Spin Polarized Electronic Transport in Quantum Nanodevices

Abstract

The Spintronics is the electronics that depend on the quantum concept of spin of electrons rather than their charge. The nanoelectronics which are based on spin require efficient ways to generate, manipulate and detect spin polarized currents and spin currents. The present thesis is designed for investigating the quantum spin transport properties of two different types of nanostructures dominating nowadays nanoelectronics devices physics research. First Model of Nanodevice: Concerning this model we investigate the spin dynamics of nanoscale junction formed of diluted ferromagnetic semiconductor as two leads and a curved semiconducting nanowire. This nanowire is ZnO nanostructure, since it exhibits piezoelectric property. We shall study the spin transport characteristics of such junction by deducing the spin current for both different orientations of spin alignments by applying the effective mass approximation and the Floquet theory. The effect of strain, generated due bending the nanowire, on the spin current is investigated. Rashba spin-orbit interaction, the influence of the frequency of the induced ac-field and magnetic field are taken into consideration. Numerical calculations show that the spin current, for both orientations of spin alignment, varies with the induced strain strongly. This variation in the spin current with strain might be due to piezoelectric effect. Also, the strain gauge factor is calculated and it is noticed that this gauge factor varies with strain and attains a maximum value  973.4 at strain equals 0.1% and  961.9 at strain equals -0.1%. These large gauges factor may find applications in different fields of nanotechnology and bionanotechnology. It is noticed from calculations that the value of the frequency associated to spin-orbit coupling is affected by strain. The results appear that the spin transport characteristics are very sensitive to strain mainly because of the shift in Fermi energy due to piezoelectric effect. Also, results appear that the variation of Young's modulus of ZnO nanowire with strain and it might be monitored by giant magnetoresistance. There are two types of strain occur simultaneously, the tensile strain at the outer surface and compressive strain at the inner surface which lead ZnO nanowire to produce piezotronic effects. Also, the present thesis studies the effect of strain on the spin transport characteristics, e.g., the conductance, spin polarization and giant magnetoresistance. The spin-orbit coupling, the ac-field's effects and magnetic field's effects also respected. Results show that the spin transport characteristics are very sensitive to strain almost because of the change in Schottky barrier height. This change in Schottky barrier height is due to the strain induced change in the band structure and the piezoelectric effect. The present results show that the junction studied could be fabricated for tips of spin polarized scanning tunneling microscopy for sensing strain in nanostructure materials. Also, this junction may find application in nanowire based nanotechnology, for example, flexible nanoelectronics and nanoelectromechanical systems (NEMS). Second Model of Nanodevice: The spin dependent conductance of graphene field effect transistor is investigated in the second model. Graphene field effect transistor model is proposed as: junction of two regions the first one is ferromagnetic graphene and the second one is superconducting graphene with -Schottky barrier at the interface of the two regions. The conductance is deduced by using Landuar-Buttiker equation, also the equation of Dirac-Bogoliubov-deGennes in one dimension was solved to deduce the corresponding spin dependent Andreev reflection and the normal reflection coefficients. Calculations are performed for the conductance for both orientations of spin alignments using numerical methods and the corresponding spin polarization and giant magnetoresistance are also calculated. In our calculations we consider two different superconducting layers. Results show that the spin-dependent specular Andreev reflection in the present studied junction plays a main role for designing such nanodevice. Also, the Schottky barrier between the ferromagnetic graphene and superconductor graphene regions might be responsible for the conductance dip for both orientations of spin alignments. The present results is very important for spin filter, superconducting qubits needed for quantum information processing at low temperatures and also it might be used as THz oscillator. Also, a spin-thermoelectric effect in graphene nanodevice is investigated. The thermoelectric parameters are expressed in terms of spin-dependent Andreev reflection and normal reflection which will be deduced by solving Dirac-Bogoliubov-deGennes equation in one dimension. Numerical calculations are performed for two different superconducting layers under the effects of both frequency of the induced ac-field and under the effect of magnetic field. Results appear that the present nanodevice operates only in narrow band of THz frequencies. Also, these results might indicate that the present nanodevice shows stability under the influence of magnetic field, which is needed for quantum information processing. The present research appears its importance in the field of spin caloritronics on the nanoscale systems and at low temperatures. A spin dependent Peltier effect in graphene nanodevice is also investigated. The Peltier coefficient is expressed in terms of spin-dependent Andreev reflection and normal reflection which will be deduced by solving Dirac-Bogoliubov-deGennes equation in one dimension. Numerical calculations are performed for two different superconducting layers under the effects of the frequency of an induced ac-field and magnetic field. Results show that the present nanodevice operates only in narrow band of THz frequencies. Also, the present results might indicate that the present nanodevice shows stability under the influence of magnetic field, which is needed for quantum information processing. The present graphene nanodevice based on Peltier effect might be used as coolers for nanoelectronic devices such as nanocontrollers and computer CPUs. The present research appears its importance in the field of spin caloritronics on the nanoscale systems and at low temperatures. Keywords: Spintronics, piezotronics, curved ZnO nanowire, ferromagnetic grapheme, superconducting grapheme, diluted magnetic semiconductor.