Optical Lattice Vibrations Research Articles

Mid-infrared thermal radiation resonating with longitudinal-optical like phonon from n++-doped GaN–semi-insulating GaN grating structure

The mid-infrared emission mechanism of line-and-space structures of metallic plates on dielectric materials is substantiated using high conductive n-doped (n++-) GaN–semi-insulating (SI-) GaN microstripe structures on an SI-GaN epitaxial layer, which was veiled when using line-and-space structures of Au plates. The present structure exhibits a few thermal emission lines originating from electric dipoles resonating with the coherent longitudinal optical (LO) phonon-like lattice vibration, which are formed by the local depolarization electric field in the surface n++-GaN/SI-GaN/n++-GaN regions. The energies of the LO-phonon-like modes shift from the original LO-phonon energy of GaN to the lower energy region, which contrasts with the LO-phonon resonant emission from the microstructures on GaAs. These emission lines have another notable feature, i.e. the observed peak energies are independent of the polar emission angle for both s- and p-polarizations, unlike the emissions by surface phonon polaritons showing a significant directive nature of peak energies. The results show that each peak energy of the present emission lines is positioned at the zero-point of the real part of the electric permittivity comprising the components of the transverse optical phonon and other electric dipoles induced by the LO-like modes, excluding the target mode. The significant peak-energy shift of the LO-like phonons is applicable to materials with wide Reststrahlen bands, which contrasts with that of the nearly LO-phonon resonating feature of materials with narrow Reststrahlen bands, such as GaAs. The peak energy shift depending on the emission direction is observed for Au–GaN stripe structures. This property is ascribed to the imperfect Au/GaN interface with surface states through the theoretical analysis of the modified electric permittivity in the surface region, numerical simulation of the local electric field via finite-difference time-domain calculation, and experimental studies on a Ti–GaN structure and emission peaks originating from an LO-like phonon of the α-Al2O3 substrate.

Coherent phonon relaxation in strontium titanate

Ultrafast decays of the intrinsic phonon modes within 260–790 cm−1 range of frequencies, which are symmetry forbidden in the first-order Raman scattering in bulk SrTiO3 crystal, have been traced using time-resolved coherent Raman technique. The effective phonon decay times are within 0.86–1.38 ps and yield corresponding linewidths of 7.7–12.3 cm−1 for optical lattice vibrations. Symmetry breakage mechanisms related to free charged carriers resulting from vacancies do not appear to play a major role.

Optical lattice vibrations in monolayer hexagonal boron nitride from macroscopic equations

Using two pairs of macroscopic equations deduced from a dipole lattice model including electronic polarization (EP) of ions and local field effects (LFEs) self-consistently, optical lattice vibrations in monolayer hexagonal boron nitride (BN) are studied theoretically for both in-plane and out-of-plane motions. Longitudinal and transverse optical (LO and TO) modes and out-of-plane (ZO) modes are derived, and explicit expressions are obtained for phonon dispersion, group velocity and density of states. The analytical phonon dispersion relations describe previous numerical results very well; the LO phonon dispersion is identical to the analytical expression of Sohier et al, which shows the degeneracy of the LO and TO modes at and the splitting at finite wavevectors due to the long-range macroscopic field. Whilst relating to microscopic quantities, the linear coefficients of the lattice equations are determined by first-principles calculated quantities (such as macroscopic susceptibilities). Therefore the EP and LFEs on the vibrational properties are studied. With no EP or LFEs, all the phonon frequencies are overestimated significantly. Including EP, the LFEs increase (decrease) the in-plane (out-of-plane) dielectric susceptibility by a factor of 2.5–3.5. Both ionic EP and LFEs should be included to obtain an accurate description of the lattice dynamics.

Anomalous Lattice Properties of Manganese Monosilicide: the Thermodynamic Approach

The temperature dependences of the thermal and elastic MnSi properties are analyzed within the limits of the self-consistent approach. The role of the anharmonicity of acoustic and optical lattice vibrations is analyzed. It is demonstrated that anomalously weak anharmonicity is characteristic for acoustic phonons. In particular, this explains the temperature dependence of the MnSi bulk compressive modulus. The one-electron contribution is estimated and the anomalous non-lattice contribution to the formation of the temperature dependences of heat capacity and volume thermal expansion coefficients.

