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Register a new userTransport scattering time probed through rf admittance of a graphene capacitor
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We have investigated electron dynamics in top gated graphene by measuring the gate admittance of a diffusive graphene capacitor in a broad frequency range as a function of carrier density. The density of states, conductivity and diffusion constant are deduced from the low frequency gate capacitance, its charging time and their ratio. The admittance evolves from an RC-like to a skin-effect response at GHz frequency with a crossover given by the Thouless energy. The scattering time is found to be independent of energy in the 0 - 200 meV investigated range at room temperature. This is consistent with a random mass model for Dirac Fermions.
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The tuning of electrical circuit resonance with a variable capacitor, or varactor, finds wide application with the most important being wireless telecommunication. We demonstrate an electromechanical graphene varactor, a variable capacitor wherein the capacitance is tuned by voltage controlled deflection of a dense array of suspended graphene membranes. The low flexural rigidity of graphene monolayers is exploited to achieve low actuation voltage in an ultra-thin structure. Large arrays comprising thousands of suspensions were fabricated to give a tunable capacitance of over 10 pF/mm$^2$, higher than that achieved by traditional micro-electromechanical system (MEMS) technologies. A capacitance tuning of 55% was achieved with a 10 V actuating voltage, exceeding that of conventional MEMS parallel plate capacitors. Capacitor behavior was investigated experimentally, and described by a simple theoretical model. Mechanical properties of the graphene membranes were measured independently using Atomic Force Microscopy (AFM). Increased graphene conductivity will enable the application of the compact graphene varactor to radio frequency systems.
We measure graphene coplanar waveguides from direct current (DC) to 13.5GHz and show that the apparent resistance (in the presence of parasitic impedances) has an quadratic frequency dependence, but the intrinsic conductivity (without the influence of parasitic impedances) is frequency-independent. Consequently, in our devices the real part of the complex alternating current conductivity is the same as the DC value and the imaginary part~0. The graphene channel is modelled as a parallel resistive-capacitive network with a frequency dependence identical to that of the Drude conductivity with momentum relaxation time~2.1ps, highlighting the influence of alternating current (AC) electron transport on the electromagnetic properties of graphene. This can lead to optimized design of high-speed analogue field-effect transistors, mixers, frequency doublers, low-noise amplifiers and radiation detectors.
We present Coulomb blockade measurements in a graphene double dot system. The coupling of the dots to the leads and between the dots can be tuned by graphene in-plane gates. The coupling is a non-monotonic function of the gate voltage. Using a purely capacitive model, we extract all relevant energy scales of the double dot system.
We report transport measurements through graphene on SrTiO3 substrates as a function of magnetic field B, carrier density n, and temperature T. The large dielectric constant of SrTiO3 screens very effectively long-range electron-electron interactions and potential fluctuations, making Dirac electrons in graphene virtually non-interacting. The absence of interactions results in a unexpected behavior of the longitudinal resistance in the N=0 Landau level, and in a large suppression of the transport gap in nano-ribbons. The bulk transport properties of graphene at B=0T, on the contrary, are completely unaffected by the substrate dielectric constant.
We study the quantization of Dirac fermions in lithographically defined graphene nanoconstrictions. We observe quantized conductance in single nanoconstrictions fabricated on top of a thin hexamethyldisilazane layer over a Si/SiO_2 wafer. This nanofabrication method allows us to obtain well defined edges in the nanoconstrictions, thus reducing the effects of edge roughness on the conductance. We prove the occurrence of ballistic transport and identify several size quantization plateaus in the conductance at low temperature. Experimental data and numerical simulations show good agreement, demonstrating that the smoothing of the plateaus is not related to edge roughness but to quantum interference effects.
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