arXiv subjects

E. H. Hwang

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

At least 37 records · Page 2Linked to original sources

arXiv2015

Transport in two-dimensional modulation doped semiconductor structures

We develop a theory for the maximum achievable mobility in modulation-doped 2D GaAs-AlGaAs semiconductor structures by considering the momentum scattering of the 2D carriers by the remote ionized dopants which must invariably be present in order to create the 2D electron gas at the GaAs-AlGaAs interface. The minimal model, assuming first order Born scattering by random quenched remote dopant ions as the only scattering mechanism, gives a mobility much lower (by a factor of 3 or more) than that observed experimentally in many ultra high-mobility modulation-doped 2D systems, establishing convincingly that the model of uncorrelated scattering by independent random remote quenched dopant ions is often unable to describe the physical system quantitively. We theoretically establish that the consideration of spatial correlations in the remote dopant distribution can enhance the mobility by (up to) several orders of magnitudes in experimental samples. The precise calculation of the carrier mobility in ultra-pure modulation-doped 2D semiconductor structures thus depends crucially on the unknown spatial correlations among the dopant ions in the doping layer which may manifest sample to sample variations even for nominally identical sample parameters (i.e., density, well width, etc.), depending on the details of the modulation-doping growth conditions.

cond-mat.mes-hall↗

arXiv2014

Inelastic carrier lifetime in a coupled graphene electron-phonon system: Role of plasmon-phonon coupling

We calculate the inelastic scattering rates and the hot electron inelastic mean free paths for both monolayer and bilayer graphene on a polar substrate. We study the quasiparticle self-energy by taking into account both electron-electron and electron-surface optical (SO) phonon interactions. In this calculation the leading order dynamic screening approximation (G$_0$W approximation) is used to obtain the quasiparticle self-energy by treating electrons and phonons on an equal footing. We find that the strong coupling between the SO phonon and plasmon leads to a new decay channel for the quasiparticle through the emission of the coupled mode, and gives rise to an abrupt increase in the scattering rate, which is absent in the uncoupled system. In monolayer graphene a single jump in the scattering rate occurs, arising from the emission of the low energy branch of the coupled plasmon-phonon modes. In bilayer graphene the emission of both low and high energy branches of the coupled modes contributes to the scattering rate and gives rise to two abrupt changes in the scattering rate. The jumps in the scattering rate can be potentially used in the hot electron device such as switching devices and oscillators.

cond-mat.mes-hall↗

arXiv2014

Carrier screening, transport, and relaxation in 3D Dirac semimetals

A theory is developed for the density and temperature dependent carrier conductivity in doped three-dimensional (3D) Dirac materials focusing on resistive scattering from screened Coulomb disorder due to random charged impurities (e.g., dopant ions and unintentional background impurities). The theory applies both in the undoped intrinsic ("high-temperature", $T \gg T_F$) and the doped extrinsic ("low-temperature", $T \ll T_F$) limit with analytical scaling properties for the carrier conductivity obtained in both regimes, where $T_F$ is the Fermi temperature corresponding to the doped free carrier density (electrons or holes). The scaling properties describing how the conductivity depends on the density and temperature can be used to establish the Dirac nature of 3D systems through transport measurements. We also consider the temperature dependent conductivity limited by the acoustic phonon scattering in 3D Dirac materials. In addition, we theoretically calculate and compare the single particle relaxation time $\tas$, defining the quantum level broadening, and the transport scattering time $\tat$, defining the conductivity, in the presence of screened charged impurity scattering. A critical quantitative analysis of the $\tat/\tas$ results for 3D Dirac materials in the presence of long-range screened Coulomb disorder is provided.

cond-mat.mes-hall↗

arXiv2014

Signatures of localization in the effective metallic regime of high mobility Si MOSFETs

