Active Lattice Vibrations Research Articles

Study of vibrational properties of Bi2 − xMnxTe3 nanocrystals in host glass: Effect of xMn‐concentration

AbstractThe Raman and infrared (IR) active lattice vibrations of xMn‐concentration Bi2 − xMnxTe3 nanocrystals (NCs) as a dopant in semiconductor Bi2Te3 nanocrystals grown in a host glass matrix, whose symmetries correspond to the space group, are investigated by Raman scattering. Transmission electron microscopy images confirm the formation of Bi2 − xMnxTe3 nanocrystals with an average size of around 5.4 nm. The observation of the six hyperfine structure lines in electron paramagnetic resonance spectra confirms the incorporation of Mn2+ ions in Bi2Te3 nanocrystals. The vibrational modes of quintuple layer (QL) of the diluted magnetic semiconductor Bi2 − xMnxTe3 nanocrystals are modified in relation to pure Bi2Te3 nanocrystals. The appearance of the (95.5 cm−1), (114 cm−1), and (103 cm−1) IR‐active modes is mainly due to quantum size effects. Indeed, the redshifts of the (95.5 cm−1), (103 cm−1), (110 cm−1), (114 cm−1), and (141 cm−1) vibrational modes give strong indications of the substitutional and interstitial incorporation of Mn2+ in the Bi2Te3 crystalline structure. In addition, the Raman spectra show the formation of Bi2OTe2 semiconductor due to the appearance of the (123 cm−1) mode. The investigation of the Bi2 − xMnxTe3 nanocrystal has promising potential to design new devices with magnetic and optical properties adjusted according to the xMn‐concentration.

Coherent modulation of theYBa2Cu3O6+xatomic structure by displacive stimulated ionic Raman scattering

We discuss the mechanism of coherent phonon generation by stimulated ionic Raman scattering, a process different from conventional excitation with near visible optical pulses. Ionic Raman scattering is driven by anharmonic coupling between a directly excited infrared-active phonon mode and other Raman modes. We experimentally study the response of $\mathrm{YB}{\mathrm{a}}_{2}\mathrm{C}{\mathrm{u}}_{3}{\mathrm{O}}_{6+x}$ to the resonant excitation of apical oxygen motions at 20 THz by midinfrared pulses, which has been shown in the past to enhance the interlayer superconducting coupling. We find coherent oscillations of four totally symmetric $({\mathrm{A}}_{\mathrm{g}})$ Raman modes and make a critical assessment of the role of these oscillatory motions in the enhancement of superconductivity.

Optically enhanced coherent transport in YBa2Cu3O6.5 by ultrafast redistribution of interlayer coupling.

Nonlinear optical excitation of infrared active lattice vibrations has been shown to melt magnetic or orbital orders and to transform insulators into metals. In cuprates, this technique has been used to remove charge stripes and promote superconductivity, acting in a way opposite to static magnetic fields. Here, we show that excitation of large-amplitude apical oxygen distortions in the cuprate superconductor YBa2Cu3O6.5 promotes highly unconventional electronic properties. Below the superconducting transition temperature (Tc=50 K) inter-bilayer coherence is transiently enhanced at the expense of intra-bilayer coupling. Strikingly, even above Tc a qualitatively similar effect is observed up to room temperature, with transient inter-bilayer coherence emerging from the incoherent ground state and similar transfer of spectral weight from high to low frequency. These observations are compatible with previous reports of an inhomogeneous normal state that retains important properties of a superconductor, in which light may be melting competing orders or dynamically synchronizing the interlayer phase. The transient redistribution of coherence discussed here could lead to new strategies to enhance superconductivity in steady state.

Phonon Frequency Spectrum HTSC YbBa2Cu3O7

Since the discovery of the high Tc superconductivity by Bednorz and Muller, several workers around the world have studied several systems to reach a really high Tc superconductor. As the strong electron-phonon coupling may be one of the possible origins of the high Tc, a knowledge about the phonon in these materials is essential. In order to investigate phonon spectra, one has to study Raman and infrared spectra of these systems. But there is a little information about the Raman spectra and infrared absorption spectra are available in the literature. Hence, a theoretical evaluation of phonon frequencies of high temperature superconductors assume importance. Due to strong covalent bonding nature in high temperature superconductors, a normal coordinate analysis using Wilson’s FG matrix is applied to evaluate Phonon frequencies. The normal coordinate analysis of optically active lattice vibrations will be useful for the theoretical interpretation of vibrational spectrum at the center of Brillouin zone in high Tc superconductors. Hence, an attempt is made to evaluate all the phonon frequencies of HTSC YbBa2Cu3O7 through normal coordinate analysis. Calculations of lattice dynamics is also performed by use of the modified three body force shell model. The present approach leads to a better understanding of phonon frequency of high Tc superconductor YbBa2Cu3O7 under investigations in the present work. This calculations yield not only the zone center phonon modes but also the stable dispersion curves through out the Brillouin zone. It also supports strong electron-phonon interactions in high Tc superconductors.

