arXiv subjects

E. H. Hwang

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

At least 55 records · Page 3Linked to original sources

arXiv2012

Effect of charged impurity correlation on transport in monolayer and bilayer graphene

We study both monolayer and bilayer graphene transport properties taking into account the presence of correlations in the spatial distribution of charged impurities. In particular we find that the experimentally observed sublinear scaling of the graphene conductivity can be naturally explained as arising from impurity correlation effects in the Coulomb disorder, with no need to assume the presence of short-range scattering centers in addition to charged impurities. We find that also in bilayer graphene correlations among impurities induce a crossover of the scaling of the conductivity at higher carrier densities. We show that in the presence of correlation among charged impurities the conductivity depends nonlinearly on the impurity density $n_i$ and can even increase with $n_i$.

cond-mat.mes-hall↗

arXiv2012

Comparison of microscopic models for disorder in bilayer graphene: Implications for the density of states and the optical conductivity

We study the effects of disorder on bilayer graphene using four different microscopic models and directly compare their results. We compute the self-energy, density of states, and optical conductivity in the presence of short-ranged scatterers and screened Coulomb impurities, using both the Born approximation and self-consistent Born approximation for the self-energy. We also include a finite interlayer potential asymmetry which generates a gap between the valence and conduction bands. We find that the qualitative behavior of the two scattering potentials are similar, but that the choice of approximation for the self-energy leads to important differences near the band edge in the gapped case. Finally, we describe how these differences manifest in the measurement of the band gap in optical and transport experimental techniques.

cond-mat.mes-hall↗

arXiv2012

2D transport and screening in topological insulator surface states

We study disorder effects on the surface states of the topological insulator Bi$_2$Se$_3$ close to the topologically protected crossing point. Close to charge neutrality, local fluctuations in carrier density arising from the random charged disorder in the environment result in electron and hole puddles that dominate the electronic properties of these materials. By calculating the polarizability of the surface state using the random phase approximation, and determining the characteristics of puddles using the self-consistent approximation, we find that band asymmetry plays a crucial role in determining experimentally measured quantities including the conductivity and the puddle autocorrelation length.

cond-mat.mes-hall↗

arXiv2012

Coulomb drag in graphene

We calculate theoretically the Coulomb drag resistivity for two graphene monolayers spatially separated by a distance "$d$". We show that the frictional drag induced by inter-layer electron-electron interaction goes asymptotically as $T^2/n^3$ and $T^2 \ln(n)/n$ in the high-density ($k_F d \gg 1$) and low-density ($k_F d \ll 1$) limits, respectively.

cond-mat.mes-hall↗

arXiv2011

Coulomb drag in monolayer and bilayer graphene

We theoretically calculate the interaction-induced frictional Coulomb drag resistivity between two graphene monolayers as well as between two graphene bilayers, which are spatially separated by a distance "$d$". We show that the drag resistivity between graphene monolayers can be significantly affected by the intralayer momentum-relaxation mechanism. For energy independent intralayer scattering, the frictional drag induced by inter-layer electron-electron interaction goes asymptotically as $ρ_D \sim T^2/n^4d^6$ and $ρ_D \sim T^2/n^2d^2$ in the high-density ($k_F d \gg 1$) and low-density ($k_F d \ll 1$) limits, respectively. When long-range charge impurity scattering dominates within the layer, the monolayer drag resistivity behaves as $ρ_D \sim T^2/n^3d^4$ and $T^2 \ln (\sqrt{n} d) /n$ for $k_F d \gg 1$ and $k_F d \ll 1$, respectively. The density dependence of the bilayer drag is calculated to be $ρ_D \propto T^2/n^{3}$ both in the large and small layer separation limit. In the large layer separation limit, the bilayer drag has a strong $1/d^4$ dependence on layer separation, whereas this goes to a weak logarithmic dependence in the strong inter-layer correlation limit of small layer separation. In addition to obtaining the asymptotic analytical formula for Coulomb drag in graphene, we provide numerical results for arbitrary values of density and layer separation interpolating smoothly between our asymptotic theoretical results.