Optical phonon modes and polaron related parameters in GaxIn1−xP

Based on a pseudopotential approach under the virtual crystal approximation that includes the effect of compositional disorder, the optical lattice vibration frequencies and polaron related parameters in zinc-blende GaxIn1−xP have been studied. Our findings showed generally reasonably good accord with data in the literature. Other case, our results are predictions. The composition dependence of longitudinal optical (LO) and transverse optical (TO) phonon modes, LO-TO splittings, Frӧhlich coupling parameter, Debye temperature of LO phonon frequency, and polaron effective mass has been analyzed and discussed. While a non-monotonic behavior has been noticed for the LO and TO phonon frequencies versus Ga concentration x, a monotonic behavior has been observed for the rest of the features of interest. The information derived from this investigation may be useful for optoelectronic technological applications.

Impact of superconducting gap on the phonon mode mixing itinerant and localized electron states

The influence of superconducting ordering on the optical lattice vibration modified through the mixing of localized states with Hubbard correlation and itinerant band states is analyzed. We consider the situation where the broad electron band is located between lower and upper Hubbard levels. The hardening or softening of phonon dynamics caused by the opening of superconducting gap in the band of itinerant carriers appears in dependence on the disposition of the electron spectrum and chemical potential. The effect varies substantially if the chemical potential approaches the edges of the band of itinerant carriers or the van Hove singularity.

Terahertz Sum-Frequency Excitation of a Raman-Active Phonon.

In stimulated Raman scattering, two incident optical waves induce a force oscillating at the difference of the two light frequencies. This process has enabled important applications such as the excitation and coherent control of phonons and magnons by femtosecond laser pulses. Here, we experimentally and theoretically demonstrate the so far neglected up-conversion counterpart of this process: THz sum-frequency excitation of a Raman-active phonon mode, which is tantamount to two-photon absorption by an optical transition between two adjacent vibrational levels. Coherent control of an optical lattice vibration of diamond is achieved by an intense terahertz pulse whose spectrum is centered at half the phonon frequency of 40THz. Remarkably, the carrier-envelope phase of the THz pulse is directly transferred into the phase of the lattice vibration. New prospects in general infrared spectroscopy, action spectroscopy, and lattice trajectory control in the electronic ground state emerge.

Effect of temperature and doping with Cu on the reflectance spectra of PbSb2Te4 crystals

For the anisotropic PbSb2Te4 single crystal, the effect of temperature and doping with Cu on the reflectance spectrum R(ν) is studied for reflection from a cleavage plane orthogonal to the trigonal axis C3. It is found that the spectrum is complex in structure and exhibits a sharp increase in the reflectance in the far-infrared region and two minima in the mid-infrared region. It is shown that the experimentally observed feature of the reflectance spectra can be quantitatively described in the context of the Drude–Lorentz theory, with the dielectric function that takes into account the contributions of hole plasma oscillations and two optical lattice vibrations at different frequencies. The static conductivity estimated from optical data is in good agreement with the results of electrical measurements of the dc conductivity.

Nonlocal electron-phonon coupling in organic semiconductor crystals: The role of acoustic lattice vibrations

We discuss, in the context of a tight-binding description, how the electronic and charge-transport properties in single crystals of molecular organic semiconductors are affected by the nonlocal electron-phonon coupling to both acoustic and optical lattice vibrations. While the nonlocal electron-phonon interactions can in general be divided into contributions from symmetric modes and antisymmetric modes, we show that only the antisymmetric coupling mechanism is operational in the case of acoustic vibrations. Interestingly, when the quantum nature of the phonons can be neglected, the effect of electron-phonon interactions with acoustic phonons is found to be equivalent to that of the electron-phonon interactions with optical phonons, in the case where contributions from symmetric and antisymmetric modes are equal.

