arXiv subjects

Wang-Kong Tse

Publications and source records attributed to Wang-Kong Tse.

49 records · Page 3Linked to original sources

arXiv2009

Drude-Interband Coupling, Screening, and the Optical Conductivity of Doped Bilayer Graphene

We present a theory of the influence of band renormalization and excitonic electron-electron interaction effects on the optical conductivity $σ(ω)$ of doped bilayer graphene. Using the Keldysh formalism, we derive a kinetic equation from which we extract numerical and approximate analytic results for $σ(ω)$. Our calculations reveal a previously unrecognized mechanism which couples the Drude and interband response and renormalizes the plasmon frequency, and suggest that screening must play an essential role in explaining the weakly renormalized conductivity seen in recent experiments.

cond-mat.mes-hall↗

arXiv2008

Energy Relaxation of Hot Dirac Fermions in Graphene

We develop a theory for the energy relaxation of hot Dirac fermions in graphene. We obtain a generic expression for the energy relaxation rate due to electron-phonon interaction and calculate the power loss due to both optical and acoustic phonon emission as a function of electron temperature $T_{\mathrm{e}}$ and density $n$. We find an intrinsic power loss weakly dependent on carrier density and non-vanishing at the Dirac point $n = 0$, originating from interband electron-optical phonon scattering by the intrinsic electrons in the graphene valence band. We obtain the total power loss per carrier $\sim 10^{-12} - 10^{-7} \mathrm{W}$ within the range of electron temperatures $\sim 20 - 1000 \mathrm{K}$. We find optical (acoustic) phonon emission to dominate the energy loss for $T_{\mathrm{e}} > (<) 200-300 \mathrm{K}$ in the density range $n = 10^{11}-10^{13} \mathrm{cm}^{-2}$.

cond-mat.mes-hall↗

arXiv2008

Chirality-induced Dynamic Kohn Anomalies in Graphene

We develop a theory for the renormalization of the phonon energy dispersion in graphene due to the combined effects of both Coulomb and electron-phonon (e-ph) interactions. We obtain the renormalized phonon energy spectrum by an exact analytic derivation of the phonon self-energy, finding three distinct Kohn anomalies (KAs) at the phonon wavevector $q = ω/v, 2k_F\pmω/v$ for LO phonons and one at $q = ω/v$ for TO phonons. The presence of these new KAs in graphene, in contrast to the usual KA $q = 2k_F$ in ordinary metals, originates from the dynamical screening of e-ph interaction (with a concomitant breakdown of the Born-Oppenheimer approximation) and the peculiar chirality of the graphene e-ph coupling.

cond-mat.mtrl-sci↗

arXiv2008

Ballistic Hot Electron Transport in Graphene

We theoretically study the inelastic scattering rate and the carrier mean free path for energetic hot electrons in graphene, including both electron-electron and electron-phonon interactions. Taking account of optical phonon emission and electron-electron scattering, we find that the inelastic scattering time $τ\sim 10^{-2}-10^{-1} \mathrm{ps}$ and the mean free path $l \sim 10-10^2 \mathrm{nm}$ for electron densities $n = 10^{12}-10^{13} \mathrm{cm}^{-2}$. In particular, we find that the mean free path exhibits a finite jump at the phonon energy $200 \mathrm{meV}$ due to electron-phonon interaction. Our results are directly applicable to device structures where ballistic transport is relevant with inelastic scattering dominating over elastic scattering.

cond-mat.mes-hall↗

arXiv2007

Phonon-induced many-body renormalization of graphene electronic properties

We develop a theory for the electron-phonon interaction effects on the electronic properties of graphene. We analytically calculate the electron self-energy, spectral function and band velocity renormalization due to phonon-mediated electron-electron interaction. We find that phonon-mediated electron-electron coupling has a large effect on the graphene band structure renormalization, and our analytic theory successfully captures the essential features of the observed graphene electron spectra in the ARPES experiments, predicting a kink at $\sim 200\mathrm{meV}$ below the Fermi level and a reduction of the band velocity by $\sim 10-20%$ at the experimental doping level.

cond-mat.mtrl-sci↗

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

Coulomb Drag in Graphene

We study the Coulomb drag between two single graphene sheets in intrinsic and extrinsic graphene systems with no interlayer tunneling. The general expression for the nonlinear susceptibility appropriate for single-layer graphene systems is derived using the diagrammatic perturbation theory, and the corresponding exact zero-temperature expression is obtained analytically. We find that, despite the existence of a non-zero conductivity in an intrinsic graphene layer, the Coulomb drag between intrinsic graphene layers vanishes at all temperatures. In extrinsic systems, we obtain numerical results and an approximate analytical result for the drag resistivity $ρ_{\textrm{D}}$, and find that $ρ_{\textrm{D}}$ goes as $T^2$ at low temperature $T$, as $1/d^4$ for large bilayer separation $d$ and $1/n^3$ for high carrier density $n$. We also discuss qualitatively the effect of plasmon-induced enhancement on the Coulomb drag, which should occur at a temperature of the order of or higher than the Fermi temperature.

