Bulk Longitudinal Optical Phonons Research Articles

Electron power loss in metal-oxide nanostructures in quantizing magnetic field

Hot-electron power loss via electron interaction with bulk longitudinal polar optical phonon in metal-oxide two dimensional nanostructures in the presence of zero and finite quantizing magnetic fields is investigated using electron temperature model. The dependence of power loss in two-dimensional nanostructures of transparent conducting oxides like ZnO, SnO2, CdO, In2O3, Ga2O3 is presented as a function of electron temperature and carrier density in zero and finite magnetic fields. Numerical calculations establish that external quantizing magnetic field enhances power loss of electrons. Further, electron power loss is proportional to the electron temperature and varies inversely with the carrier density. The electron magneto-power loss qualitatively differs from the oscillating dependence of the electron-phonon scattering on the quantizing magnetic field.

Electron-LO-phonon intrasubband scattering rates in a hollow cylinder under the influence of a uniform axial applied magnetic field

Scattering rates arising from the interactions of electrons with bulk longitudinal optical (LO) phonon modes in a hollow cylinder are calculated as functions of the inner radius and the uniform axial applied magnetic field. Now, the specific nature of electron-phonon interactions mainly depends on the character of the energy spectrum of electrons. As is well known, in cylindrical quantum wires, the application of a parallel magnetic field lifts the double degeneracy of the non-zero azimuthal quantum number states; \(m \not = 0\); irrespective of all electron’s radial quantum number l states. In fact, this Zeeman splitting is such that the \(m < 0\) electron’s energy subbands initially decrease with the increase of the parallel applied magnetic field. In a solid cylinder, the lowest-order; {\(l = 1;\,m = 0\)} subband is always the ground state. In a hollow cylinder, however, as the axial applied magnetic field is increased, the electron’s energy subbands take turns at becoming the ground state; following the sequence \(\lbrace m=0,-1,-2...~ -N\rbrace\) of azimuthal quantum numbers. Furthermore, in a hollow cylinder, in general, the electron’s energy separations between any two subbands are less than the LO phonon energy except for exceptionally high magnetic fields, and some highest-order quantum number states. In view of this, the discussion of the energy relaxation here is focused mainly on intrasubband scattering of electrons and only within the lowest-order {\(l = 1;\,m = 0\)} electron’s energy subband. The intrasubband scattering rates are found to be characterized by shallow minima in their variations with the inner radius, again, for a fixed outer radius. This feature is a consequence of a balance between two seemingly conflicting effects of the electron’s confinement by the inner and outer walls of the hollow cylinder. First; increased confinement of the charge carriers generally leads to the enhancement of the rates. Second; the presence of a hole in a hollow cylinder leads to a significant suppression of the scattering rates. The intrasubband scattering rates also show a somewhat parabolic increase in their variations with the applied magnetic field; an increase which is more pronounced in a relatively thick hollow cylinder.

Cyclotron-phonon resonance power absorption in free standing nanostructure of transparent conducting oxides

A theory of cyclotron-phonon resonance power absorption (CPRPA) in a free-standing nanostructure (FSNS) of transparent conducting oxides (TCO) is presented. An analytical expression for optical power absorption is obtained based on quantum mechanical perturbation technique for the case when electrons are scattered by bulk longitudinal polar optical phonons. The optical power absorption spectrum consists of a series of peaks due to electron transition between Landau levels with simultaneous absorption of photon and emission of phonon. The intensity and location of absorption peaks depends on the material parameters such as electron effective mass and LO-phonon energy. Frequency, temperature and thickness dependence of absorption power and the variation of full-width at half maximum (FWHM) of absorption peaks as a function of magnetic field, temperature, and number of Landau levels are presented. The FWHM of CPRPA peaks increases linearly with square root of magnetic field. Further, FWHM is independent of temperature and thickness of FSNS. The numerical results obtained for all TCOs confirm that CPRPA measurements would be useful in understanding optical absorption characteristics of TCOs in finite and zero magnetic fields.

Phonon-assisted emission and absorption of individual color centers in hexagonal boron nitride

Defect centers in hexagonal boron nitride represent room-temperature single-photon sources in a layered van der Waals material. These light emitters appear with a wide range of transition energies ranging over the entire visible spectrum, which renders the identification of the underlying atomic structure challenging. In addition to their eminent properties as quantum light emitters, the coupling to phonons is remarkable. Their photoluminescence exhibits significant side band emission well separated from the zero phonon line (ZPL) and an asymmetric broadening of the ZPL itself. In this combined theoretical and experimental study we show that the phonon side bands can be well described in terms of the coupling to bulk longitudinal optical (LO) phonons. To describe the ZPL asymmetry we show that in addition to the coupling to longitudinal acoustic (LA) phonons also the coupling to local mode oscillations of the defect center with respect to the entire host crystal has to be considered. By studying the influence of the emitter’s wave function dimensions on the phonon side bands we find reasonable values for the size of the wave function and the deformation potentials. We perform photoluminescence excitation measurements to demonstrate that the excitation of the emitters is most efficient by LO-phonon assisted absorption.

