Zone-center Longitudinal Optical Phonon Research Articles

A tunable time-resolved spontaneous Raman spectroscopy setup for probing ultrafast collective excitation and quasiparticle dynamics in quantum materials.

We present a flexible and efficient ultrafast time-resolved spontaneous Raman spectroscopy setup to study collective excitation and quasi-particle dynamics in quantum materials. The setup has a broad energy tuning range extending from the visible to near infrared spectral regions for both the pump excitation and Raman probe pulses. Additionally, the balance between energy and time-resolution can be controlled. A high light collecting efficiency is realized by high numerical aperture collection optics and a high-throughput flexible spectrometer. We demonstrate the functionality of the setup with a study of the zone-center longitudinal optical phonon and hole continuum dynamics in silicon and discuss the role of the Raman tensor in time-resolved Raman scattering. In addition, we show an evidence for unequal phonon softening rates at different high symmetry points in the Brillouin zone of silicon by means of detecting pump-induced changes in the two-phonon overtone spectrum. Demagnetization dynamics in the helimagnet Cu2OSeO3 is studied by observing softening and broadening of a magnon after photo-excitation, underlining the unique power of measuring transient dynamics in the frequency domain, and the feasibility to study phase transitions in quantum materials.

Lyman spectra of holes bound to Cu, Ag, and Au acceptors in ZnTe and CdTe

The group IB impurities (Cu, Ag, and Au) incorporated into the II-VI zinc blende hosts, ZnTe and CdTe, exhibit in their infrared absorption spectra well-resolved excitation lines followed by a photoionization continuum. They are due to electric dipole transitions from the ``$1s$-like'' ground state to the various ``$p$-like'' excited states characteristic of a hole bound to a Coulomb center. Their spacings agree well with those predicted in the effective mass theory for single acceptors as expected for group IB elements substitutionally replacing the group IIB cations of the host. The simultaneous excitation of a Lyman transition in combination with the zone center longitudinal optical phonon, hence lying in the photoionization continuum, displays Fano-like asymmetry. The large chemical shift of the $1s$ ground states results in the pronounced shift of a Lyman line of a given acceptor with respect to the corresponding Lyman line of another acceptor in the same host.

Monte Carlo study of phonon dynamics in III-V compounds

The Monte Carlo (MC) stochastic method is used to solve the phonon Boltzmann transport equation. A solution scheme taking into account all the different individual types of Normal and Umklapp processes which respect energy and momentum conservation rules is presented. The use of the common relaxation time approximation in the MC solver is thus avoided. A generalized Ridley theoretical scheme is used instead to calculate three-phonon scattering rates, with the Grüneisen constant as the only adjustable parameter. A method for deriving adequate adjustable parameters is presented. Polarization branches with real nonlinear dispersion relations for transverse or longitudinal optical and acoustic phonons are considered. Zone-center longitudinal optical (LO) phonon lifetimes are extracted from the MC simulations for GaAs, InP, InAs, and GaSb. Decay channels contributions to zone-center LO phonon lifetimes are investigated using the calculated scattering rates. Vallée-Bogani’s channel is found to have a negligible contribution in all studied materials, notably GaAs. A comparison of phonons behavior between the different materials indicates that the previously reported LO phonon lifetimes in InAs and GaSb were quite underestimated.

Ultra-violet resonant Raman scattering studies of dilute GaPN alloys

We report a resonant Raman scattering study of GaP1 xNx in the ultraviolet spectral range. A strong intensity resonance near the E1 transition energy exhibiting a zone-center longitudinal optical phonon provides evidence that the L-point conduction band edge of GaP1 xNx is strongly perturbed by nitrogen isoelectronic impurities and slowly moves up in energy with increasing nitrogen concentration. This result, combined with results of previous studies, strongly suggests that GaP1 xNx is best described as an impurity band system.

