arXiv subjects

E. H. Hwang

Publications and source records attributed to E. H. Hwang.

At least 91 records · Page 5Linked to original sources

arXiv2008

Acoustic phonon scattering limited carrier mobility in 2D extrinsic graphene

We theoretically calculate the phonon scattering limited electron mobility in extrinsic (i.e. gated or doped with a tunable and finite carrier density) 2D graphene layers as a function of temperature $(T)$ and carrier density $(n)$. We find a temperature dependent phonon-limited resistivity $ρ_{ph}(T)$ to be linear in temperature for $T\agt 50 K$ with the room temperature intrinsic mobility reaching values above $10^5$ cm$^2/Vs$. We comment on the low-temperature Bloch-Grüneisen behavior where $ρ_{ph}(T) \sim T^4$ for unscreened electron-phonon coupling.

cond-mat.mes-hall↗

arXiv2008

Single particle relaxation time versus transport scattering time in a 2D graphene layer

We theoretically calculate and compare the single-particle relaxation time ($τ_s$) defining quantum level broadening and the transport scattering time ($τ_t$) defining Drude conductivity in 2D graphene layers in the presence of screened charged impurities scattering and short-range defect scattering. We find that the ratio $τ_t/τ_s$ increases strongly with increasing $k_F z_i$ and $κ$ where $k_F$, $z_i$, and $κ$ are respectively the Fermi wave vector, the separation of the substrate charged impurities from the graphene layer, and the background lattice dielectric constant. A critical quantitative comparison of the $τ_t/τ_s$ results for graphene with the corresponding modulation-doped semiconductor structures is provided, showing significant differences between these two 2D carrier systems.

cond-mat.mes-hall↗

arXiv2007

Inelastic carrier lifetime in graphene

We consider hot carrier inelastic scattering due to electron--electron interactions in graphene, as functions of carrier energy and density. We calculate the imaginary part of the zero-temperature quasiparticle self-energy for doped graphene, utlizing the $G_0W$ and random phases approximations. Using the full dynamically screened Coulomb interaction, we obtain the inelastic quasiparticle lifetimes and associated mean free paths. The linear dispersion of graphene gives lifetime energy dependences that are qualitatively different from those of parabolic-band semiconductors. We also get good agreement with data from angle-resolved photoemission spectroscopy experiments.

cond-mat.mes-hall↗

arXiv2007

Density dependent exchange contribution to $\partialμ/\partial n$ in extrinsic graphene

We calculate $\partialμ/\partial n$ in extrinsic graphene as a function of carrier density $n$ at zero temperature by obtaining the electronic self-energy within the Hartree-Fock approximation. The exchange-driven Dirac-point logarithmic singularity in the quasiparticle velocity of intrinsic graphene disappears in the extrinsic case. The calculated renormalized $\partialμ/\partial n$ in extrinsic graphene has the same qualitative $n^{-\frac12}$ density dependence as the inverse bare density of states with a 20% enhancement from the corresponding bare value, a relatively weak effect compared to the corresponding parabolic-band case.

cond-mat.mes-hall↗

arXiv2007

Transport in chemically doped graphene in the presence of adsorbed molecules

Motivated by a recent experiment reporting on the possible application of graphene as sensors, we calculate transport properties of 2D graphene monolayers in the presence of adsorbed molecules. We find that the adsorbed molecules, acting as compensators that partially neutralize the random charged impurity centers in the substrate, enhance the graphene mobility without much change in the carrier density. We predict that subsequent field-effect measurements should preserve this higher mobility for both electrons and holes, but with a voltage induced electron-hole asymmetry that depends on whether the adsorbed molecule was an electron or hole donor in the compensation process. We also calculate the low density magnetoresistance and find good quantitative agreement with experimental results.

cond-mat.mes-hall↗

arXiv2007

Transport and percolation in a low-density high-mobility two-dimensional hole system

We present a study of the temperature and density dependence of the resistivity of an extremely high quality two-dimensional hole system grown on the (100) surface of GaAs. For high densities in the metallic regime ($p\agt 4 \times 10^{9}$ cm$^{-2}$), the nonmonotonic temperature dependence ($\sim 50-300$ mK) of the resistivity is consistent with temperature dependent screening of residual impurities. At a fixed temperature of $T$= 50 mK, the conductivity vs. density data indicates an inhomogeneity driven percolation-type transition to an insulating state at a critical density of $3.8\times 10^9$ cm$^{-2}$.

