Temperature dependent transport characteristics of graphene/n-Si diodes

Realizing an optimal Schottky interface of graphene on Si is challenging, as the electrical transport strongly depends on the graphene quality and the fabrication processes. Such interfaces are of increasing research interest for integration in diverse electronic devices as they are thermally and chemically stable in all environments, unlike standard metal/semiconductor interfaces. We fabricate such interfaces with n-type Si at ambient conditions and find their electrical characteristics to be highly rectifying, with minimal reverse leakage current ($<$10$^{-10}$ A) and rectification of more than $10^6$. We extract Schottky barrier height of 0.69 eV for the exfoliated graphene and 0.83 eV for the CVD graphene devices at room temperature. The temperature dependent electrical characteristics suggest the influence of inhomogeneities at the graphene/n-Si interface. A quantitative analysis of the inhomogeneity in Schottky barrier heights is presented using the potential fluctuation model proposed by Werner and G\"{u}ttler.Comment: 5 pages, 5 figure

Surfaces roughness effects on the transmission of Gaussian beams by anisotropic parallel plates

Influence of the plate surfaces roughness in precise ellipsometry experiments is studied. The realistic case of a Gaussian laser beam crossing a uniaxial platelet is considered. Expression for the transmittance is determined using the first order perturbation theory. In this frame, it is shown that interference takes place between the specular transmitted beam and the scattered field. This effect is due to the angular distribution of the Gaussian beam and is of first order in the roughness over wavelength ratio. As an application, a numerical simulation of the effects of quartz roughness surfaces at normal incidence is provided. The interference term is found to be strongly connected to the random nature of the surface roughness.Comment: 18 pages, Journal of Physics D: Applied Physics, volume 36, issue 21, pages 2697 - 270

Spin dependent quantum interference in non-local graphene spin valves

Spin dependent electron transport measurements on graphene are of high importance to explore possible spintronic applications. Up to date all spin transport experiments on graphene were done in a semi-classical regime, disregarding quantum transport properties such as phase coherence and interference. Here we show that in a quantum coherent graphene nanostructure the non-local voltage is strongly modulated. Using non-local measurements, we separate the signal in spin dependent and spin independent contributions. We show that the spin dependent contribution is about two orders of magnitude larger than the spin independent one, when corrected for the finite polarization of the electrodes. The non-local spin signal is not only strongly modulated but also changes polarity as a function of the applied gate voltage. By locally tuning the carrier density in the constriction we show that the constriction plays a major role in this effect and indicates that it can act as a spin filter device. Our results show the potential of quantum coherent graphene nanostructures for the use in future spintronic devices

High flux polarized gamma rays production: first measurements with a four-mirror cavity at the ATF

The next generation of e+/e- colliders will require a very intense flux of gamma rays to allow high current polarized positrons to be produced. This can be achieved by converting polarized high energy photons in polarized pairs into a target. In that context, an optical system consisting of a laser and a four-mirror passive Fabry-Perot cavity has recently been installed at the Accelerator Test Facility (ATF) at KEK to produce a high flux of polarized gamma rays by inverse Compton scattering. In this contribution, we describe the experimental system and present preliminary results. An ultra-stable four-mirror non planar geometry has been implemented to ensure the polarization of the gamma rays produced. A fiber amplifier is used to inject about 10W in the high finesse cavity with a gain of 1000. A digital feedback system is used to keep the cavity at the length required for the optimal power enhancement. Preliminary measurements show that a flux of about $4\times10^6 \gamma$/s with an average energy of about 24 MeV was generated. Several upgrades currently in progress are also described

The role of soil carbon in natural climate solutions

Acknowledgements. This study was made possible by funding from the Craig and Susan McCaw Foundation. Data Deposition A global spatial dataset of reforestation opportunities is available on Zenodo (https://zenodo.org/record/883444). Figures 1 and 2 have associated raw data that can be made available upon request.Peer reviewedPostprin

