Searcharxiv⌕ Search

arXiv subjects

E. H. Hwang

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

At least 127 records · Page 7Linked to original sources
arXiv2002

Temperature dependent resistivity of spin-split subbands in GaAs 2D hole system

We calculate the temperature dependent resistivity in spin-split subbands induced by the inversion asymmetry of the confining potential in GaAs 2D hole systems. By considering both temperature dependent multisubband screening of impurity disorder and hole-hole scattering we find that the strength of the metallic behavior depends on the symmetry of the confining potential (i.e., spin-splitting) over a large range of hole density. At low density above the metal-insulator transition we find that effective disorder reduces the enhancement of the metallic behavior induced by spin-splitting. Our theory is in good qualitative agreement with existing experiments.

cond-mat.str-el↗
arXiv2002

Optical Conductivity of Ferromagnetic Semiconductors

The dynamical mean field method is used to calculate the frequency and temperature dependent conductivity of dilute magnetic semiconductors. Characteristic qualitative features are found distinguishing weak, intermediate, and strong carrier-spin coupling and allowing quantitative determination of important parameters defining the underlying ferromagnetic mechanism.

cond-mat.mtrl-sci↗
arXiv2001

The relative importance of electron-electron interactions compared to disorder in the two-dimensional "metallic" state

The effect of substrate bias and surface gate voltage on the low temperature resistivity of a Si-MOSFET is studied for electron concentrations where the resistivity increases with increasing temperature. This technique offers two degrees of freedom for controlling the electron concentration and the device mobility, thereby providing a means to evaluate the relative importance of electron-electron interactions and disorder in this so-called ``metallic'' regime. For temperatures well below the Fermi temperature, the data obey a scaling law where the disorder parameter ($k_{\rm{F}}l$), and not the concentration, appears explicitly. This suggests that interactions, although present, do not alter the Fermi-liquid properties of the system fundamentally. Furthermore, this experimental observation is reproduced in results of calculations based on temperature-dependent screening, in the context of Drude-Boltzmann theory.

cond-mat.mes-hall↗
arXiv2001

Analysis of the resistance in p-SiGe over a wide temperature range

The temperature dependence of a system exhibiting a `metal-insulator transition in two dimensions at zero magnetic field' (MIT) is studied up to 90K. Using a classical scattering model we are able to simulate the non-monotonic temperature dependence of the resistivity in the metallic high density regime. We show that the temperature dependence arises from a complex interplay of metallic and insulating contributions contained in the calculation of the scattering rate $1/\td(E,T)$, each dominating in a limited temperature range.

cond-mat.str-el↗
arXiv2001

Plasmon dispersion in dilute 2D electron systems: Quantum-Classical and Wigner Crystal-Electron Liquid Crossover

We theoretically calculate the finite wave vector plasmon dispersion in a low density 2D electron layer taking into account finite temperature, finite layer width, and local field corrections. We compare our theoretical results with recent Raman scattering spectroscopic experimental 2D plasmon dispersion data in GaAs quantum wells at very low carrier densities ($r_s > 10$) and large wave vectors ($q \ge k_F$). We find good agreement with the experimental data, providing an explanation for why the experimentally measured dispersion seems to obey the simple classical long wavelength 2D plasmon dispersion formula. We also provide a critical discussion on the observable manifestations of the quantum-classical and the Wigner crystal - electron liquid crossover behavior in the 2D plasmon properties as a function of electron density and temperature in GaAs quantum well systems.

cond-mat.str-el↗
arXiv2000

Correlation induced phonon softening in low density coupled bilayer systems

We predict a possible phonon softening instability in strongly correlated coupled semiconductor bilayer systems. By studying the plasmon-phonon coupling in coupled bilayer structures, we find that the renormalized acoustic phonon frequency may be softened at a finite wave vector due to many-body local field corrections, particularly in low density systems where correlation effects are strong. We discuss experimental possibilities to search for this predicted phonon softening phenomenon.

cond-mat.str-el↗
arXiv2000

Parallel magnetic field induced giant magnetoresistance in low density {\it quasi}-two dimensional layers

We provide a possible theoretical explanation for the recently observed giant positive magnetoresistance in high mobility low density {\it quasi}-two dimensional electron and hole systems. Our explanation is based on the strong coupling of the parallel field to the {\it orbital} motion arising from the {\it finite} layer thickness and the large Fermi wavelength of the {\it quasi}-two dimensional system at low carrier densities.

cond-mat.str-el↗
arXiv2000

Interface charged impurity scattering in semiconductor MOSFETs and MODFETs: temperature dependent resistivity and 2D "metallic" behavior

We present the results on the anomalous 2D transport behavior by employing Drude-Boltzmann transport theory and taking into account the realistic charge impurity scattering effects. Our results show quantitative agreement with the existing experimental data in several different systems and address the origin of the strong and non-monotonic temperature dependent resistivity.

cond-mat.str-el↗
arXiv2000

Calculated temperature-dependent resistance in low density 2D hole gases in GaAs heterostructure

We calculate the low temperature resistivity in low density 2D hole gases in GaAs heterostructures by including screened charged impurity and phonon scattering in the theory. Our calculated resistance, which shows striking temperature dependent non-monotonicity arising from the competition among screening, nondegeneracy, and phonon effects, is in excellent agreement with recent experimental data.

cond-mat.str-el↗
arXiv1999

Role of confined phonons in thin film superconductivity

We calculate the critical temperature $T_c$ and the superconducting energy gaps $Δ_n$ of a thin film superconductor system, where $Δ_n$ is the superconducting energy gap of the $n$-th subband. Since the quantization of both the electron energy and phonon spectrum arises due to dimensional confinement in one direction, the effective electron-electron interaction mediated by the quantized confined phonons is different from that mediated by the bulk phonon, leading to the modification of $T_c$ in the thin film system. We investigate the dependence of $T_c$ and $Δ_n$ on the film thickness $d$ with this modified interaction.