Calculation of Anharmonic Coupling Constants between Phonon Modes in GaAs

Anharmonic coupling between phonon modes of a crystal determine many of its properties, thermal conductivity being a standard example. Although anharmonicity-related effects observed in bulk crystals (decay of the zone-center longitudinal optical (LO) phonon or acoustic wave attenuation) rely only on a small subset of anharmonic couplings, the full set of such constants is needed for a complete description of phonon-induced dephasing and relaxation of electrons and excitons in quantum dots [1–4]. The reason for this is that breaking of the translational symmetry and appearance of a quantized spectrum completely changes the energy-momentum conservation conditions and opens decoherence (relaxation and dephasing) paths which are not allowed in a bulk crystal. So far, only the anharmonic couplings between acoustic branches have been completely described [5], while the only known coupling involving optical phonons is that between the LO phonon and two longitudinal acoustic (LA) phonons, which seems to be responsible for the decay of the zone-center LO phonon in a bulk crystal [6] and is therefore accessible experimentally [7]. In this paper, we derive third-order anharmonic coupling constants between various phonon modes in a zinc-blende crystal. We use a model combining the

Electron–phonon coupling effect on quantum transport in carbon nanotubes using time-dependent wave-packet approach

We investigate the electron–phonon coupling effects on the electronic transport properties of the metallic ( 5 , 5 ) -carbon nanotube using the time-dependent wave-packet approach based on the Kubo–Greenwood formula within a tight-binding approximation. We consider the zone-center longitudinal optical phonon among various phonon modes, and calculate the conductance for the 10 μ m carbon nanotubes at room temperature. We show that the electron–phonon coupling largely decreases the conductance around the Fermi energy, which is explained by the time-dependent behaviors of propagating length and the electronic band structure.

Residual strains in cubic silicon carbide measured by Raman spectroscopy correlated with x-ray diffraction and transmission electron microscopy

Cubic (3C) silicon carbide (SiC) epilayers grown on Si substrates by chemical vapor deposition, characterized using transmission electron microscopy (TEM), high-resolution x-ray diffraction (HRXRD), and Raman spectroscopy, reveal the presence of biaxial in-plane strain. Defect (stacking faults, twins, dislocations) distributions revealed by TEM are correlated with peak widths obtained from HRXRD measurements and Raman shifts of the zone center longitudinal optical phonon line. TEM showed defect densities decreasing with increasing distance from SiC∕Si interface as the lattice mismatch stress is relaxed. Structural defect densities show the most significant reduction within the first 2μm of the epilayer. TEM observations were correlated with a monotonic decrease in HRXRD peak width (full width at half maximum) from 780arcsec (1.5μm thick epilayer) to 350arcsec (10μm thick epilayer). Raman spectroscopy indicates that the residual biaxial in-plane strain decreases with increasing epilayer thickness initially, but becomes essentially constant between 6 and 10μm. Differences in the observed behavior between HRXRD and Raman spectroscopy are discussed in terms of the fundamental interactions of incident radiation with the 3C-SiC epilayers.

Optical properties of Si-doped InN grown on sapphire (0001)

The carrier concentration dependence of the interaction between free carriers and longitudinal optical (LO) phonons of InN is studied by Raman scattering and Fourier transform infrared measurements. InN is grown on a sapphire (0001) surface by plasma-assisted molecular beam epitaxy. The carrier concentration is varied from $1.8\ifmmode\times\else\texttimes\fi{}{10}^{18}$ to $1.5\ifmmode\times\else\texttimes\fi{}{10}^{19}{\mathrm{cm}}^{\ensuremath{-}3}$ by Si doping. The infrared reflection spectra, to which the vibration in the $a\ensuremath{-}b$ plane contributes, reveal a linear coupling between the ${\mathrm{E}}_{1}(\mathrm{LO})$ phonon and the plasma oscillation of the free carriers. From the plasma frequency the electron effective mass is estimated to be ${m}_{e\ensuremath{\perp}}^{*}{=0.085m}_{0}$ for the intrinsic InN. The Raman spectra, to which the vibration along the c axis contributes, reveal that the ${\mathrm{A}}_{1}(\mathrm{LO})$ phonon and free carriers couple nonlinearly, where a Fano interference between the zone-center LO phonon and the quasicontinuum electronic state along the c axis is prominent. With these results, the anisotropic electronic structure of InN is discussed.

