arXiv subjects

E. H. Hwang

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

At least 19 recordsLinked to original sources

arXiv2026

Commensuration torques in double-moiré twisted trilayer hexagonal boron nitride and graphene heterostructures

We study commensuration-driven torques and angle locking in double-moiré trilayer hexagonal boron nitride (hBN) and graphene heterostructures using large-scale atomistic relaxations. In twisted trilayer hBN (t3BN) homostructures, double-moiré commensuration ($θ_{12} = -θ_{23}$) give rise to local energy minima accompanied by torque sign reversals, signaling a restoring tendency toward the commensurate configuration. The corresponding binding energies are $\sim$0.2-0.3 meV/atom, originating from enhanced overlap of low-energy stacking domains, although the system is globally stable at zero twist. In contrast, in graphene/hBN heterolayers systems the global energy minimum can coincide with the double-moiré commensuration angle, particularly near $\sim$0.6$^{\circ}$, reflecting competition between lattice mismatch and interfacial relaxation. Incommensurate atomic structures have reduced stabilization due to suppressed overlap of low-energy stacking and have enhanced superlubricity due to spatial averaging of interfacial energies. These results establish double-moiré commensuration as a general, system-dependent mechanism for twist-angle stabilization, whose angular stability is characterized by the torque magnitude and binding energy. Coulomb electrostatic interactions further enhance the stabilization energy without changing the underlying physics.

cond-mat.mes-hall↗

arXiv2026

Magnetoplasmons in $N$-layer structures

We provide a systematic framework to investigate the magnetoplasmons of multilayer two-dimensional electron systems by using the Kac--Murdock--Szegő (KMS) Toeplitz matrix to consider interlayer Coulomb interactions. In the absence of interlayer tunneling, we show that the single-layer magnetoplasmon branch splits into $N$ collective modes -- one in-phase mode and $N-1$ out-of-phase modes -- and derive their asymptotic behaviors in the long-wavelength limit, as well as in the limit of large layer separation and strong magnetic fields. When interlayer tunneling is present, we clarify the magnetoplasmon dispersion, both qualitatively and quantitatively, by identifying the magnetoplasmon mode associated with each interband transition, as well as tunneling magnetoplasmons arising from interband transitions with the same Landau level index. Our study presents the hybridization between the modes governed by underlying symmetries, along with an enhanced tunneling magnetoplasmon gap exceeding the associated interband gap. The KMS-based analytic formalism thus provides a comprehensive physical understanding of magnetoplasmons in multilayer structures.

cond-mat.mes-hall↗

arXiv2025

Plasmons in N-layer systems

In multilayer structures, the coupling between layers gives rise to unique plasmon modes, but analytic solutions are typically available only for bilayers due to the increasing complexity as the number of layers increases. We investigate plasmons in multilayer structures, including the effects of interlayer tunneling. By introducing the Coulomb eigenvector basis for multilayer systems, which can be solved exactly using Kac-Murdock-Szegő Toeplitz matrices, we analytically derive the long-wavelength plasmon dispersions both with and without interlayer tunneling. In the $N$-layer systems, we find that, in the absence of interlayer tunneling, the out-of-phase acoustic or charge neutral plasmon modes with linear dispersions ($ω_α\propto q/\sqrt{{1-\cos{\left(\frac{α-1}{N}π\right)}}}$ for $α= 2, 3, \cdots, N$) exist, while the in-phase classical plasmon mode exhibits its conventional dispersion ($ω_1\propto \sqrt{q}$). When interlayer tunneling is present, the out-of-phase modes develop plasmon gaps that are governed by specific interband transitions, whereas the classical mode remains unaffected. These findings have broad applicability to general coupled-layer structures.