cond-mat.mes-hall↗

arXiv2007

Scattering mechanisms and Boltzmann transport in graphene

Different scattering mechanisms in graphene are explored and conductivity is calculated within the Boltzmann transport theory. We provide results for short-range scattering using the Random Phase Approximation for electron screening, as well as analytical expressions for the dependence of conductivity on the dielectric constant of the substrate. We further examine the effect of ripples on the transport using a surface roughness model developed for semiconductor heterostructures. We find that close to the Dirac point, σ\sim n^β, where β=1,0,-2 for Coulomb, short-range and surface roughness respectively; implying that Coulomb scattering dominates over both short-range and surface roughness scattering at low density.

cond-mat.mes-hall↗

arXiv2007

A self-consistent theory for graphene transport

We demonstrate theoretically that most of the observed transport properties of graphene sheets at zero magnetic field can be explained by scattering from charged impurities. We find that, contrary to common perception, these properties are not universal but depend on the concentration of charged impurities $n_{\rm imp}$. For dirty samples ($250 \times 10^{10} {\rm cm}^{-2} < n_{\rm imp} < 400 \times 10^{10} {\rm cm}^{-2}$), the value of the minimum conductivity at low carrier density is indeed $4 e^2/h$ in agreement with early experiments, with weak dependence on impurity concentration. For cleaner samples, we predict that the minimum conductivity depends strongly on $n_{\rm imp}$, increasing to $8 e^2/h$ for $n_{\rm imp} \sim 20 \times 10^{10}{\rm cm}^{-2}$. A clear strategy to improve graphene mobility is to eliminate charged impurities or use a substrate with a larger dielectric constant.

cond-mat.mes-hall↗

arXiv2007

Electron-Electron Interactions in Graphene

We discuss the validity (or not) of the ring-diagram approximation (i.e. RPA) in the calculation of graphene self-energy in the weak-coupling ($r_s \ll 1$) limit, showing that RPA is a controlled and valid approximation for \textit{extrinsic} graphene where the Fermi level is away from the Dirac point.

cond-mat.mtrl-sci↗

arXiv2007

The quasiparticle spectral function in doped graphene

We calculate the real and imaginary electron self-energy as well as the quasiparticle spectral function in doped graphene taking into account electron-electron interaction in the leading order dynamically screened Coulomb coupling. Our theory provides the basis for calculating {\it all} one-electron properties of extrinsic graphene. Comparison with existing ARPES measurements shows broad qualitative agreement between theory and experiment. We also calculate the renormalized graphene momentum distribution function, finding a typical Fermi liquid discontinuity at k_F. We also provide a critical discussion of the relevant many body approximations (e.g. RPA) for graphene.

cond-mat.mes-hall↗

arXiv2007

Many-body exchange-correlation effects in graphene

We calculate, within the leading-order dynamical-screening approximation, the electron self-energy and spectral function at zero temperature for extrinsic (or gated/doped) graphene. We also calculate hot carrier inelastic scattering due to electron-electron interactions in graphene. We obtain the inelastic quasiparticle lifetimes and associated mean free paths from the calculated self-energy. The linear dispersion and chiral property of graphene gives energy dependent lifetimes that are qualitatively different from those of parabolic-band semiconductors.

cond-mat.mes-hall↗

arXiv2007

Measurement of Scattering Rate and Minimum Conductivity in Graphene

The conductivity of graphene samples with various levels of disorder is investigated for a set of specimens with mobility in the range of $1-20\times10^3$ cm$^2$/V sec. Comparing the experimental data with the theoretical transport calculations based on charged impurity scattering, we estimate that the impurity concentration in the samples varies from $2-15\times 10^{11}$ cm$^{-2}$. In the low carrier density limit, the conductivity exhibits values in the range of $2-12e^2/h$, which can be related to the residual density induced by the inhomogeneous charge distribution in the samples. The shape of the conductivity curves indicates that high mobility samples contain some short range disorder whereas low mobility samples are dominated by long range scatterers.

cond-mat.mes-hall↗

arXiv2007

Dielectric function, screening, and plasmons in 2D graphene

The dynamical dielectric function of two dimensional graphene at arbitrary wave vector $q$ and frequency $ω$, $ε(q,ω)$, is calculated in the self-consistent field approximation. The results are used to find the dispersion of the plasmon mode and the electrostatic screening of the Coulomb interaction in 2D graphene layer within the random phase approximation. At long wavelengths ($q\to 0$) the plasmon dispersion shows the local classical behavior $ω_{cl} = ω_0 \sqrt{q}$, but the density dependence of the plasma frequency ($ω_0 \propto n^{1/4}$) is different from the usual 2D electron system ($ω_0 \propto n^{1/2}$). The wave vector dependent plasmon dispersion and the static screening function show very different behavior than the usual 2D case.

