다중혈관 관상동맥 환자에서 y-문합을 이용하여 양쪽 내흉동맥만을 사용한 우회술의 조기 성적

This article reviews the basic theoretical aspects of graphene, a one-atom-thick allotrope of carbon, with unusual two-dimensional Dirac-like electronic excitations. The Dirac electrons can be controlled by application of external electric and magnetic fields, or by altering sample geometry and/or topology. The Dirac electrons behave in unusual ways in tunneling, confinement, and the integer quantum Hall effect. The electronic properties of graphene stacks are discussed and vary with stacking order and number of layers. Edge (surface) states in graphene depend on the edge termination (zigzag or armchair) and affect the physical properties of nanoribbons. Different types of disorder modify the Dirac equation leading to unusual spectroscopic and transport properties. The effects of electron-electron and electron-phonon interactions in single layer and multilayer graphene are also presented.

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The electronic properties of graphene

Single-layer metal dichalcogenides are two-dimensional semiconductors that present strong potential for electronic and sensing applications complementary to that of graphene.

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Electronics and optoelectronics of two-dimensional transition metal dichalcogenides.

We report the measurement of the thermal conductivity of a suspended single-layer graphene. The room temperature values of the thermal conductivity in the range ∼(4.84 ± 0.44) × 103 to (5.30 ± 0.48) × 103 W/mK were extracted for a single-layer graphene from the dependence of the Raman G peak frequency on the excitation laser power and independently measured G peak temperature coefficient. The extremely high value of the thermal conductivity suggests that graphene can outperform carbon nanotubes in heat conduction. The superb thermal conduction property of graphene is beneficial for the proposed electronic applications and establishes graphene as an excellent material for thermal management.

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Superior Thermal Conductivity of Single-Layer Graphene

Graphene has been attracting great interest because of its distinctive band structure and physical properties. Today, graphene is limited to small sizes because it is produced mostly by exfoliating graphite. We grew large-area graphene films of the order of centimeters on copper substrates by chemical vapor deposition using methane. The films are predominantly single-layer graphene, with a small percentage (less than 5%) of the area having few layers, and are continuous across copper surface steps and grain boundaries. The low solubility of carbon in copper appears to help make this growth process self-limiting. We also developed graphene film transfer processes to arbitrary substrates, and dual-gated field-effect transistors fabricated on silicon/silicon dioxide substrates showed electron mobilities as high as 4050 square centimeters per volt per second at room temperature.

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Large-Area Synthesis of High-Quality and Uniform Graphene Films on Copper Foils

We compute the phase diagram of a biased graphene bilayer. The existence of a ferromagnetic phase is discussed with respect to both carrier density and temperature. We find that the ferromagnetic transition is first-order, lowering the value of U relatively to the usual Stoner criterion. We show that in the ferromagnetic phase the two planes have unequal magnetization and that the electronic density is holelike in one plane and electronlike in the other.

Low-density ferromagnetism in biased bilayer graphene.

We study the optical properties of double-layer graphene for linearly polarized evanescent modes and discuss the in-phase and out-of-phase plasmon modes for both, longitudinal and transverse polarization, and for inhomogeneous dielectric media. We find an energy for which reflection is zero, leading to exponentially amplified transmitted modes similar to what happens in left-handed materials. For layers with equal densities $n={10}^{12}\phantom{\rule{0.28em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$, we find a typical layer separation of $d\ensuremath{\approx}500\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{m}$ to detect this amplification for transverse polarization, which may serve as an indirect observation of transverse plasmons. When the two graphene layers lie on different chemical potentials, the exponential amplification either follows the in-phase or the out-of-phase plasmon mode depending on the order of the low- and high-density layers. This opens up the possibility of a tunable near-field amplifier or switch.

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Plasmons and near-field amplification in double-layer graphene

This work was supported by Deutsche Forschungsgemeinschaft via Grant No. GRK 1570, by FCT under Grants No. PTDC/FIS/101434/2008 and No. PTDC/FIS/113199/2009, and MIC under Grant No. FIS2010-21883-C02-02

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Plasmons and screening in a monolayer of MoS2

Using a semiclassical approach and input from experiments on the conductivity of graphene, we determine the electronic density dependence of the electronic transport coefficients---conductivity, thermal conductivity, and thermopower---of doped graphene. Also, the electronic density dependence of the optical conductivity is obtained. Finally, we show that the classical Hall effect (low field) in graphene has the same form as for the independent electron case, characterized by a parabolic dispersion, as long as the relaxation time is proportional to the momentum.

Phenomenological study of the electronic transport coefficients of graphene

We compute the dc and the optical conductivity of graphene for finite values of the chemical potential by taking into account the effect of disorder, due to midgap states (unitary scatterers) and charged impurities, and the effect of both optical and acoustic phonons. The disorder due to midgap states is treated in the coherent-potential approximation (a self-consistent approach based on the Dyson equation) whereas that due to charged impurities is also treated via the Dyson equation with the self-energy computed using second-order perturbation theory. The effect of the phonons is also included via the Dyson equation with the self-energy computed using first-order perturbation theory. The self-energy due to phonons is computed both using the bare electronic Green's function and the full electronic Green's function although we show that the effect of disorder on the phonon propagator is negligible. Our results are in qualitative agreement with recent experiments. Quantitative agreement could be obtained if one assumes water molecules under the graphene substrate. We also comment on the electron-hole asymmetry observed in the dc conductivity of suspended graphene.

Tobias Stauber

Papers

Low-density ferromagnetism in biased bilayer graphene.

Plasmons and near-field amplification in double-layer graphene

Plasmons and screening in a monolayer of MoS2

Phenomenological study of the electronic transport coefficients of graphene

Conductivity of suspended and non-suspended graphene at finite gate voltage