Using the real-time diagrammatic technique and taking into account both the sequential and cotunneling processes, we analyze the transport properties of single-wall metallic carbon nanotubes coupled to nonmagnetic and ferromagnetic leads in the full range of parameters. In particular, considering the two different shell filling schemes of the nanotubes, we discuss the behavior of the differential conductance, tunnel magnetoresistance and the shot noise. We show that in the Coulomb diamonds corresponding to even occupations, the shot noise becomes super-Poissonian due to bunching of fast tunneling processes resulting from the dynamical channel blockade, whereas in the other diamonds the noise is roughly Poissonian, in agreement with recent experiments. The tunnel magnetoresistance is very sensitive to the number of electrons in the nanotube and exhibits a distinctively different behavior depending on the shell filling sequence of the nanotube.

We present theoretical study of shot noise in single wall metallic carbon nanotubes weakly coupled to either nonmagnetic or ferromagnetic leads. Using the real-time diagrammatic technique, we calculate the current, Fano factor and tunnel magnetoresistance in the sequential tunneling regime. It is shown that the differential conductance displays characteristic four-fold periodicity, indicating single-electron charging. Such a periodicity is also visible in tunnel magnetoresistance of the system as well as in the Fano factor. The present studies elucidate the impact of ferromagnetic (vs. nonmagnetic) contacts on the transport characteristics under consideration.

Oscillations of the giant magnetoresistance (GMR) and thermo-electric power (TEP) vs. both the thickness of the non-magnetic spacer and also that of the ferromagnetic slabs are studied in the current-perpendicular-to-plane (CPP) geometry of magnetic trilayer systems, in terms of a singleband tight-binding model without impurities. The spin-dependent conductance has been calculated from the Kubo formula by means of a recursion Green's function method and the semi-infinite ideal-lead wires trick. Additionally the TEP is obtained directly from Mott's formula. In general, the thickness oscillations of the GMR and the TEP may have just one or two (short and long) oscillations. The long period, related to spectacular beats, is apparently of non-RKKY type. The TEP oscillations are strongly enhanced with respect to those of the GMR, have the same periods, but different phases and a negative bias.The phenomenon of giant magnetoresistance (GMR) in magnetic multilayers has been intensively studied both experimentally [1,2] and theoretically [3][4][5][6][7] for more than five years now. After the paper [5] was published it has become clear that one can expect a large magnetoresistance even in systems which have no impurities and no structural defects. As pointed out in [7], the GMRoscillations have not only RKKY-type components, but may additionally reveal large particular non-RKKY oscillations (see below). For the case of semi-infinite ferromagnetic slabs, a period of those extra oscillations with the spacer thickness in the CPP (current perpendicularto-plane) geometry has been shown in [7] to originate from values of the in-plane wavenumber k at which at least one spectral density vanishes at the ferromagnetic interface.In the present letter we study the CPP-GMR (currentperpendicular-to-plane) behaviour of layered systems of the type W/F 1 /S/F 2 /W , where W stands for a semiinfite ideal lead wire, F 1 and F 2 for different ferromagnets, generally with different thicknesses, and S for the non-magnetic spacer. Thus, apart from the leads, we are dealing with the usual magnetic trilayer systems. Our aim is to show that very pronounced beats may also, under some circumstances, appear as a function of the thicknesses of the ferromagnetic slabs. Besides the giant magneto-resistance effect, we also calculate the corresponding effect for the thermo-electric power (TEP).The calculation technique, we have developed, is based on the Green's function recursion method [8,6]; it is close to that of [6] with one essential exception, namely instead of working with finite systems in the in-plane space (x, y directions), we have reduced the whole problem to one dimension by performing the Fourier transform in the infinite x-y plane. Hence, our Green's functions fulfil the following equation:where for the simple cubic lattice t z,z ′ = t δ z ′ ,z±1 andIn eqn. (2), E F is the Fermi energy; t (< 0) is the nearest-neighbour hopping integral; a is the lattice constant, and V σ (z) is the spin-dependent atomic potential, which we assume to be c...

We report on the giant magnetoresistance (GMR) of multiwall carbon nanotubes with ultra small diameters. In particular, we consider the effect of the inter-wall interactions and the lead/nanotube coupling. Comparative studies have been performed to show that in the case when all walls are well coupled to the electrodes, the so-called inverse GMR can appear. The tendency towards a negative GMR depends on the inter-wall interaction and on the nanotube length. If, however, the inner nanotubes are out of contact with one of the electrodes, the GMR remains positive even for relatively strong inter-wall interactions regardless of the outer nanotube length. These results shed additional light on recently reported experimental data, where an inverse GMR was found in some multiwall carbon nanotube samples.

By ab initio LMTO calculations in the atomic sphere approximation we study the oscillatory thickness dependence of the exchange coupling between ferromagnetic CO slabs grown epitaxially on the (001) surface of a f.c.c. Cu spacer in a d,-Co/&-Cu multilayer structure and find not only the well-known oscillations with the spacer thickness 4 , but-as a new result-also a pronounced oscillation with the magnetic thickness dl . Thus, the experimentalist should not only vary, as usual, the spacer thickness 4 , but also the magnetic thickness dl , when trying to optimize the exchange coupling of the system for applications.

In a recent paper Liang et al. [Nature 411, 665 (2001)] showed experimentally, that metallic nanotubes, strongly coupled to external electrodes, may act as coherent molecular waveguides for electronic transport. The experimental results were supported by theoretical analysis based on the scattering matrix approach. In this paper we analyze theoretically this problem using a real-space approach, which makes it possible to control quality of interface contacts. Electronic structure of the nanotube is taken into account within the tight-binding model. External electrodes and the central part (sample) are assumed to be made of carbon nanotubes, while the contacts between electrodes and the sample are modeled by appropriate on-site (diagonal) and hopping (off-diagonal) parameters. Conductance is calculated by the Green function technique combined with the Landauer formalism. In the plots displaying conductance vs. bias and gate voltages, we have found typical diamond structure patterns, similar to those observed experimentally. In certain cases, however, we have found new features in the patterns, like a double-diamond sub-structure.

Based on a tight-binding model and a recursive Green's function technique, spin-depentent ballistic transport through tinny graphene sheets (flakes) is studied. The main interest is focussed on: electrical conductivity, giant magnetoresistance (GMR) and shot noise. It is shown that when graphene flakes are sandwiched between two ferromagnetic electrodes, the resulting GMR coefficient may be quite significant. This statement holds true both for zigzag and armchair chiralities, as well as for different aspect (width/length) ratios. Remarkably, in absolute values the GMR of the armchair-edge graphene flakes is systematically greater than that corresponding to the zigzag-edge graphene flakes. This finding is attributed to the different degree of conduction channel mixing for the two chiralities in question. It is also shown that for big aspect ratio flakes, 3-dimensional end-contacted leads, very much like invasive contacts, result in non-universal behavior of both conductivity and Fano factor.

We report on theoretical studies of transport through graphene quantum dots weakly coupled to external ferromagnetic leads. The calculations are performed by exact diagonalization of a tightbinding Hamiltonian with finite Coulomb correlations for graphene sheet and by using the real-time diagrammatic technique in the sequential and cotunneling regimes. The emphasis is put on the role of graphene flake shape and spontaneous edge magnetization in transport characteristics, such as the differential conductance, tunneling magnetoresistance (TMR) and the shot noise. It is shown that for certain shapes of the graphene dots a negative differential conductance and nontrivial behavior of the TMR effect can occur.