Carbon; Current voltage characteristics; Difference equations; Energy gap; Green's function; Molecules; Polyacetylenes; Strain; Threshold voltage, Bending angle; Bent molecules; Electronic transport; Local strains; Strained molecules; Structural and electronic properties; Theoretical study; Trans-polyacetylene, Electronic properties
PUBLISHER: Elsevier

Adelifard, M., Salamatizadeh, R., Ketabi, S.A. Fabrication and characterization of nanostructural WS2/WO3 binary compound semiconductors prepared by the sulfurization of sprayed thin films (2016) 27 (5), pp. 5243-5250.

DOI: 10.1007/s10854-016-4420-x

We have investigated the impact of sulfurization and changing the metal salt ratio ([s]/[w] = 6–24 at.%) in the precursor solution on the structural, morphological, optical, electrical and photoconductivity properties of WS2/WO3 binary thin films grown on glass substrate by the spray pyrolysis technique. X–ray diffraction analysis showed that while the as-deposited samples present an amorphous nature, annealing these films in sulfur atmosphere generate a mix phase of WS2/WO3. FESEM study revealed the changes in surface morphology with increase in sulfur concentration in the spray solution. UV–Vis optical measurements analysis showed that these binary films have a relatively high absorption coefficient ~2 × 104–8 × 104 cm−1 in the visible spectrum with two distinct inflexions indicative of two transitions correspond to WS2/WO3 phases in agreement with the XRD analysis. The electrical studies showed that all these samples have a p-type conductivity; and the resistivity and photoconductivity decreases with increasing the sulfur to tungsten molar ratio. © 2016, Springer Science+Business Media New York.

INDEX KEYWORDS: Absorption spectroscopy; Amorphous films; Optical data processing; Photoconductivity; Semiconducting films; Spray pyrolysis; Substrates; Tungsten compounds, Absorption co-efficient; Diffraction analysis; Fabrication and characterizations; Optical measurement; P type conductivity; Precursor solutions; Spray-pyrolysis techniques; Sulfur concentrations, Thin films
PUBLISHER: Springer New York LLC

Mirzakhani, M., Zarenia, M., Ketabi, S.A., Da Costa, D.R., Peeters, F.M. Energy levels of hybrid monolayer-bilayer graphene quantum dots (2016) 93 (16), art. no. 165410, .

DOI: 10.1103/PhysRevB.93.165410

Often real samples of graphene consist of islands of both monolayer and bilayer graphene. Bound states in such hybrid quantum dots are investigated for (i) a circular single-layer graphene quantum dot surrounded by an infinite bilayer graphene sheet and (ii) a circular bilayer graphene quantum dot surrounded by an infinite single-layer graphene. Using the continuum model and applying zigzag boundary conditions at the single-layer-bilayer graphene interface, we obtain analytical results for the energy levels and the corresponding wave spinors. Their dependence on perpendicular magnetic and electric fields are studied for both types of quantum dots. The energy levels exhibit characteristics of interface states, and we find anticrossings and closing of the energy gap in the presence of a bias potential. © 2016 American Physical Society.

PUBLISHER: American Physical Society

Ketabi, S.A. Structural Parameters and Spin Filtering Properties of Ga1−x(M)xP compound (2016) 29 (4), pp. 959-964.

DOI: 10.1007/s10948-016-3396-3

In this work, we have explored the structural and magnetic properties of GaP-based diluted magnetic semiconductors (DMSs). Based on first-principle density functional theory (DFT) calculations and using a full potential linearized augmented plane wave (FP-LAPW) method in generalized gradient approximation (GGA), some significant structural and magnetic properties of Ga 1−x(M)xP compound as DMS are investigated. In this compound, M is a transition element such as vanadium (V), manganese (Mn), cobalt (Co), and copper (Cu) with a concentration of X. We have calculated the structural parameters such as the equilibrium lattice constant and bulk modulus of the compound. Furthermore, the spin polarization and magnetic moments are studied. We have found that by increasing the atomic number of the transition element, the lattice constant reduces, except for that of Cu, and compressibility improved in comparison with GaP. Moreover, with X=25 %, the Ga0.75(M)0.25P compound becomes more stable by increasing the atomic number of the transition element M. The study of the electronic properties of the compound indicates that the main contribution in total density of states near Fermi level is related to the 3d orbitals of the transition elements and the highest magnetic moment is for Mn-doped GaP. © 2016, Springer Science+Business Media New York.

AUTHOR KEYWORDS: DFT; Diluted magnetic semiconductor; Spin polarization; Transition elements
INDEX KEYWORDS: Atoms; Density functional theory; Diluted magnetic semiconductors; Electronic properties; Gallium phosphide; III-V semiconductors; Indium compounds; Lattice constants; Magnetic moments; Magnetic properties; Magnetic semiconductors; Spin polarization; Transition metals; Vanadium compounds, Diluted magnetic semiconductors (DMSs); First-principle density-functional theories; Full potential linearized augmented plane wave method; Generalized gradient approximations; Structural and magnetic properties; Structural parameter; Total density of state; Transition element, Structural properties
PUBLISHER: Springer New York LLC

Ketabi, S.A., Nakhaee, M. Influence of soliton distributions on the spin-dependent electronic transport through polyacetylene molecule (2016) 86 (3), pp. 669-680.

DOI: 10.1007/s12043-015-1077-6

In this paper, a detailed numerical study of the role of selected soliton distributions on the spin-dependent transport through trans-polyacetylene (PA) molecule is presented. The molecule is attached symmetrically to magnetic semi-infinite three-dimensional electrodes. Based on Su- Schrieffer-Heeger (SSH) Hamiltonian and using a generalized Green's function formalism, we calculate the spin-dependent currents, the electronic transmission and tunnelling magnetoresistance (TMR). We found that the presence of a uniform distribution of the soliton centres along the molecular chain reduced the size of the band gap of trans-PA molecule. Moreover, a sublattice of the correlated solitons as binary clusters, which are randomly distributed along the chain, can induce extended electronic states in the band gap of the molecule. In this case, the band gap of the molecule is suppressed and at lower voltages, the TMR bandwidth is narrowed. The current-voltage characteristic then shows an ohmic-like behaviour. © Indian Academy of Sciences.

