Two-phonon Absorption Spectra Research Articles

Two-phonon Absorption Spectra in the Layered Honeycomb Compound α-RuCl3

We performed infrared absorption spectroscopy for α-RuCl3 with a two-dimensional honeycomb lattice of Ru3+ ions. In the mid-infrared range, we observed three absorption peaks corresponding to intra-atomic d–d transitions among Ru3+ ions in accordance with a previous report. In the far-infrared range, we observed conspicuous broad absorption structures near 600 cm−1 in addition to several sharp single phonon peaks. Our first-principles calculations for the isolate sandwich model together with the factor group analysis revealed that the peak structures near 600 cm−1 are absorptions related to a two-phonon process. Our results stimulate further study on the spin–lattice coupling in α-RuCl3 as well as the optical detection of Majorana excitations in Kitaev spin liquids.

Two-phonon absorption spectra in wurtzite GaN

Two-phonon absorption spectrum was measured for wurtzite GaN and was compared with two-phonon density of states of zinc-blende GaN obtained by lattice-dynamical calculations with an adiabatic bond-charge model. All structures in the spectrum were well explained by phonon structures of zinc-blende GaN. As a method to analyze two-phonon absorption spectra of wurtzite materials, proposed was the utilization of phonon dispersion curves of corresponding zinc-blende modifications, and the method was found to be useful.

Lattice vibrations in CuInSe2 crystals

Infrared reflection spectra of CuInSe2 crystals are investigated for the polarizations E∥c and E⊥c. Contours of the reflection spectra are calculated and phonon parameters and dielectric constants are determined. The effective charges of the Cu, In, and Se ions in these materials are determined. The two-phonon absorption spectra in CuInSe2 are investigated and the absorption bands are identified from the selection rules.

Lattice vibrations in CuIn 1− xGa xSe 2 crystals

Infrared reflectivity spectra of CuInSe 2, CuGaSe 2 and CuIn 1− x Ga x Se 2 solid solutions have been investigated for the polarizations E| c and E⊥ c. Reflectivity spectrum contours are calculated and phonon parameters and dielectric constant are determined. The dependence of the longitudinal-transverse splitting, the oscillator strength and the damping factor on x composition are shown. Raman scattering of CuIn 1− x Ga x Se 2 solid solution has been investigated. Effective ionic charges for ions of these materials are determined. Two-phonon absorption spectra in CuGaSe 2 and CuInSe 2 have been investigated and absorption bands have been identified in accordance with selection rules.

Lattice dynamics of CuAlS 2 and CuAlSe 2

We have measured the first-order Raman spectra of CuAlSe 2 and the second-order Raman spectra of CuAlS 2 and CuAlSe 2, and reexamined two-phonon absorption spectra of CuAlS 2. Lattice dynamical calculations have been performed using a rigid ion model. Force constants and charges as fitting parameters have been determined so as to reproduce zone-center phonon frequencies and the two-phonon spectra observed. Calculated two-phonon density of states shows reasonable agreements with those spectra.

Two-photon absorption in GaAs/AlGaAs multiple quantum wells.

Photoconductivity measurements revealing two-photon absorption in GaAs/AlGaAs multiple quantum wells are presented. It is shown that both the forbidden 1S and the allowed 2P excitons can be observed. The energy separation between 1S and 2P excitons is determined directly from the absorption spectra to high accuracy. The data give evidence for two different absorption mechanisms: One is the two-photon absorption and the other one is the absorption of the second-harmonic wave which is generated inside the sample by nonlinear optical processes. By means of this additional absorption mechanism, the observation of the 1S excitions in two-phonon absorption spectra can be explained qualitatively.

Raman scattering and far-infrared studies of the vanadium dihalides with layered structure

The vanadium dihalides with layered structure, V${\mathrm{Cl}}_{2}$, V${\mathrm{Br}}_{2}$, and V${\mathrm{I}}_{2}$, have been investigated by Raman and infrared spectroscopy. A compilation of the measured optical frequencies is given. The new lines appearing in the Raman spectrum of V${\mathrm{I}}_{2}$ below its N\'eel temperature ${T}_{N}$ are attributed to zone-boundary-phonon Raman scattering induced by Bragg scattering from the spin superstructure. We discuss the symmetry properties and selection rules for the proposed spin-dependent electron-phonon coupling mechanism due to a phonon modulation of the exchange interaction. The measured two-phonon absorption spectra of the three materials show a shift to lower frequencies with respect to the calculated two-phonon density of states. Some possible reasons for this unusually large shift are discussed. The Raman spectra of V${\mathrm{Cl}}_{2}$ and V${\mathrm{Br}}_{2}$ show additional features at low temperature, different from the sharp lines observed in V${\mathrm{I}}_{2}$ below ${T}_{N}$. This additional scattering is also shifted to lower frequencies compared to the calculated one-phonon density of states. We explain the appearance of the additional Raman scattering by a spin-phonon coupling via the exchange modulation mechanism, whereas the shift in the Raman spectra should be due to a self-energy renormalization by the coupling mechanism.

Far-infrared phonon absorption in InSb

Far-infrared absorption due to single-phonon and two-phonon processes was investigated in undoped InSb using a high-resolution Fourier-transform spectrometer. Single-phonon absorption by both transverse- and longitudinal-optical modes at the Brillouin-zone center was observed. Two-phonon absorption spectra correlated well with two-phonon density-of-states curves calculated using parameters derived from inelastic-neutron-scattering experiments. Brillouin-zone locations of critical points that gave rise to prominent features on the two-phonon density-of-states curve were identified by investigating phonon frequency contours on symmetry planes. Critical points at (0.6, 0, 0), $X$, and $L$ for phonon combinations involving transverse modes and on the (111) boundary plane of the Brillouin zone for combinations containing the longitudinal-acoustical phonon gave rise to almost all the prominent features observed on the two-phonon absorption spectra. Derived frequencies for phonon modes at (0.6, 0, 0), $X$, and $L$ agreed well with values determined directly from inelastic-neutron-scattering experiments. Although strong features, arising from 2TA overtones at $X$ and $L$, appear in the calculated spectrum, no absorption corresponding to these features was observed.

Optical and dielectric properties and lattice dynamics of some fluorite structure ionic crystals

Some optical, dielectric and lattice properties of three alkaline earth fluorides (calcium, strontium and barium fluorides) and of two other isomorphous fluorides (cadmium and lead fluoride) are reported. Results obtained from experimental studies on infrared and Raman spectra give the fundamental transverse and longitudinal optical lattice mode frequencies, the two-phonon absorption spectra, and the spectra associated with U centre absorptions. Calculations of phonon dispersion curves, density of states and density of com­bined states functions, have been made for all five crystals and these are compared with the experimental results.

Two-phonon absorption spectrum of NaNO 3

From the low temperature two-phonon absorption spectrum of single crystals of NaNO 3 some information has been obtained about the wave vector dispersion of the phonon branches for the inner (NO - 3)-vibrations and the lattice vibrations.