One‐phonon resonant electron Raman scattering in a cylindrical GaAs/AlAs quantum dot

We have presented a theoretical calculation of the differential cross section for the electron Raman scattering process associated with the interface optical phonon modes in cylindrical GaAs quantum dots (QDs) with a AlAs matrix. We consider the Fröhlich electron–phonon interaction in the framework of the dielectric continuum approach. The selection rules for the processes are studied. Singularities are found to be sensitively size‐dependent, and, by varying the size of the QDs, it is possible to control the frequency shift in the Raman spectra. A discussion of the phonon behavior for QDs with different size is presented. Copyright © 2013 John Wiley & Sons, Ltd.     

Numerical evaluation of energy loss rate for hot carriers in quantum wells

Using the Fröhlich potential associated with realistic optical phonon modes in quantum well systems, the energy loss rates of hot electrons, holes, and electron–hole pairs are calculated, with special emphasis on the effects of carrier density, hot phonon population, quantum well width, and phonon dispersion on the hot-carrier relaxation process in quasi-two-dimensional systems.     

Polaronic effects in one-dimensional quantum antidot arrays

We study the effect of polaronic corrections arising from theelectron-longitudinal optical phonon interaction on the energyspectrum of a two-dimensional electron system with a one-dimensionalperiodic antidot array geometry created by a weak electrostaticmodulation potential, and subjected to a weak magnetic fieldmodulation as well as a uniform strong perpendicular staticmagnetic field. To incorporate the effects of electron-phononinteractions within the framework of Fröhlich polaron theory, wefirst apply a displaced-oscillator type unitary transformation todiagonalise the relevant Fröhlich Hamiltonian, and we thendetermine the parameters of this transformation together with theparameter included in the electronic trial wave function . On thebasis of this technique, it has been shown that the polaroniccorrections have non-negligible effects on the electronic spectrumof a two-dimensional electron system with a quantum antidot array,since switching such an interaction results in shifting thedegeneracy restoring points of Landau levels wherein the flatbandcondition is fulfilled, thus suppressing the Weiss oscillations.     

One-phonon resonant electron Raman scattering in a cylindrical semiconductor quantum dot

We have presented a theoretical calculation of the differential cross section (DCS) for the electron Raman scattering (ERS) process associated with the surface optical (SO) phonon modes in semiconductor quantum dots (QDs). We consider the Fröhlich electron-phonon interaction in the framework of the dielectric continuum approach. Different scattering configurations are discussed and the selection rules for the processes are also studied. Singularities are found to be sensitively size-dependent and by varying the size of the QDs, it is possible to control the frequency shift in the Raman spectra. A discussion of the phonon behavior for QDs with large and small size is presented. The numerical results are also compared with that of experiments.     

Electron-phonon interaction in non-polar quantum dots induced by the amorphous polar environment

We propose a Fröhlich-type electron-phonon interaction mechanism for carriers confined in a non-polar quantum dot surrounded by an amorphous polar environment. Carrier transitions under this mechanism are due to their interaction with the oscillating electric field induced by the local vibrations in the surrounding amorphous medium. We estimate the corresponding energy relaxation rate for electrons in Si nanocrystals embedded in a SiO₂ matrix as an example. When the nanocrystal diameter is larger than 4 nm then the gaps between the electron energy levels of size quantization are narrow enough to allow for transitions accompanied by emission of a single local phonon having the energy about 140 meV. In such Si/SiO₂ nanocrystals the relaxation time is in nanosecond range.     

Investigation of hot electrons and hot phonons generated within an AlN/GaN high electron mobility transistor

We review our recent results obtained on an AlN/GaN-based high-electron-mobility transistor. The temperature of the electrons drifting under a relatively-high electric field is significantly higher than the lattice temperature (i.e., the hot electrons are generated). These hot electrons are produced through the Fröhlich interaction between the drifting electrons and long-lived longitudinal-optical phonons. By fitting electric field vs. electron temperature deduced from the measurements of photoluminescence spectra to a theoretical model, we have deduced the longitudinal-optical-phonon emission time for each electron is to be on the order of 100 fs. We have also measured the decay time constant for LO phonons to be about 4.2 ps. An electric field present in a GaN/AlN heterostructure can bring both the first-order and second-order Raman scattering processes into strong resonances. The resonant Stokes and anti-Stokes Raman scattering results in the increase and decrease of non-equilibrium longitudinal-optical phonon temperatures, respectively. Moreover, the phonon temperature measured from the Raman scattering is increased with an applied electric field at a much higher rate than the lattice temperature due to the presence of field-induced non-equilibrium longitudinal-optical phonons.     