Electronic Spin Transport in Dual-Gated Bilayer Graphene

The elimination of extrinsic sources of spin relaxation is key in realizing the exceptional intrinsic spin transport performance of graphene. Towards this, we study charge and spin transport in bilayer graphene-based spin valve devices fabricated in a new device architecture which allows us to make a comparative study by separately investigating the roles of substrate and polymer residues on spin relaxation. First, the comparison between spin valves fabricated on SiO2 and BN substrates suggests that substrate-related charged impurities, phonons and roughness do not limit the spin transport in current devices. Next, the observation of a 5-fold enhancement in spin relaxation time in the encapsulated device highlights the significance of polymer residues on spin relaxation. We observe a spin relaxation length of ~ 10 um in the encapsulated bilayer with a charge mobility of 24000 cm2/Vs. The carrier density dependence of spin relaxation time has two distinct regimes; n<4 x 1012 cm-2, where spin relaxation time decreases monotonically as carrier concentration increases, and n>4 x 1012 cm-2, where spin relaxation time exhibits a sudden increase. The sudden increase in the spin relaxation time with no corresponding signature in the charge transport suggests the presence of a magnetic resonance close to the charge neutrality point. We also demonstrate, for the first time, spin transport across bipolar p-n junctions in our dual-gated device architecture that fully integrates a sequence of encapsulated regions in its design. At low temperatures, strong suppression of the spin signal was observed while a transport gap was induced, which is interpreted as a novel manifestation of impedance mismatch within the spin channel

Design of a Polarised Positron Source Based on Laser Compton Scattering

We describe a scheme for producing polarised positrons at the ILC from polarised X-rays created by Compton scattering of a few-GeV electron beam off a CO2 or YAG laser. This scheme is very energy effective using high finesse laser cavities in conjunction with an electron storage ring.Comment: Proposal submitted to the ILC workshop, Snowmass 2005. v2: note number adde

A Compact Ring for Thom X-Ray Source

International audienceThe goal of X-ray sources based on Compton back scattering processes is to develop a compact device, which could produce an intense flux of monochromatic X-rays. Compton back-scattering resuls from collisions between laser pulses and relativistic electron bunches. Due to the relative low value of the Compton cross section, a high charge electron beam, a low emittance and a high focusing at the interaction point are required for the electron beam. In addition, the X-ray flux is related to the characteristics of the electron beam, which are themselves dynamically affected by the Compton interaction. One possible configuration is to inject frequently into a storage ring with a low emittance linear accelerator without waiting for the synchrotron equilibrium. As a consequence, the optics should be designed taking into account the characteristics of the electron beam from the linear accelerator. The accelerator ring design for a 50 MeV electron beam, aiming at producing a flux higher than 1013 ph/s, will be presented

Non-planar four-mirror optical cavity for high intensity gamma ray flux production by pulsed laser beam Compton scattering off GeV-electrons

As part of the R&D toward the production of high flux of polarised Gamma-rays we have designed and built a non-planar four-mirror optical cavity with a high finesse and operated it at a particle accelerator. We report on the main challenges of such cavity, such as the design of a suitable laser based on fiber technology, the mechanical difficulties of having a high tunability and a high mechanical stability in an accelerator environment and the active stabilization of such cavity by implementing a double feedback loop in a FPGA

The vaccine potential of Bordetella pertussis biofilm-derived membrane proteins

Pertussis is an infectious respiratory disease of humans caused by the gram-negative pathogen Bordetella pertussis. The use of acellular pertussis vaccines (aPs) which induce immunity of relative short duration and the emergence of vaccine-adapted strains are thought to have contributed to the recent resurgence of pertussis in industrialized countries despite high vaccination coverage. Current pertussis vaccines consist of antigens derived from planktonic bacterial cultures. However, recent studies have shown that biofilm formation represents an important aspect of B. pertussis infection, and antigens expressed during this stage may therefore be potential targets for vaccination. Here we provide evidence that vaccination of mice with B. pertussis biofilm-derived membrane proteins protects against infection. Subsequent proteomic analysis of the protein content of biofilm and planktonic cultures yielded 11 proteins which were ≥ three-fold more abundant in biofilms, of which Bordetella intermediate protein A (BipA) was the most abundant, surface-exposed protein. As proof of concept, mice were vaccinated with recombinantly produced BipA. Immunization significantly reduced colonization of the lungs and antibodies to BipA were found to efficiently opsonize bacteria. Finally, we confirmed that bipA is expressed during respiratory tract infection of mice, and that anti-BipA antibodies are present in the serum of convalescent whooping cough patients. Together, these data suggest that biofilm proteins and in particular BipA may be of interest for inclusion into future pertussis vaccines.Facultad de Ciencias ExactasCentro de Investigación y Desarrollo en Fermentaciones Industriale