A Graphene Field-Effect Device

In this letter, a top-gated field effect device (FED) manufactured from monolayer graphene is investigated. Except for graphene deposition, a conventional top-down CMOS-compatible process flow is applied. Carrier mobilities in graphene pseudo-MOS structures are compared to those obtained from top-gated Graphene-FEDs. The extracted values exceed the universal mobility of silicon and silicon-on-insulator MOSFETs.Comment: 12 pages, 3 figure

CB17: Inferring the dynamical history of a prestellar core with chemo-dynamical models

We present a detailed theoretical study of the isolated Bok globule CB17 (L1389) based on spectral maps of CS, HCO$^+$, C$^{18}$O, C$^{34}$S, and H$^{13}$CO$^+$ lines. A phenomenological model of prestellar core evolution, a time-dependent chemical model, and a radiative transfer simulation for molecular lines are combined to reconstruct the chemical and kinematical structure of this core. We developed a general criterion that allows to quantify the difference between observed and simulated spectral maps. By minimizing this difference, we find that very high and very low values of the effective sticking probability $S$ are not appropriate for the studied prestellar core. The most probable $S$ value for CB17 is 0.3--0.5. The spatial distribution of the intensities and self-absorption features of optically thick lines is indicative of UV irradiation of the core. By fitting simultaneously optically thin and optically thick transitions, we isolate the model that reproduces all the available spectral maps to a reasonable accuracy. The line asymmetry pattern in CB17 is reproduced by a combination of infall, rotation, and turbulent motions with velocities $\sim0.05$ km s$^{-1}$, $\sim0.1$ km s$^{-1}$, and $\sim0.1$ km s$^{-1}$, respectively. These parameters corresponds to energy ratios $E_{\rm rot}/E_{\rm grav}\approx0.03$, $E_{\rm therm}/E_{\rm grav}\approx0.8$, and $E_{\rm turb}/E_{\rm grav}\approx0.05$ (the rotation parameters are determined for $i=90^\circ$). The chemical age of the core is about 2 Myrs. In particular, this is indicated by the central depletion of CO, CS, and HCO$^+$. Based on the angular momentum value, we argue that the core is going to fragment, i.e., to form a binary (multiple) star. (abridged)Comment: ApJ, in pres

Non-volatile switching in graphene field effect devices

The absence of a band gap in graphene restricts its straight forward application as a channel material in field effect transistors. In this letter, we report on a new approach to engineer a band gap in graphene field effect devices (FED) by controlled structural modification of the graphene channel itself. The conductance in the FEDs is switched between a conductive "on-state" to an insulating "off-state" with more than six orders of magnitude difference in conductance. Above a critical value of an electric field applied to the FED gate under certain environmental conditions, a chemical modification takes place to form insulating graphene derivatives. The effect can be reversed by electrical fields of opposite polarity or short current pulses to recover the initial state. These reversible switches could potentially be applied to non-volatile memories and novel neuromorphic processing concepts.Comment: 14 pages, 4 figures, submitted to IEEE ED

Heat to Electricity Conversion by a Graphene Stripe with Heavy Chiral Fermions

A conversion of thermal energy into electricity is considered in the electrically polarized graphene stripes with zigzag edges where the heavy chiral fermion (HCF) states are formed. The stripes are characterized by a high electric conductance Ge and by a significant Seebeck coefficient S. The electric current in the stripes is induced due to a non-equilibrium thermal injection of "hot" electrons. This thermoelectric generation process might be utilized for building of thermoelectric generators with an exceptionally high figure of merit Z{\delta}T \simeq 100 >> 1 and with an appreciable electric power densities \sim 1 MW/cm2.Comment: 8 pages, 3 figure

Mobility in Graphene Double Gate Field Effect Transistors

In this work, double-gated field effect transistors manufactured from monolayer graphene are investigated. Conventional top-down CMOS-compatible processes are applied except for graphene deposition by manual exfoliation. Carrier mobilities in single- and double gated graphene field effect transistors are compared. Even in double-gated graphene FETs, the carrier mobility exceeds the universal mobility of silicon over nearly the entire measured range. At comparable dimensions, reported mobilities for ultra thin body silicon-on-insulator MOSFETs can not compete with graphene FET values.Comment: 7 pages, 9 figure

Quantum interference and Klein tunneling in graphene heterojunctions

The observation of quantum conductance oscillations in mesoscopic systems has traditionally required the confinement of the carriers to a phase space of reduced dimensionality. While electron optics such as lensing and focusing have been demonstrated experimentally, building a collimated electron interferometer in two unconfined dimensions has remained a challenge due to the difficulty of creating electrostatic barriers that are sharp on the order of the electron wavelength. Here, we report the observation of conductance oscillations in extremely narrow graphene heterostructures where a resonant cavity is formed between two electrostatically created bipolar junctions. Analysis of the oscillations confirms that p-n junctions have a collimating effect on ballistically transmitted carriers. The phase shift observed in the conductance fringes at low magnetic fields is a signature of the perfect transmission of carriers normally incident on the junctions and thus constitutes a direct experimental observation of ``Klein Tunneling.''Comment: 13 pages and 6 figures including supplementary information. The paper has been modified in light of new theoretical results available at arXiv:0808.048

Исследование сварного соединения, выполненного из сталей 12Х1МФ и 12Х18Н9Т для паропровода энергетического котла

Проводится рентгенофазовый и рентгенофлюоресцентный анализ зоны сварного шва при термоциклировании с целью предотвращения трещинообразования вследствие воздействия термического циклаThe X-ray phase and X-ray fluorescent analysis of a zone of a welded connection during thermocycling for the purpose of prevention of a fracturing owing to thermal cycle influenc

Photoconductivity of biased graphene

Graphene is a promising candidate for optoelectronic applications such as photodetectors, terahertz imagers, and plasmonic devices. The origin of photoresponse in graphene junctions has been studied extensively and is attributed to either thermoelectric or photovoltaic effects. In addition, hot carrier transport and carrier multiplication are thought to play an important role. Here we report the intrinsic photoresponse in biased but otherwise homogeneous graphene. In this classic photoconductivity experiment, the thermoelectric effects are insignificant. Instead, the photovoltaic and a photo-induced bolometric effect dominate the photoresponse due to hot photocarrier generation and subsequent lattice heating through electron-phonon cooling channels respectively. The measured photocurrent displays polarity reversal as it alternates between these two mechanisms in a backgate voltage sweep. Our analysis yields elevated electron and phonon temperatures, with the former an order higher than the latter, confirming that hot electrons drive the photovoltaic response of homogeneous graphene near the Dirac point

Photocurrent measurements of supercollision cooling in graphene

The cooling of hot electrons in graphene is the critical process underlying the operation of exciting new graphene-based optoelectronic and plasmonic devices, but the nature of this cooling is controversial. We extract the hot electron cooling rate near the Fermi level by using graphene as novel photothermal thermometer that measures the electron temperature ($T(t)$) as it cools dynamically. We find the photocurrent generated from graphene $p-n$ junctions is well described by the energy dissipation rate $C dT/dt=-A(T^3-T_l^3)$, where the heat capacity is $C=\alpha T$ and $T_l$ is the base lattice temperature. These results are in disagreement with predictions of electron-phonon emission in a disorder-free graphene system, but in excellent quantitative agreement with recent predictions of a disorder-enhanced supercollision (SC) cooling mechanism. We find that the SC model provides a complete and unified picture of energy loss near the Fermi level over the wide range of electronic (15 to $\sim$3000 K) and lattice (10 to 295 K) temperatures investigated.Comment: 7pages, 5 figure