Browsing by Subject "quantum transport"

Now showing 1 - 3 of 3

Open Access
Bias voltage control of a molecular spin valve
(2009) Can, Duygu
With the discovery of giant magneto resistance a new field called spintronics is emerged. Utilizing spin-degree of freedom of the electron as well as its charge, high-speed devices which consumes low energy can be designed. One of the main concerns of spintronics is creating spin polarized currents. Half-metallic materials, which conduct electrons of one spin state but behave as an insulator for the other spin state, are ideal candidates for this purpose. In a way they function as spinvalves, and the current passing through these materials will be spin polarized. The half-metallic property of periodic atomic chains of carbon-transition metal compounds and spin-valve property of transition metal caped finite carbon linear chains motivated our study. In this work, we analyzed the spin dependent transport properties of CrCnCr atomic chains. We connected the magnetic CrCnCr molecules to appropriate electrodes and studied their electronic and magnetic properties under applied bias. All the calculations are carried out using a method which combines density functional theory (DFT) with non-equilibrium Green’s function (NEGF) technique. For CrCnCr molecules with odd n we observed cumulenic bond lengths, while the C−C bonds are in polyynic nature for even n. In these structures Cr atoms induce net magnetic moments on C atoms. The magnetic moment on Cr atoms favors anti-parallel (AF) alignment for even n and parallel (FM) alignment for odd n. This situation is inverted when the molecules are connected to the electrodes. Two-probe conductance calculations of such systems reveal that their conductance properties are also n dependent. Finite bias voltages which create non-equilibrium conditions within the device region, causes the spin-degenerate molecular levels of the device to be separated from each other. Then conductance properties of the device become spin dependent. We observe that the ground state CrCnCr two-probe systems with odd n changes from AF to FM at a critical voltage. Thus, we have a spinvalve which is initially in ”off-state” turned on with applied bias. We achieved to control spin-polarization of the current transmitted through a molecular spinvalve with applied bias voltage. We showed that they are molecular analogues of GMR devices. These molecular spin-valve devices function without any need of an external magnetic field as it is required in conventional GMR devices.
Open Access
Electro-magnetic properties and phononic energy dissipation in graphene based structures
(2008) Sevinçli, Haldun
With the synthesis of a single atomic plane of graphite, namely graphene honeycomb structure, active research has been focused on the massless Dirac fermion behavior and related artifacts of the electronic bands crossing the linearly at the Fermi level. This thesis presents a theoretical study on the electronic and magnetic properties of graphene based structures, and phononic energy dissipation. First, functionalization of these structures by 3d-transition metal (TM) atoms is investigated. The binding energies, electronic and magnetic properties have been investigated for the cases where TM-atoms adsorbed to a single side and double sides of graphene. It is found that 3d-TM atoms can be adsorbed on graphene with binding energies ranging between 0.10 to 1.95 eV depending on their species and coverage density. Upon TM-atom adsorption graphene becomes a magnetic metal. TM-atoms can also be adsorbed to graphene nanoribbons with armchair edge shapes (AGNRs). Binding of TM-atoms to the edge hexagons of AGNR yield the minimum energy state for all TM-atom species examined in this work and in all ribbon widths under consideration. Dependingon the ribbon width and adsorbed TM-atom species, AGNR, a non-magnetic semiconductor, can either be a metal or a semiconductor with ferromagnetic or anti-ferromagnetic spin alignment. Interestingly, Fe or Ti adsorption makes certain AGNRs half-metallic with a 100% spin polarization at the Fermi level. These results indicate that the properties of graphene and graphene nanoribbons can be strongly modified through the adsorption of 3d TM atoms. Second, repeated heterostructures of zigzag graphene nanoribbons of different widths are shown to form multiple quantum well structures. Edge states of specific spin directions can be confined in these wells. The electronic and magnetic state of the ribbon can be modulated in real space. In specific geometries, the absence of reflection symmetry causes the magnetic ground state of whole heterostructure to change from antiferromagnetic to ferrimagnetic. These quantum structures of different geometries provide novel features for spintronic applications. Third, as a possible device application, a resonant tunnelling double barrier structure formed from a finite segment of armchair graphene nanoribbon with varying widths has been proposed based on first-principles transport calculations. Highest occupied and lowest unoccupied states are confined in the wider region, whereas the narrow regions act as tunnelling barriers. These confined states are identified through the energy level diagram and isosurface charge density plots which give rise to sharp peaks originating from resonant tunnelling effect. Finally, we studied dynamics of dissipation of local vibrations to the surrounding substrate. A model system consisting of an excited nano-particle which is weakly coupled with a substrate is considered. Using three different methods, the dynamics of energy dissipation for different types of coupling between the nano-particle and the substrate is studied, where different types of dimensionality and phonon densities of states were also considered for the substrate. Results of this theoretical analysis are verified by a realistic study. To this end the phonon modes and interaction parameters involved in the energy dissipation from an excited benzene molecule to the graphene are calculated performing first-principles calculations.
Open Access
A time-dependent study of bistability in resonant tunneling structures
(1997) Keçecioğlu, Ersin
A comjDutational time-dependent study of the bistability in resonant tunneling structures including the electron-electron interactions is presented. A new computational method for the investigation of many jDarticle interacting systems for the study of quantum transport in small systems is introduced. The timedependence of the wave-function in the Schrödinger equation is studied by discretizing the energy spectrum and the time steps. A simple model for a double barrier resonant tunneling structure is introduced. The method is then applied to this simple model of double barrier resonant tunneling structure, and this geometry is investigated systematically in terms of inter-pcirticle interaction strength and number of particles. By applying the method to this simple geometry it is shown that there exists instabilities which occur a.s oscillcitions in the current-voltage characteristics of the model geometry.