cond-mat.supr-con↗
arXiv1999

Collective charge density fluctuations in superconducting layered systems with bilayer unit cells

Collective modes of bilayered superconducting superlattices (e.g., YBCO) are investigated within the conserving gauge-invariant ladder diagram approximation including both the nearest interlayer single electron tunneling and the Josephson-type Cooper pair tunneling. By calculating the density-density response function including Coulomb and pairing interactions, we examine the two collective mode branches corresponding to the in-phase and out-of-phase charge fluctuations between the two layers in the unit cell. The out-of-phase collective mode develops a long wavelength plasmon gap whose magnitude depends on the tunneling strength with the mode dispersions being insensitive to the specific tunneling mechanism (i.e., single electron or Josephson). We also show that in the presence of tunneling the oscillator strength of the out-of-phase mode overwhelms that of the in-phase-mode at $k_{\|} = 0$ and finite $k_z$, where $k_z$ and $k_{\|}$ are respectively the mode wave vectors perpendicular and along the layer. We discuss the possible experimental observability of the phase fluctuation modes in the context of our theoretical results for the mode dispersion and spectral weight.

cond-mat.supr-con↗
arXiv1999

Charged impurity scattering limited low temperature resistivity of low density silicon inversion layers

We calculate within the Boltzmann equation approach the charged impurity scattering limited low temperature electronic resistivity of low density $n$-type inversion layers in Si MOSFET structures. We find a rather sharp quantum to classical crossover in the transport behavior in the $0 - 5$K temperature range, with the low density, low temperature mobility showing a strikingly strong non-monotonic temperature dependence, which may qualitatively explain the recently observed anomalously strong temperature dependent resistivity in low-density, high-mobility MOSFETs.

cond-mat.dis-nn↗
arXiv1998

Collective charge density excitations in two-component one-dimensional quantum plasmas: Phase fluctuation mode dispersion and spectral weight in semiconductor quantum wire nanostructures

Collective charge density excitation spectra of both a spatially separated two-component quasi-one dimensional (1D) quantum plasma, as existing in a semiconductor double quantum wire structure, and a 1D homogeneous electron-hole plasma, as appropriate for a photoexcited semiconductor quantum wire system, are calculated within the two-component random-phase-approximation. We also investigate the effect of impurity scattering on the collective mode dispersion and damping, and calculate the collective mode spectral weight by obtaining the dynamical structure factor. We find that both OP and AP modes are overdamped by impurity scattering below some critical wave vector. We find that in the long wavelength limit the spectral weight is carried mostly by the OP, but the spectral weight of the undamped AP mode at finite (but not too large) wave vectors is comparable with that of the OP mode, making it viable to observe the AP mode in semiconductor quantum wire systems. The effect of the interwire electron tunneling in a biwire system on the collective charge density excitation spectra is also studied. We discuss the mode dispersion and damping from an effective Luttinger liquid perspective as well and in some cases include local field corrections in our calculations.

cond-mat.str-el↗
arXiv1998

Plasmons in coupled bilayer structures

We calculate the collective charge density excitation dispersion and spectral weight in bilayer semiconductor structures {\it including effects of interlayer tunneling}. The out-of-phase plasmon mode (the ``acoustic'' plasmon) develops a long wavelength gap in the presence of tunneling with the gap being proportional to the square root (linear power) of the tunneling amplitude in the weak (strong) tunneling limit. The in-phase plasmon mode is qualitatively unaffected by tunneling. The predicted plasmon gap should be a useful tool for studying many-body effects.

cond-mat.str-el↗
arXiv1998

Band gap renormalization in photoexcited semiconductor quantum wire structures in the GW approximation

We investigate the dynamical self-energy corrections of the electron-hole plasma due to electron-electron and electron-phonon interactions at the band edges of a quasi-one dimensional (1D) photoexcited electron-hole plasma. The leading-order $GW$ dynamical screening approximation is used in the calculation by treating electron-electron Coulomb interaction and electron-optical phonon Fröhlich interaction on an equal footing. We calculate the exchange-correlation induced band gap renormalization (BGR) as a function of the electron-hole plasma density and the quantum wire width. The calculated BGR shows good agreement with existing experimental results, and the BGR normalized by the effective quasi-1D excitonic Rydberg exhibits an approximate one-parameter universality.

cond-mat.mes-hall↗
arXiv1998

On the c-axis optical reflectivity of layered cuprate superconductors

Using a conventional BCS -- Fermi liquid model we calculate the c-axis optical reflectivity of the layered high temperature cuprate superconductors by obtaining the finite temperature dynamical dielectric function in a microscopic self-consistent gauge invariant formalism. We get good semi-quantitative agreement with all the existing experimental data by using the measured normal state $dc$ resistivities as the input parameters in obtaining the c-axis hopping amplitude and the normal state level broadening in our microscopic calculation.

cond-mat.supr-con↗
arXiv1996

Plasmon-pole approximation for semiconductor quantum wire electrons

We develop the plasmon-pole approximation for an interacting electron gas confined in a semiconductor quantum wire. We argue that the plasmon-pole approximation becomes a more accurate approach in quantum wire systems than in higher dimensional systems because of severe phase-space restrictions on particle-hole excitations in one dimension. As examples, we use the plasmon-pole approximation to calculate the electron self-energy due to the Coulomb interaction and the hot-electron energy relaxation rate due to LO-phonon emission in GaAs quantum wires. We find that the plasmon-pole approximation works extremely well as compared with more complete many-body calculations.

cond-mat↗