Optical phonon modes and transmissivity in BaWO4 single crystal

AbstractBarium tungstate (BaWO4) single crystal has been grown using Czochralski technique. It belongs to the scheelite structure, forming the space group I 41/a at room temperature and the primitive cell contains two molecular units. The polarized Raman spectra were recorded by a micro‐Raman spectrophotometer system in the backscattering geometry. All the observed Raman modes were assigned. The Raman mode at 924 cm–1, which belongs to the totally symmetrical Ag optical modes, has the strongest intensity and its linewidth is 4.6 cm–1. The infrared active lattice vibrations have been studied, eight optical modes were observed and assigned. The ultraviolet absorption edge is at 256 nm and the optical transparency range is up to 2500 nm at room temperature. The energy gap Eg of this crystal was obtained from the optical transmission spectra. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Single crystal raman spectroscopy of cerussite

Raman and infrared active modes of cerussite were assigned according to their symmetry species and compared to other aragonite group minerals. Small satellite bands at 823 and 1031 cm-1 on the low-wavenumber side of the fundamental vibrations ν2 and ν1, respectively, have been assigned to the isotopic substitutions of 13C and 18O. The Raman active ν1 and ν2 carbonate modes are observed at 1051 and 835 cm-1. The absence of the B2g component of the ν1 and ν2 vibrations has been explained by the small coupling between the Ag and B2g modes. The Raman active ν3 carbonate anti-symmetric stretching mode is observed at 1361(Ag), 1376 (B1g), 1419 (B3g), and 1477 (B2g) cm-1, while the corresponding infrared active bands are observed at 1396, 1432, and 1456 cm-1. The Raman active ν4 carbonate bending mode is observed at 673 (Ag), 668 (B2g), 681 (B1g), and 694 (B2g) cm-1. The corresponding infrared bands are observed at 670, 679, and 698 cm-1. In both ν3 and ν4 the factor group splitting between the B1g and B3g modes is 1 to 3 times smaller than the separation of the Ag and B2g modes. Raman active lattice vibrations are detected at 120 (B3g), 132 (Ag), 148 (B1g), 152(B2g), 174 (B2g), 179 (B1g), 213 (Ag), 226 (B3g), and 243 cm-1 (B2g). Corresponding infrared active bands are detected at 573, 543, 573, 423, 375, 290, 205, 165, 146, and 134 cm-1. Raman bands at 949, 966, 989, 1000, and 1104 cm-1 and at 922, 946, 967, 988, 996, and 1007 cm-1 in the infrared spectra are assigned to combination and overtone bands. Raman bands at 1676 (Ag), 1689 (Ag), 1730 (B3g), and 1740 (B1g) cm-1 are ascribed to combination modes of ν1 + ν4 with bands at 2052 and 2092 cm-1 assigned to 2ν1. Corresponding infrared bands are observed at 1729 and 1740 cm-1 (ν1 + ν3). Bands at 2359, 2409, 2471, and 2521 cm-1 are ascribed to ν1 + ν3, with broad bands at 1246 and 1323 cm-1 assigned to 2ν4 modes.

Infrared spectroscopic ellipsometry—a new tool for characterization of semiconductor heterostructures

Spectroscopic ellipsometry (SE) for infrared wavelengths is presented as a novel technique for contactless and nondestructive measurement of free-carrier and crystal-structure properties of complex semiconductor heterostructures for device applications. Infrared (IR)-active lattice vibrations and LO phonon–plasmon coupled modes dominate the infrared dielectric response of semiconductor materials. Analysis of ellipsometry data from 2 to 33 μm can precisely determine thin-film dielectric functions (DF) without numerical Kramers–Kronig analysis and thus provides information on phonon mode frequencies and broadening parameters, static dielectric constants, and free-carrier parameters, even for films with thicknesses only a fraction of the probing wavelengths. Alloy composition, film strain, and crystal quality of sample constituents in thin-film heterostructures can be derived. An infrared dielectric function database, which was established by analysis of simple heterostructures, is used for the investigation of complex device structures. As an example, we demonstrate the characterization of a laser diode (LD) structure based on group-III-nitride materials, where information such as concentration and mobility of free carriers in the n- and p-type regions, thickness, alloy composition, and quality of device constituents are accessible.