Lyman spectrum of holes bound to substitutional 3d transition metal ions in a III-V host:GaAs(Mn2+,Co2+,orCu2+),GaP(Mn2+),andInP(Mn2+)

The 3d-transition metal impurities (Mn, Co, and Cu) incorporated into III-V zinc blende hosts (GaAs, GaP, or InP) exhibit well resolved excitation lines followed by a photoionization continuum in their infrared absorption spectra. They are associated with transitions from a ``1s-like'' ground state to various ``p-like'' excited states characteristic of a hole bound to a Coulomb center. Their spacings agree remarkably well with those predicted in the effective mass theory for single acceptors [A. Baldereschi and N. O. Lipari, Phys. Rev. B 9, 1525 (1974)] as expected for 3d-transition metal ions substitutionally replacing the group III cations of the host. The shape of the photoionization spectra, the occurrence of the simultaneous excitation of the Lyman transitions in combination with the zone center longitudinal optical phonon and hence lying in the photoionization continuum and displaying Fano-like asymmetries, the additional continuum excitations to and beyond the ${p}_{1/2}$ valence band maximum, are the features described and interpreted. Cu acceptors in GaAs show Lyman doublets, indicative of two independent centers; it is speculated that they could arise from He-like ${3d}^{10}{4s}^{2}{4p}^{1},$ ${\mathrm{Cu}}^{2\ensuremath{-}},$ and H-like ${3d}^{9}{4s}^{2}{4p}^{1},$ ${\mathrm{Cu}}^{\ensuremath{-}},$ Coulomb centers.

Infrared observation of longitudinal optical modes in MBE-grown polar semiconductors films

The feasibility of observing the zone-center longitudinal optical phonon (LO) of a crystal as a minimum in its infrared transmission spectrum, when measured with radiation obliquely incident on a thin film of the crystal, i.e., the Berreman effect, is demonstrated with an MBE′-grown zinc-blende MnTe epilayer. Whereas the zone-center transverse optic phonon (TO) appears irrespective of the polarization of the incident radiation for both normal and oblique incidence, the LO can be detected only in the latter with light polarized in the plane of incidence. We have verified the Berreman effect in a variety of MBE-grown II–VI epilayers and heterostructures.

Magnetopolaron in a quantum well: LO-phonon confinement effects

Magnetopolarons in a polar-crystal slab are studied considering electron coupling with confined LO-phonons. The electron self-energy is calculated by use of a standard perturbation theory treatment. It is demonstrated that phonon confinement lowers polaron effects. The influence of electron coupling to interface phonons on the polaron shift is discussed. Special attention is paid to the magnetophonon resonance. Phonon dispersion is shown to be important for the interpretation of the pinning of the transition frequencies in high magnetic fields to values much lower than the Brillouin zone center longitudinal optical phonon frequency.

Magnetopolaron in a Quantum Well. LO‐Phonon Confinement Effects

AbstractMagnetopolarons in a polar‐crystal slab are studied considering electron coupling with confined LO‐Phonons. The electron self‐energy is calculated by using the standard perturbation theory treatment. Screening effects are included within the static Thomas‐Fermi approximation. It is demonstrated that phonon confinement lowers the polaron effects. Special attention is paid to the magnetophonon resonance. Phonon dispersion is shown to be important for the interpretation of the interpretation of the pinning of the transition frequencies for high magnetic fields to values much lower than the Brillouin zone center longitudinal optical phonon frequency ω0.

Dielectric function of SmS: II. Infrared response: LO(Γ) phonon softening

The zone-center longitudinal optical phonon of semiconducting SmS shows a 10% softening with respect to that of EuS. This result confirms the lattice-dynamical model prediction and provides first experimental evidence for the existence of an anomalous dipolar charge deformability of the Sm 2+ ion in SmS.

Band Structure Parameters Deduced from Tunneling Experiments

Measurements of the voltage and temperature dependence of tunneling in Ge and GaSb are presented which confirm the close proximity of the (000) and (111) conduction band edges in these materials. In the case of Ge, the energy separation of these edges is found to increase with increasing donor concentration. Tunneling in the indirect semiconductor GaP shows no evidence for indirect (phonon-assisted) tunneling transitions. It is believed that tunneling in the junctions which we studied proceeds via deep-level impurities rather than between conduction and valence bands directly, thereby eliminating the requirement of wave number conservation. Revised values for the zone-center longitudinal optical phonon energies as deduced from tunneling data in 3–5 and lead salt semiconductors are presented.