A Study of the Long-Wavelength Optical Lattice Vibrations in Quaternary AlxInyGa1-x-yN Alloys

A Study of the Long-Wavelength Optical Lattice Vibrations in Quaternary AlxInyGa1-x-yN Alloys

Effect of in Concentration on the Optical Lattice Vibrations in Quaternary Al<sub>x</sub>In<sub>y</sub>Ga<sub>1-x-y</sub>N Alloys

Fourier transform infrared (FTIR) spectroscopy has been utilized to measure long-wavelength optical lattice vibrations of high-quality quaternary AlxlnyGa1-x-yN thin films at room temperature. The AlxlnyGa1-x-yN films were grown on c-plane (0001) sapphire substrates with AlN as buffer layers using plasma assisted molecular beam epitaxy (PA-MBE) technique with indium (In) mole fraction y = 0.0 to 0.10 and constant aluminium (Al) mole fraction x = 0.06. The experimental results indicated that the AlxlnyGa1-x-yN alloys had two-mode behavior, for the A1 (LO) and E1 (TO) modes.

Multiphonon Resonant Raman Scattering (MRRS) of Semiconductor Nanomaterials for Biodetection

Robust, rapid and sensitive detection of specific molecules at ultra low concentrations is becoming increasing important, especially for medical diagnosis, food safety and environmental protection. Various spectroscopic techniques have been developed to apply in analysis and assay fields, including fluorescent spectroscopy, infrared spectroscopy and surface-enhanced Raman spectroscopy (SERS), etc. Multiphonon resonant Raman scattering (MRRS) technique is usually used to investigate optical lattice vibrations and the interaction between phonon and electron in semiconductor nanomaterials. Recently, it exhibits fascinating prospects in biodetection due to its significant specific spectroscopic and material advantages. The multiple-order phonon lines are material intrinsic and thus can be used as a characteristic fingerprint signal to label the biomolecules. The strong anti-interference capacity to external environment allows an accurate detection. Narrow bandwidth makes the technique suitable for multiplexed analysis. Proper choice of nontoxic inorganic semiconductor materials facilitates in-vivo detection. This review focuses on the current theoretical and technical developments in MRRS of semiconductor nanomaterials and highlights their applications in bioanalysis. The basic principle, merits, challenges and future perspectives of MRRS detection technique are discussed.

Experimental investigation of long-wavelength optical lattice vibrations in quaternary Al xIn yGa 1− x−yN alloys and comparison with results from the pseudo-unit cell model

Raman and Fourier transform infrared (FTIR) spectroscopies have been utilized to measure long-wavelength optical lattice vibrations of high-quality quaternary Al x In y Ga 1− x−y N thin films at room temperature. The Al x In y Ga 1− x−y N films were grown on c-plane (0 0 0 1) sapphire substrates with AlN as buffer layers using plasma assisted molecular beam epitaxy (PA-MBE) technique with aluminum (Al) mole fraction x ranging from 0.0 to 0.2 and constant indium (In) mole fraction y=0.1. Pseudo unit cell (PUC) model was applied to investigate the phonons frequency, mode number, static dielectric constant, and high frequency dielectric constant of the Al x In y Ga 1− x−y N mixed crystals. The theoretical results were compared with the experimental results obtained from the quaternary samples by using Raman and FTIR spectroscopies. The experimental results indicated that the Al x In y Ga 1− x−y N alloy had two-mode behavior, which includes A 1(LO), E 1(TO), and E 2(H). Thus, these results are in agreement with the theoretical results of PUC model, which also revealed a two-mode behavior for the quaternary nitride. We also obtained new values of E 1(TO) and E 2(H) for the quaternary nitride samples that have not yet been reported in the literature.

Anomalous properties of optical lattice vibrations in HgTe: A double-well model of the lattice potential for a Hg atom

A double-well potential model has been proposed for a Hg atom in the HgTe crystal lattice in which the Hg atom either occupies the center of the anion tetrahedron or is located at the off-center position. The fundamental TO mode of the Hg-Te vibrations at a frequency of ∼120 cm−1 corresponds to the vibrations of the off-center Hg atom, and the additional mode is associated with the vibrations of the Hg atom located at the center of the anion tetrahedron. The model has made it possible to explain both the strong increase in the intensity of the additional mode with increasing temperature, which, at T = 300 K, is comparable to the intensity of the fundamental mode, and the anomalous change (decrease) in the damping parameter of the fundamental TO mode with increasing pressure, which corresponds to a more ordered crystal structure.