AUTHOR KEYWORDS: Polyacetylene; Soliton; Spin-dependent current; Tunnelling magnetoresistance
INDEX KEYWORDS: Current voltage characteristics; Energy gap; Hamiltonians; Molecules; Polyacetylenes; Solitons, Electronic transmissions; Generalized Green's function; Randomly distributed; Spin dependent electronic transport; Spin dependent transport; Three-dimensional electrode; Trans-polyacetylene; Uniform distribution, Tunnelling magnetoresistance
PUBLISHER: Indian Academy of Sciences

Sadeghi, N., Ketabi, S.A., Shahtahmassebi, N., Abolhassani, M.R. Spin-polarized electron transport through magnetic poly-BIPO molecule: the role of soliton–antisoliton separation (2016) 90 (2), pp. 195-200.

DOI: 10.1007/s12648-015-0746-0

Spin-polarized transport through a one-dimensional metal/poly-BIPO/metal model junction with the soliton–antisoliton separation is investigated. Nonlinear spin and charge densities are considered in magnetic poly-BIPO molecule, as a neutral soliton and charged antisoliton with different separations. The calculations are performed based on Su–Schrieffer–Heeger Hamiltonian which is extended with Heisenberg and Hubbard Hamiltonians to include the spin and electron–electron interactions. The spin-dependent transport properties are obtained within the framework of the Landauer–Büttiker formalism based on Green’s function theory. This study demonstrates the reduction in current and spin polarization as the separation between soliton and antisoliton centers is increased. We have found that when the soliton–antisoliton separation is less than 14 sites, the spin polarization is almost 100 % plato, over the voltage ranges more than 0.3 V. Also the energy differences between the soliton–antisoliton mid-gap states for up- and down-spin electrons and the Fermi energy of the system are reduced. However, for the soliton–antisoliton separation lengths more than 14 sites, these quantities tend to constant values with enhancement of the distance between the excitation centers. © 2015, Indian Association for the Cultivation of Science.

AUTHOR KEYWORDS: Green’s function method; Polyacetylene; Soliton–antisoliton pair; Spin-polarized transport
PUBLISHER: Scientific Publishers

Ketabi, S.A. Effect of radiation on the electronic transport through polyacetylene-based junctions (2016) 481, pp. 165-171.

DOI: 10.1016/j.physb.2015.11.009

A theoretical study is presented to investigate the electronic transport properties of polyacetylene-based junctions irradiated by time-dependent electromagnetic field (EF). Making use of tight-binding procedure and within the framework of time-dependent Green's function formalism, the current-voltage (I-V) characteristics of trans-PA molecule and the electronic transmission through the junction are studied. At low temperatures, our results indicate the EF causes a broad tail inside the gap region and induces the extended electronic states in the band gap of the molecule give rise to the enhancement of the hopping conduction through the junction. Furthermore, the size of the molecular gap is proportional to the incident angles of radiation and at the incident angles between θ=30° and θ=45° the band gap of the molecule suppressed and the I-V characteristics show metallic-like behavior. © 2015 Elsevier B.V.

AUTHOR KEYWORDS: Electronic transport; Polyacetylene; Radiation; Time-dependent Green's function
INDEX KEYWORDS: Electromagnetic fields; Green's function; Heat radiation; Molecules; Polyacetylenes, Electronic transmissions; Electronic transport; Electronic transport properties; Hopping conduction; IV characteristics; Low temperatures; Theoretical study; Time dependent, Energy gap
PUBLISHER: Elsevier

Ahmadi Fouladi, A., Ketabi, S.A. Electronic properties of Z-shaped graphene nanoribbon under uniaxial strain (2015) 74, art. no. 12077, pp. 475-480.

DOI: 10.1016/j.physe.2015.08.018

Abstract Based on tight-binding approximation and a generalized Green's function method, the effect of uniaxial strain on the electron transport properties of Z-shaped graphene nanoribbon (GNR) composed of an armchair GNR sandwiched between two semi-infinite metallic armchair GNR electrodes is numerically investigated. Our results show that the increase of uniaxial strain enhances the band gap and leads to a metal-to-semiconductor transition for Z-shaped GNR. Furthermore, in the Landauer-Büttiker formalism, the current-voltage characteristics, the noise power resulting from the current fluctuations and Fano factor of strained Z-shaped GNR are explored. It is found the threshold voltage for the current and the noise power increased so that with reinforcement of the uniaxial strain parameter strength, the noise power goes from the Poisson limit to sub-Poisson region at higher bias voltages. © 2015 Elsevier B.V.

AUTHOR KEYWORDS: Graphene nanoribbon; Green's function method; Noise power; Uniaxial strain
INDEX KEYWORDS: Current voltage characteristics; Electron transport properties; Electronic properties; Energy gap; Graphene; Green's function; Threshold voltage, Current fluctuations; Generalized Green's function; Graphene nano-ribbon; Graphene nanoribbon (GNR); Metal-to-semiconductor transition; Noise power; Tight-binding approximations; Uni-axial strains, Nanoribbons
PUBLISHER: Elsevier

Sadeghi, N., Ketabi, S.A., Shahtahmassebi, N., Abolhassani, M.R. Influence of Soliton Spin Density on the Spin Filtering Properties of Magnetic Poly-BIPO Molecule (2015) 28 (7), pp. 2203-2208.

DOI: 10.1007/s10948-015-3004-y

Spin-polarized transport through the magnetic polyacetylene (poly-BIPO) in the presence of a soliton spin density distribution is numerically investigated in the metal/poly-BIPO/metal model junction. The study was performed using the modified Su–Schrieffer–Heeger Hamiltonian with the extended Hubbard model for the electron–electron interaction in the case of finite electron correlation. The spin transport properties were calculated within the framework of Green’s function theory and Landauer–Büüttiker formalism. Our results indicate that the presence of neutral solitons gives rise to the extension of spin density distribution which induces the extended soliton states in the middle of an energy gap of the molecule such that the band gap width of the molecule reduced It is also shown that the spin transport depends on the relative direction of the spin of soliton and the spin of radicals, and also, the maximum polarization happens when the spin direction of the radicals and soliton is in the opposite directions. © 2015, Springer Science+Business Media New York.

AUTHOR KEYWORDS: Green’s function method; Polyacetylene; Soliton; Spin filtering
INDEX KEYWORDS: Energy gap; Molecules; Polyacetylenes; Spin polarization, Band gap width; Electron interaction; Extended Hubbard model; Neutral solitons; S function; Spin density distributions; Spin filtering; Spin polarized transport, Solitons
PUBLISHER: Springer New York LLC

Sarverstani, N.K., Ketabi, S.A., Yazdani, A. The Effect of RKKY Interaction on Chemical Pressure in Formation of Nanocrystalline Structure for Some Gd Intermetallic Compounds (2015) 28 (3), pp. 999-1002.