cond-mat.mes-hall↗

arXiv2022

Diffusive density response of electrons in anisotropic multiband systems

We explicitly calculate the density-density response function with conserving vertex corrections for anisotropic multiband systems in the presence of impurities including long-range disorder. The direction-dependence of the vertex corrections is correctly considered to obtain the diffusion constant which is given by the combination of the componentwise transport relaxation times and velocities on the Fermi surface. We also investigate the diffusive density response of various anisotropic systems, propose some empirical rules for the corresponding diffusion constant, and demonstrate that it is crucial to consider the component-dependence of the transport relaxation times to correctly interpret the transport properties of anisotropic systems, especially various topological materials with a different power-law dispersion in each direction.

cond-mat.mes-hall↗

arXiv2020

Impurity scattering induced carrier transport in twisted bilayer graphene

We theoretically calculate the impurity-scattering induced resistivity of twisted bilayer graphene at low twist angles where the graphene Fermi velocity is strongly suppressed. We consider, as a function of carrier density, twist angle, and temperature, both long-ranged Coulomb scattering and short-ranged defect scattering within a Boltzmann theory relaxation time approach. For experimentally relevant disorder, impurity scattering contributes a resistivity comparable to (much larger than) the phonon scattering contribution at high (low) temperatures. Decreasing twist angle leads to larger resistivity, and in general, the resistivity increases (decreases) with increasing temperature (carrier density). Inclusion of the van Hove singularity in the theory leads to a strong increase in the resistivity at higher densities, where the chemical potential is close to a van Hove singularity, leading to an apparent density-dependent plateau type structure in the resistivity, which has been observed in recent transport experiments. We also show that the Matthissen's rule is strongly violated in twisted bilayer graphene at low twist angles.

cond-mat.mes-hall↗

arXiv2019

Infrared spectroscopic study of carrier scattering in gated CVD graphene

We measured Drude absorption of gated CVD graphene using far-infrared transmission spectroscopy, and determined carrier scattering rate (g) as function of the varied carrier density (n). The n-dependent g(n) was obtained for a series of conditions systematically changed as (10 K, vacuum) -> (300 K, vacuum) -> (300 K, ambient pressure), which reveals that (1) at low-T, charged impurity (=A/sqrt(n)) and short-range defect (=B*sqrt(n)) are the major scattering sources which constitute the total scattering g=A/sqrt(n)+B*sqrt(n), (2) among various kinds of phonons populated at room-T, surface polar phonon of the SiO2 substrate is the dominantly scattering source, (3) in air, the gas molecules adsorbed on graphene play a dual role in carrier scattering as charged impurity center and resonant scattering center. We present the absolute scattering strengths of those individual scattering sources, which provides the complete map of scattering mechanism of CVD graphene for the first time. This scattering map allows us find out practical measures to suppress the individual scatterings, the mobility gains accompanied by them, and finally the ultimate attainable carrier mobility for CVD graphene.

cond-mat.mtrl-sci↗

arXiv2019

Linear-in-T resistivity in dilute metals: A Fermi liquid perspective

We consider a short-range deformation potential scattering model of electron-acoustic phonon interaction to calculate the resistivity of an ideal metal as a function of temperature (T) and electron density (n). We consider both 3D metals and 2D metals, and focus on the dilute limit, i.e., low effective metallic carrier density of the system. The main findings are: (1) a phonon scattering induced linear-in-T resistivity could persist to arbitrarily low T in the dilute limit independent of the Debye temperature ($T_D$) although eventually the low-T resistivity turns over to the expected Bloch-Gruneisen (BG) behavior with $T^5$ ($T^4$) dependence, in 3D (2D) respectively; (2) because of low values of n, the phonon-induced resistivity could be very high in the system; (3) the resistivity shows an intrinsic saturation effect at very high temperatures (for $T>T_D$), and in fact, decreases with increasing T above a high crossover temperature with this crossover being dependent on both $T_D$ and n in a non-universal manner. We also provide high-T linear-in-T resistivity results for 2D and 3D Dirac materials. Our work brings out the universal features of phonon-induced transport in dilute metals, and we comment on possible implications of our results for strange metals, emphasizing that the mere observation of a linear-in-T metallic resistivity at low temperatures or a very high metallic resistivity at high temperatures is not necessarily a reason to invoke an underlying quantum critical strange metal behavior. We discuss the temperature variation of the effective transport scattering rate showing that the scattering rate could be below or above $k_BT$, and in particular, purely coincidentally, the calculated scattering rate happens to be $k_BT$ in normal metals with no implications for the so-called Planckian behavior.