cond-mat.mes-hall↗

arXiv2011

Temperature-dependent compressibility in graphene and two-dimensional systems

We calculate the finite temperature compressibility for two-dimensional semiconductor systems, monolayer graphene, and bilayer graphene within the Hartree-Fock approximation. We find that the calculated temperature dependent compressibility including exchange energy is non-monotonic. In 2D systems at low temperatures the inverse compressibility decreases first with increasing temperature, but after reaching a minimum it increases as temperature is raised further. At high enough temperatures the negative compressibility of low density systems induced by the exchange energy becomes positive due to the dominance of the finite temperature kinetic energy. The inverse compressibility in monolayer graphene is always positive and its temperature dependence appears to be reverse of the 2D semiconductor systems, i.e., it increases first with temperature and then decreases at high temperatures. The inverse compressibility of bilayer graphene shows the same non-monotonic behavior as ordinary 2D systems, but at high temperatures it approaches a constant which is smaller than the value of the non-interacting bilayer graphene. We find the leading order temperature correction to the compressibility within Hartree-Fock approximation to be $T^2 \ln T$ at low temperatures for all three systems.

cond-mat.mes-hall↗

arXiv2011

dmu/dn In suspended bilayer graphene: the interplay of disorder and band gap

We present an interpretation of recent experimental measurements of dmu/dn in suspended bilayer graphene samples. We demonstrate that the data may be quantitatively described by assuming a spatially varying band gap induced by local electric fields. We demonstrate that the gap fluctuations vary amongst different samples and that the gap fluctuations are correlated with the associated charge density fluctuations, indicating that the mechanism causing this effect is likely to be an extrinsic effect. We also provide predictions for the optical conductivity and mobility of suspended bilayer graphene samples with small band gaps.

cond-mat.mes-hall↗

arXiv2011

Theory of 2D transport in graphene for correlated disorder

We theoretically revisit graphene transport properties as a function of carrier density, taking into account possible correlations in the spatial distribution of the Coulomb impurity disorder in the environment. We find that the charged impurity correlations give rise to a density dependent graphene conductivity, which agrees well qualitatively with the existing experimental data. We also find, quite unexpectedly, that the conductivity could increase with increasing impurity density if there is sufficient inter-impurity correlation present in the system. In particular, the linearity (sublinearity) of graphene conductivity at lower (higher) gate voltage is naturally explained as arising solely from impurity correlation effects in the Coulomb disorder.

cond-mat.mes-hall↗

arXiv2011

Disorder-induced temperature-dependent transport in graphene: Puddles, impurities, activation, and diffusion

We theoretically study the transport properties of both monolayer and bilayer graphene in the presence of electron-hole puddles induced by charged impurities which are invariably present in the graphene environment. We calculate the graphene conductivity by taking into account the non-mean-field two-component nature of transport in the highly inhomogeneous density and potential landscape, where activated transport across the potential fluctuations in the puddle regimes coexists with regular metallic diffusive transport. The existence of puddles allows the local activation at low carrier densities, giving rise to an insulating temperature dependence in the conductivity of both monolayer and bilayer graphene systems. We also critically study the qualitative similarity and the quantitative difference between monolayer and bilayer graphene transport in the presence of puddles. Our theoretical calculation explains the non-monotonic feature of the temperature dependent transport, which is experimentally generically observed in low mobility graphene samples. We establish the 2-component nature (i.e., both activated and diffusive) of graphene transport arising from the existence of potential fluctuation induced inhomogeneous density puddles. The temperature dependence of the graphene conductivity arises from many competing mechanisms, even without considering any phonon effects, such as thermal excitation of carriers from the valence band to the conduction band, temperature dependent screening, thermal activation across the potential fluctuations associated with the electron-hole puddles induced by the random charged impurities in the environment, leading to very complex temperature dependence which depends both on the carrier density and the temperature range of interest.