Hot electron effects in optically-pumped mid-infrared intersubband semiconductor laser

A theoretical investigation of an optically-pumped mid-infrared intersubband semiconductor laser is presented. The influence of electrons and dopant ions on the conduction band structure is simulated with a self-consistent Poisson–Schrödinger solver. Electron-polar optical phonon interactions are calculated by using a macroscopic phonon model with electromagnetic boundary conditions. In order to assess the influence of the electronic temperature on the device optical performances, electron dynamics under optical pumping are investigated within a rate equation model where particle and energy flow equations are derived from Boltzmann's equation with Fermi statistics. Our calculations show that population inversion between the first and second excited states can occur at 77 K under intersubband optical excitation.     

Bond stretching phonon anomalies due to incommensurate charge density wave instabilities in high-Tc cuprates

Phonon anomalies observed in various high T_c cuprates are analyzed theoretically within the Hubbard-Holstein model in the limit of strong local electron correlations and in presence of long-range Coulomb interaction. The phonon self-energy is evaluated by taking into account the charge collective modes that become critical upon doping approaching an instability towards an incommensurate charge density wave (ICDW) driven by electron correlations. The doping dependence of phonon softening features and the highly distinctive phonon self-energy dependence on the wave vector agree with experiments. We discuss relevance of dynamical corrections to the density correlation function to achieve a sizeable bond-stretching phonon softening with a kink-like profile away from the zone boundary.     

Enhanced Fröhlich interaction of semiconductor cuprous oxide films determined by temperature‐dependent Raman scattering and spectral transmittance

Anomalous low temperature behaviors in cuprous oxide (Cu₂O) film grown on quartz substrate have been investigated by temperature‐dependent Raman and transmittance spectra. The longitudinal optical components of two Γ₁₅‐ phonon modes become sharper and more intense at a low temperature. It can be found that the highest‐order electronic transition located at 6.4 eV exhibits a minimum transmittance near 200 K. Correspondingly, the variations from phonon intensity ratios reveal obvious anomalies with the decreasing temperature, indicating the existence of strong electron–phonon coupling mediated by Fröhlich interaction in the Cu₂O films below the temperature of 200 K. Copyright © 2012 John Wiley & Sons, Ltd.     

On the self-trapping of an electron due to lattice interaction

The problem of the sharpness of the polaron self-trapping transition for a single electron placed in the conduction band of an insulator and interacting with the phonons of the underlying lattice is investigated within the Fröhlich Hamiltonian. It is shown that this phenomenon depends sensitively on the lattice dimensionality, the nature of the electron-phonon coupling, the nature of the phonon dispersion; and on the discrete or the continuum nature of the applicable Hamiltonian.     

Electronic and optical properties of layered chalcogenide FeIn2Se4

We investigated the spectral-dependent dielectric function and temperature-dependent bandgap energy of layered chalcogenide FeIn₂Se₄ crystals. The critical-point energy and Lorentzian broadening were analyzed by fitting second-derivative spectra using the standard critical point model. The temperature effect of the bandgap energy was analyzed based on an analytical model considering both thermal lattice expansion and electron–phonon interactions. We also extensively analyzed the temperature dependence of absorption tails and identified their possible origins. The dielectric functions and absorption coefficient in the photon-energy range of 0.75–4.75 eV were obtained. The results also showed that optical phonon modes associated with the electron–phonon interactions could be closely related to the average phonon energy.     

Quadrupole splitting and Eu partial lattice dynamics in europium orthophosphate EuPO <Subscript>4</Subscript>

Hyperfine interactions in europium orthophosphate EuPO₄ were investigated using ¹⁵¹Eu Mössbauer spectroscopy from 6 to 300 K. The value of the quadrupole splitting and the asymmetry parameter were refined and further substantiated by nuclear forward scattering data obtained at room temperature. The temperature dependence of the relative absorption was modeled with an Eu specific Debye temperature of 221(1) K. Eu partial lattice dynamics were probed by means of nuclear inelastic scattering and the mean force constant, the Lamb-Mössbauer factor, the internal energy, the vibrational entropy, the average phonon group velocity were calculated using the extracted density of phonon states. In general, Eu specific vibrations are characterized by rather small phonon energies and contribute strongly to the total entropy of the system. Although there is no classical Debye like behavior at low vibrational energies, the average phonon group velocity can be reasonably approximated using a linear fit.     

Electron- and phonon-polaron effects in TCNQ

The effects of electron correlation and electron-optical phonon interaction were investigated in the case of the valence- and conduction bands of pure TCNQ crystals using the electron-polaron model of Toyozawa and the Fröhlich electron-phonon interaction Hamiltonian. The original Hartree-Fock bandwidths were reduced by a factor of ～3–5 to 0.034 and 0.090 eV, respectively, and the forbidden gap decreased to ～2.7 eV due to polaron formation.     

