Chang Sub Kim

Classical analogy of Fano resonances

We present an analogy of Fano resonances in quantum interference to classical resonances in the harmonic oscillator system. It has a manifestation as a coupled behaviour of two effective oscillators associated with propagating and evanescent waves. We illustrate this point by considering a classical system of two coupled oscillators and interfering electron waves in a quasi-one-dimensional narrow constriction with a quantum dot. Our approach provides a novel insight into Fano resonance physics and provides a helpful view in teaching Fano resonances.

Theory of coherent acoustic phonons in In x Ga 1 − x N / GaN multiple quantum wells

A microscopic theory for the generation and propagation of coherent LA phonons in pseudomorphically strained wurzite (0001) InGaN/GaN multi-quantum well (MQW) p-i-n diodes is presented. The generation of coherent LA phonons is driven by photoexcitation of electron-hole pairs by an ultrafast Gaussian pump laser and is treated theoretically using the density matrix formalism. We use realistic wurzite bandstructures taking valence-band mixing and strain-induced piezo- electric fields into account. In addition, the many-body Coulomb ineraction is treated in the screened time-dependent Hartree-Fock approximation. We find that under typical experimental conditions, our microscopic theory can be simplified and mapped onto a loaded string problem which can be easily solved.

Characteristics of transmission resonance in a quantum-dot superlattice

We investigate phase-coherent electron transport through height-varying potential barriers in a quantum-dot superlattice. Due to the aspect ratio variations of two alternating potential heights in the quantum channel, well-arranged resonant peaks in the first miniband of each plateau are divided into the paired peaks of two groups, which produce an extra gap inside each miniband. In addition, for a five barrier case, the second and third resonant peaks in the miniband are no longer distinguishable at a critical aspect ratio, and the amplitude of this degenerate peak becomes smaller than one and eventually approaches zero. The mean lifetimes of the resonant peaks whose amplitudes remain unity are studied. We also examine the resonant tunneling with under-unity transmission in the one-dimensional superlattice system with alternating potential barriers. Finally, it is found that the “quasi-resonance” appears in a quantum-dot superlattice with 13 barriers consisting of 2 alternating potential heights.

Time-dependent density-functional theory for the stopping power of an interacting electron gas for slow ions

Based on the time-dependent density-functional theory, we have derived a rigorous formula for the stopping power of an interacting electron gas for ions in the limit of low projectile velocities. If dynamical correlation between electrons is not taken into account, this formula recovers the corresponding stopping power of noninteracting electrons in an effective Kohn-Sham potential. The correlation effect, specifically the excitonic one in electron-hole pair excitations, however, is found to considerably enhance the stopping power for intermediately charged ions, bringing our theory into good agreement with experiment.

Collapse of resonance in quasi-one-dimensional quantum channels

We study the resonance structure of the conductance (transmissivity) of a quasi-one-dimensional channel that contains an attractive impurity of finite dimensions and derive an exact expression for the scattering matrix. We show that an impurity of finite dimensions may cause a set of Fano resonances to appear in the transmissivity. We also find that due to the coherent interaction the Fano resonances can collapse and discrete levels may appear in the continuum. Finally, we establish the wave function of the discrete levels and study the channel transmissivity in the critical regime.

Femtosecond four-wave mixing experiments on GaAs quantum wells using two independently tunable lasers

Femtosecond two beam and three beam ( v1, v1; v2) four-wave mixing (FWM) experiments on GaAs quantum wells have been performed using two partially synchronized, independently tunable lasers with external jitter compensation. Heavy and light hole beatings are observed with these two mutually incoherent lasers. FWM signals are observed when v2 is completely below the exciton energies, with no spectral overlap with the absorption profile. These off-resonant signals are stronger than the interband continuum signals for equivalent detunings. [S0031-9007(98)06123-7]

Coherent resonant transmission in temporally periodically driven potential wells: the Fano mirror

The electron transmission over an oscillating quantum well is studied perturbatively and numerically in the phase-coherent regime. The dynamically induced multichannel situation gives rise to a pronounced asymmetric resonance and antiresonance structure. The Fano line obtained is attributed to the interaction of the raised virtual discrete energy level with the continuum. Interestingly, the unitarity condition imposes the restriction that the transmission coefficients vanish altogether at the zero energies, leading to the concept of an electronic mirror.

Exchange and correlation effects on plasmon dispersions and Coulomb drag in low-density electron bilayers

We investigate the effect of exchange and correlation (xc) on the plasmon spectrum and the Coulomb drag between spatially separated low-density two-dimensional electron layers. We adopt a new approach, which employs dynamic xc kernels in the calculation of the bi-layer plasmon spectra and of the plasmon-mediated drag, and static many-body local field factors in the calculation of the particle-hole contribution to the drag. The spectrum of bi-layer plasmons and the drag resistivity are calculated in a broad range of temperatures taking into account both intra- and inter-layer correlation effects. We observe that both plasmon modes are strongly affected by xc corrections. After the inclusion of the complex dynamic xc kernels, a decrease of the electron density induces shifts of the plasmon branches in opposite directions. And this is in stark contrast to the tendency obtained within the RPA that both optical and acoustical plasmons move away from the boundary of the particle-hole continuum with a decrease in the electron density. We find that the introduction of xc corrections results in a significant enhancement of the transresistivity and qualitative changes in its temperature dependence. In particular, the large high-temperature plasmon peak that is present in the random phase approximation is found to disappear when the xc corrections are included. Our numerical results at low temperatures are in good agreement with the results of recent experiments by M. Kellogg {\it et al.}, Solid State Commun. \textbf{123}, 515 (2002).

Tunneling through a quantum channel with impurities: An exactly solvable model

Abstract The quasi-one-dimensional nanochannel with two attractive impurities are considered theoretically to investigate the electron localization and transmission. We solve the single-electron problem exactly within the short-range interaction and the phase-coherent limit. Consequently, we determine the wave functions and the energy eigenvalues of the special electron states. The special states are the discrete levels in the continuum, thus forming an artificial molecule in the electron waveguide. Further, we derive the analytical expression for the transmission amplitude and present a detailed analysis of the electron transmission through the quantum waveguide.

Cyclotron resonance of interface magnetopolarons at finite temperatures

Taking into account the interaction of an electron with interface optical phonons as well as bulk longitudinal optical phonons, we study the cyclotron resonance of magnetopolarons at the interfaces of polar crystals. We consider both the absorption and emission processes in our calculation using the Greens function method at finite temperatures. Consequently, different temperature characteristics of the cyclotron resonance mass and resonance frequency are found on either side of the off-resonance magnetic field region.