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

Coulomb Drag and Spin Coulomb Drag in the presence of Spin-orbit Coupling

Employing diagrammatic perturbation theory, we calculate the (charge) Coulomb drag resistivity $ρ_D$ and spin Coulomb drag resistivity $ρ_{\uparrow\downarrow}$ in the presence of Rashba spin-orbit coupling. Analytical expressions for $ρ_D$ and $ρ_{\uparrow\downarrow}$ are derived, and it is found that spin-orbit interaction produces a small enhancement to $ρ_D$ and $ρ_{\uparrow\downarrow}$ in the ballistic regime while $ρ_D$ is unchanged in the diffusive regime. This enhancement in the ballistic regime is attributed to the enhancement of the nonlinear susceptibility (i.e. current produced through the rectification of the thermal electric potential fluctuations in the passive layer) while the lack of enhancement in the diffusive regime is due to the suppression by disorder.

cond-mat.mes-hall↗

arXiv2006

Theory of Light Emission in Sonoluminescence as Thermal Radiation

Based on the model proposed by Hilgenfeldt {\it at al.} [Nature {\bf 398}, 401 (1999)], we present here a comprehensive theory of thermal radiation in single-bubble sonoluminescence (SBSL). We first invoke the generalized Kirchhoff's law to obtain the thermal emissivity from the absorption cross-section of a multilayered sphere (MLS). A sonoluminescing bubble, whose internal structure is determined from hydrodynamic simulations, is then modelled as a MLS and in turn the thermal radiation is evaluated. Numerical results obtained from simulations for argon bubbles show that our theory successfully captures the major features observed in SBSL experiments.

cond-mat.soft↗

arXiv2006

Intrinsic Spin Hall Effect in the presence of Extrinsic Spin-Orbit Scattering

Intrinsic and extrinsic spin Hall effects are considered together on an equal theoretical footing for the Rashba spin-orbit coupling in two-dimensional (2D) electron and hole systems, using the diagrammatic method for calculating the spin Hall conductivity. Our analytic theory for the 2D holes shows the expected lowest-order additive result for the spin Hall conductivity. But, the 2D electrons manifest a very surprising result, exhibiting a non-analyticity in the Rashba coupling strength $α$ where the strictly extrinsic spin Hall conductivity (for $α= 0$) cannot be recovered from the $α\to 0$ limit of the combined theory. The theoretical results are discussed in the context of existing experimental results.

cond-mat.mes-hall↗

arXiv2005

Spin Accumulation in the Extrinsic Spin Hall Effect

The drift-diffusion formalism for spin-polarized carrier transport in semiconductors is generalized to include spin-orbit coupling. The theory is applied to treat the extrinsic spin Hall effect using realistic boundary conditions. It is shown that carrier and spin diffusion lengths are modified by the presence of spin-orbit coupling and that spin accumulation due to the extrinsic spin Hall effect is strongly and qualitatively influenced by boundary conditions. Analytical formulas for the spin-dependent carrier recombination rates and inhomogeneous spin densities and currents are presented.

cond-mat.mes-hall↗

arXiv2005

Spin Hall Effect in Doped Semiconductor Structures

In this Letter we present a microscopic theory of the extrinsic spin Hall effect based on the diagrammatic perturbation theory. Side-jump (SJ) and skew-scattering (SS) contributions are explicitly taken into account to calculate the spin Hall conductivity, and we show their effects scale as $σ_{xy}^{SJ}/σ_{xy}^{SS} \sim (\hbar/τ)/ε_F$, with $τ$ being the transport relaxation time. Motivated by recent experimental work we apply our theory to n- and p-doped 3D and 2D GaAs structures, obtaining $σ_s/σ_c \sim 10^{-3}-10^{-4}$ where $σ_{s(c)}$ is the spin Hall (charge) conductivity, which is in reasonable agreement with the recent experimental results of Kato \textit{et al}. [Science \textbf{306}, 1910 (2004)] in n-doped 3D GaAs system.

cond-mat.mes-hall↗