cond-mat.str-el↗

arXiv2018

Plasmon-pole approximation for many-body-effects in extrinsic graphene

We develop the plasmon-pole approximation (PPA) theory for calculating the carrier self-energy of extrinsic graphene as a function of doping density within analytical approximations to the $GW$ random phase approximation ($GW$-RPA). Our calculated self-energy shows excellent quantitative agreement with the corresponding full $GW$-RPA calculation results in spite of the simplicity of the PPA, establishing the general validity of the plasmon-pole approximation scheme. We also provide a comparison between the PPA and the hydrodynamic approximation in graphene, and comment on the experimental implications of our findings.

cond-mat.mes-hall↗

arXiv2018

Diluted magnetic Dirac-Weyl materials: Susceptibility and ferromagnetism in three-dimensional chiral gapless semimetals

We theoretically investigate the temperature-dependent static susceptibility and long-range magnetic coupling of three-dimensional (3D) chiral gapless electron-hole systems (semimetals) with arbitrary band dispersion [i.e., $\varepsilon(k) \sim k^N$, where $k$ is the wave vector and $N$ is a positive integer]. We study the magnetic properties of these systems in the presence of dilute random magnetic impurities. Assuming carrier-mediated Ruderman-Kittel-Kasuya-Yosida indirect exchange interaction, we find that the magnetic ordering of intrinsic 3D chiral semimetals in the presence of dilute magnetic impurities is ferromagnetic for all values of $N$. Using finite-temperature self-consistent field approximation, we calculate the ferromagnetic transition temperature ($T_{\rm c}$). We find that $T_{\rm c}$ increases with increasing $N$ due to the enhanced density of states, and the calculated $T_{\rm c}$ is experimentally accessible assuming reasonable coupling between the magnetic impurities and itinerant carriers.

cond-mat.mes-hall↗

arXiv2018

Dimensionally mixed coupled collective modes

We develop the diagrammatic formulation of the many-body theory for the coupled collective modes in interacting electron systems of different dimensions. The formalism is then applied in detail to a two-dimensional system coupled to a three-dimensional electron gas. We find two dimensionally-mixed plasmon modes which in the long wavelength limit are respectively 3D-like and acoustic in nature, but are strongly coupled at larger wave vectors. The same formalism can be applied to any dimensional combinations, and we also present the results for 1D-2D and 1D-3D coupled systems.

cond-mat.str-el↗

arXiv2017

Large negative transconductance in multilayer graphene: The role of intersubband scattering

We calculate the transport properties of multilayer graphene, considering the effect of multisubband scattering in a high density regime, where higher subbands are occupied by charge carriers. To calculate the conductivity of multilayer graphene, we use the coupled multiband Boltzmann transport theory while fully incorporating the multiband scattering effects. We show that the allowed scattering channels, screening effects, chiral nature of the electronic structure, and type of impurity scatterings determine the transport behavior of multilayer graphene. We find that the conductivity of multilayer graphene shows a sudden change when the carriers begin to occupy the higher subbands, and therefore a large negative transconductance (NTC) appears as the carrier density varies. These phenomena arise mostly from the intersubband scattering and the change in the density of states at the band touching density. Based on our results, it is possible to build novel devices utilizing the large NTC in multilayer graphene.