DOI: 10.1007/s10948-014-2868-6

Although there is no discrimination between cluster and nanoparticle, the formation of cluster is usually investigated by the short-range exchange between the groups of magnetic ions that are closer to each other than the nearest neighbors. The phenomenon is considered by the decrease of symmetry that is related to the increase of conduction electron concentration (chemical pressure), where the compressibility is increased while the strength of exchange interaction is decreased. This reduction may be the main reason for the formation of intra-cluster interactions, which can be also the base of isolated nanosized formation. The decrease of symmetry is the result of the electrical field gradient calculated by WIEN2k package, which can be supposed to vary between 0 and 1. The formation of defined cluster may be geometrized by a dynamic equilibrium between two triangle antiferromagnetic islands’ nanopercolations in the correlation length of RC, 3.16Å≤RC≤3.6∘Å,with eight nearest neighbors. © 2014, Springer Science+Business Media New York.

AUTHOR KEYWORDS: Asymmetry parameter; Chemical pressure; Electrical field gradient; Nanocluster
INDEX KEYWORDS: Gadolinium; Intermetallics; Nanoclusters; Nanomagnetics, Antiferromagnetics; Asymmetry parameter; Chemical pressures; Conduction electrons; Correlation lengths; Electrical field gradients; Nano-crystalline structures; Short-range exchange, Chemical compounds
PUBLISHER: Springer New York LLC

Sarverstani, N.K., Yazdani, A., Ketabi, S.A. Investigating the Cause of Quantum Phase Transition in Gd Intermetallic Compounds (2015) 28 (3), pp. 1003-1006.

DOI: 10.1007/s10948-014-2673-2

Quantum phase transition (QPT), a consequence of quantum critical point (QCP), was considered for the suppression of unstable double phase transition of Gd intermetallic compounds to the paramagnetic state, despite the lack of both hybridization and crystal field effect (CFE) which are fundamental in the existence of QCP. The phenomenon is supposed to be due to the exchange fluctuation character in terms of competition between exchange and kinetic energies of conduction electrons (c.e) during the strong spin-induced polarization of c.e defined by DFT calculations. All electronic structure calculations in this work were carried out by WIEN2K package in the framework of PBE approximation. We found evidences on the tuning of KF in direction to shift the topological magnetic ions to minimize the correlation length RC=2KFRij at the critical point. At this point, the system is close to the critical value near the double ferromagnetic-antiferromagnetic percolation threshold. Experiment shows that the broad dynamic instability of the Curie temperature can lead the sample behavior to the antiferromagnetic state by induced stress motion due to the sample shape (powder to needle) and to the paramagnetic state by imposing magnetic field. The calculated cohesive energy implies that the exchange energy can adopt the magneto-crystalline system as a more stable symmetrical configuration by developing much more negative cohesive energy depending on the yttrium compound in direction to reduce the bulk modulus, which results in the vanishing of antiferromagnetic ordering. We found evidences on the quantum phase (QP) coherence of the Fermi wave vector λF, which contributes to the shear stress σ in direction to stabilize both magnetic and crystalline structures in the paramagnetic state. © 2014, Springer Science+Business Media New York.

AUTHOR KEYWORDS: Conduction electron; Correlation length; Exchange energy; Quantum phase transition
INDEX KEYWORDS: Antiferromagnetism; Crystalline materials; Electronic structure; Intermetallics; Kinetic energy; Magnetism; Paramagnetism; Phase transitions; Shear flow; Shear stress; Solvents, Antiferromagnetic orderings; Antiferromagnetic state; Conduction electrons; Correlation lengths; Electronic structure calculations; Exchange energy; Quantum critical points; Quantum phase transitions, Quantum theory
PUBLISHER: Springer New York LLC

Ketabi, S.A., Khouzestani, H.F. Electronic transport through dsDNA based junction: A Fibonacci model (2015) 14 (3), pp. 67-72.

A numerical study is presented to investigate the electronic transport properties through a synthetic DNA molecule based on a quasiperiodic arrangement of its constituent nucleotides. Using a generalized Green's function technique, the electronic conduction through the poly(GACT)-poly(CTGA) DNA molecule in a metal/DNA/metal model structure has been studied. Making use of a renormalization scheme we transform the Hamiltonian of double-stranded DNA (dsDNA) molecule to an effective Hamiltonian corresponding to a one-dimensional chain in which the effective on-site energies are arranged as a quasiperiodic lattice according to Fibonacci sequence. The room temperature current-voltage characteristic of dsDNA has been investigated in this Fibonacci model and compared with those corresponding to poly(GACT)-poly(CTGA) DNA molecule. Our results indicate the main effect of the quasiperiodic arrangement of the nucleotides as the Fibonacci sequence on the electronic spectrum structure of the dsDNA is that the energy band gaps of the molecule have a tendency for suppression. The room temperature I-V characteristic of the DNA Fibonacci model shows a linear and ohmic-like behavior. © 2014, Isfahan University of Technology. All rights reserved.

AUTHOR KEYWORDS: Current-voltage characteristic; DNA; Fibonacci model; Green's function
PUBLISHER: Isfahan University of Technology

Sarvestani, N.K., Ketabi, S.A., Yazdani, A. Electro-mechanical character of Gd and Gd2In and possibility of a Kondo-like behavior in Gd2In (2014) 613, pp. 62-67.

DOI: 10.1016/j.jallcom.2014.05.197

The elastic constants of Gd-element and Gd2In have been determined by density functional theory (DFT). Mechanical properties have been specified and compared with available experimental results. In this work, a detailed ab initio study on Gd and Gd2In are presented in order to investigate the magneto-elastic and electronic properties and to consider the tune parameters involved in these values. The relationship between these data and the possibility of Kondo-like effect in Gd2In were discussed. © 2014 Elsevier Ltd. All rights reserved.

