## Publications

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**ABSTRACT:**Spin-dependent electron transport in an open double quantum ring, when each ring is made up of four quantum dots and threaded by a magnetic flux, is studied. Two independent and tunable gate voltages are applied to induce Rashba spin-orbit effect in the quantum rings. Using non-equilibrium Green’s function formalism, we study the effects of electron-electron interaction on spin-dependent electron transport and show that although the electron-electron interaction induces an energy gap, it has no considerable effect when the bias voltage is sufficiently high. We also show that the double quantum ring can operate as a spin-filter for both spin up and spin down electrons. The spin-polarization of transmitted electrons can be tuned from �1 (pure spin-down current) to þ1 (pure spin-up current) by changing the magnetic flux and/or the gates voltage. Also, the double quantum ring can act as AND and NOR gates when the system parameters such as Rashba coefficient are properly adjusted.Journal of Applied Physics 02/2014; 115(8):084307. · 2.21 Impact Factor - [Show abstract] [Hide abstract]

**ABSTRACT:**The spin-dependent electron transport properties of zinc- and manganese-doped adenine molecules connected to zigzag graphene leads are studied in the zero bias regime using the non-equilibrium Green’s function method. The conductance of the adenine molecule increased and became spin-dependent when a zinc or manganese atom was doped into the molecules. The effects of a transverse electric field on the spin-polarization of the transmitted electrons were investigated and the spin-polarization was controlled by changing the transverse electric field. Under the presence of a transverse electric field, both the zinc- and manganese-doped adenine molecules acted as spin-filters. The maximum spin-polarization of the manganese-doped adenine molecule was greater than the molecule doped with zinc.Journal of Applied Physics 01/2014; 115(044701):1-7. · 2.21 Impact Factor - [Show abstract] [Hide abstract]

**ABSTRACT:**Spin-polarization in double-stranded DNA is studied in the presence of a magnetic field applied along its helix axis using the non-equilibrium Green's function method. The spin-polarization could be tuned by changing the magnetic field. In some special cases, the double-stranded DNA behaved as a perfect spin-filter. Furthermore, the dependency of the spin-polarization on the spin-orbit strength and dephasing strength is studied.Journal of Applied Physics 01/2014; 115(20):204701-204701-5. · 2.21 Impact Factor - [Show abstract] [Hide abstract]

**ABSTRACT:**We study the proximity-induced superconductivity effect in a double-stranded DNA by solving the Bogoliubov-de Gennes equations and taking into account the effect of thermal fluctuations of the twist angle between neighboring base pairs. We show that the electron conductance is spin-dependent and the conductance of spin up (down) increases (decreases) due to the spin-orbit coupling. It is found that, for T<100K, the band gap energy is temperature-independent and it decreases due to the SOC. In addition, by solving the Bogoliubov-de Gennes equations and local gap parameter equation self-consistently, we find the critical temperature at which transition to superconductivity can take place.Journal of Applied Physics 12/2013; 115(5). · 2.21 Impact Factor - [Show abstract] [Hide abstract]

**ABSTRACT:**By omitting some carbon atoms from middle of a zigzag graphene cluster, and Hydrogen termination of sp2 orbital, we make the different porous zigzag graphene clusters, and investigate the electron transport properties of the structures by the non-equilibrium Green function method at zero bias regime. It is shown that, the conductance of porous clusters depends on the final symmetry of porous cluster and the local imbalance number (nA-nB), in which nA and nB are the number of omitted atoms from A-sublattice and B-sublattice respectively. Also it is shown that, if three carbon atoms (one type-A and two type-B sites) are omitted the conductance for |E-Ef|≥5 eV is significantly higher than the conductance of original zigzag graphene cluster due to the increment in less affected conducting channels. We show that, spin flipping occurs under Rashba spin orbit interaction at E=Ef, when three atoms are omitted from the original cluster. Therefore the local imbalance number and final symmetry of porous graphene cluster can be used as a rule for designing porous graphene devices and the device can be used in spintronic applications.Physica E Low-dimensional Systems and Nanostructures 12/2013; · 1.86 Impact Factor -
##### Article: Dispersion relation and growth rate in a Cherenkov free electron laser: Finite axial magnetic field

