Temperature-dependent phonon linewidths and shifts in bismuth from inelastic neutron scattering

Raman spectra of TlGaSe2 crystal at different temperatures are discussed. The temperature dependence of frequency shifts and linewidths of the Raman peaks in the frequency region of 10-320 cm-1 have been measured in the range from 50 to 320 K. The analysis of the experimental data showed that the temperature dependencies of phonon frequencies and linewidths are well described by considering the contributions from thermal expansion and lattice anharmonicity. The anharmonic contribution (phonon-phonon coupling) is found to be due to three-phonon processes. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Spectroscopy of water vapor in the 720-nm wavelength region: Line strengths, self-induced pressure broadenings and shifts, and temperature dependence of linewidths and shifts

Temperature dependence of phonon linewidth in β-AgI

Temperature dependence of the raman linewidth and frequency shift in Ge and Si

Mid-infrared region of the spectrum is rich with vibrational-rotational lines of molecular pollutants within the atmosphere. Differential absorption lidar (DIAL) technique has commonly been used to remotely detect the low concentrations of the pollutants.1-3 This technique depends critically on the assumed molecular absorption line frequency, line strength, linewidth, pressure shift of line frequency and temperature dependence of line width. In the present study, a novel and compact IR tunable laser system with narrow linewidth and wide tunability 4 in the spectral range from 9.1 to 12.5 μm has been built. Our objective is to measure the line parameters of the atmospheric gases by taking advantage of the high power and wide tunability of this sideband laser. Those parameters are important for remote sensing applications between 9.1-12.5 μm. This paper presents the measurements of absorption line frequencies, frequency shifts due to pressure, line strengths and self-broadened Lorentz linewidths of NH3 and C2H4 transitions which were covered by the CO2 laser used in the experiment.

Homogeneous IR line shape of matrix isolated molecules in the debye approximation

Phonon effects on sharp luminescence lines of Nd 3+ in Gd 3Sc 2Ga 3O 12 garnet (GSGG)

We study the temperature dependence of phonon frequency shifts and phonon linewidths in diamond and graphite using tight-binding molecular dynamics simulations. The calculation of one-phonon spectral intensities of several modes through velocity - velocity correlation functions allows a quantitative and nonperturbative study of these anharmonic effects. Our results for the zone-centre optical mode of diamond, for which experimental data are available, agree very well with first-order Raman-scattering measurements.

The Differential Absorption Lidar (DIAL) technique can be used to remotely measure profiles of atmospheric gases such as water vapor. The application of this technique requires accurate water vapor spectroscopic data on line strength, pressure broadening, pressure shifts, and linewidth temperature dependence in the wavelength region of interest. As part of NASA's atmospheric research program, the NASA Langley Research Center is currently involved in developing a DIAL system for the Lidar Atmospheric Sensing Experiment (LASE)(1). This DIAL system will use tunable Alexandrite lasers and operate from a high-altitude ER-2 (extended range U-2) aircraft for the measurement of tropospheric water vapor distributions. To support this experiment, a high- resolution spectroscopy setup consisting of a cw ring dye laser and two White cells was used to provide measurements of needed water vapor line parameters. Measurements were made of the absorption line strength, width, and pressure shift in air, and the temperature dependence of linewidth and shift for 270 absorption lines lying between 717 and 738 nm. The line shapes were also studied, and deviations from the standard Voigt profile were found resulting from the Dicke-narrowing effect. In addition, linewidths and shifts have been investigated in various buffer gases including nitrogen, oxygen, argon, and xenon.

Coupling between the infinite and finite clusters of a spin system with the reentrant transition and spin-glass state has been studied as a function of temperature in the range from 4 K to 300 K. Thin films of ${\mathrm{CdCr}}_{2\mathit{x}}$${\mathrm{In}}_{2\mathrm{\ensuremath{-}}2\mathit{x}}$${\mathrm{Se}}_{4}$ with 0.8\ensuremath{\le}x\ensuremath{\le}0.95 were investigated. The magnetic properties of ${\mathrm{CdCr}}_{2\mathit{x}}$${\mathrm{In}}_{2\mathrm{\ensuremath{-}}2\mathit{x}}$${\mathrm{Se}}_{4}$ spinel are very sensitive to the substitution of Cr by In, which leads to a random distribution of magnetic interactions. As-deposited samples had a multilayered structure of Cr/Cd-Cr-In-Se/Cr. After heat treatment we have obtained a uniform single film of controlled composition. The temperature dependence of linewidth and line shift of the ferromagnetic resonance, in parallel geometry, was used to compute the coupling parameters: M, which describes the resonance interaction between the finite and infinite clusters, and D, which is related to the relaxation processes. The models of randomly distributed spins proposed by Anderson and Continentino were applied. The model of a two-level system with asymmetric double-well potentials predicts a frequency shift due to the coupling between the infinite ferromagnetic network and finite clusters. We assume that the height of a barrier introduced in this model is a linear function of the temperature. The significant temperature dependence of coupling parameters was obtained for samples in the spin-glass state with Cr concentration x=0.85 and 0.8. We have found that M and D are not single-valued functions of indium concentration (dilution level). We have also determined the temperature dependence of (M-D), which has an intuitive meaning as being responsible for the interaction of nondamped magnons of the infinite cluster with a two-level system. The expression (M-D) was found to be a linear function of the temperature.