Effect of hydrogen-like impurity on a qubit in quantum pseudodot at finite temperature

Influences of temperature on the qubit's properties in a quantum pseudodot (QPD) are studied by using the variational method of Pekar type (VMPT) and quantum statistics theory (QST). The formation of qubit requires two special conditions: One is the strong-coupling between the electron and the bulk longitudinal optical (LO) phonon, and the other is the hydrogen-like impurity (HLI) locating at the center of the QPD. So, we choose strong-coupling RbCl crystal as an example and calculate the electron probability density (EPD) and the oscillating period (OP) of the QPD qubit changing with the temperature and the Coulombic impurity potential (CIP).

Effects of Temperature and Magnetic Field on the Coherence Time of a RbCl Parabolic Quantum Dot Qubit

Employing the Pekar variational method, quantum statistics theory and the Fermi golden rule, the temperature and magnetic field effects on the qubit in rubidium chloride (RbCl) parabolic quantum dots (PQDs) are investigated. We then obtain the eigenenergies and corresponding eigenfunctions of ground and first-excited states coupled strongly to an electron to bulk longitudinal optical phonons in a RbCl PQD with applied magnetic field. A two-level system of PQDs may be regarded as a single qubit. The spontaneous emission of phonons causes the qubit decoherence. The numerical results indicate that the coherence time decreases with elevating temperature. The coherence increases the effective confinement length, whereas there is a decrease of the magnetic field’s cyclotron frequency.

The effect of electric field on the coherence time of a 2D RbCl parabolic quantum dot qubit

The effects of the electric field on the coherence time of a 2D RbCl parabolic quantum dot (PQD) qubit are studied by using the variational method of Pekar type (VMPT) and the Fermi Golden Rule. We calculate the excitation energy of an electron strongly coupled to bulk longitudinal optical (LO) phonons in the 2D RbCl PQD under an applied electric field. The phonon spontaneous emission causes the decoherence of the qubit. The investigated results indicate that the coherence time increases with increasing strength of the electric field and the effective confinement length, whereas it decreases with increasing polaron radius. Our research results would be useful for the design and implementation of the solid-state quantum computation.

Effect of temperature on the coherence time of a parabolic quantum dot qubit

The effects of the temperature on the coherence time of a parabolic quantum dot (PQD) qubit are investigated by using the variational method of Pekar type. We obtain the ground and the first excited states’ eigenenergies and the corresponding eigenfunctions of an electron strongly coupled to bulk longitudinal optical phonons in the PQD. This two-level PQD system may be employed as a single qubit. The phonon spontaneous emission causes the decoherence of the qubit. We find that the coherence time will decrease with increasing temperature. It is an increasing function of the effective confinement length, whereas it is decreasing one of the polaron radius. We find that by changing the temperature, the effective confinement length and the polaron radius one can adjust the coherence time. Our research results would be useful for the design and implementation of the solid-state quantum computation.

Effects of Electric Field and Temperature on RbCl Asymmetry Quantum Dot Qubit

The effects of the electric field and temperature on a RbCl asymmetry quantum dot (AQD) qubit are investigated by using a variational method of the Pekar type. We first study the eigenenergies and eigenfunctions of the ground and first excited states of an electron strongly coupled to bulk longitudinal optical (LO) phonons with a three-dimensional (3D) harmonic potential under the influence of an electric field. This AQD two-level system may serve as a single qubit. We then obtain the time evolution of the electron’s probability density when the electron is in the superposition state of the ground and first excited states. Finally, we investigate the effects of the electric field and temperature on the oscillating period, the time evolution of the electron’s probability density and the coherence time. The numerical results show that the electron’s probability density and its oscillatory period will increase (decrease) with increasing temperature in the low (high) temperature regime. The electron’s probabi...

Influence of an anisotropic parabolic potential on the quantum dot qubit

To study the influence of an anisotropic parabolic potential (APP) on the properties of a quantum dot (QD) qubit, we obtain the eigenenergies and eigenfunctions of the ground and first excited state of an electron, which is strongly coupled to the bulk longitudinal optical (LO) phonons, in a QD under the influence of an APP by the celebrated Lee—Low—Pines (LLP) unitary transformation and the Pekar type variational (PTV) methods. Then, this kind of two-level quantum system can be excogitated to constitute a single qubit. When the electron locates at the superposition state of its related eigenfunctions, we get the time evolution of the electron's probability density. Finally, the influence of an APP on the QD qubit is investigated. The numerical calculations indicate that the probability density will oscillate periodically and it is a decreasing function of the effective confinement lengths of the APP in different directions. Whereas its oscillatory period is an increasing one and will diminish with enhancing the electron—phonon (EP) coupling strength.