Combining experimental data, numerical transport calculations, and theoretical analysis, we study the temperature-dependent resistivity of high-mobility 2D Si MOSFETs to search for signatures of weak localization induced quantum corrections in the effective metallic regime above the critical density of the so-called two-dimensional metal-insulator transition (2D MIT). The goal is to look for the effect of logarithmic insulating localization correction to the metallic temperature dependence in the 2D conductivity so as to distinguish between the 2D MIT being a true quantum phase transition versus being a finite-temperature crossover. We use the Boltzmann theory of resistivity including the temperature dependent screening effect on charged impurities in the system to fit the data. We analyze weak perpendicluar field magnetoresistance data taken in the vicinity of the transition and show that they are consistent with weak localization behavior in the strongly disordered regime $k_F\ell\gtrsim1$. Therefore we supplement the Botzmann transport theory with a logarithmic in temperature quantum weak localization correction and analyze the competition of the insulating temperature dependence of this correction with the metallic temperature dependence of the Boltzmann conductivity. Using this minimal theoretical model we find that the logarithmic insulating correction is masked by the metallic temperature dependence of the Botzmann resistivity and therefore the insulating $\log T$ behavior may be apparent only at very low temperatures which are often beyond the range of temperatures accessible experimentally. Analyzing the low-$T$ experimental Si MOSFET transport data we identify signatures of the putative insulating behavior at low temperature and density in the effective metallic phase.

cond-mat.mes-hall↗

arXiv2014

Effective field theory, three-loop perturbative expansion, and their experimental implications in graphene many-body effects

Many-body electron-electron interaction effects are theoretically considered in monolayer graphene from a continuum effective field-theoretic perspective by going beyond the standard leading-order perturbative renormalization group (RG) analysis. Given that the bare fine structure constant in graphene is of order unity, which is neither small to justify a perturbative expansion nor large enough for strong-coupling theories to be applicable, the problem is a difficult one, with some similarity to 2+1-dimensional strong-coupling quantum electrodynamics (QED). In this work, we take a systematic and comprehensive analytical approach, working primarily at the Dirac point (intrinsic graphene), by going up to three loops in the diagrammatic expansion to both ascertain the validity of perturbation theory and to estimate quantitatively higher-order renormalization effects. While no direct signatures for non-Fermi liquid behavior at the Dirac point have yet been observed experimentally, there is ample evidence for the interaction-induced renormalization of the graphene velocity as the carrier density approaches zero. We provide a critical comparison between theory and experiment, using both bare- and screened-interaction (RPA) calculations. We find that while the one-loop RG analysis gives reasonable agreement with the experimental data, especially when screening and finite-density effects are included through the RPA, the two-loop analysis reveals a strong-coupling critical point in suspended graphene, signifying either a quantum phase transition or a breakdown of the weak-coupling RG approach. We show that the latter is more likely by adapting Dyson's argument for the breakdown of perturbative QED to the case of graphene. We propose future experiments and theoretical directions to make further progress on this important and difficult problem.

cond-mat.mes-hall↗

arXiv2014

Short-range disorder effects on electronic transport in 2D semiconductor structures

We study theoretically the relative importance of short-range disorder in determining the low-temperature 2D mobility in GaAs-based structures with respect to Coulomb disorder which is known to be the dominant disorder in semiconductor systems. We give results for unscreened and screened short-range disorder effects on 2D mobility in quantum wells and heterostructures, comparing with the results for Coulomb disorder and finding that the asymptotic high-density mobility is always limited by short-range disorder which, in general, becomes effectively stronger with increasing `carrier density' in contrast to Coulomb disorder. We also predict an intriguing re-entrant metal-insulator transition at very high carrier densities in Si-MOSFETs driven by the short-range disorder associated with surface roughness scattering.

cond-mat.mes-hall↗

arXiv2014

Mobility versus quality in 2D semiconductor structures

We consider theoretically effects of random charged impurity disorder on the {\it quality} of high-mobility two dimensional (2D) semiconductor structures, explicitly demonstrating that the sample mobility is not necessarily a reliable or universal indicator of the sample quality in high-mobility modulation-doped 2D GaAs structures because, depending on the specific system property of interest, mobility and quality may be controlled by different aspects of the underlying disorder distribution, particularly since these systems are dominated by long-range Coulomb disorder from both near and far random quenched charged impurities. We show that in the presence of both channel and remote charged impurity scattering, which is a generic situation in modulation-doped high-mobility 2D carrier systems, it is quite possible for higher (lower) mobility structures to have lower (higher) quality as measured by the disorder-induced single-particle level broadening. In particular, we establish that there is no reason to expect a unique relationship between mobility and quality in 2D semiconductor structures as both are independent functionals of the disorder distribution, and are therefore, in principle, independent of each other. Using a simple, but reasonably realistic, ``2-impurity'' minimal model of the disorder distribution, we provide concrete examples of situations where higher (lower) mobilities correspond to lower (higher) sample qualities. We discuss experimental implications of our theoretical results and comment on possible strategies for future improvement of 2D sample quality.

