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2. The corresponding energy sensitivity was estimated to be δE = ΔΕC* δq2/e2 = 30ħ, where EC denotes the charging energy. While this charge sensitivity is slightly better compared to our earlier Coulomb blockade region rf-SET, the energy sensitivity is slightly lower. However, since our measurements were conducted at 4.2 K, we can expect an improved behavior for lower temperatures.

NOISE IN GRAPHENE

M. Craciun, R. Danneau, P. Hakonen, A. Morpurgo, J. Salmilehto, M. Tomi, S. Russo, and F. Wu

Owing to its unique structure, graphene conduction occurs via massless Dirac fermions. Graphene is a gapless semiconductor: the conduction and the valence band are touching at two inequivalent points (K and K', usually called Dirac points) where the density of state vanishes. Nevertheless, the conductivity at the Dirac point remains finite as charge is carried by evanescent waves, i.e. by tunneling between the leads. These unique properties are also reflected in the shot noise properties of graphene. In perfect short and wide graphene strips (W/L ≥ 3), for heavily doped graphene leads, at the Dirac point both minimum conductivity and Fano factor are expected to reach universal values of 4e2/h and 1/3 respectively. Astonishingly, the transmission coefficients at the Dirac point in perfect graphene show similar form as those found in diffusive systems.

We have studied shot noise in short and large graphene strips (with different width over length ratio W/L) prepared by Prof. A. Morpurgo and his collaborators in Delft. The samples were made of graphene sheets exfoliated from natural graphite and deposited on top of Si/SiO2 wafer, where the substrate is used as a back gate. Our measurements show that for strips with large W/L, both minimum conductivity and Fano factor reach universal values of 4e2/h and 1/3 respectively. We find that the Fano factor has a maximum at the Dirac point and it diminishes with increasing carrier density. We also see that for smaller W/L ratios, the Fano factor is lowered as expected by the theory. While the ballistic nature of conduction in graphene is strongly debated, our findings support the view that transport at the Dirac point occurs via evanescent waves, i.e. that carriers can propagate without scattering.

NOISE IN MULTITERMINAL MWNTS

P. Hakonen, T. Heikkilä, T. Tsuneta, P. Virtanen, and F. Wu

We have investigated shot noise in multiterminal, disordered multiwalled carbon nanotubes (MWNTs) at 4.2 K over the frequency f = 600 - 850 MHz. Quantitative comparison of our data to semiclassical theory, based on non-equilibrium distribution functions, indicates that a major part of the noise is caused by a non-equilibrium state imposed by the contacts. Our data exhibits non-local shot noise across weakly transmitting contacts while a low-impedance contact eliminates such noise almost fully. When neglecting inelastic effects, we obtain Ftube < 0.03 for the intrinsic Fano factor of our MWNTs. The reason for this behavior is unclear at present; it cannot be explained by the presence of ballistic channels.

Annual Report 2007

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