AUTHOR KEYWORDS: Ab initio calculation; DFT; Kondo effect; Mechanical properties; Rare-earth intermetallic compounds
INDEX KEYWORDS: Electronic properties; Kondo effect; Mechanical properties, Ab initio calculations; Ab initio study; DFT; Electro-mechanical; Magneto-elastic; Rare earth intermetallic compounds, Gadolinium
PUBLISHER: Elsevier Ltd

Kamali Sarvestani, N., Yazdani, A., Ketabi, S.A. The effect of pressure-induced structural transition on exchange interaction function and electronic structure in Gd-element (2014) 16 (45), pp. 25191-

DOI: 10.1039/c4cp04242e

In the present work, two models based on the mean field approximation and density functional theory are developed for two independent subsystems - the “local-spin exchange” and “conduction band”- in order to analysis the elimination of exchange anisotropy, where the possibility of Kondo-like behavior in gadolinium-element can be investigated. These models allow us to describe the coupled spin-lattice subsystems in direction to remove the intra-layer loop of exchange of “hexagonal” to lower symmetry of “rhombohedral” (crystallography slip). The intra-layer “a-b” loop exchange, which is the cause of exchange anisotropy, was calculated by the exchange eigenvalue-eigenfunction Jij(R-R′) between two completely separate magnetic ions (Rij≥ 3.6 Å>R4f≈ 0.36 Å) in the metallic Gd-element, where there is no crystal field effect (L = 0) and to a good approximation no notable hybridization in the mean field approximation. In this regard, the pressure induced phase transition of Gd from hexagonal to rhombohedral as the result of the first principle density functional theory by using the Wien2K package within the PBE + U approximation, is investigated. We observed the leakage of d orbitals into f orbitals in the electronic structure of the Gd rhombohedral phase, as well as the coincidence of all three principal directions in the eigenvalue (λmin(K)). Both phenomena can predict the appearance of Kondo-like behavior in Gd. © the Partner Organisations 2014.

Ashhadi, M., Ketabi, S.A. Quasi-particle energies and excitonic effects in bilayer of hexagonal boron nitride (2014) 187, pp. 1-4.

DOI: 10.1016/j.ssc.2014.01.022

First-principles calculations based on the density functional theory (DFT), GW and Bethe-Salpeter equations are performed on the bilayer of hexagonal boron nitride (bilayer h-BN) to investigate the electronic structure and optical properties. Due to the quantum confinement effect and the less efficient electronic screening, the value of direct (indirect) band gap increases from a value of 4.45 eV (4.37 eV) within the density functional theory to a value of 6.74 eV (6.67 eV) within the GW many-body Green×s function theory. The calculated optical absorption spectra are dominated by exciton states with a binding energy about of 1.67 eV. The enhanced excitonic effects in bilayer h-BN have the potential to be used in optoelectronic and excitonic devices. © 2014 Published by Elsevier Ltd.

AUTHOR KEYWORDS: A. Nanostructures; D. Dielectric response; D. Electron-electron interaction; D. Optical properties

Ahmadi Fouladi, A., Ketabi, S.A. Influence of quantum interference on the tunnel magnetoresistance in a molecular junction in the presence of in-plane electric field (2014) 27 (4), pp. 1105-1110.

DOI: 10.1007/s10948-013-2393-z

Quantum interference effects on the spin-dependent transport in a molecular spin-valve, as FM/Benzene/FM model junction, in which a benzene molecule attached to ferromagnetic (FM) three-dimensional leads is numerically investigated. Using a generalized Green's function method and in the framework of Landauer-Büttiker formalism, the variation of interference conditions is determined by rotation of the benzene molecule, and replacement of the connection configurations of the molecule to the FM leads from ortho to meta and then para configuration. We have found that transport characteristics, including the spin-dependent current and tunnel magnetoresistance (TMR) are strongly influenced by the quantum interference effects. Besides, the effect of an in-plane electric field on the spin-dependent transport characteristics is treated. © 2013 Springer Science+Business Media New York.

AUTHOR KEYWORDS: Benzene molecule; Green's function; Quantum interference; Spin-dependent transport; TMR
INDEX KEYWORDS: Benzene; Electric fields; Green's function; Magnetoresistance; Molecules; Quantum interference devices, Benzene molecules; Generalized Green's function; Quantum interference; Quantum interference effects; Spin dependent transport; TMR; Transport characteristics; Tunnel magnetoresistance, Tunnelling magnetoresistance
PUBLISHER: Springer New York LLC

Ketabi, S.A., Ashhadi, M. Influence of inelastic electron-phonon interaction on the noise power through a molecular junction (2013) 53, pp. 150-154.

DOI: 10.1016/j.physe.2013.05.007

The effect of the inelastic electron-phonon interaction on the spin-dependent transport through a ferromagnetic/molecular structure as FM/trans-PA/FM model junction is numerically investigated. Based on a generalized Green's function formalism and also the mapping technique, which transforms the many-body electron-phonon interaction problem into a multi-channels single-electron scattering problem, we found that in the presence of the electron-phonon interaction, the currents are generally reduced and the step-like structure of the current-voltage characteristic are deformed. In addition, our results indicate that the main influence of the inclusion of the electron-phonon interaction is the reduction of the conductance gap of the molecule. Furthermore, it is shown that due to inelastic interactions, the noise power increase and the significant change occurs in the Fano factor. © 2013 Elsevier B.V.

AUTHOR KEYWORDS: Electron-phonon interaction; Fano factor; Green's function formalism; Molecular junction
INDEX KEYWORDS: Conductance gap; Fano factor; Generalized Green's function; Inelastic interaction; Mapping techniques; Molecular junction; Scattering problems; Spin dependent transport, Green's function, Electron-phonon interactions

Ahmadi Fouladi, A., Ketabi, S.A. Spin-dependent fano factor in FM/DNA/FM molecular junction (2013) 26 (2), pp. 469-475.

DOI: 10.1007/s10948-012-1768-x

Based on a tight-binding model and a generalized Green's function method in the Landauer-Büttiker formalism, the current-voltage characteristics, the noise power resulting of the spin-dependent current fluctuations and Fano factor of poly(G)-poly(C) DNA molecule sandwiched between ferromagnetic three-dimensional electrodes (FM/DNA/FM) are numerically investigated. It is found when the bias voltage increased, the current and noise power increase and the Fano factor decreases. Our results show the spin-dependent transport properties are significantly influenced by the electrode/molecule coupling strength so that the reinforcement of the electrode/molecule coupling strength give rise to increasing of the current and noise power so that the Fano factor decreased. © 2012 Springer Science+Business Media, LLC.

AUTHOR KEYWORDS: Deoxyribonucleic acid (DNA); Fano factor; Green's function; Spin-dependent transport
INDEX KEYWORDS: Current fluctuations; Fano factor; Generalized Green's function; Molecular junction; Spin dependent transport; Spin-dependent transport properties; Three-dimensional electrode; Tight binding model, DNA; Green's function; Three dimensional; Transport properties, Nucleic acids

Ashhadi, M., Ketabi, S.A. Electronic transport properties through ZGNR/BNAM/ZGNR (2012) 46, pp. 250-253.