We have calculated the implicit shift for various modes of frequency in a pure graphene sheet. Thermal expansion and Grüneisen parameter which are required for implicit shift calculation have already been studied and reported. For this calculation, phonon frequencies are obtained using force constants derived from dynamical matrix calculated using VASP code where the density functional perturbation theory (DFPT) is used in interface with phonopy software. The implicit phonon shift shows an unusual behavior as compared to the bulk materials. The frequency shift is large negative (red shift) for ZA and ZO modes and the value of negative shift increases with increase in temperature. On the other hand, blue shift arises for all other longitudinal and transverse modes with a similar trend of increase with increase in temperature. The q dependence of phonon shifts has also been studied. Such simultaneous red and blue shifts in transverse or out plane modes and surface modes, respectively leads to speculation of surface softening in out of plane direction in preference to surface melting.

A spectral energy density based formalism is implemented to probe the temperature dependent frequency shift, linewidth, structural stability and coupling of normal modes of vibrations of free-standing graphene using a combination of lattice dynamics and molecular dynamics (MD). The in-plane lattice parameter shows a thermal contraction upto 1300 K and it expands thereafter. Frequency of the bending mode (ZA) becomes imaginary in the quasi-harmonic dispersion at higher temperatures, suggestive of a structural instability. However, the frequency of the ZA mode becomes real in the dispersion obtained from MD. Dynamical stability to the structure is restored by strong anharmonic coupling of phonon modes which is automatically incorporated in the MD simulations, whereas it is ignored in the quasi-harmonic dispersion. The mode resolved phonon spectra at Γ point show a blue-shift of degenerate longitudinal and transverse (LO/TO) optic modes. The blue-shift observed in canonical (NVT) and isobaric–isothermal (NPT) ensembles are more prominent than the shift predicted by quasi-harmonic approximation (QHA) due to the additional contribution from phonon–phonon coupling. The out-of-plane optic (ZO) mode frequencies are red-shifted in the QHA due to membrane-effect, whereas MD simulations show that the strong phonon–phonon coupling dominates the membrane effect leading to a blue-shift. The linewidth of LO/TO and ZO modes increases non-monotonically with temperature. The anharmonic coupling of normal modes at high symmetry points in the Brillouin zone is also discussed.

We present comprehensive temperature dependent Raman measurements for chemical vapor deposition grown horizontally aligned layered MoS2 in a temperature range of 4–330 K under a resonance condition. Our analysis of temperature dependent phonon frequency shift and linewidth suggests a finite role of three and four phonon anharmonic effect. We observe Davydov splitting of the out-of-plane (A 1g ) and in-plane ( E2g1 ) modes for both three layer (3L) and few layer (FL) systems. The number of Davydov splitting components are found more in FL compared to 3L MoS2, which suggests that it increases with an increasing number of layers. Further, Davydov splitting is analyzed as a function of temperature. Temperature evaluation of the Raman spectra shows that the Davydov splitting, especially for A 1g mode, is very strong and well resolved at low temperature. We observe that A 1g mode shows the splitting at low temperature, while E2g1 mode is split even at room temperature, which suggests a prominent role of A 1g mode in the interlayer interaction at low temperature. Further, an almost 60-fold increase in the intensity of the phonon modes at low temperature clearly shows the temperature dependent tuning of the resonance effect.

We present a combined experimental and computational study of two-dimensional molybdenum disulfide and the effect of temperature on the frequency shifts of the Raman-active E2g and A1g modes in the monolayer. While both peaks show an expected red-shift with increasing temperature, the frequency shift is larger for the A1g mode than for the E2g mode. This is in contrast to previously reported bulk behavior, in which the E2g mode shows a larger frequency shift with temperature. The temperature dependence of these phonon shifts is attributed to the anharmonic contributions to the ionic interaction potential in the two-dimensional system.

We report the optical phonon shifts induced by phase transition effects of vanadium dioxide (VO2) in monolayer molybdenum disulfide (MoS2) when interfacing with a VO2 film showing a metal-insulator transition coupled with structural phase transition (SPT). To this end, the monolayer MoS2 directly synthesized on a SiO2/Si substrate by chemical vapor deposition was first transferred onto a VO2/c-Al2O3 substrate in which the VO2 film was prepared by a sputtering method. We compared the MoS2 interfaced with the VO2 film with the as-synthesized MoS2 by using Raman spectroscopy. The temperature-dependent Raman scattering characteristics exhibited the distinct phonon behaviors of the E2g1 and A1g modes in the monolayer MoS2. Specifically, for the as-synthesized MoS2, there were no Raman shifts for each mode, but the enhancement in the Raman intensities of E2g1 and A1g modes was clearly observed with increasing temperature, which could be interpreted by the significant contribution of the interface optical interference effect. In contrast, the red-shifts of both the E2g1 and A1g modes for the MoS2 transferred onto VO2 were clearly observed across the phase transition of VO2, which could be explained in terms of the in-plane tensile strain effect induced by the SPT and the enhancement of electron-phonon interactions due to an increased electron density at the MoS2/VO2 interface through the electronic phase transition. This study provides further insights into the influence of interfacial hybridization for the heterogeneous integration of 2D transition-metal dichalcogenides and strongly correlated materials.