cond-mat.str-el↗

arXiv2007

Carrier transport in 2D graphene layers

Carrier transport in gated 2D graphene monolayers is theoretically considered in the presence of scattering by random charged impurity centers with density $n_i$. Excellent quantitative agreement is obtained (for carrier density $n > 10^{12} \rm{cm}^{-2}$) with existing experimental data (Ref. \onlinecite{kn:novoselov2004, kn:novoselov2005, kn:zhang2005, kn:kim2006, kn:fuhrer2006}). The conductivity scales linearly with $n/n_i$ in the theory, and shows extremely weak temperature dependence. The experimentally observed asymmetry between electron and hole conductivities is explained by the asymmetry in the charged impurity configuration in the presence of the gate voltage, while the high-density saturation of conductivity for the highest mobility samples is explained as a crossover between the long-range and the point scattering dominated regimes. We argue that the experimentally observed saturation of conductivity at low density arises from the charged impurity induced inhomogeneity in the graphene carrier density which becomes severe for $n \lesssim n_i \sim 10^{12} \rm{cm}^{-2}$.

cond-mat.mes-hall↗

arXiv2007

Is Graphene a Fermi Liquid?

We answer the question posed in the title above by considering theoretically the electron-electron interaction induced many-body effects in undoped (`intrinsic') and doped (`extrinsic') 2D graphene layers. We find that (1) intrinsic graphene is a marginal Fermi liquid with the imaginary part of the self-energy, ${Im}Σ(ω)$, going as linear in energy $ω$ for small $ω$, implying that the quasiparticle spectral weight vanishes at the Dirac point as $({ln}ω)^{-1}$; and, (2) extrinsic graphene is a well-defined Fermi liquid with ${Im}Σ(ω)\sim ω^2\mathrm{ln}ω$ near the Fermi surface similar to 2D carrier systems with parabolic energy dispersion. We provide analytical and numerical results for quasiparticle renormalization in graphene, concluding that all experimental graphene systems are ordinary 2D Fermi liquids since any doping automatically induces generic Fermi liquid behavior.

cond-mat.mes-hall↗

arXiv2006

Transport properties of two-dimensional electron systems on silicon (111) surfaces

We theoretically study transport properties of a two-dimensional electron system on a hydrogen-passivated Si(111) surface in the field-effect-transistor (FET) configuration. We calculate the density and temperature dependent mobility and resistivity for the recently fabricated Si(111)-vacuum FET by using a semiclassical Boltzmann theory including screened charged impurity scattering. We find reasonable agreement with the corresponding experimental transport properties, indicating that the screened disorder potential from random charged impurities is the main scattering mechanism. We also find that the theoretical results with the valley degeneracy $g_v=2$ give much better agreement with experiment than the $g_v=6$ situation indicating that the usual bulk six-valley degeneracy of Si is lifted in this system.

cond-mat.mtrl-sci↗

arXiv2006

Hall coefficient and magnetoresistance of 2D spin-polarized electron systems

Recent measurements of the 2D Hall resistance show that the Hall coefficient is independent of the applied in-plane magnetic field, i.e., the spin-polarization of the system. We calculate the weak-field Hall coefficient and the magnetoresistance of a spin polarized 2D system using the semi-classical transport approach based on the screening theory. We solve the coupled kinetic equations of the two carrier system including electron-electron interaction. We find that the in-plane magnetic field dependence of the Hall coefficient is suppressed by the weakening of screening and the electron-electron interaction. However, the in-plane magnetoresistance is mostly determined by the change of the screening of the system, and can therefore be strongly field dependent.

cond-mat.mes-hall↗

arXiv2005

Low density spin-polarized transport in 2D semiconductor structures: The enigma of temperature dependent magnetoresistance of Si MOSFETs in an in-plane applied magnetic field

The temperature dependence of 2D magnetoresistance in an applied in-plane magnetic field is theoretically considered for electrons in Si MOSFETs within the screening theory for long-range charged impurity scattering limited carrier transport. In agreement with recent experimental observations we find an essentially temperature independent magnetoresistivity for carrier densities well into the 2D metallic regime due to the field-induced lifting of spin and, perhaps, valley degeneracies. In particular the metallic temperature dependence of the ballistic magnetoresistance is strongly suppressed around the zero-temperature critical magnetic field ($B_s$) for full spin-polarization, with the metallic temperature dependence strongest at B=0, weakest around $B \sim B_s$, and intermediate at $B \gg B_s$.

cond-mat.dis-nn↗