Conductivity from charge or spin density waves

Below a Peierls transition the coupled electron phonon collective mode plays an important role in the conductivity of one-dimensional metal models such as have been recently postulated for various organic compounds. Within the jellium model, or in an incommensurate situation, the mode frequency goes to zero for q → 0 and is responsible for the infinite conductivity first proposed by Fröhlich. Impurities, lattice commensurability and three dimensional ordering introduce a gap into the mode spectrum. The low frequency conductivity and a large dielectric constant are predicted. Similar effects are predicted for a spin density wave.     

Effect of electron–phonon interaction on the energy spectrum of a quantum antidot in the presence of a uniform strong magnetic field

We have studied the effect of electron–phonon interaction for small electron–phonon coupling on the electronic energy spectrum of an electron confined by a parabolic potential and a repulsive antidot potential in the presence of a uniform strong magnetic field and an Aharonov–Bohm flux field by using a variational procedure. We have shown that the presence of the antidot potential removes degeneracy of the Landau levels and electron–phonon interaction has nonnegligible effects on these levels.     

Phonon-assisted intersubband transitions in wurtzite GaN/InxGa1-xN quantum wells

A detailed numerical calculation on the phonon-assisted intersubband transition rates of electrons in wurtzite GaN/In_xGa_1-xN quantum wells is presented. The quantum-confined Stark effect induced by the built-in electric field and the ternary mixed crystal effect are considered. The electron states are obtained by iteratively solving the coupled Schrödinger and Poisson equations and the dispersion property of each type of phonon modes is considered in the derivation of Fermi's golden rule to evaluate the transition rates. It is indicated that the interface and half-space phonon scattering play an important role in the process of 1-2 radiative transition. The transition rate is also greatly reduced by the built-in electric field. The present work can be helpful for the structural design and simulation of new semiconductor lasers.     

Electronic structure and energy relaxation in strained InAs/GaAs quantum pyramids

We perform experimental and theoretical studies of the electronic structure and relaxation processes in pyramid shaped InAs/GaAs quantum dots (QDs), grown by molecular beam epitaxy in the Stranski-Krastanow growth mode. Structural properties are characterized with plan view and cross section transmission electron microscopy.Finite difference calculations of the strain and the 3D Schrödinger equation, taking into account piezoelectric and excitonic effects, agree with experimental results on transition energies of ground and excited states, revealed in luminescence and absorption spectra. We find as relative standard deviation of the size fluctuation ξ=0.04; the pyramid shape fluctuates between {101} and {203} side facets.Carrier capture into the QD ground state after carrier excitation above barrier is a very efficient process. No luminescence from excited states is observed at low excitation density. Energy relaxation processes in the zero-dimensional energy states are found to be dominated by phonon energy selection rules. However, multi-phonon emission (involving GaAs barrier, InAs wetting layer, InAs QD and interface modes) allows for a large variety of relaxation channels and thus a phonon bottleneck effect does not exist here.     

Magnetpolaron effect in two-dimensional anisotropic parabolic quantum dot in a perpendicular magnetic field

We study the two-dimensional weak-coupling Fr o¨hlich polaron in a completely anisotropic quantum dot in a perpendicular magnetic field. By performing a unitary transformation, we first transform the Hamiltonian into a new one which describes an anisotropic harmonic oscillator with new mass and trapping frequencies interacting with the same phonon bath but with different interaction form and strength. Then employing the second-order Rayleigh–Schr o¨dinger perturbation theory, we obtain the polaron correction to the ground-state energy. The magnetic field and anisotropic effects on the polaron correction to the ground-state energy are discussed.     

Electron and phonon relaxation in metal films perturbed by a femtosecond laser pulse

A theoretical investigation of the electron and phonon time-dependent distributions in an Ag film subjected to a femtosecond laser pulse has been carried out. A system of two coupled time-dependent Boltzmann equations, describing electron and phonon dynamics, has been numerically solved. In the electron Boltzmann equation, electron–electron and electron–phonon collision integrals are considered together with a source term for laser perturbation. In the phonon Boltzmann equation, only electron–phonon collisions are considered, neglecting laser perturbation and phonon–phonon collisions. Screening of the interactions has been accounted for in both the electron–electron and the electron–phonon collisions. The results show the simultaneous electron and phonon time-dependent distributions from the initial non-equilibrium behaviour up to the establishment of a new final equilibrium condition. PACS 72.10.-d; 71.10.Ca; 63.20.Kr