DOI: 10.1016/j.physe.2012.08.010

We present a theoretical study of electron transport properties through boron-nitride aromatic molecules (BNAMs) embedded between two zig-zag graphene nanoribbons (ZGNRs), which are considered as electrodes. The work is based on a tight-binding Hamiltonian model within the framework of a generalized Green's function technique and relies on the Landauer-Bütikker formalism as the basis for studying the current-voltage characteristic of this system. It is shown that the current can decrease at a finite value of voltage and the electron transport can open a conduction gap in the ZGNR/BNAM/ZGNR structure. © 2012 Elsevier B.V.

INDEX KEYWORDS: Aromatic molecules; Conduction gap; Electron transport; Electronic transport properties; Generalized Green's function; Graphene nanoribbons; Theoretical study; Tight-binding Hamiltonians; Zig-zag, Boron nitride; Electron transport properties; Graphene; Nitrides; Transport properties, Hamiltonians

Fouladi, A., Ketabi, S.A., Elahi, S.M., Sebt, S.A. Tunnel magnetoresistance of the heterocyclic molecular junctions: A green's function approach (2012) 25 (6), pp. 1965-1970.

DOI: 10.1007/s10948-012-1544-y

We explore the spin-dependent transport properties in a molecular junction model made of heterocyclic molecules such as poythiophene, polyfuran, and polypyrrole sandwiched between ferromagnetic three-dimensional electrodes, hereafter named FM/h-molecule/FM junction, based on a tight-binding model and a generalized Green's function method in the Landauer-Büttiker formalism. The coherent spin-dependent transport through the energy levels of heterocyclic molecules, the transmittance, current-voltage (I-V ) characteristics, and the tunnel magnetoresistance (TMR) of the FM/h-molecule/FM junction are numerically investigated. It is found that the spin-dependent transport properties are importantly influenced by the heteroatoms in the heterocyclic molecules. It is shown that the TMR of the molecular junction can be quite large (over 69 %) depending on the applied voltage and the molecular field of the FM electrodes. © Springer Science+Business Media, LLC 2012.

AUTHOR KEYWORDS: Green's function; Heterocyclic molecules; Spin-dependent transport; TMR
INDEX KEYWORDS: Applied voltages; Generalized Green's function; Heteroatoms; Heterocyclic molecules; Molecular fields; Molecular junction; Polyfuran; Spin dependent transport; Spin-dependent transport properties; Three-dimensional electrode; Tight binding model; TMR; Tunnel magnetoresistance, Electric resistance; Electrodes; Green's function; Magnetoresistance; Molecules; Organic compounds; Polypyrroles; Three dimensional; Transport properties; Tunnel junctions, Current voltage characteristics

Ahmadi Fouladi, A., Ketabi, S.A., Elahi, S.M., Sebt, S.A. Effects of dephasing on the spin-dependent currents and noise power in a molecular junction (2012) 85 (5), art. no. 163, .

DOI: 10.1140/epjb/e2012-30056-8

The role of dephasing reservoirs on the spin-dependent transport through a polythiophene (PT) molecule sandwiched between ferromagnetic 3-dimensional electrodes as a FM/PT/FM junction is numerically investigated. Our calculations are performed based on a tight-binding model and a generalized Green's function method in the well-known Landauer-Büttiker formalism. We investigate the influence of dephasing reservoirs on the spin dependent currents, noise power, Fano factor and tunnel magnetoresistance (TMR) of the junction. Our results illustrate that the presence of dephasing reservoirs give rise to increase the currents and noise power due to reduced destructive interference resulting from the Büttiker probes. We found the significant change in the Fano factor and TMR of the FM/PT/FM junction in the presence of dephasing reservoirs. Furthermore, we explore the influence of the electrode/molecule coupling strength on the transport properties of the FM/PT/FM junction. It is shown that the electrode/molecule coupling strength may control the spin-dependent transport properties and so it is a significant parameter for designing of the efficient molecular spintronic devices. © EDP Sciences, Società Italiana di Fisica, Springer-Verlag 2012.

INDEX KEYWORDS: 3-dimensional; Coupling strengths; Dephasing; Destructive interference; Fano factor; Generalized Green's function; Molecular junction; Noise power; Spin dependent transport; Spin-dependent transport properties; Spintronic device; Tight binding model; Tunnel magnetoresistance, Electric resistance; Interference suppression; Magnetoelectronics, Transport properties

Ashhadi, M., Ketabi, S.A. Electric field effects on tunnel magnetoresistance in FM-organic molecule-FM junction (2011) 44 (3), pp. 605-608.

DOI: 10.1016/j.physe.2011.10.011

The electronic properties of an organic molecule under external electric field are investigated based on a single band tight-binding model Hamiltonian and the Greens function approach with the Landauer-Büttiker formalism. These properties are studied for a chain of benzene rings (oligophenylene). A self-consistent calculation is adopted to analyze the external electric field effects on the electronic properties of system. It is shown that variation in electron density and HOMOLUMO gap of the junction are direction depended on the external electric field strength. Our results show that in the presence of external electric field the transmission, current and tunnel magnetoresistance (TMR) decrease, the negative differential resistance occurs. © 2011 Elsevier B.V.

INDEX KEYWORDS: Benzene ring; External electric field; External electric-field effect; HOMO-LUMO gaps; Negative differential resistances; Oligophenylenes; Organic molecules; Self-consistent calculation; Single band; Tight binding model; Tunnel magnetoresistance, Benzene; Electric properties; Electric resistance; Electronic properties; Engineering exhibitions; Hamiltonians; Magnetoelectronics; Magnetoresistance, Electric field effects

Ardyanian, M., Ketabi, S.A. Temperature dependence of continuum and time resolved photoluminescence of germanium nanostructures (2011) 11 (3), .

Germanium nanostructures were generated in the post annealed germanium oxide thin films. Visible and near infrared photoluminescence bands were observed in the samples annealed at 350°C and 400°C, respectively. These different luminescence ranges are attributed to the presence of the defects in oxide matrix and quantum confinement effect in the germanium nanostructures, respectively. Decay time and temperature dependence of the luminescence for different bands were investigated, which confirmed our idea about the origin of the luminescence.

AUTHOR KEYWORDS: Germanium; Nanostructures; Photoluminescence; Temperature dependence

Ketabi, S.A., Bahoosh, S.G., Shahtahmasebi, N. Temperature dependence of the giant magnetoresistance in Fe/DNA/Fe structure (2011) 11 (10), pp. 8943-8946.