cond-mat.mes-hall↗

arXiv2011

Non-monotonic temperature dependent transport in graphene grown by Chemical Vapor Deposition

Temperature-dependent resistivity of graphene grown by chemical vapor deposition (CVD) is investigated. We observe in low mobility CVD graphene device a strong insulating behavior at low temperatures and a metallic behavior at high temperatures manifesting a non-monotonic in the temperature dependent resistivity.This feature is strongly affected by carrier density modulation. To understand this anomalous temperature dependence, we introduce thermal activation of charge carriers in electron-hole puddles induced by randomly distributed charged impurities. Observed temperature evolution of resistivity is then understood from the competition among thermal activation of charge carriers, temperature-dependent screening and phonon scattering effects. Our results imply that the transport property of transferred CVD-grown graphene is strongly influenced by the details of the environment

cond-mat.mes-hall↗

arXiv2011

Optical and transport gaps in gated bilayer graphene

We discuss the effect of disorder on the band gap measured in bilayer graphene in optical and transport experiments. By calculating the optical conductivity and density of states using a microscopic model in the presence of disorder, we demonstrate that the gap associated with transport experiments is smaller than that associated with optical experiments. Intrinsic bilayer graphene has an optical conductivity in which the energy of the peaks associated with the interband transition are very robust against disorder and thus provide an estimate of the band gap. In contrast, extraction of the band gap from the optical conductivity of extrinsic bilayer graphene is almost impossible for significant levels of disorder due to the ambiguity of the transition peaks. The density of states contains an upper bound on the gap measured in transport experiments, and disorder has the effect of reducing this gap which explains why these experiments have so far been unable to replicate the large band gaps seen in optical measurements.

cond-mat.mes-hall↗

arXiv2011

Scattering Mechanisms in a High Mobility Low Density Carbon-Doped (100) GaAs Two-Dimensional Hole System

We report on a systematic study of the density dependence of mobility in a low-density Carbon-doped (100) GaAs two-dimensional hole system (2DHS). At T= 50 mK, a mobility of 2.6 x 10^6 cm^2/Vs at a density p=6.2 x 10^10 cm^- was measured. This is the highest mobility reported for a 2DHS to date. Using a back-gated sample geometry, the density dependence of mobility was studied from 2.8 x 10^10 cm^-2 to 1 x 10^11 cm^-2. The mobility vs. density cannot be fit to a power law dependence of the form mu ~ p^alpha using a single exponent alpha. Our data indicate a continuous evolution of the power law with alpha ranging from ~ 0.7 at high density and increasing to ~ 1.7 at the lowest densities measured. Calculations specific to our structure indicate a crossover of the dominant scattering mechanism from uniform background impurity scattering at high density to remote ionized impurity scattering at low densities. This is the first observation of a carrier density-induced transition from background impurity dominated to remote dopant dominated transport in a single sample.

cond-mat.str-el↗

arXiv2011

Chirality-dependent phonon-limited resistivity in multiple layers of graphene

We develop a theory for the temperature and density dependence of phonon-limited resistivity $ρ(T)$ in bilayer and multilayer graphene, and compare with the corresponding monolayer result. For the unscreened case, we find $ρ\approx C T$ with $C \propto v_{\rm F}^{-2}$ in the high-temperature limit, and $ρ\approx A T^4$ with $A \propto v_{\rm F}^{-2} k_{\rm F}^{-3}$ in the low-temperature Bloch-Grüneisen limit, where $v_{\rm F}$ and $k_{\rm F}$ are Fermi velocity and Fermi wavevector, respectively. If screening effects are taken into account, $ρ\approx C T$ in the high-temperature limit with a renormalized $C$ which is a function of the screening length, and $ρ\approx A T^6$ in the low-temperature limit with $A \propto k_{\rm F}^{-5}$ but independent of $v_{\rm F}$. These relations hold in general with $v_{\rm F}$ and a chiral factor in $C$ determined by the specific chiral band structure for a given density.