cond-mat.mes-hall↗

arXiv2013

Two-dimensional metal-insulator-transition as a potential fluctuation driven semiclassical transport phenomenon

We theoretically consider the carrier density tuned (apparent) two-dimensional (2D) metal-insulator-transition (MIT) in semiconductor heterostructure-based 2D carrier systems as arising from a classical percolation phenomenon in the inhomogeneous density landscape created by the long-range potential fluctuations induced by random quenched charged impurities in the environment. The long-range Coulomb disorder inherent in semiconductors produces strong potential fluctuations in the 2D system where a fraction of the carriers gets trapped or classically localized, leading to a mixed 2-component semiclassical transport behavior at intermediate densities where a fraction of the carriers is mobile and another fraction immobile. At high carrier density, all the carriers are essentially mobile whereas at low carrier density all the carriers are essentially trapped since there is no possible percolating transport path through the lake-and-mountain inhomogeneous potential landscape. The low-density situation with no percolation would mimic an insulator whereas the high-density situation with allowed percolating paths through the lake-and-mountain energy landscape would mimic a metal with the system manifesting an apparent MIT in between. We calculate the transport properties as a function of carrier density, impurity density, impurity location, and temperature using a 2-component (trapped and mobile carriers) effective medium theory. Our theoretically calculated transport properties are in good qualitative agreement with the experimentally observed 2D MIT phenomenology in 2D electron and hole systems. We find a high (low) density metallic (insulating) temperature-dependence of the 2D resistivity, and an intermediate-density crossover behavior which could be identified with the experimentally observed 2D MIT.

cond-mat.mes-hall↗

arXiv2013

Universal density scaling of disorder-limited low-temperature conductivity in high-mobility two-dimensional systems

We theoretically consider the carrier density dependence of low-temperature electrical conductivity in high-quality and low-disorder two-dimensional (2D) `metallic' electronic systems such as 2D GaAs electron or hole quantum wells or gated graphene. Taking into account resistive scattering by Coulomb disorder arising from quenched random charged impurities in the environment, we show that the 2D conductivity σ(n) varies as σ~ n^{β(n)} as a function of the 2D carrier density n where the exponent β(n) is a smooth, but non-monotonic, function of density with possible nontrivial extrema. In particular, the density scaling exponent β(n) depends qualitatively on whether the Coulomb disorder arises primarily from remote or background charged impurities or short-range disorder, and can, in principle, be used to characterize the nature of the dominant background disorder. A specific important prediction of the theory is that for resistive scattering by remote charged impurities, the exponent βcan reach a value as large as 2.7 for k_F d ~ 1, where k_F ~\sqrt{n} is the 2D Fermi wave vector and d is the separation of the remote impurities from the 2D layer. Such an exponent β(>5/2) is surprising because unscreened Coulomb scattering by remote impurities gives a limiting theoretical scaling exponent of β= 5/2, and naively one would expect β(n) \le 5/2 for all densities since unscreened Coulomb scattering should nominally be the situation bounding the resistive scattering from above. We find numerically and show theoretically that the maximum value of α(β), the mobility (conductivity) exponent, for 2D semiconductor quantum wells is around 1.7 (2.7) for all values of d (and for both electrons and holes) with the maximum αoccurring around k_F d ~ 1. We discuss experimental scenarios for the verification of our theory.

cond-mat.mes-hall↗

arXiv2013

Velocity renormalization and anomalous quasiparticle dispersion in extrinsic graphene

Using many-body diagrammatic perturbation theory we consider carrier density- and substrate-dependent many-body renormalization of doped or gated graphene induced by Coulombic electron-electron interaction effects. We quantitatively calculate the many-body spectral function, the renormalized quasiparticle energy dispersion, and the renormalized graphene velocity using the leading-order self-energy in the dynamically screened Coulomb interaction within the ring diagram approximation. We predict experimentally detectable many-body signatures, which are enhanced as the carrier density and the substrate dielectric constant are reduced, finding an intriguing instability in the graphene excitation spectrum at low wave vectors where interaction completely destroys all particle-like features of the noninteracting linear dispersion. We also make experimentally relevant quantitative predictions about the carrier density and wave-vector dependence of graphene velocity renormalization induced by electron-electron interaction. We compare on-shell and off-shell self-energy approximations within the ring diagram approximation, finding a substantial quantitative difference between their predicted velocity renormalization corrections in spite of the generally weak-coupling nature of interaction in graphene.