First-order Raman scattering in cylindrical wurtzite nanowire

Abstract We have presented a theoretical study on electron resonant Raman scattering (ERRS) process associated with the bulk longitudinal optical (LO), surface optical (SO) and quasi-confined (QC) phonon modes in a free-standing wurtzite nanowire (NW). We consider the Frohlich electron–phonon interaction in the framework of the dielectric continuum model. Numerical calculations on the GaN material reveal that differential cross-section (DCS) is sensitive to the wire size. The bulk LO and high-frequency quasi-confined (QC+) phonons make main contributions to the DCS and the impact of the SO phonon can be negligible in the ERRS process. Moreover, scattering intensity of the bulk LO phonon is strongly enhanced as the incident photon energy approaches the energy band-gap of the GaN.

Exciton–phonon interaction in cylindrical semiconductor quantum dots

The exciton–longitudinal optical phonon interaction is theoretically investigated for the case of polar semiconductor cylindrical quantum dots embedded in semiconductor matrix. The theory is developed within the dielectric continuum model considering the Fröhlich interaction between electrons and confined bulk longitudinal optical phonons for a configurational interaction model of quantum dot. Representative longitudinal optical phonon mode for the exciton–phonon interaction is predicted for cylindrical InAs/GaAs quantum dots.

Resonant Raman scattering in CdSxSe1−xnanocrystals: effects of phonon confinement, composition, and elastic strain

AbstractOptical phonon modes, confined in CdSxSe1−xnanocrystal (NC) quantum dots (≈2 nm in radius) grown in a glass matrix by the melting‐nucleation method, were studied by resonant Raman scattering (RRS) spectroscopy and theoretical modeling. The formation of nanocrystalline quantum dots (QDs) is evidenced by the observation of absorption peaks and theoretically expected resonance bands in the RRS excitation spectra. This system, a ternary alloy, offers the possibility to investigate the interplay between the effects of phonon localization by disorder and phonon confinement by the NC/matrix interface. Based on the concept of propagating optical phonons, which is accepted for two‐mode pseudo‐binary alloys in their bulk form, we extended the continuous lattice dynamics model, which has successfully been used for nearly spherical NCs of binary materials, to the present case. After determining the alloy composition for NCs (that was evaluated with only 2–3% uncertainty using the bulk longitudinal optical phonon wavenumbers) and the NC size (using atomic force microscopy and optical absorption data), the experimental RRS spectra were described rather well by this theory, including the line shape and polarization dependence of the scattering intensity. Even though the presence of a compressive strain in the NCs (introduced by the matrix) masks the expected downward shift owing to the phonons' spatial quantization, the asymmetric broadening of both Raman peaks is similar to that characteristic of NCs of pure binary materials. Although with some caution, we suggest that both CdSe‐like and CdS‐like optical phonon modes indeed are propagating within the NC size unless the alloy is considerably heterogeneous. Copyright © 2011 John Wiley &amp; Sons, Ltd.

INFLUENCES OF THE TEMPERATURE AND THE PHONONS EFFECT ON THE PROPERTIES OF THE STRONG-COUPLING EXCITON IN A QUANTUM WELL

The influences of the temperature and the phonons effect on the properties of the exciton, which is strongly coupled with interface optical (IO) phonons and weakly coupled with bulk longitudinal-optical (LO) phonons, in a quantum well are studied by means of Huybrechts' linear-combination operator and the Lee–Low–Pines variational method. The expressions for the induced potential of the ground state, the energy shift of the ground state, and the first internal excited state of the exciton were derived. Numerical results are illustrated for AgBr / AgCl quantum well. The results indicate that the induced potential of the ground state, the energy shift of the ground state, and the energy shift of the first internal excited state of the exciton produced by the exciton strongly coupling with IO phonons increase with increasing temperature; however, the induced potential of the ground state, the energy shift of ground state, and the energy shift of the first internal excited state of the exciton produced by the exciton weakly coupling with bulk LO phonons decrease with increasing temperature. The influence of temperature is greater to the changes of the induced potential and the energy shift of the exciton with well widths and the distances between electron and hole.

