Resonant magnetopolaron effects in GaAs/AlGaAs multiple quantum well structures

A detailed experimental study of electron cyclotron resonance (CR) has been carried out at 4.2 K in three modulation-doped GaAs/Al0.3Ga0.7As multiple quantum well samples in fields up to 30 T. A strong avoided-level-crossing splitting of the CR energies due to resonant magnetopolaron effects is observed for all samples near the GaAs reststrahlen region. Resonant splittings in the region of AlAs-like interface phonon modes of the barriers are observed in two samples with narrower well width and smaller doping concentration. The interaction between electrons and the AlAs interface optical phonon modes has been calculated for our specific sample structures in the framework of the memory-function formalism. The calculated results are in good agreement with the experimental results, which confirms our assignment of the observed splitting near the AlAs-like phonon region is due to the resonant magnetopolaron interaction of electrons in the wells with AlAs-like interface phonons. (C) 1998 Elsevier Science B.V. All rights reserved.

Resonant magnetopolaron effects in GaAs/AlGaAs multiple quantum well structures

A detailed experimental study of electron cyclotron resonance (CR) has been carried out at 4.2 K in three modulation-doped GaAs/Al0.3Ga0.7As multiple quantum well samples in fields up to 30 T. A strong avoided-level-crossing splitting of the CR energies due to resonant magnetopolaron effects is observed for all samples near the GaAs reststrahlen region. Resonant splittings in the region of AlAs-like interface phonon modes of the barriers are observed in two samples with narrower well width and smaller doping concentration. The interaction between electrons and the AlAs interface optical phonon modes has been calculated for our specific sample structures in the framework of the memory-function formalism. The calculated results are in good agreement with the experimental results, which confirms our assignment of the observed splitting near the AlAs-like phonon region is due to the resonant magnetopolaron interaction of electrons in the wells with AlAs-like interface phonons. (C) 1998 Elsevier Science B.V. All rights reserved.

Enhancement of Exciton-Phonon Interaction in InGaN Quantum Wells Induced by Electron-Beam Irradiation

InGaN/GAN multiple quantum wells grown by metal-organic chemical vapor deposition were irradiated with the electron beam from a low energy accelerator. The electron irradiation induced a redshift by 50 meV in the photoluminescence spectra of the electron-irradiated InGaN/GaN quantum wells, irrespective of the exposure time to the electron beam which ranges from 10 to 1000s. The localization parameter extracted from the temperature-dependent photoluminescence spectra was found to increase in the Irradiated samples. Analysis of the intensity of the longitudinal optical phonon sidebands showed the enhancement of the exciton-phonon coupling, indicating that the excitons are more strongly localized in the irradiated InGaN wells. The change in the pholotuminescence spectra. In the irradiated InGa/GAN quantum wells were explained in terms of the increase of indium concentration in indium rich clusters induced by the electron irradiation (C) 2009 The Japan Society of Applied Physics

High-field cyclotron resonance and electron-phonon interaction in modulation-doped multiple quantum well structures

A systematic study of electron cyclotron resonance (CR) in two sets of GaAs/Al0.3Ga0.7As modulation-doped quantum-well samples (well widths between 12 and 24 nm) has been carried out in magnetic fields up to 30 T. Polaron CR is the dominant transition in the region of GaAs optical phonons for the set of lightly doped samples, and the results are in good agreement with calculations that include the interaction with interface optical phonons. The results from the heavily doped set are markedly different. At low magnetic fields (below the GaAs reststrahlen region), all three samples exhibit almost identical CR which shows little effect of the polaron interaction due to screening and Pauli-principle effects. Above the GaAs LO-phonon region (B > similar to 23 T), the three samples behave very differently. For the most lightly doped sample (3 x 10(11) cm(-2)) only one transition minimum is observed, which can be explained as screened polaron CR. A sample of intermediate density (6 x 10(11) cm(-2)) shows two lines above 23 T; the higher frequency branch is indistinguishable from the positions of the single line of the low density sample. For the most heavily, doped sample (1.2 x 10(12) cm(-2)) there is no evidence of high frequency resonance, and the strong, single line observed is indistinguishable from the lower branch observed from sample with intermediate doping density. We suggest that the low frequency branch in our experiment is a magnetoplasmon resonance red-shifted by disorder, and the upper branch is single-particle-like screened polaron CR. (C) 1998 Elsevier Science B.V. All rights reserved.