DOI: 10.1166/jnn.2011.3485

A numerical study is presented to investigate the spin-dependent transport through poly(dG)-poly(dC) DNA molecule sandwiched between ferromagnetic contacts in the absence and in the presence of environmental effects. Making use of tight-binding procedure and within the framework of a generalized Green's function technique, the room temperature current-voltage characteristics of DNA molecule and the giant magnetoresistance (GMR) of Electrode/DNA/Electrode structure, with iron (Fe) as the electrode are studied. It is found that the GMR to be lower than 12% for small applied bias and about 35% for applied bias larger than 2.5 volts. Considering the environmental effects, the GMR would be increased up to 13% at a bias lower than 2 volts and decreases up to 23% for the bias about 2.5 volts. In addition, our calculations indicate that for applied biases around 2.5 volts, the GMR decreases when the temperature increased. Copyright © 2011 American Scientific Publishers. All rights reserved.

AUTHOR KEYWORDS: DNA; GMR; Green's Function Technique; Spin-Dependent Transport; Spintronics
INDEX KEYWORDS: Applied bias; DNA molecules; Ferromagnetic contacts; Generalized Green's function; GMR; Green's Function Technique; Numerical studies; Room temperature; Spin dependent transport; Temperature dependence; Tight binding, DNA; Galvanomagnetic effects; Giant magnetoresistance; Magnetoelectronics, Current voltage characteristics, DNA; iron, article; chemistry; electrode; instrumentation; magnetism; methodology; temperature; thermodynamics, DNA; Electrodes; Iron; Magnetics; Temperature; Thermodynamics

Ketabi, S.A., Kazemi, A.S., Bagheri-Mohagheghi, M.M. The effect of complexing agent on the crystallization of ZnO nanoparticles (2011) 77 (4), pp. 679-688.

DOI: 10.1007/s12043-011-0135-y

In this work, some structural and optical properties of the zinc oxide (ZnO) nanoparticles were studied. The highly crystalline ZnO nanoparticles were produced by the hydrothermal and sol - gel methods. The analyses of the XRD patterns, STEM images and UV spectroscopy showed that the size of the nanoparticles prepared by oxalic acid was smaller than the ones by urea. The properties of oxalic acid and urea were also investigated to determine the most effective crystallization process of ZnO nanoparticles. It has been shown that pH, decomposition temperature and activity coefficient of the complexing agent have certain effects on crystallization process. © Indian Academy of Sciences.

AUTHOR KEYWORDS: Hydrothermal method; Sol-gel method; ZnO nanoparticles
INDEX KEYWORDS: Complexing agents; Crystallization process; Decomposition temperature; Effect of complexing agents; Gel method; Hydrothermal methods; Oxalic Acid; Sol-gel methods; STEM images; Structural and optical properties; XRD patterns; ZnO; ZnO nanoparticles, Crystallization; Gels; Metabolism; Optical properties; Organic acids; pH effects; Sol-gel process; Sol-gels; Sols; Ultraviolet spectroscopy; Urea; Zinc oxide, Nanoparticles

Ashhadi, M., Ketabi, S.A. Effect of inelastic electronphonon interaction on tunneling magnetoresistance through ferromagneticorganic moleculeferromagnetic junction(2011) 151 (18), pp. 1236-1239.

DOI: 10.1016/j.ssc.2011.05.046

We have studied the effect of inelastic electronphonon interaction on the spin-dependent transport properties of a molecule, trans-polyacetylene (trans-PA), as a molecular bridge sandwiched between two ferromagnetic (FM) electrodes. The work is based on a tight-binding Hamiltonian model within the framework of a generalized Green's function technique and relies on the LandauerBttiker formalism as the basis for studying the currentvoltage characteristic of this system. We use the wide-band approximation for FM electrodes. It is shown that due to inelastic interactions, the spin currents increase in a finite value of voltage and tunnel magnetoresistance (TMR) decreases compared with TMR obtained in the absence of phonons. © 2011 Elsevier Ltd. All rights reserved.
AUTHOR KEYWORDS: A. Polyacetylene molecule; B. Green's function method; D. Electronphonon interaction; D. Tunnel magnetoresistance
INDEX KEYWORDS: A. Polyacetylene molecule; B. Green's function method; D. Tunnel magnetoresistance; Electron phonon; Generalized Green's function; Inelastic interaction; Molecular bridges; Spin currents; Spin-dependent transport properties; Tight-binding Hamiltonians; Trans-polyacetylene; Tunnel magnetoresistance; Tunneling magnetoresistance; Wide-band, Acetylene; Differential equations; Electrodes; Green's function; Hamiltonians; Magnetoelectronics; Magnetoresistance; Transport properties, Electric resistance

Ashhadi, M., Ketabi, S.A. Tunnel magnetoresistance of FMorganic moleculeFM junction: A Greens function approach (2011) 43 (6), pp. 1208-1212.

DOI: 10.1016/j.physe.2011.02.001

Coherent spin-dependent electronic transport is investigated in a molecular junction made of organic molecule attached to two the semi-infinite ferromagnetic (FM) electrodes with finite cross sections. The conductance properties of the spin system are studied for a chain of benzene rings (oligophenylene). The work is based on a single band tight-binding model Hamiltonian and the Greens function approach with the LandauerBüttiker formalism. It is shown that tunnel magnetoresistance (TMR) of molecular junction can be large (over 60%) by adjusting the related parameters, and depends on: (i) the applied voltages and (ii) the number of benzene rings. © 2011 Elsevier B.V. All rights reserved.
INDEX KEYWORDS: Applied voltages; Benzene ring; Conductance properties; Cross section; Electronic transport; Molecular junction; Oligophenylenes; Organic molecules; Single band; Spin systems; Tight binding model; Tunnel magnetoresistance, Benzene; Electric resistance; Magnetoelectronics; Magnetoresistance, Hamiltonians

Ashhadi, M., Ketabi, S.A., Shahtahmasebi, N., Fakhrabad, D.V., Askari, M. The role of impurities on the properties of electron transport through the metal/trans-PA/metal system: Greens function approach (2011) 43 (4), pp. 924-928.

DOI: 10.1016/j.physe.2010.11.015

In this work, we present a theoretical study on the conductance of trans-polyacetylene (trans-PA) nanowire in the presence of impurities and use of trans-PA as a molecular bridge sandwiched between two semi-infinite metallic electrodes with a simple cubic structure and square cross section. We use a tight-binding Hamiltonian model within the framework of a generalized Greens function technique and rely on the LandauerBtikker formalism as the basis for studying the currentvoltage characteristic of this system. Our calculations indicate that the presence of impurities gives rise to significant enhancement in the density of states within the bandgap and large enhancement in conductance and the currentvoltage characteristic. Also our results show that the electronic properties of the system are sensitive to the molecule-to-impurity coupling strength. © 2010 Elsevier B.V. All rights reserved.
INDEX KEYWORDS: Band gaps; Coupling strengths; Density of state; Electron transport; Metallic electrodes; Molecular bridges; Simple cubic; Square cross section; Theoretical study; Tight-binding Hamiltonians; Trans-polyacetylene, Acetylene; Electronic properties; Impurities; Nanowires, Hamiltonians

Ketabi, S.A., Vahedi, D., Khozestani, H.F. TMR and spin-dependent transport of polyacetylene-based magnetic junctions (2011) 83 (1), art. no. 015802, .