Infrared optical properties of Mn1.56Co0.96Ni0.48O4 spinel films sputter deposited in an oxygen partial pressure series

Mn 1.56 Co 0.96 Ni 0.48 O 4 spinel films were sputter deposited onto silicon substrates using a series of oxygen partial pressures. Fourier transform infrared transmission and reflectance, and Raman scattering measurements were made. The 1–25 μm wavelength range was examined using these optical techniques. The complex index of refraction was calculated for this entire wavelength range. Infrared active vibrations were analyzed using multiple oscillator analysis, Kramers–Kronig analysis, and derivative reflectance spectroscopy. The Raman and infrared active lattice vibrations were observed to shift with increasing oxygen partial pressure during film deposition, and were consistent with the earlier published shift in Debye frequency calculated from resistivity data. The films were shown to have an optically transparent window from 6 to 14 μm wavelength, with the multiphonon cutoff occurring at 14 μm. The frequency of the multiphonon cutoff was also observed to shift to higher frequency with the oxygen partial pressure during sputtering. These studies, and the earlier work on the variation of the thermopower and resistivity with oxygen partial pressure and film temperature, are consistent with a change in the ratio of Mn3+ to Mn4+ cations with oxygen, with small polaron hopping as the charge transport mechanism.

Resistivity, thermopower and the correlation to infrared active vibrations of Mn1.56Co0.96Ni0.48O4 spinel films sputtered in an oxygen partial pressure series

Mn 1.56 Co 0.96 Ni 0.48 O 4 spinel was sputter deposited using a series of oxygen partial pressures. Electrical resistivity versus temperature and thermopower versus temperature measurements at each oxygen partial pressure were made. The variations of the thermopower and resistivity with oxygen partial pressure are consistent with a change in the ratio of Mn3+to Mn4+ cations, which occurs due to changes of oxygen content of the material. The weak temperature dependence of the thermopower indicates small polaron hopping is the charge transport mechanism. Combining the models of Mott and Schnakenberg to analyze the transport data, we find that the Debye temperature (or frequency) is an increasing function of the oxygen partial pressure used during sputtering. The calculated shift in the Debye frequency from the resistivity is consistent with the observed shift in the fundamental infrared active lattice vibrations from Fourier transform infrared spectroscopy and Raman spectroscopy.

Fe impurities inL-alanine: An EPR, luminescence, and Raman study

We present a study of low concentration Fe(III) doping in crystalline L-alanine by means of electron paramagnetic resonance, resonant Raman scattering, and photoluminescence. The magnetic resonance experiments show that Fe(III) occupies two inequivalent sites of rhombic symmetry in the L-alanine crystal. We obtain for the principal directions of the effective giromagnetic tensor the values ${g}_{z}=1.881(5),$ ${g}_{y}=4.641(8),$ and ${g}_{x}=7.238(5).$ In the optical experiments we find a selective resonant enhancement of some of the Raman active lattice vibrations comprising displacements of the hydrogen bonds between the carboxyl and ammonium groups of neighboring molecules. Based on these results we discuss the possible localization of the impurity in the crystal.

Concentration dependent vibrational mode behaviour in the mixed crystal system SnS xSe 2- x

The laser Raman and far-infrared spectra of CdI 2 type mixed crystals SnS x Se 2- x where 0⩽ x⩽2 were measured in the region up to 500 cm −1. The spectra rule out any possibility of one mode behaviour of the optically active lattice vibrations and indicate a two mode behaviour in the series. A modified random element isodisplacement (MREI) model is applicable.

Optically Active Vibrations of Cesium Actinide(V) Hexafluorides (CsUF6, CsNpF6, and CsPuF6)

Normal coordinate analysis of the optically active vibrations of CsUF6 has been made, with the assumption of an idealized structural model. On the basis of the results of this treatment, all of the observed bands so far reported have been successfully assigned to the intramolecular vibrations (UF6−) as well as the optically active lattice vibrations (the motions of Cs+ against UF6− octahedra, abbreviated to Cs+—UF6−). The force constants concerning U-F and Cs-F bonds have been obtained within the framework of the modified valence force field. These force constants have been transferred to predict fundamental vibrational frequencies of CsNpF6 and CsPuF6. It has been found that the present analysis based on a whole crystal is powerful not only for assigning the optically active lattice vibrations but also for elucidating the vibronic structures of the electronic absorption spectra obtained at low-temperature measurements.

Optically Active Vibrations of Lithium and α-Sodium Uranium Hexafluorides (LiUF6 and α-NaUF6)

Normal coordinate analyses of the optically active vibrations of LiUF6 and α-NaUF6 have been made, with the assumption of an idealized structural model. On the basis of the results of this treatment, all of the observed bands so far reported have been successfully assigned to the intramolecular vibrations (UF6−) as well as the optically active lattice vibrations [M+(Li, Na) … UF6−]. The force constants concerning U-F and M-F bonds have been obtained within the framework of the modified valence force field. It has been found that the present analysis, based on a whole crystal, is powerful not only for assigning the optically active lattice vibrations but also for elucidating the vibronic structure of the electronic absorption spectra obtained at low-temperature measurements.