High-field cyclotron resonance and electron-phonon interaction in modulation-doped multiple quantum well structures

A systematic study of electron cyclotron resonance (CR) in two sets of GaAs/Al0.3Ga0.7As modulation-doped quantum-well samples (well widths between 12 and 24 nm) has been carried out in magnetic fields up to 30 T. Polaron CR is the dominant transition in the region of GaAs optical phonons for the set of lightly doped samples, and the results are in good agreement with calculations that include the interaction with interface optical phonons. The results from the heavily doped set are markedly different. At low magnetic fields (below the GaAs reststrahlen region), all three samples exhibit almost identical CR which shows little effect of the polaron interaction due to screening and Pauli-principle effects. Above the GaAs LO-phonon region (B > similar to 23 T), the three samples behave very differently. For the most lightly doped sample (3 x 10(11) cm(-2)) only one transition minimum is observed, which can be explained as screened polaron CR. A sample of intermediate density (6 x 10(11) cm(-2)) shows two lines above 23 T; the higher frequency branch is indistinguishable from the positions of the single line of the low density sample. For the most heavily, doped sample (1.2 x 10(12) cm(-2)) there is no evidence of high frequency resonance, and the strong, single line observed is indistinguishable from the lower branch observed from sample with intermediate doping density. We suggest that the low frequency branch in our experiment is a magnetoplasmon resonance red-shifted by disorder, and the upper branch is single-particle-like screened polaron CR. (C) 1998 Elsevier Science B.V. All rights reserved.

Electron-phonon interaction in atomic-scale conductors: Einstein oscillators versus full phonon modes

Two extreme pictures of electron-phonon interactions in nanoscale conductors are compared: one in which the vibrations are treated as independent Einstein atomic oscillators, and one in which electrons are allowed to couple to the full, extended phonon modes of the conductor. It is shown that, under a broad range of conditions, the full-mode picture and the Einstein picture produce essentially the same net power at any given atom in the nanojunction. The two pictures begin to differ significantly in the limit of low lattice temperature and low applied voltages, where electron-phonon scattering is controlled by the detailed phonon energy spectrum. As an illustration of the behaviour in this limit, we study the competition between trapped vibrational modes and extended modes in shaping the inelastic current-voltage characteristics of one-dimensional atomic wires.

Electron-Phonon Interaction Within the Framework of the Fluctuating Valence of Copper Atoms — a Theoretical Model for High Temperature Superconductivity

Electron-phonon interaction is considered within the framework of the fluctuating valence of Cu atoms. Anderson's lattice Hamiltonian is suitably modified to take this into account. Using Green's function technique tbe possible quasiparticle excitations' are determined. The quantity 2delta k(O)/ kB Tc is calculated for Tc= 40 K. The calculated values are in good agreement with the experimental results.

Effects of confinement on the electron-phonon interaction in Al(0.18)Ga(0.82)As/GaAs quantum wells

Photoluminescence measurements at different temperatures have been performed to investigate the effects of confinement on the electron-phonon interaction in GaAs/AlGaAs quantum wells (QWs). A series of samples with different well widths in the range from 150 up to 750 A was analyzed. Using a fitting procedure based on the Passler-p model to describe the temperature dependence of the exciton recombination energy, we determined a fit parameter which is related to the strength of the electron-phonon interaction. On the basis of the behavior of this fit parameter as a function of the well width thickness of the samples investigated, we verified that effects of confinement on the exciton recombination energy are still present in QWs with well widths as large as 450 angstrom. Our findings also show that the electron-phonon interaction is three times stronger in GaAs bulk material than in Al(0.18)Ga(0.82)As/GaAs QWs.