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**ABSTRACT:**A theoretical analysis is presented for dispersion relation and growth rate in a Cherenkov free electron laser with finite axial magnetic field. It is shown that the growth rate and the resonance frequency of Cherenkov free electron laser increase with increasing axial magnetic field for low axial magnetic fields, while for high axial magnetic fields, they go to a saturation value. The growth rate and resonance frequency saturation values are exactly the same as those for infinite axial magnetic field approximation. The effects of electron beam self-fields on growth rate are investigated, and it is shown that the growth rate decreases in the presence of self-fields. It is found that there is an optimum value for electron beam density and Lorentz relativistic factor at which the maximum growth rate can take place. Also, the effects of velocity spread of electron beam are studied and it is found that the growth rate decreases due to the electron velocity spread.11/2013; 20(12). - [Show abstract] [Hide abstract]

**ABSTRACT:**A theory is presented for exciton formation in a graphene sheet using the center-of-mass approximation. The energy levels and wavefunctions of exciton are calculated analytically which show that the exciton can form if the band gap of graphene is not zero. We show that the energy gap of graphene plays the role of the mass which if not zero, leads to formation of the excitons. It is shown that the main quantum number of the exciton ground state changes with the graphene dielectric constant. Also, all of the states are found to be four-fold degenerate. The binding energy of exciton can reach as high as 1/4 of the energy gap of graphene which is notable among the conventional quasi-2D systems. This result can play an important rule in the photonics of graphene.Journal of Applied Physics 08/2013; 114(7). · 2.21 Impact Factor - Journal of Computational and Theoretical Nanoscience, vol. 10, issue 5, pp. 1196-1201. 05/2013; 10(5):1196-1201.
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**ABSTRACT:**We investigate the effect of electron-electron interaction on voltage distribution, charge distribution and current-voltage curve of two dimensional nano-MOSFETs with dimension equal to 1 × 1 nm2, 3 × 3 nm2, and 6 × 6 nm2 by using non-equilibrium Green function method. It is shown that the turn on voltage increases by decreasing the size of sample because of size quantization. Also we show that for a critical drain-source voltage a negative resistance is seen at current-voltage curve of 1 × 1 nm2 sample because of electron-electron interaction, and in consequence it can tolerate lower gate voltage in real practical applications.AIP Advances. 03/2013; 3(3). - Journal of Computational and Theoretical Nanoscience, vol. 10, issue 3, pp. 587-590. 03/2013; 10(3):587-590.
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**ABSTRACT:**We consider a double-stranded DNA molecule connected to non-magnetic and magnetic (nickel (Ni) and iron (Fe)) leads and study its spintronic properties using tight binding non-equilibrium Green function method. By considering the combination of the spin-orbit coupling, the environment-induced dephasing, and the helical symmetry, it is shown that although the hopping parameter of spin up electrons is higher than spin down electrons for both Ni and Fe leads, the spin up (spin down) current is significantly higher than spin down (spin up) current when the lead is Ni (Fe) and for non-magnetic leads the difference is not high with respect to magnetic leads, i.e., there should be a matching process rule between leads and dsDNA. Also, we show that by applying an AC electric (gating) field perpendicular to helix-axis, the spin current is approximately zero at half period and will be maximized for specific gate voltage at other half period and therefore dsDNA habits as a field effect transistor (FET). It is found that there are both p-channel and n-channel FET and the logical cell can be designed theoretically. We think that the results of present paper could motivate further experimental studies on DNA spintronics.Journal of Applied Physics 02/2013; 113(7). · 2.21 Impact Factor - [Show abstract] [Hide abstract]

**ABSTRACT:**In this paper, spin-filtering properties of transmitted electrons through a quantum ring in the presence of Rashba spin-orbit interaction and magnetic flux are studied. To investigate the effects of coupling between the leads and ring on the spin-filtering, the S-matrix method is used. It is shown that by tuning the Rashba spin-orbit strength and the magnetic flux, the quantum ring can act as a perfect spin-filter with high efficiency. The spin-filtering can be changed from spin up to spin down and vice versa by changing the Rashba strength when the magnetic flux is held constant or by changing the magnetic flux when the Rashba strength is held constant. In addition, the effect of the angle between the leads on spin-filtering properties is taken into account and the angles at which the spin-filtering can occur are determined. The spin-filtering can take place in narrow ranges of electron energy for weak coupling, while for strong coupling it can take place in a wide range of electron energy.Journal of Applied Physics 01/2013; 113(4):044316. · 2.21 Impact Factor - [Show abstract] [Hide abstract]