Assignment of the Raman active lattice vibrations in various phases of 1,2,4,5-tetrabromobenzene and 1,2,4,5-tetrachlorobenzene crystals

Results of polarized Raman spectroscopy are reported and used to obtain complete assignment of the optical lattice modes of 1,2,4,5-tetrabromobenzene crystals for both the beta and gamma phases. Similar information is provided for the beta phase of 1,2,4,5-tetrachlorobenzene crystal. The results are compared with lattice-dynamics calculations. Discrepancies with previously reported measurements and calculations are discussed and rationalized.

Optically active vibrations of lanthanum copper oxide compound (La2CuO4)

Normal coordinate analysis of the optically active vibrations of lanthanum copper oxide has been made, with the assumption of a perovskite K 2 NiF 4 structural model. On the basis of the results of this treatment, all of the observed bands so far reported have been successfully assigned to the intramolecular CuO vibrations and the optically active lattice vibrations (the motions of La against CuO 6 octahedra, abbreviated to La---CuO 6 ). The force constants concerning CuO and LaO bonds have been obtained within the framework of the modified valence force fields. It has been found that the present normal coordinate analysis based on a whole crystal is useful for assigning the optically active vibrations (normal modes) at the center of the Brillouin zone in La 2 CuO 4 .

The FTIR study of uranium oxides by the method of light pipe reflection spectroscopy

Light pipe infrared reflection spectra of UO2, UO3, U3O8 have been studied by using an FTIR spectrometer. The uranium oxide powders were ground to ensure fine particle size and distributed on the inner surface of a straight glass pipe with gold coating. The infrared beam from the interferometer was focused into one end of the pipe at 45° incidence and then the transmitted beam was refocused by a pair of Cassegrainian type mirrors. The resultant spectra show the infrared characteristics of the ⋯-U-O-U-O-⋯, uranyl ion UO22+ bond vibration and the active lattice vibrations predicted by group theory calculations. In comparison to the transmission spectra measured by authors or reported in literature, this 45° incident light pipe method as well as the previous light pipe method offer advantages of sensitivity, ease of acquisition and interpretation, and require a very small sample. It confirms the power of the light pipe method for studying powders and its special utility for the infrared studies of hazardous materials.

Optical Transitions of Jahn‐Teller Local Centres

AbstractThe multiphonon optical transitions between degenerate electronic local states are considered taking into account a strong interaction of the local centre (LC) with dynamic Jahn‐Teller active lattice vibrations. The results of numerical evaluation of the band shape are listed at different values of Stokes' loss constants for T ⊗ (a + e) → E ⊗ (a + e) transition of the cubic LC.

Infrared reflection spectra of mixed crystals HfS 2-xTe x

The infrared reflection spectra of the CdI 2-type mixed crystals HfS 2- x Te x were measured between 30 and 4000 cm -1. The spectra of the sulfur rich alloys show a two-mode behavior of the optically active lattice vibrations, whereas a plasma mode of free carriers determines the spectra of the tellurium rich alloys. A sharp increase of the plasma resonance frequency occurs near the composition HfS 0.5Te 1.5, which seems to indicate a semiconductor-semimetal transition in the HfS 2- x Te x alloys.

Raman scattering and lattice vibrations of Ni2SiO4 spinel at elevated temperature

Raman spectra of Ni2SiO4 spinel (Oh7Z=8) have been measured in the temperature range from 20 to 600 °C and the Raman active vibrations (A1g+Eg+3F2g) have been assigned. A calculation of the optically active lattice vibrations of this spinel has been made, assuming a potential function which combines general valence and short range force constants. The values of the force constants at 20 and 500 °C have been calculated from the vibrational frequencies of the observed Raman spectra and infrared (IR) spectral data.

Effect of Hydrostatic and Uniaxial Pressure on Structural Properties and Raman Active Lattice Vibrations in Bi2Te3

AbstractThe lattice parameters of the hexagonal unit cell, the atomic parameters, and the shift of the Raman active vibrations are measured of p‐Bi2Te3 under uniaxial and hydrostatic pressure. The structural investigations are performed by neutron diffraction, and the lattice vibrations are studied by Raman scattering. The results of the diffraction experiments constitute a direct experimental confirmation of the weak bonding between the sandwiches in this narrow gap semiconductor with sandwich structure. As a consequence, some Raman active phonon modes show a rather strong dependence upon uniaxial pressure up to Δṽ/Δp = 1.9 × 10−8cm−1/Pa.