cond-mat.mes-hall↗

arXiv2013

Surface polar optical phonon interaction induced many-body effects and hot-electron relaxation in graphene

We theoretically study various aspects of the electron-surface optical phonon interaction effects in graphene on a substrate made of polar materials. We calculate the electron self-energy in the presence of the surface phonon-mediated electron-electron interaction focusing on how the linear chiral graphene dispersion is renormalized by the surface phonons. The electron self-energy as well as the quasiparticle spectral function in graphene are calculated, taking into account electron-polar optical phonon interaction by using a many body perturbative formalism. The scattering rate of free electrons due to polar interaction with surface optical phonons in a dielectric substrate is calculated as a function of the electron energy, temperatures, and carrier density. Effects of screening on the self-energy and scattering rate are discussed. Our theory provides a comprehensive quantitative (and qualitative) picture for surface phonon interaction induced many-body effects and hot electron relaxation in Dirac materials.

cond-mat.mes-hall↗

arXiv2013

Density dependent electrical conductivity in suspended graphene: Approaching the Dirac point in transport

We theoretically consider, comparing with the existing experimental literature, the electrical conductivity of gated monolayer graphene as a function of carrier density, temperature, and disorder in order to assess the prospects of accessing the Dirac point using transport studies in high-quality suspended graphene. We show that the temperature dependence of graphene conductivity around the charge neutrality point provides information about how close the system can approach the Dirac point although competition between long-range and short-range disorder as well as between diffusive and ballistic transport may considerably complicate the picture. We also find that acoustic phonon scattering contribution to the graphene resistivity is always relevant at the Dirac point in contrast to higher density situations where the acoustic phonon contribution to the resistivity is strongly suppressed at the low temperature Bloch-Grüneisen regime. We provide detailed numerical results for temperature and density dependent conductivity for suspended graphene.

cond-mat.mes-hall↗

arXiv2012

Gate tunable quantum transport in double layer graphene

We analyze the effect of screening provided by the additional graphene layer in double layer graphene heterostructures (DLGs) on transport characteristics of DLG devices in the metallic regime. The effect of gate-tunable charge density in the additional layer is two-fold: it provides screening of the long-range potential of charged defects in the system, and screens out Coulomb interactions between charge carriers. We find that the efficiency of defect charge screening is strongly dependent on the concentration and location of defects within the DLG. In particular, only a moderate suppression of electron-hole puddles around the Dirac point induced by the high concentration of remote impurities in the silicon oxide substrate could be achieved. A stronger effect is found on the elastic relaxation rate due to charged defects resulting in mobility strongly dependent on the electron denisty in the additional layer of DLG. We find that the quantum interference correction to the resistivity of graphene is also strongly affected by screening in DLG. In particular, the dephasing rate is strongly suppressed by the additional screening that supresses the amplitude of electron-electron interaction and reduces the diffusion time that electrons spend in proximity of each other. The latter effect combined with screening of elastic relaxation rates results in a peculiar gate tunable weak-localization magnetoresistance and quantum correction to resistivity. We propose suitable experiments to test our theory and discuss the possible relevance of our results to exisiting data.

cond-mat.mes-hall↗

arXiv2012

Electronic transport in two dimensional Si:P $δ$-doped layers

We investigate theoretically 2D electronic transport in Si:P $δ$-doped layers limited by charged-dopant scattering. Since the carrier density is approximately equal to the dopant impurity density, the density dependent transport shows qualitatively different behavior from that of the well-studied 2D Si-MOSFETs where the carrier density is independent of the impurity density. We find that the density dependent mobility of the Si:P system shows non-monotonic behavior which is exactly opposite of the non-monotonicity observed in Si-MOSFETs --- in the Si:P system the mobility first decreases with increasing density and then it increases slowly with increasing density above a typical density $10^{14}$ cm$^{-2}$ (in contrast to Si MOSFETs where the mobility typically increases with density first and then slowly decreases at high density as surface roughness scattering dominates). In the low density limit (or strong screening limit) mobility decreases inversely with increasing density, but in the high density limit (or weak screening limit) it slowly increases due to the finite width effects of the 2D layer. In the intermediate density regime ($1/a < 2 k_F < q_{TF}$, where $a$, $k_F$, and $q_{TF}$ are the confinement width of the $δ$-layer, Fermi wave vector, and Thomas-Fermi screening wave vector, respectively) the density dependent mobility is approximately a constant at the minimum value. However, the calculated mean free path increase monotonically with density. We also compare the transport scattering time relevant to the mobility and the single particle relaxation time relevant the quantum level broadening, finding that the transport scattering time could be much larger than the single-particle scattering time unlike in Si MOSFETs where they are approximately equal.

