Soft-mode spectroscopy

Petzelt J, Ostapchuk T (2001) Far-infrared ferroelectric soft mode spectroscopy on thin films. Ferroelectrics 249 81... 

Petzelt J, Ostapchuk T (2002) Soft-mode spectroscopy in SrTiOa, BaTiOa, and BST films... 

F.J. Scott Soft-mode spectroscopy Experimental studies of structural phase transitions. Rev. Mod. 

Many of the ferroelectric materials exhibit softening of certain vibrational modes. Soft-mode behaviour of ferroelectric materials has been investigated in detail by employing Raman spectroscopy and neutron scattering, and the subject has been reviewed by Blinc Zeks (1974). 

The typical and much discussed effect of phase transitions is a so-called soft mode, A soft mode is a vibration, the frequency of which nears zero as the physical parameter (mostly the temperature but sometimes also the pressure or the external electric field) approaches its critical point. One of the fir.st soft modes was observed by Raman et al. in the a / quartz transition (Raman and Nedungadi, 1940). The theory of these modes was proposed by Cochran (Cochran, 1960, 1961). It turns out that the soft mode is simply the vibration that, due to its form, allows the transformation from one phase to the other. At the transition point, the restoring forces disappear and the frequency approaches zero. Extensive reviews of the application of spectroscopy in connection with the investigation of phase transitions have been provided by Rao and Iqbal (Rao and Rao, 1978 Iqbal, 1986). 

Neutron spectroscopy is becoming a principal tool for the study of protein dynamics (Cusack, 1986, 1989 Middendorf, 1984 Middendorf et al., 1984). Current instruments cover motions with characteristic times from 10 to 10 sec. This range embraces essentially all protein modes excited at room temperature (the soft modes), including motions of the solvent shell and also low-frequency large-scale domain motions, like the hinge-bending motion of the lysozyme domains that form the... 

Sirenko AA, Akimov lA, Pox JR, Clark AM, Li H-C, Si W, Xi XX (1999) Observation of the first-order Raman scattering in SrTi03 thin films. Phys Rev Lett 82 4500 Akimov lA, Sirenko AA, Clark AM, Hao J-H, Xi XX (2000) Electric-field induced soft-mode hardening in SrTiOa films. Phys Rev Lett 84 4625 99. Tenne DA, Clark AM, James AR, Chen K, Xi XX (2001) Soft phonon modes in Bao.5Sro.5Ti03 thin films studied by Raman spectroscopy. Appl Phys Lett 79 3836-3838... 

Spectroscopy algorithm, as wiU be outlined in the next Section. VSCF and CC-VSCF can fail for strongly anharmonic soft modes such as torsions. This is related to the fact that for large amplitude torsional motions, normal mode coordinates are not suitable, and when used may give rise to extremely large coupling due to their geometric nature. Other than such exceptions, the method seems in most cases of satisfactory accuracy. 

Ha)nvard et al. (2005) performed a comprehensive investigation of LaAlOs in the temperature range of 10-750 K and determined the crystal structure, dielectric relaxation, specific heat, birefringence, and the frequencies of the two soft modes via Raman spectroscopy. While all these experiments show that the behaviour at the critical point around Tc = 813 K is consistent with a second-order transition, some evidence for an additional anomalous behaviour below 730 K have been shown. This anomaly was explained by a biquadratic coupling between the primary order parameter of the transition and the hopping of intrinsic oxygen vacancies. 

Using dielectric relaxation spectroscopy, it is possible to determine the values of the rotational viscosity tensor corresponding to the three Euler angles in the chiral smectic C and A phases. These viscosity coefficients (Ye, Yq>, 7i) active in the tilt fluctuations (the soft mode), the phase fluctuations (the Goldstone mode), and the molecular re-... 

The strucmre remains cubic at any temperature. However, for temperatures less than Tj, diffusion halos in the x-ray diffraction spectra show the existence of a disorder on the atomic scale. Furthermore, no domain strucmre is seen and no soft mode is detected by infrared spectroscopy. 

The structures for small Si clusters were verified in several experiments. Small clusters can be soft-landed on an inert gas matrix, permitting Raman4 or IR5 spectroscopy. This confirmed the Sin geometries for n < 7. Further proof came from vibrationally resolved photoelectron spectroscopy (PES),6 which exposed some normal modes of anions as overtones on the photoelectron spectra. The electronic transitions also contain structural information about anions.7 PES established a major difference between Sig and Si6, revealing the charge-dependence of Si cluster geometries (Fig. 2).7... 

There are three main detection modes for EAPFS within the appearance potential spectroscopy (APS) technique./31/ First, one may monitor soft-x-ray emission due to the decay of the core hole left by the primary process. This is called SXAPS-EAPFS (Figure le). Second, it is also possible to monitor Auger electrons due to the same core-hole decay, as in AEAPS-EAPFS and AMEFS-EAPFS, cf. Figure If. Third, one may measure the remaining total intensity of... 

In order to characterize the MgO sites where the Pd atoms are stabilized after deposition by soft-landing techniques, we used CO as a probe molecule [61]. The adsorption energy, Eb, of CO has been computed and compared with results form thermal desorption spectroscopy (TDS). The vibrational modes, (o, of the adsorbed CO molecules have been determined and compared with Fourier transform infrared (FTTR) spectra. From this comparison one can propose a more realistic hypothesis on the MgO defect sites where the Pd atoms are adsorbed. 

MAS is routinely used in solid-state NMR spectroscopy for eliminating the elfects of chemical shift anisotropy, heteronuclear dipolar interactions and first-order quadrupolar interactions. In this method the sample is rotated abont the axis inclined at 54.74° with respect to the external magnetic field Bo, so that the average of the geometric term in nnclear spin interactions (3cos 0 — 1) = Except for some specific applications of static wide-line solid-state NMR spectroscopy (i.e., for or for the determination of CSA patterns), the MAS is an essential feature of solid-state NMR spectroscopy and can also be nsed in variable temperature mode. The MAS methodology can also be nsed for high-resolntion studies of viscous hquids and soft solids. 

Fujimori H, Yashima M, Kakihana M, Yoshimura M (2000) Phase transition and soft phonon modes in SrZrOs around 1200°C by ultraviolet laser Raman spectroscopy. Phys Rev B 61 3971... 

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