cond-mat.mes-hall↗

arXiv2017

Ferromagnetism in chiral multilayer 2D semimetals

We calculate the temperature dependent long-range magnetic coupling in the presence of dilute concentrations of random magnetic impurities in chiral multilayer two-dimensional semimetals, i.e., undoped intrinsic multilayer graphene. Assuming a carrier-mediated indirect RKKY exchange interaction among the well-separated magnetic impurities with the itinerant carriers mediating the magnetic interaction between the impurities, we investigate the magnetic properties of intrinsic multilayer graphene using an effective chiral Hamiltonian model. We find that due to the enhanced density of states in the rhombohedral stacking sequence of graphene layers, the magnetic ordering of multilayer graphene is ferromagnetic in the continuum limit. The ferromagnetic transition temperature is calculated using a finite-temperature self-consistent field approximation and found to be within the experimentally accessible range for reasonable values of the impurity-carrier coupling.

cond-mat.mes-hall↗

arXiv2016

Collective modes in multi-Weyl semimetals

We investigate collective modes in three dimensional (3D) gapless multi-Weyl semimetals with anisotropic energy band dispersions (i.e., $E\sim \sqrt{ k_{\parallel}^{2J} + k_z^2}$, where $k_{\parallel}$ and $k_z$ are wave vectors and $J$ is a positive integer). For comparison, we also consider the gapless semimetals with the isotropic band dispersions (i.e., $E\sim k^J$). We calculate analytically long-wavelength plasma frequencies incorporating interband transitions and chiral properties of carriers. For both the isotropic and anisotropic cases, we find that interband transitions and chirality lead to the depolarization shift of plasma frequencies. For the isotropic parabolic band dispersion (i.e., $N=2$, $E\sim k^2$), the long-wavelength plasma frequencies lie outside the single particle excitation regions for all carrier densities, and thus the plasmons do not decay via Landau damping. For the higher-order band dispersions ($N \ge 3$) the long-wavelength plasmons experience damping below a critical density. For systems with the anisotropic dispersion the density dependence of the long-wavelength plasma frequency along the direction of non-linear dispersion behaves like that of the isotropic linear band model ($N=1$), while along the direction of linear dispersion it behaves like that of the isotropic non-linear model ($N \ge 2$). Plasmons along both directions remain undamped over a broad range of densities due to the chirality induced depolarization shift. Our results provide a comprehensive picture of how band dispersion and chirality affect plasmon behaviors in 3D gapless chiral systems with the arbitrary band dispersion.

cond-mat.mes-hall↗

arXiv2016

Stacking dependence of carrier-interactions in multilayer graphene systems

We identify qualitative trends in the stacking sequence dependence of carrier-carrier interaction phenomena in multilayer graphene. Our theory is based on a new approach which explicitly exhibits the important role in interaction phenomena of the momentum-direction dependent intersite phases determined by the stacking sequence. Using this method, we calculate and compare the self-energies, density--density response functions, collective modes, and ground-state energies of several different few layer graphene systems. The influence of electron--electron interactions on important electronic properties can be understood in terms of competition between intraband exchange, interband exchange and correlation contributions that vary systematically with stacking arrangement.

cond-mat.mes-hall↗

arXiv2015

Screening and transport in 2D semiconductor systems at low temperatures

Low temperature carrier transport properties in two-dimensional (2D) semiconductor systems can be theoretically well-understood within a mean-field type RPA-Boltzmann theory as being limited by scattering from screened Coulomb disorder arising from random quenched charged impurities in the environment. In the current work, we derive a number of simple analytical formula, supported by realistic numerical calculations, for the relevant density, mobility, and temperature range where 2D transport should manifest strong intrinsic (i.e., arising purely from electronic effects and not from phonon scattering) metallic temperature dependence in different semiconductor materials arising entirely from the 2D screening properties, thus providing an explanation for why the strong temperature dependence of the 2D resistivity can only be observed in high-quality and low-disorder (i.e., high-mobility) 2D samples and also why some high-quality 2D materials (i.e., n-GaAs) manifest much weaker metallicity than other materials. We also discuss effects of interaction and disorder on the 2D screening properties in this context as well as compare 2D and 3D screening functions to comment why such a strong intrinsic temperature dependence arising from screening cannot occur in 3D metallic carrier transport. Experimentally verifiable predictions are made about the quantitative magnitude of the maximum possible low-temperature metallicity in 2D systems and the scaling behavior of the temperature scale controlling the quantum to classical crossover where the system reverses the sign of the temperature derivative of the 2D resistivity at high temperatures.

