Photon-assisted shot noise in graphene in the Terahertz range

When subjected to electromagnetic radiation, the fluctuation of the electronic current across a quantum conductor increases. This additional noise, called photon-assisted shot noise, arises from the generation and subsequent partition of electron-hole pairs in the conductor. The physics of photon-assisted shot noise has been thoroughly investigated at microwave frequencies up to 20 GHz, and its robustness suggests that it could be extended to the Terahertz (THz) range. Here, we present measurements of the quantum shot noise generated in a graphene nanoribbon subjected to a THz radiation. Our results show signatures of photon-assisted shot noise, further demonstrating that hallmark time-dependant quantum transport phenomena can be transposed to the THz range.Comment: includes supplemental materia

Adsorption of O-2 and C2Hn (n=2, 4, 6) on the Al9Co2(001) and o-Al13Co4(100) complex metallic alloy surfaces

International audienceOxidation of the Al9CO2( 0 0 1) and Al13Co4(1 0 0) surfaces has been performed in a wide range of temperatures at 2 x 10(-8) or 1 x 10(-7) mbar oxygen pressure. Only Al-O bonding is observed. The oxidation kinetics are controlled by the quantity of oxygen sticking on the surface. Oxidation results from a competition between several effects: formation of an oxide film, dissolution of the film, oxygen desorption and oxygen dissolution into the bulk. For temperatures lower than 710 K for the Al9Co2(0 0 1) surface and 925 K for the Al13Co4(1 0 0) surface, a similar to 5 angstrom thick oxide film is formed which does not show any long-range order and desorbs upon annealing. When oxidation is performed at higher temperatures, oxygen diffusion into the bulk is observed. A poorly ordered oxide film having a sixton structure is formed on the Al9Co2(0 0 1) surface when oxidation is performed at 775 K, which is dissolved when annealing at higher temperatures. On the Al13Co4(1 0 0) surface, only a weak streaky polar circle is observed following annealing at 925 K the film formed at room temperature, which corresponds to an hexagonal network of O atoms into small ultrathin oxide layers domains. The oxidation behaviour of the Al9Co2(0 0 1) and Al13Co4(1 0 0) surfaces has been ascribed to the strong covalent character of bonds present in these Al-Co phases, which prevents aluminium diffusion. C2Hn molecules (n = 2, 4, 6) do not adsorb on the Al13Co4(1 0 0) surface in the experimental conditions used in this study, thus suggesting that this surface might not be the active one in the semi-hydrogenation of acetylene

Electron swarm coefficients and critical field strength of the gaseous ternary mixtures CF3I-SF6-N2 and CF3I-SF6-CO2

The electron drift velocity, longitudinal diffusion and effective ionization coefficients in the gaseous ternary mixtures CF3I-SF6-N2 and CF3I-SF6-CO2 have been measured using a pulsed Townsend apparatus over the density-normalized electric field intensity, E/N, from 230 to 470 Td and 230 to 430 Td, respectively (1 Td = 10-17 V cm2). Several ternary gas mixture compositions containing 10% to 50% CF3I or SF6 and either N2 or CO2 have been studied. Also, we have measured the critical field strength, E/Ncrit, for all of these gaseous mixtures, finding out that the 40% CF3I-40% SF6-20% N2 and 40% CF3I-40% SF6-20% CO2 have an E/Ncrit value smaller by only 4.0% and 1.0%, respectively than that of pure SF6 (360 Td). Provided that both CF3I, CO2 and N2 are environmentally friendly, its addition to SF6 may be highly advantageous over other binary SF6 mixtures with gases having relatively high warming potentials, since these mixtures have only 40% SF6 and behave, from the swarm physics point of view as good candidates to ameliorate impact of SF6 when released into the atmosphere. An interesting synergetic effect has been found for the CF3I-SF6 mixture when N2 added and has been interpreted in terms of the efficient low-energy electron scattering of N2 at low collision energies up to 2 eV.Fil: González Magaña, Olmo. Universidad Nacional Autónoma de México; MéxicoFil: Colorado, N. R.. Universidad Nacional Autónoma de México; México. Universidad Autónoma Metropolitana; MéxicoFil: Basurto, Eduardo. Instituto Nacional de Electricidad y Energías Limpias; MéxicoFil: Serkovic Loli, Laura Natalia. Universidad Nacional Autónoma de México; México. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Juárez, A. M.. Universidad Nacional Autónoma de México; MéxicoFil: Hernández Ávila, J. L.. Instituto Nacional de Electricidad y Energías Limpias; MéxicoFil: De Urquijo, J.. Universidad Nacional Autónoma de México; Méxic

Growth and structure of ultrathin alumina films on the (110) surface of gamma-Al4Cu9 complex metallic alloy

