Giant Magnetoresistance And Thermoelectric Effect In Graphene-like Materials

We have investigated the electric transport, magnetotransport and thermoelectric effects in graphene-like nanoribbons using the non-equilibrium Green’s function method combined with the density functional theory or the tight-binding model. The effects of width parity and edge doping on conductance, magnetoresistance(MR) and thermoelectric coefficients of zigzag a-graphyne nanoribbons(ZaGNRs) are studied in detail. Analytical and numerical analysis about the dependence of transport on characteristic parameters based on the tight-binding model have given better physical understanding and extended the results for ZaGNRs to other graphene-like materials. The main points of my thesis are list as follows: 1. The spin-dependent electron transport of ferromagnetic ZaGNRs has been simulatedusing the non-equilibrium Green’s function method combined with the density-functional theory. A giant magnetoresistance(GMR) as high as 106% has beenpredicted in the pristine even-width ZaGNRs though MR in pristine odd-widthZaGNRs is expected very small. By analyzing the wave functions of states near theFermi energy, we have understood physically how the transversal geometry symmetryof even-width ZaGNRs introduces the odd-even effect. In anti-parallel(AP)magnetization configuration of the two electrodes, each spin conductance spectrumof odd-width pristine ZaGNRs shows a plateau of height 20 G =e h at the Fermi energywhile a gap of 0.15 e V emerges there in even-width ones. Interestingly, if the edge Catom at the position where the magnetization inverses is substituted by an atom of otherelements, the above mentioned conductances of even- and odd-width nanoribbon swap.With the help of scattering states at Fermi energy in the AP configuration, it is foundthat the impurity atom blocks the extension of the states and decrease the conductancein odd-with ZaGNRs. In contrast, in even-width ZaGNRs, the impurity atom breaksthe transversal symmetry of the system, removes the symmetry restriction on theelectron transport and increases the conductance. In the parallel(P) magnetizationconfiguration of the electrodes, the odd-even effects and the doping effects onconductance are weak, therefore the GMR appears only in even-width pristineZaGNRs and odd-width doped ZaGNRs. This suggests that the magnetoresistance canbe manipulated in a wide range by the dopants on edges of ZaGNRs. 2. Doping enhancement of spin thermoelectric effects in the linear regime have beensystematically investigated for various two-probe n-ZaGNRs(n=3, 4, 5, and 6) in the Pand AP configurations. It is difficult to realize strong spin thermoelectric effects or tohave a larger spin Seebeck coefficient CS than the charge Seebeck coefficient SS inpristine nanoribbons. Edge doping can break the geometry symmetry of the systemsand enhance the thermoelectric effect. The enhancement depends on the width ofnanoribbon and the type and position of the dopant. In the P configurations, S CS >Scan be realized in a wide temperature range with 10 S CS >S at room temperature for N-doped 5-ZaGNR and 6-ZaGNR. In the AP configuration, when the doping position isaway from the magnetization transition point, S CS >S can be observed in doped even-width ZaGNRs. SS is always larger than CS at low temperature and a pure-spin-current thermospin device can be achieved at specific temperatures. 3. Observing the similarity of graphene-like materials and the effective manupilating oftransport by various chemical functionalization in nanoribbons, we have tried toestablish a general model to understand the effects of atomic disorder on the transportand thermoelectric properties in graphene-like nanoribbons based on the tight-bindingmodel combined with the Green’s function method. Applying a disorder potential to thetight-binding model of ferromagnetic pristine graphene nanoribbons, we have studied the effects of atomic edge disorder on the properties of the nanoribbons. It is possible to observe the process from quantitative to qualitative changes of the systems in the existence of disorder. Our result shows that the above results obtained from first principles simulations are applicable to other graphene-like materials. The atomistic disorder can switch on(off) the GMR in odd-(even-) width zigzag nanoribbons of graphene-like materials. The conductance of ferromagnetic nanoribbons is insensitive to edge disorders in the P magnetization configuration but becomes sensitive in the AP one. The Seebeck coefficients of the two spins have opposite signs when disorders are localized away from the magnetization transition position in the AP configuration. The material can then produce spin current from temperature gradient. Analytical expressions of conductance for 2-ZGNRs have been obtained to physically understand the effects of disorder on transport.