Dipolar vibrational modes confined in ZnSe nanocrystals

AbstractOptical phonons confined in highly monodisperse and nearly spherical ZnSe quasi‐zero‐dimensionals (QDs) produced in the form of a colloidal solution in porous matrix have been studied experimentally and theoretically. Far‐infrared transmission spectra and Raman spectra show a broad band between the bulk transverse optical (TO) and longitudinal optical (LO) phonon frequencies. The data are discussed by calculating the frequencies of the spatially quantized phonon modes in the framework of a continuum model.

Influences of temperature and low-dimensional interface phonons on the effective mass of a polaron in a quantum well

The temperature dependence of the effective mass of a polaron, which is weakly coupled with bulk longitudinal optical (LO) phonons and strongly coupled with interface optical (IO) phonons in an infinite quantum well (QW), are studied by means of the Tokuda modified linear-combination operator method and the modified Lee-Low-Pines variational method. The expressions for the effective mass of the polaron as a function of the well width and temperature are derived. Numerical results show that the total effective mass of the polaron is composed of two parts: one part is the polaron effect induced by the interaction of the electron with LO-phonons, and the other part is the effective mass of the polaron caused by the interaction of the electron with IO-phonons. In particular, among four branches of IO phonons, only two branches interacting with electrons with the frequencies ω + + and ω - + make contributions to the effective mass of the polaron. Numerical result for KI/AgCl/KI QW shows that the IO phonons dispersion has an obvious influence on the interaction between the electron and the phonons. The contributions of the interactions between the electron and the different branches of phonons to the effective mass of the polaron and the changes of the effective mass with well width and temperature are markedly different.

Effects of electron– and impurity-ion–LO phonon couples on the impurity states in cylindrical quantum wires

The variational method and the effective mass approximation are used to calculate the phonon effects on the hydrogenic impurity states in a cylindrical quantum wire with finite deep potential by taking both the couplings of the electron-confined bulk longitudinal optical (LO) phonons and the impurity-ion–LO phonons into account. The binding energies and the phonon contributions are calculated as functions of the transverse dimension of the quantum wire. The results show that the polaronic effect induced by the electron–LO phonon coupling and the screening effect induced by the impurity-ion–LO phonon coupling tend to compensate each other and the total effects reduce the impurity binding energies.

Stark effects on bound polarons in polar cylindrical quantum wires with finite confining potential

The electric field effects on bound polarons in polar quantum wires with a finite confining potential are studied by a variational method. Both the confined bulk longitudinal optical phonons and the interface optical phonons are taken into account in considering the electron-phonon as well as the impurity-ion phonon couplings. The binding energies are calculated as functions of the transverse dimension of the quantum wire, and the donor-impurity position for different electric fields. The numerical results for the GaAs quantum wire are given and discussed as an example. A peak of the binding energy is found at a certain radius and shift toward a larger radius with applying an electric field. The calculated results confirm that the electron-phonon interaction reduces both the binding energies of impurity states and the Stark shifts and cannot be neglected.

Temperature effects on interface polarons in a strained (111)-oriented zinc-blende GaN/AlGaN heterojunction under pressure

The properties of interface polarons in a strained (111)-oriented zinc-blende GaN/AlxGa1−xN heterojunction at finite temperature under hydrostatic pressure are investigated by adopting a modified LLP variational method and a simplified coherent potential approximation. Considering the effect of hydrostatic pressure on the bulk longitudinal optical phonon mode, two branches interface-optical phonon modes and strain, respectively, we calculated the polaronic self-trapping energy and effective mass as functions of temperature, pressure and areal electron density. The numerical result shows that both of them near linearly increase with pressure but the self-trapping energies are nonlinear monotone increasing with increasing of the areal electron density. They are near constants below a range of temperature whereas decrease dramatically with increasing temperature beyond the range. The contributions from the bulk longitudinal optical phonon mode and one branch of interface optical phonon mode with higher frequency are important whereas the contribution from another branch of interface optical phonon mode with lower frequency is extremely small so that it can be neglected in the further discussion.

Phonon effect on binding energies of impurity states in cylindrical quantum wires of polar semiconductors under an electric field

Phonon effect on hydrogenic impurity states in cylindrical quantum wires of polar semiconductors under an applied electric field is studied theoretically by a variational approach. The binding energies are calculated as functions of the transverse dimension of the quantum wire, and the donor-impurity position under different fields. The electron–phonon interaction is considered in the calculations by taking both the confined bulk longitudinal optical phonons and interface optical phonons as well as the impurity-ion–phonon coupling. The numerical results for the CdTe and GaAs quantum wires are given and discussed as examples. It is confirmed that the electron–phonon interaction obviously reduces both the binding energy and the Stark energy-shift of the bound polarons in quantum wires.