**ABSTRACT:**We study spin-resolved electron transport in a double quantum ring in the presence of Rashba spin-orbit interaction and a magnetic flux using quantum waveguide theory. We show that, at the proper values of the system parameters such as the Rashba coupling constant, the radius of the rings, and the angle between the leads, the double quantum ring can act as a perfect electron spin-inverter with very high efficiency. Also, the double quantum ring can work as a spin switch. The spin polarization of transmitted electrons can be controlled and changed from −1 to +1 by using a magnetic flux.Journal of Applied Physics 01/2013; 113:014303. · 2.21 Impact Factor - Journal of Computational and Theoretical Nanoscience, vol. 10, issue 1, pp. 194-198. 01/2013; 10(1):194-198.
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**ABSTRACT:**We study the spin-dependent electron transport through a double-stranded DNA (dsDNA) using the Bogoliubov-de Gennes equations and non-equilibrium Green's function method. We calculate the spin-dependent electron conductance and spin-polarization for different lengths, helix angles, twist angles of dsDNA, the environment-induced dephasing factors, and hopping integral. It is shown that the conductance decreases by increasing the length and dephasing factor. Also, we show that the spin-polarization depends on the helical symmetry and the length of DNA. It is shown that the double-stranded DNA can act as a perfect spin filter. Finally, we show that the sign of spin polarization can be inverted from +1 (-1) to -1 (+1) for some values of hopping integral.Journal of Applied Physics 01/2013; 114(19):194706-194706-4. · 2.21 Impact Factor - [Show abstract] [Hide abstract]

**ABSTRACT:**In this paper, we study the spin-dependent electron transport properties of graphene n-p-n junction in the presence of Rashba spin-orbit interaction using transfer matrix method. It is found that for a graphene n-p-n junction, the spin-resolved electron conductance depends on the strength of Rashba spin-orbit interaction and the built-in potential of graphene junctions. For an appropriate value of Rashba strength, perfect electron spin-inversion with high conductance can take place when the graphene junction is biased with sufficient built-in potential. In this case, the graphene junction can be used as a spin-inverter which is an important device in spintronic. Finally, the spin-dependent conductance for a graphene n-n-n junction is investigated and compared with that of n-p-n junction.Journal of Applied Physics 11/2012; 112(10). · 2.21 Impact Factor - Applied Physics. 11/2012;
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**ABSTRACT:**Chaotic behavior of an electron motion in a free-electron laser with realistic helical wiggler and ion-channel guiding is studied using Poincare surface-of-section maps. The effects of a realistic electron beam density on chaotic electron dynamics are investigated by considering an electron beam with Gaussian density profile in radial distance. The effects of self-fields on chaotic electron dynamics are investigated for different Gaussian beam parameters, and the results are compared with those of uniform electron beam. It is shown that the electron chaotic behavior can be controlled by changing the Gaussian beam parameter. Also, the chaotic behavior can be controlled by increasing the ion-channel and/or the electron beam densities.Physics of Plasmas 11/2012; 19(11). · 2.38 Impact Factor - [Show abstract] [Hide abstract]

**ABSTRACT:**Electron acceleration in the inverse free electron laser (IFEL) with a helical wiggler in the presence of ion-channel guiding and axial magnetic field is investigated in this article. The effects of tapering wiggler amplitude and axial magnetic field are calculated for the electron acceleration. In free electron lasers, electron beams lose energy through radiation while in IFEL electron beams gain energy from the laser. The equation of electron motion and the equation of energy exchange between a single electron and electromagnetic waves are derived and then solved numerically using the fourth order Runge-Kutta method. The tapering effects of a wiggler magnetic field on electron acceleration are investigated and the results show that the electron acceleration increases in the case of a tapered wiggler magnetic field with a proper taper constant.Chinese Physics C 09/2012; 36(9):879. · 0.34 Impact Factor - [Show abstract] [Hide abstract]

**ABSTRACT:**Electronic and spin transport properties of a benzene molecule connected to semi-infinite armchair and zigzag graphene nano-ribbon leads are calculated using non-equilibrium Green function (NEGF) method at zero bias regime. It is shown that, the molecule is conductive in a specific range of energy for both armchair and zigzag leads. This behavior is similar for both kinds of leads. In presence of Rashba spin–orbit interaction, the molecule shows spin filtering properties for both kinds of leads but these properties are not similar for armchair and zigzag leads. Also, a spin-polarized current is seen in the molecule connected to zigzag leads. The benzene molecule with Rashba spin–orbit interaction can be considered as an excellent candidate for molecular electronic and spintronic devices which can have very small dimensions relative to the conventional semiconductor devices.physica status solidi (b) 09/2012; 249(9):1735-1743. · 1.49 Impact Factor

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