International audienceThe first stages of oxidation of the (1 1 0) surface of a gamma-Al4Cu9 complex metallic alloy were investigated by combining x-ray photoemission spectroscopy, low energy electron diffraction and scanning tunnel microscopy studies. Oxidation at room temperature in the 2 x 10(-8) to 2 x 10(-7) mbar oxygen pressure range occurs in two steps: a fast regime is followed by a much slower one, leading to the formation of a thin aluminium oxide film showing no long range order. Cu-O bonds are never observed, due to fast oxygen induced aluminium segregation. The low value of the estimated activation energy for aluminium diffusion (0.65 +/- 0.12 eV at(-1)) was ascribed to the presence of two vacancies in the gamma-Al4Cu9 structure. Annealing at 925K the oxide film formed at room temperature leads to the formation of small crystallized domains with a sixton structure similar to structures reported in the literature following the oxidation of Cu-9% Al(1 1 1), NiAl (1 1 0) and FeAl(1 1 0) surfaces as well as ultrathin Al films deposited onto Cu(1 1 1) or Ni(1 1 1) surfaces. Two contributions were observed in the O1s peaks, which have been ascribed to loosely bound oxygen species and oxygen belonging to the sixton structure respectively

Assessment of the self-consistency of electron-THF cross sections using electron swarm techniques: Mixtures of THF-Ar and THF-N2

The pulsed Townsend technique has been used to measure transport coefficients in mixtures of tetrahydrofuran (THF) with N2 and argon. These measurements are the first investigations of swarm transport in gas mixtures with THF, with the drift velocity and effective Townsend ionization coefficient reported for a range of reduced electric fields between 0.23 and 800 Td (1 Td = 10−21 V m2). These transport coefficients are compared with those calculated using a multiterm kinetic theory, using the cross section set developed in our previous studies [N. A. Garland et al., Phys. Rev. A 88, 062712 (2013) and M. J. E. Casey et al., J. Chem. Phys. 147, 195103 (2017)]. The swarm technique of iteratively adjusting cross sections to reproduce experimental transport measurements is subsequently utilized in this study to address the deficiencies in the earlier cross section sets, exposed by the gas mixture measurements. Refinement of the low-energy extrapolation of the quasielastic database cross section and the low-energy extrapolation and magnitude of the dissociative electron attachment cross section are detailed, as well as the adjustments to the two previously proposed neutral dissociation cross sections. These refinements were necessary in order to minimize differences between our measured and calculated transport coefficients

Self-consistency of electron-THF cross sections using electron swarm techniques

The drift velocity and first Townsend ionization coefficient of electrons in gaseous tetrahydrofuran are measured over the range of reduced electric fields 4-1000 Td using a pulsed-Townsend technique. The measured drift velocities and Townsend ionization coefficients are subsequently used, in conjunction with a multi-term Boltzmann equation analysis, as a further discriminative assessment on the accuracy and completeness of a recently proposed set of electron-THF vapor cross sections. In addition, the sensitivity of the transport coefficients to uncertainties in the existing cross sections is presented. As a result of that analysis, a refinement of the momentum transfer cross section for electron-THF scattering is presented, along with modifications to the neutral dissociation and dissociative electron attachment cross sections. With these changes to the cross section database, we find relatively good self-consistency between the measured and simulated drift velocities and Townsend coefficients

Ordering and dynamical properties of superbright C-60 molecules on Ag(111)

International audienceC-60 monolayers grown on Ag(111) at room temperature form incommensurate lattices that convert into a commensurate (2 root 3 x 2 root 3)R30 degrees phase upon annealing. The C-60 molecules in the commensurate phase have been observed to exist in three different states on Ag(111), namely bright, dim, and superbright (SB). All three species are in dynamical equilibrium at 280 < T < 440 K. The bright and dim species were the subject of an earlier low-energy electron diffraction study that determined their geometries on the surface and the dynamics of the switching between those two states. The study presented here takes a closer look at the SB molecules, which appear and disappear at temperature-dependent rates with a measured activation barrier of 1.5 eV. The SB molecules in the commensurate phase comprise about 0.5% of the molecules and have a spatially random distribution. The evidence suggests that the formation of the three different states of C-60 on Ag(111) is a result of stress imposed by the substrate as the C-60 adopts the commensurate spacing that is slightly smaller than its natural spacing. In the incommensurate phases, there is no bright-dim contrast, but SB C-60 molecules form and organize into ordered arrays that appear to correspond to the moire patterns that are produced by the mutually incommensurate lattices. This suggests that the substrate responds to the nonuniform forces imposed by the C-60 molecules by producing raised islands of Ag atoms at the vertices of the moire structure. A similar island structure may account for the SB molecules in the commensurate phase