Phonon Coupling Effects Research Articles

The study of far-infrared phonon spectra on Hg1-xCdxTe

The phonon spectra of mixed-crystal Hg1-xCdxTe with composition x=0.18 to 0.45 have been investigated by means of far-infrared transmission and reflection spectroscopy at temperatures from 4.5 to 300 K. In addition to low-frequency TA mode absorption and two-phonon combination absorption in transmission spectroscopy the fine structures of the CdTe-like optical phonon reflection band and the complex structure of the HgTe-like band, as well as the plasmon-LO phonon coupling effect have been observed and investigated both experimentally and theoretically.

Infrared absorption and far-infrared diffuse reflection spectra in Pb-doped Bi?Sr?Ca?Cu?O high-temperature superconductor

In this paper we study the room-temperature infrared transmission spectra (400–1600cm−1) and far-infrared diffuse reflection spectra (50–450cm−1) in Pb-doped Bi−Sr−Ca−Cu−O (2223) single phase (Tc=107k, sp1), multiphase (Tc=110k, sp2) and nonsuperconducting samples (sp3). The spectral features in superconductor are totally different from those in nonsuperconductor, which show the different crystal structure. The correlation existing between a factor group analysis of the phonons in (2223) and (2212) compounds affords a tentative assignment of ir-active modes in Pb-doped (2223) single phase by comparison with reported data in (2212) materials. The Cu−O stretching Eu vibration (605cm−1) of CuO2 layers is the characteristic vibrational mode related perovskitelike crystal structures. Two phonon coupling effect emerges in the infrared transmission spectra in Pb-doped superconductor. The Ca−O vibration A2u (254cm−1) might be related to superconductivity of Bi-based family.

Van Hove excitons and high-Tc superconductivity. II. Toward a calculation of Tc

For pt.I see ibid., vol.1, p.8911 (1989). Tc is estimated for the excitonic model developed in an earlier paper. It is shown that, by inclusion of transverse phonon coupling and retardation effects, the high values of Tc and small isotope effects can readily be understood. The connection of the excitons with covalent bond formation is stressed, as well as the relation of high-Tc oxide superconductors to other exotic superconducting materials.

Phonon ringing and anharmonicity effects in core spectra.

We show that the soft x-ray emission spectrum of the boron K core exciton in ${\mathrm{B}}_{2}$${\mathrm{O}}_{3}$ displays the largest phonon coupling effects yet observed in core spectroscopies. We interpret the observed double-peaked spectrum as the first clear example of phonon ringing. It is necessary to modify the existing theory of this effect by including anharmonicity in order to obtain an accurate description of the experimental results.

Far infrared studies of lattice and free carrier effects in Cd xHg 1−xSe

Far infrared reflection spectra of Cd xHg 1−xSe are measured in the region 20 to 600 cm -1 at temperature between 5 and 77 K. Reflection spectra are explained well by the dynamic dielectric function which is constructed as a sum of interband, intraband and phonon contributions. The plasma-LO phonon coupling and free carrier effects are discussed.

Libron–phonon coupling effect on the infrared absorption spectra of molecules trapped in matrices

The libron–phonon coupling (LPC) model is used to calculate the IR absorption coefficient of diatomic molecules trapped in solid matrices at low temperatures. The coupling between the collective motions of the crystal and the molecule libration-type motion is at the basis of the LPC model. This coupling results in temperature dependent shifts and broadening of the spectral lines even in the low temperature regime. The near IR experimental spectra of CO in Ne, Ar, Kr, and Xe matrices are successfully interpreted with the present theory. The strength of the libron–phonon coupling and other constants pertaining to the theory are reported. The far IR spectra is predicted as well.

Far infrared reflection spectra in narrow-gap semiconductors

Far infrared reflection spectra of CdXHg1−X Se are measured in the region 20 to 600 cm−1 at temperatures between 5 and 77 K. Reflection spectra are explained well by the dynamic dielectric function which is constructed as a sum of Γ8−Γ8 interband, intraband and phonon contributions. The plasmon-LO phonon coupling effect is also discussed.