DOI: 10.1088/0031-8949/83/01/015802

A numerical study is presented that investigates the spin-dependent transport through a trans-polyacetylene (trans-PA) molecule sandwiched between ferromagnetic (FM) contacts. Using the tight-binding procedure and in the framework of a generalized Green's function technique, the room temperature current-voltage characteristics of the trans-PA molecule and the tunnel magnetoresistance (TMR) of the electrode/trans-PA/electrode structure, with iron (Fe) as the electrode, are studied. It is found that the parallel arrangement of magnetic moments in FM electrodes causes a much higher current through the polyacetylene molecule than does the anti-parallel arrangement. Also, our results indicate that TMR has its maximum value (more than 60%) at low bias voltages. © 2011 The Royal Swedish Academy of Sciences.
INDEX KEYWORDS: Generalized Green's function; Low bias voltage; Magnetic junctions; Maximum values; Numerical studies; Parallel arrangement; Room temperature; Spin dependent transport; Tight binding; Trans-polyacetylene; Tunnel magnetoresistance, Acetylene; Electric resistance; Green's function; Magnetic field effects; Magnetic moments; Magnetoresistance; Molecules, Current voltage characteristics

Kazemi, A., Ketabi, S.A., Bagheri-Mohagheghi, M.M., Abadyan, M. The effect of activity coefficient on growth control of ZnO nanoparticles (2011) 83 (1), art. no. 015801, .

DOI: 10.1088/0031-8949/83/01/015801

In this paper, the relative importance of effective parameters such as the activity coefficient, thermal decomposition and pH of chemical additives is investigated on the control procedure of ZnO nanoparticle growth. It is found that the activity coefficient is of greater importance compared with other parameters and should not be neglected in nanosynthesis any longer. This effect of activity coefficient of additives may also improve the fabrication and properties of other applicable nanostructures. © 2011 The Royal Swedish Academy of Sciences.
INDEX KEYWORDS: Chemical additive; Control procedures; Effective parameters; Growth control; Relative importance; Thermal decompositions; ZnO nanoparticles, Nanoparticles; Pyrolysis; Zinc oxide, Activity coefficients

Kazemi, A., Abadyan, M., Ketabi, S.A. Controlled structural and optical properties of ZnO nano-particles (2010) 82 (3), art. no. 035801, .

DOI: 10.1088/0031-8949/82/03/035801

In this work, we have analyzed two synthesis procedures through experimental characterizations, where one provides a main temperature region for the control of the shape and size of ZnO nano-particles in comparison to the other. We have found that the complexing agent has a significant role in showing such a control region. This effect might also improve the fabrication and properties of other interesting and applicable nano-structures. © 2010 The Royal Swedish Academy of Sciences.
INDEX KEYWORDS: Complexing agents; Control region; Experimental characterization; Shape and size; Structural and optical properties; Synthesis procedure; Temperature regions; ZnO nanoparticles, Nanoparticles; Zinc oxide, Optical properties

Kalhor, D., Zahiri, R., Ketabi, S.A., Ebrahimzad, A. The effect of Ag intermediate layer on crystalline, optical and electrical properties of nano-structured thin film (2010) 84 (5), pp. 539-546.

ITO thin films and ITO/Ag/ITO multilayered films were prepared on glass substrate by reactive thermal evaporation technique without intentionally heating the substrate. After deposition the films were annealed in air at three different temperatures (300°C, 420°C and 540°C). The thickness of each layer in the ITO/Ag/ITO films was kept constant at 50 nm/10 nm/40 nm. The opto-electrical and structural properties of ITO/Ag/ITO multilayered films were compared with conventional ITO single-layer films. Although both films had identical thickness, 100 nm, the ITO/Ag/ITO films showed a lower resistivity. XRD spectra showed that Ag intermediate layer had a small effect on crystalline properties of ITO/Ag/ITO films. © 2010 IACS.
AUTHOR KEYWORDS: Annealing; Indium-tin-oxide; Multilayered films; Thermal evaporation

Ketabi, S.A., Ghane, T., Shahtahmasebi, N. Influence of solitons on the conductance properties of double-stranded deoxyribonucleic acid (2010) 74 (1), pp. 143-152.

DOI: 10.1007/s12043-010-0016-9

A numerical study is presented to investigate the role of solitons in the electronic states of double-stranded DNA (dsDNA) molecule in the metal/DNA/metal system. Based on tight-binding Hamiltonian model and within the framework of a generalized Green's function technique, we consider a ladder model for poly(dG)-poly(dC) DNA molecule containing M cells with four sites (two base pair sites and two backbone sites) in each cell. In the presence of a sublattice of solitons, our results show that the homogeneous soliton distributions induce the electronic states in the band gap of DNA molecule. In addition, the room temperature current{voltage characteristic of the system shows a linear and ohmic-like behaviour. © Indian Academy of Sciences.
AUTHOR KEYWORDS: Deoxyribonucleic acid; Electronic transmission; Green's function; Ladder model; Soliton
INDEX KEYWORDS: Band gaps; Base pairs; Conductance properties; Deoxyribonucleic acids; DNA molecules; Double-stranded DNA (ds-DNA); Electronic transmission; Electronic transmissions; Generalized Green's function; Ladder model; Numerical studies; Room temperature; Sub-lattices; Tight-binding Hamiltonians; Voltage characteristics, Binding sites; DNA; Electronic states; Genes; Ladders; Molecules; Organic acids; Solitons, Green's function

Vahedi Fakhrabad, D., Shahtahmassebi, N., Askari, M., Ashhadi, M., Ketabi, S.A. Tunnel magnetoresistance of the system of ferromagnetic electrode/polyacetylene/ferromagnetic electrode: A Green's function approach (2010) 43 (2), pp. 620-624.