Optical-Phonon-Limited High-Field Transport in Layered Materials

An optical-phonon-limited velocity model has been employed to investigate high-field transport in a selection of layered 2-D materials for both, low-power logic switches with scaled supply voltages, and high-power, high-frequency transistors. Drain currents, effective electron velocities, and intrinsic cutoff frequencies as a function of carrier density have been predicted, thus providing a benchmark for the optical-phonon-limited high-field performance limits of these materials. The optical-phonon-limited carrier velocities for a selection of multi-layers of transition metal dichalcogenides and black phosphorus are found to be modest compared to their n-channel silicon counterparts, questioning the utility of biasing these devices in the source-injection dominated regime. h-BN, at the other end of the spectrum, is shown to be a very promising material for high-frequency, high-power devices, subject to the experimental realization of high carrier densities, primarily due to its large optical-phonon energy. Experimentally extracted saturation velocities from few-layer MoS2 devices show reasonable qualitative and quantitative agreement with the predicted values. The temperature dependence of the measured v(sat) is discussed and compared with the theoretically predicted dependence over a range of temperatures.

Polaron effects on the optical absorptions in asymmetrical semi-parabolic quantum wells

The linear and nonlinear optical absorptions considering the weak-coupling electron-LO-phonon interaction in asymmetrical semiparabolic quantum wells are theoretically investigated. The numerical results for the typical GaAs/AlxGa1-xAs material show that the factors of Al content x, the relaxation time and the photon energy have great influence on the optical absorption coefficients. Moreover, the theoretical values of the optical absorptions are more than a factor of 2-3 higher than the one in the structure without considering the electron-LO-phonon interaction by calculating. (C) 2007 Elsevier B.V. All rights reserved.

NEAR-RESONANCE RAMAN-SCATTERING OF LONGITUDINAL OPTICAL PHONON MODES AND INTERFACE MODES IN GAAS/ALAS SUPERLATTICES

The near-resonance Raman scattering of GaAs/AlAs superlattices is investigated at room temperature. Owing to the resonance enhancement of Frohlich interaction, the scattering intensity of even LO confined modes with A1 symmetry becomes much stronger than that of odd modes with B2 symmetry. The even modes were observed in the polarized spectra, while the odd modes appear in the depolarized spectra as in the off-resonance case. The second-order Raman spectra show that the polarized spectra are composed of the overtone and combinations of even modes, while the depolarized spectra are composed of the combinations of one odd mode and one even mode. The results agree well with the selection rules predicted by the microscopic theory of Raman scattering in superlattices, developed recently by Huang and co-workers. In addition, the interface modes and the combinations of interface modes and confined modes are also observed in the two configurations.

Acoustic and optical phonon dynamics from femtosecond time-resolved optical spectroscopy of the superconducting iron pnictide Ca(Fe0.944Co0.056)(2)As-2

We report the temperature evolution of coherently excited acoustic and optical phonon dynamics in the superconducting iron pnictide single crystal Ca(Fe0.944Co0.056)(2)As-2 across the spin density wave transition at T-SDW similar to 85 K and the superconducting transition at T-SC similar to 20 K. The strain pulse propagation model applied to the generation of the acoustic phonons yields the temperature dependence of the optical constants, and longitudinal and transverse sound velocities in the temperature range from 3.1 K to 300 K. The frequency and dephasing times of the phonons show anomalous temperature dependence below T-SC indicating a coupling of these low-energy excitations with the Cooper-pair quasiparticles. A maximum in the amplitude of the acoustic modes at T similar to 170 is seen, attributed to spin fluctuations and strong spin-lattice coupling before T-SDW. Copyright (c) EPLA, 2012