Strong electron-phonon interaction effects in modulated transient reflectance spectra of Ga.50In.50P

Time-resolved modulated reflection spectrum near the fundamental edge of MBE-grown direct-gap GaxIn1−xP (x = 0.5) has been studied by using sensitive picosecond excite-probe methods. The spectrum at 77K shows considerable structure in comparison with steady-state photoluminescence data. In particular, a strong optical phonon coupling effect has been observed in the transient spectra, suggestive of impurity intermediated bound polaron behavior.

A comparison of the impurity superlattice model and DLSO results for the EL2 level in GaAs

Presents results for the case of a 4*4 impurity superlattice model of deep levels in semiconductors. In particular the authors consider the case of gallium arsenide and make comparison with deep level optical spectroscopy measurements for the EL2 level and show the degree of agreement to be good. They also consider the effects of phonon coupling and show them to be important, in particular in the case of photoionisation to the valence band.

Strong electron-phonon interaction effects in modulated transient reflectance spectra of Ga .50In .50P

Time-resolved modulated reflection spectrum below the fundamental edge of MBE-grown direct-gap Ga xIn 1-xP (x = 0.5) has been studied by using sensitive picosecond excite-probe methods. The spectrum at 77K shows considerable structure in comparison with previous steady-state photoluminescence data. In particular, a strong optical phonon coupling effect has been observed in the excitation spectrum, suggestive of an acceptor bound polaron.

Electron-phonon coupling and resonant magneto-phonon effect in optical behaviour of two-dimensionally confined charge carriers

Very recently enhanced electron-optical phonon coupling effects have been reported in the two-dimensionally confined carriers in GaAs/Al 1− x Ga x As superlattices and quantum wells. We report here the results of a study of several electron-optical phonon couplings possible in such systems. Where appropriate, zone-folding effects are explicitly taken into account. The relevance of some of these interactions to III–V compound semiconductor inversion/accumulation layers is noted. Certain special features of two-dimensional confinement in these systems make it possible to use resonant magneto—absorption studies as an unambiguous probe of these enhanced electron-phonon couplings. Results for the resonant splitting of the confined Landau level—optical phonon states are obtained within an approximation that retains only the resonant self-energy term of the upper Landau level. The resonant splitting is found to be proportional to B 3 4 for two-dimensionally confined carriers, in contrast to the B 2 3 behavior found in three-dimensional bulk semiconductors. Influence of damping and electron-light vertex correction is also discussed.

The photoionisation cross section of deep-level impurities in semiconductors

A simple analytic expression for the photoionisation cross section of deep-level impurities in semiconductors is obtained in a form that is explicitly related to the electron-scattering strength of the impurity. It also includes the dependence on size and charge, and the character of the localised-state Bloch functions. All of these quantities affect the spectral dependence immediately above threshold. The model introduces a simple billiard-ball wavefunction and the shape functions near threshold are derived, for neutral and charged (plus or minus) centres which are either donor-like or acceptor-like. The effect of phonon coupling is discussed in the context of a single-frequency model. At low temperatures the spectral shape near the shoulder of the curve is determined by the charge, and the effect of phonon coupling is to raise the apparent threshold.

Evidence for an imaginary spin-orbit term in the proton optical potential

Evidence is given that there is a positive imaginary spin-orbit potential in the proton optical potential for 40Ca at around 30 MeV, dropping with energy and apparently becoming negative at high energies. This is consistent with probable phonon-coupling effects but inconsistent at lower energies with ab initio OM calculations. Similar behaviour is found for 16O but with generally much smaller values of the imaginary potential. The authors comment on the need for suitable data to enable them to establish the general properties of Wso, the imaginary spin-orbit strength. The determination of an imaginary spin-orbit potential is made possible since the l-dependent potential used permits unprecedented quality of fit for angular distributions for these nuclei.

Path integral approach to the compressible Ising model

The path integral method is used to decouple the spin-photon variables in the Wagner-Swift Hamiltonian for a compressible Ising model where the exchange interaction is expanded up to terms quadratic in displacements. A brief review is given of the earlier work. The approximations made by the previous workers to obtain the effect of the spin phonon coupling on the Debye- Waller factor and the phonon frequencies are shown to correspond to the lowest-order terms in the cumulant expansion in the present approach. Finally, the procedure to extend the formalism to anharmonic crystals is outlined.