cond-mat.mes-hall↗

arXiv2012

Valley-dependent 2D transport in Si-MOSFETs

Motivated by interesting recent experimental results, we consider theoretically charged-impurity scattering-limited 2D electronic transport in (100), (110), and (111)-Si inversion layers at low temperatures and carrier densities, where screening effects are important. We show conclusively that, given the same bare Coulomb disorder, the 2D mobility for a given system increases monotonically with increasing valley degeneracy. We also show that the temperature and the parallel magnetic field dependence of the 2D conductivity is strongly enhanced by increasing valley degeneracy. We analytically consider the low temperature limit of 2D transport, particularly its theoretical dependence on valley degeneracy, comparing with our full numerical results and with the available experimental results. We make qualitative and quantitative predictions for the parallel magnetic field induced 2D magnetoresistance in recently fabricated high-mobility 6-valley Si(111)-on-vacuum inversion layers. We also provide a theory for 2D transport in ultrahigh mobility Si(111) structures recently fabricated in the laboratory, discussing the possibility of observing the fractional quantum Hall effect in such Si(111) structures.

cond-mat.mes-hall↗

arXiv2012

Polarizability and Screening in Chiral Multilayer Graphene

We calculate the static polarizability of multilayer graphene and study the effect of stacking arrangement, carrier density, and onsite energy difference on graphene screening properties. At low densities, the energy spectrum of multilayer graphene is described by a set of chiral two-dimensional electron systems and the associated chiral nature determines the screening properties of multilayer graphene showing very different behavior depending on whether the chirality index is even or odd. As density increases, the energy spectrum follows that of the monolayer graphene and thus the polarizability approaches that of monolayer graphene. The qualitative dependence of graphene polarizability on chirality and layering indicates the possibility of distinct graphene quantum phases as a function of the chirality index.

cond-mat.mtrl-sci↗

arXiv2012

Disorder by order in graphene

We predict the existence of an intriguing "disorder by order" phenomenon in graphene transport where higher quality (and thus more ordered) samples, while having higher mobility at high carrier density, will manifest more strongly insulating (and thus effectively more disordered) behavior as the carrier density is lowered compared with lower quality samples (with higher disorder) which exhibit an approximate resistivity saturation phenomenon at low carrier density near the Dirac point. This predicted behavior simulating a metal-insulator transition, which we believe to have recently been observed in an experiment at Manchester University [L. A. Ponomarenko et al., Nat. Phys. 7, 958 (2011)], arises from the suppression of Coulomb disorder induced inhomogeneous puddles near the charge neutrality point in high quality graphene samples.

cond-mat.mes-hall↗

arXiv2012

Interplay between phonon and impurity scattering in 2D hole transport

We investigate temperature dependent transport properties of two-dimensional p-GaAs systems taking into account both hole-phonon and hole-impurity scattering effects. By analyzing the hole mobility data of p-GaAs in the temperature range 10 K$<T<$100 K, we estimate the value of the appropriate deformation potential for hole-phonon coupling. Due to the interplay between hole-phonon and hole-impurity scattering the calculated temperature-dependent resistivity shows interesting nonmonotonic behavior. In particular, we find that there is a temperature range (typically 2 K$<T<$10 K) in which the calculated resistivity becomes independent of temperature due to a subtle cancellation between the temperature dependent resistive scattering contributions arising from impurities and phonons. This resistivity saturation regime appears at low carrier densities when the increasing resistivity due to phonon scattering compensates for the decreasing resistivity due to the nondegeneracy effect. This temperature-independent flat resistivity regime is experimentally accessible and may have already been observed in a recent experiment.

cond-mat.mes-hall↗