DOI: 10.1016/j.physe.2010.10.002

Based on a π-orbital tight-binding model Hamiltonian and the Green's function approach together with the Landauer formalism, we studied the coherent spin-dependent transport properties of ferromagnetic (FM)-molecule-FM junctions for the molecule to be trans-polyacetylene (t-PA). The conductance properties of the spin system are studied for a t-PA molecule of fixed length. It is shown that tunnel magnetoresistance (TMR) of the molecular junction can be quite large (over 60%) and the TMR value depends on applied voltage. The TMR maximum value for a t-PA junction is obtained at a low applied voltage (Va<0.7). © 2010 Elsevier B.V.
INDEX KEYWORDS: Electrodes; Ferromagnetic materials; Ferromagnetism; Molecules; Tunnelling magnetoresistance, Conductance properties; Ferromagnetic electrodes; Green's function approaches; Landauer formalisms; Low applied voltages; Spin-dependent transport properties; Trans-polyacetylene; Tunnel magnetoresistance, Hamiltonians
PUBLISHER: Elsevier B.V.

Mozafari, E., Ketabi, S.A., Shahtahmassebi, N. Temperature dependence of I-V characteristics of C60 molecule: A Green's function approach (2009) art. no. 5394559, pp. 94-97.

Making use of a generalized Green's function technique and Landauer formalism, the temperature depended current-voltage (I-V) characteristics of C60 molecule, sandwiched between two metallic electrodes are numerically investigated. In addition, the influence of the electron-phonon coupling strength on the electronic properties of the molecule is studied. The I-V characteristics of the molecule are determined in two temperature limits, T=3K and T=300K. Our results indicate that the molecule primarily act as a semiconductor in lower temperatures but moves toward becoming an ohmic-like conductor when the temperature increased to the higher magnitudes. © 2009 IEEE NANO Organizers.
AUTHOR KEYWORDS: C60 molecule; Current-voltage characteristic; Electron-phonon coupling; Landauer formalism
INDEX KEYWORDS: Electron phonon couplings; Electron-phonon coupling strengths; Generalized Green's function; IV characteristics; Landauer; Landauer formalism; Metallic electrodes; Temperature dependence; Two-temperature, Electron correlations; Electron-phonon interactions; Electronic properties; Green's function; Molecules; Nanotechnology, Current voltage characteristics

Ketabi, S.A., Fouladi, A.A. Transport properties of poly(GACT) - Poly(CTGA) deoxyribonucleic acid: A ladder model approach (2009) 72 (6), pp. 1023-1036.

DOI: 10.1007/s12043-009-0082-z

In this paper, based on the tight-binding Hamiltonian model and within the framework of a generalized Green's function technique, the electronic conduction through the poly(GACT) - poly(CTGA) DNA molecule in SWNT/DNA/SWNT structure has been numerically investigated. In a ladder model, we consider DNA as a planar molecule containing M cells and four further sites (two base pair sites and two backbone sites) in each cell, sandwiched between two semi-infinite single-walled carbon nanotubes (SWNT) as the electrodes. Having relied on Landauer formalism, we focussed on studying the current-voltage characteristics of DNA, the effect of the coupling strength of SWNT/DNA interface and the role of tube radius of nanotube contacts on the electronic transmission through the foregoing structure. Finally, a characteristic time was calculated for the electron transmission, which measures the delay caused by the tunnelling through the SWNT/DNA interface. The results clearly show that the calculated characteristic time and also the conductance of the system are sensitive to the coupling strength between DNA molecule and nanotube contacts. © Indian Academy of Sciences.
AUTHOR KEYWORDS: Carbon nanotube; Deoxyribonucleic acid; Electronic transmission; Green's function method; Ladder model
INDEX KEYWORDS: Base pairs; Characteristic time; Coupling strengths; Deoxyribonucleic acid; Deoxyribonucleic acids; DNA molecules; Electron transmission; Electronic conduction; Electronic transmission; Electronic transmissions; Generalized Green's function; Green's function method; Ladder model; Landauer; Nanotube contacts; Planar molecules; Semi-infinite; Tight-binding Hamiltonians, Carbon nanotubes; Cell membranes; Current voltage characteristics; DNA; Electric potential; Genes; Ladders; Molecules; Nucleic acids; Organic acids; Single-walled carbon nanotubes (SWCN); Transport properties, Green's function

Ketabi, S.A., Moghaddam, H.M., Shahtahmasebi, N. Electron transport through SWNT/trans-PA/ SWNT structure (the role of solitons): A t-matrix technique (2007) 69 (4), pp. 661-668.

DOI: 10.1007/s12043-007-0164-8

Using a tight-binding model and a transfer-matrix technique, we numerically investigate the effects of the coupling strength and the role of solitons on the electronic transmission through a system in which inms-polyacetylene (trans-PA) molecule is sandwiched between two semi-infinite single-walled carbon nanotubes (SWNT). We rely on Landauer formalism as the basis for studying the conductance properties of this system. Our calculations show that the solitons play an important role in the response of this system causing a large enhancement in the conductance. Also our results suggest that the conductance is sensitive to the CNT/molecule coupling strength. © Indian Academy of Sciences.
AUTHOR KEYWORDS: Carbon nanotube; Electronic transmission; Soliton; t-matrix; Trans-polyacetylene
INDEX KEYWORDS: Computational methods; Electric conductance; Electron transport properties; Polyacetylenes; Solitons, Coupling strength; Electronic transmissions; t-matrix; Trans polyacetylene; Transfer-matrix technique, Single-walled carbon nanotubes (SWCN)

Moghaddam, H.M., Ketabi, S.A., Shahtahmasebi, N. The role of solitons on the properties of electron transport through the CNT/t-PA/CNT system - A Green's function approach (2007) 19 (11), art. no. 116211, .

DOI: 10.1088/0953-8984/19/11/116211

Using a tight-binding model and methods based on Green's function theory and Löwdin's partitioning technique, we numerically investigate the effects of the coupling strength, the role of the solitons and the presence of a linear applied voltage drop across the molecule on the electronic transmission through a system in which trans-polyacetylene (t-PA) as the molecule is sandwiched between two semi-infinite single-walled carbon nanotubes (CNT). We rely on Landauer formalism as the basis for studying the conductance properties of this system. Our calculations show that the solitons play an important role in this system, causing a large enhancement in the conductance. Also, our results suggest that the conductance is sensitive to the CNT/molecule coupling strength, and the presence of an applied voltage considerably increases the conductance. © 2007 IOP Publishing Ltd.
INDEX KEYWORDS: Electric conductance; Electric potential; Electron transport properties; Electronic properties; Single-walled carbon nanotubes (SWCN); Voltage control, Coupling strength; Electronic transmission; Trans-polyacetylene (t-PA), Solitons

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