Bounds on the Minimal Energy of Translation Invariant N-Polaron Systems

For systems of $N$ charged fermions (e.g. electrons) interacting with longitudinal optical quantized lattice vibrations of a polar crystal we derive upper and lower bounds on the minimal energy within the model of H. Fr\"ohlich. The only parameters of this model, after removing the ultraviolet cutoff, are the constants $U>0$ and $\alpha>0$ measuring the electron-electron and the electron-phonon coupling strengths. They are constrained by the condition $\sqrt{2}\alpha<U$, which follows from the dependence of $U$ and $\alpha$ on electrical properties of the crystal. We show that the large $N$ asymptotic behavior of the minimal energy $E_N$ changes at $\sqrt{2}\alpha=U$ and that $\sqrt{2}\alpha\leq U$ is necessary for thermodynamic stability: for $\sqrt{2}\alpha > U$ the phonon-mediated electron-electron attraction overcomes the Coulomb repulsion and $E_N$ behaves like $-N^{7/3}$.

Optical properties of the Si-doped GaN/Al2O3 films

Morphological and optical studies of the Si-doped GaN films (doping level NSi = 1.5 × 1019 cm−3) grown by vapor-phase epitaxy from metalorganic compounds on a sapphire substrate oriented along the c axis are conducted. For the grown GaN films, the following characteristics are obtained: energy of electron transition E0, absorption coefficient α, refractive index n, and frequencies of transverse and longitudinal optical lattice vibrations characteristic of the crystalline GaN films.

Interaction of Charge Carriers with Lattice Vibrations in Organic Molecular Semiconductors: Naphthalene as a Case Study

Recent theoretical studies suggest that the modulation of the electronic couplings (transfer integrals) between adjacent molecules by lattice vibrations, i.e., the so-called nonlocal electron−phonon coupling, plays a key role in the charge-transport properties of molecular organic semiconductors. However, a detailed understanding of this mechanism is still missing. Here, we combine density functional theory calculations and molecular mechanics simulations and use a chemistry-based insight to derive the nonlocal electron−phonon coupling constants due to the interaction of charge carriers with the optical lattice vibrations in the naphthalene crystal. The results point to a very strong coupling to both translational and librational intermolecular vibrational modes as well as to intramolecular modes. Along some crystal directions, the nonlocal interactions are found to be dominated by nontotally symmetric vibrational modes which lead to an alternation (Peierls-type dimerization) pattern. Importantly, we intr...

Low-frequency optical lattice vibrations in Hg1 − x Cd x Te alloys

The lattice reflection spectra of the Hg1 − x Cd x Te (x = 0.06–0.70) alloys measured in the low-frequency range of optical vibrations (the region of the anomalous mode of Hg-Te vibrations in HgTe) at room temperature are interpreted. The low-frequency modes observed at frequencies of 98, 105, and 112 cm−1 for all compositions of the Hg1 − x Cd x Te alloy are assigned to the modes of Hg-Te vibrations, as was previously done for modes of Cd-Te vibrations in the quasi-molecular approximation. According to the double-well potential model for the Hg atom in the crystal lattice of the alloy, the Hg atom either can occupy the center of the anion tetrahedron or can be located in the off-center position. The fundamental strong mode of Hg-Te vibrations at a frequency of about 120 cm−1 (at T = 300 K) corresponds to the vibrations of the off-center Hg atom, and the low-frequency vibration modes correspond to the vibrations of the Hg atom located at the center of the anion tetrahedron.

10.1007/s11700-008-1013-1

It is shown that total positron scattering on acoustic and optical lattice vibrations in atomic semiconductors and ionic crystals causes total positron thermalization before annihilation at both high and low (T≤10 K) temperatures.

Alloy composition fluctuation and band edge energy structure of In‐rich InxGa1–xN layers investigated by systematic spectroscopy

AbstractAfter the finding of the bandgap energy of InN as less than 1 eV, the re‐examination of the bandgap of InGaN alloys and its bowing parameter is an issue of interest. We study the absorption spectra of In‐rich InGaN layers by taking account of the Burstein‐Moss effect and bandgap renormalization with the analysis of photoluminescence and infrared reflectance spectra. We find the far larger broadening of optical transition and lattice vibration energies than those expected from the alloy composition fluctuation based on the x‐ray diffraction patterns. We estimate that local potential fluctuation by the alloy composition fluctuation give rise to the strong localization of carriers and relaxation of the momentum conservation rule in optical transition. The average bandgap energies of the films show the bowing parameter less than 1.4 eV; the value down to 0.7 eV is possible. (© 2007 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)