cond-mat.mtrl-sci↗

arXiv2011

Compressibility of graphene

We develop a theory for the compressibility and quantum capacitance of disordered monolayer and bilayer graphene including the full hyperbolic band structure and band gap in the latter case. We include the effects of disorder in our theory, which are of particular importance at the carrier densities near the Dirac point. We account for this disorder statistically using two different averaging procedures: first via averaging over the density of carriers directly, and then via averaging in the density of states to produce an effective density of carriers. We also compare the results of these two models with experimental data, and to do this we introduce a model for inter-layer screening which predicts the size of the band gap between the low-energy conduction and valence bands for arbitary gate potentials applied to both layers of bilayer graphene. We find that both models for disorder give qualitatively correct results for gapless systems, but when there is a band gap at charge neutrality, the density of states averaging is incorrect and disagrees with the experimental data.

cond-mat.mes-hall↗

arXiv2011

Correlations, Plasmarons, and Quantum Spectral Function in Bilayer Graphene

We theoretically study the many-body effects of electron electron interaction on the single particle spectral function of doped bilayer graphene. Using random phase approximation, we calculate the real and imaginary part of the self-energy and hence the spectral function. The spectral function near the Fermi surface shows the usual quasiparticle peak, establishing doped bilayer graphene, in contrast to the unstable neutral system, to be a Fermi liquid. Away from the Fermi surface, an additional broad plasmaron peak is visible in the spectral function. From the low energy behaviour of the self-energy we calculate the quasiparticle residue and the effective mass of the quasiparticles as a function of carrier density. We present results for both the on-shell and the off-shell approximation for the quasiparticle renormalization.

cond-mat.mes-hall↗

arXiv2010

Conductivity of graphene on boron nitride substrates

We calculate theoretically the disorder-limited conductivity of monolayer and bilayer graphene on hexagonal boron nitride (h-BN) substrates, comparing our theoretical results with the recent experimental results. The comparison leads to a direct quantitative estimate of the underlying disorder strength for both short-range and long-range disorder in the graphene on h-BN system. We find that the good interface quality between graphene and h-BN leads to strongly suppressed charged impurity scattering compared with the corresponding SiO$_2$ substrate case, thus producing very high mobility for the graphene on h-BN system.

cond-mat.mes-hall↗

arXiv2010

Collective modes of monolayer, bilayer, and multilayer fermionic dipolar liquid

Motivated by recent experimental advances in creating polar molecular gases in the laboratory, we theoretically investigate the many body effects of two-dimensional dipolar systems with the anisotropic and $1/r^3$ dipole-dipole interactions. We calculate collective modes of 2D dipolar systems, and also consider spatially separated bilayer and multilayer superlattice dipolar systems. We obtain the characteristic features of collective modes in quantum dipolar gases. We quantitatively compare the modes of these dipolar systems with the modes of the extensively studied usual two-dimensional electron systems, where the inter-particle interaction is Coulombic.

cond-mat.quant-gas↗

arXiv2010

Electronic transport in two dimensional graphene

We provide a broad review of fundamental electronic properties of two-dimensional graphene with the emphasis on density and temperature dependent carrier transport in doped or gated graphene structures. A salient feature of our review is a critical comparison between carrier transport in graphene and in two-dimensional semiconductor systems (e.g. heterostructures, quantum wells, inversion layers) so that the unique features of graphene electronic properties arising from its gap- less, massless, chiral Dirac spectrum are highlighted. Experiment and theory as well as quantum and semi-classical transport are discussed in a synergistic manner in order to provide a unified and comprehensive perspective. Although the emphasis of the review is on those aspects of graphene transport where reasonable consensus exists in the literature, open questions are discussed as well. Various physical mechanisms controlling transport are described in depth including long- range charged impurity scattering, screening, short-range defect scattering, phonon scattering, many-body effects, Klein tunneling, minimum conductivity at the Dirac point, electron-hole puddle formation, p-n junctions, localization, percolation, quantum-classical crossover, midgap states, quantum Hall effects, and other phenomena.

cond-mat.mes-hall↗