cond-mat.mes-hall↗

arXiv2015

The 2D metal-insulator transition as a strong localization induced crossover phenomenon

Low-disorder and high-mobility 2D electron (or hole) systems undergo an apparent metal-insulator-transition (MIT) at low temperatures as the carrier density (n) is varied. In some situations, the 2D MIT can be caused at a fixed low carrier density by changing an externally applied in-plane magnetic field parallel to the 2D layer. dρ/dT changes its sign at some nonuniversal sample-dependent critical carrier density n_c separating an effective 2D metal (dρ/dT >0) for n>n_c from an effective 2D insulator (dρ/dT<0) for n<n_c. We study the 2D MIT phenomenon as a possible strong localization induced crossover process controlled by the Ioffe-Regel criterion, k_F l=1. Calculating the quantum mean free path (l) in the effective metallic phase from a realistic transport theory including disorder scattering effects, we solve the integral equation defined by the Ioffe-Regel criterion to obtain the nonuniversal critical density n_c as a function of the applicable physical experimental parameters including disorder strength, in-plane magnetic field, spin and valley degeneracy, background dielectric constant and carrier effective mass, and temperature. The key physics underlying the nonuniversal parameter dependence of the n_c is the temperature and density dependence of the Coulomb disorder. Our calculated results for the crossover n_c appear to be in qualitative and semi-quantitative agreement with the available experimental data in different 2D semiconductor systems lending credence to the possibility that the apparent 2D MIT signals the onset of the strong localization crossover in disordered 2D systems. We provide some results for graphene where a low-temperature 2D MIT becomes possible in the presence of intervalley scattering. We also provide an extensive comparison with the theoretical results obtained on the basis of 2D MIT being considered as a percolation transition.

cond-mat.mes-hall↗

arXiv2015

Strongly metallic electron and hole 2D transport in an ambipolar Si-vacuum field effect transistor

We report experiment and theory on an ambipolar gate-controlled Si-vacuum field effect transistor (FET) where we study electron and hole (low-temperature 2D) transport in the same device simply by changing the external gate voltage to tune the system from being a 2D electron system at positive gate voltage to a 2D hole system at negative gate voltage. The electron (hole) conductivity manifests strong (moderate) metallic temperature dependence with the conductivity decreasing by a factor of 8 (2) between 0.3 K and 4.2 K with the peak electron mobility ($\sim 18$ m$^2$/Vs) being roughly 20 times larger than the peak hole mobility (in the same sample). Our theory explains the data well using RPA screening of background Coulomb disorder, establishing that the observed metallicity is a direct consequence of the strong temperature dependence of the effective screened disorder.

cond-mat.mes-hall↗

arXiv2015

Charge transport in gapless chiral electron systems with arbitrary band dispersion

Using the semiclassical Boltzmann transport theory, we analytically consider dc charge transport in gapless electron-hole (both chiral and non-chiral) systems in the presence of resistive scattering due to static disorder arising from random quenched impurities in the background. We obtain the dependence of the Boltzmann conductivity on carrier density and temperature for arbitrary band dispersion in arbitrary dimensionality assuming long-range ($\sim 1/r$) Coulomb disorder and zero-range white noise disorder [$\sim δ(r)$]. We establish that the temperature and the density dependence of the Boltzmann conductivity manifests scaling behaviors determining respectively the intrinsic semimetallic or the extrinsic metallic property of the gapless system. Our results apply equally well to both chiral and non-chiral gapless systems, and provide a qualitative understanding of the dependence of the Boltzmann conductivity on the band dispersion in arbitrary dimensionality.

cond-mat.dis-nn↗