1g Mode Research Articles

The effect of Cu substitution on the A 1g mode of La 0.7Sr 0.3MnO 3 manganites

We report on the first Raman data of Cu substituted La 1− y Sr y Mn 1− x Cu x O 3 (0≤ x≤0.10 and 0.17≤ y≤0.3, accordingly in order to have the same Mn 4+/[Mn 4++Mn 3+] ratio), collected in the frequency range 100–900 cm −1 and at room temperature, with parallel ( e i∥ e s) and crossed ( e i⊥ e s) polarizations of the incident ( e i) and scattered ( e s) light. Spectra were fitted with a Drude–Lorentz model, and peaks at 190–220 and 430 cm −1, together with two broad structures centered at near 500 and 670 cm −1, have been found. We also have observed that the A 1g mode is substantially shifted with increasing Cu substitution. The A 1g phonon shift is a linear function of the tolerance factor t and the rhombohedral angle α r, thus following the structural changes of the MnO 6 octahedra in the system.

Boron nitride nanotubes synthesized in the temperature range 1000–1200 °C

Boron nitride nanotubes were synthesized on the iron-deposited alumina substrates by a catalytic reaction of the ball-milled boron and boron nitride powder mixture with ammonia in the temperature range 1000–1200 °C. The diameter of nanotubes is in the range of 40–100 nm. The nanotubes grown below 1100 °C possess exclusively a bamboo-like structure. As the temperature increases to 1200 °C, almost all nanotubes show a cylindrical structure in which the boron nitride sheets are tilted to the tube axis by an angle of about 25°. Electron energy-loss spectroscopy identifies that the ratio of boron and nitrogen is almost one. The Raman scattering peak associated with the E 2g mode shifts to the higher frequency and narrows as the growth temperature increases. The results indicate that the growth temperature can be a crucial growth parameter in controlling the structure and crystallinity of boron nitride nanotubes. On the basis of the structural features, we suggest a base-growth mechanism for both bamboo-like and cylindrical boron nitride nanotubes.

High pressure raman study of Bi-2212

The hydrostatic pressure dependence of the Raman spectra of Bi2Sr2CaCu2O8 single crystals has been investigated. The energy of the A g and B 1g modes was found to increase with pressure in agreement with previously reported measurements, except the strong mode at ∼465 cm−1, which softens with pressure, while another peak at ∼458 cm−1 appears more pronounced at low temperatures and high pressures. The energy of both modes does not seem to change with increasing pressure, up to ∼5 GPa, although the average energy of the wide band has been found to soften, which is in disagreement with previously published results. Based on the modifications observed in some phonons at ∼1.8 GPa, which correlate with the reduction of T c and the deformations of the CuO5 pyramids, we attribute the mode at ∼465 cm−1 to the vibrations of the apex oxygen atoms. All modes due to the oxygen atoms were found to be strongly anharmonic.

MgB2 and AlB2 at high pressures

We have studied the effect of pressure on vibrational and electronic properties of MgB2 and AlB2 by ab initio calculations and Raman spectroscopy. The comparison between the calculations and the Raman data puts the common assignment of a broad spectral feature near 600 cm−1 in MgB2 to the E 2g phonon into question. At variance with MgB2 the Raman spectra of AlB2 exhibit a well-defined E 2g mode indicating that the anomalous Raman spectrum encountered in MgB2 is not related to the metallicity of the samples nor is it intrinsic to crystals of the AlB2 structure type. A theoretical estimate of the pressure dependence of T c in MgB2 shows that the experimentally observed decrease of T c under pressure is predominantly due to phonon frequency shifts.

Study of Raman active phonons in NdMnO 3

A g and B 2g Raman active modes in NdMnO 3 single crystals, grown by flux and floating zone techniques, have been studied in the temperature range from 4.2 to 300 K. Similar to orthorhombic LaMnO 3, the NdMnO 3 structure distorted by a static Jahn–Teller effect is consistent with the D 2h 16 space group. The observed phonon frequencies, intensities and bandwidths are influenced by the temperature evolution of the Mn 3+ magnetic sublattice and, particularly, the most intense B 2g mode (∼601 cm −1) softens below T N∼75 K following the paramagnetic to canted-antiferromagnetic phase transition.

Analysis of the absorption and luminescence spectra of U3+:Cs2NaYBr6 single crystals

Abstract Uranium(3+) doped Cs 2 NaYBr 6 single crystals with a 2.1 and 0.09 mol% U 3+ concentration have been obtained by the Bridgman–Stockbarger method. The U 3+ ions replaced the Y 3+ ions at an octahedral site of O h symmetry. Luminescence spectra of the crystal were recorded at 4.2 K. The emission bands observed in the visible and near infrared regions have been assigned to transitions from the lowest components of the 4 G 7/2 and 4 F 3/2 multiplets to the crystal-field components of the 4 I 9/2 ground state. Absorption spectra were recorded in the 3900–35,000 cm −1 range at 4.2 K. The analysis of these spectra enabled the assignement of 28 energy levels, which were fitted to seven parameters of an empirical Hamiltonian representing the combined atomic and crystal-field interactions, with a rms deviation of 43 cm −1 . The energies of the most prominent vibronic features have been measured and assigned to the specific vibrational modes of the UCl 6 3− moiety or to lattice modes. Above 14,000 cm −1 vibronic transitions associated with the electric dipole allowed 5f 3 –5f 2 6d 1 transitions were observed. The unresolved zero-phonon lines are accompanied by a number of vibrational progressions combined with the even S 1 (a 1g ) mode.

Pressure dependence of the Raman spectrum, lattice parameters and superconducting critical temperature of MgB 2: evidence for pressure-driven phonon-assisted electronic topological transition

We overview recent high-pressure studies of high-temperature superconductor MgB 2 by Raman scattering technique combined with measurements of superconducting critical temperature T c and lattice parameters up to 57 GPa. An anomalously broadened Raman band at 620 cm −1 is observed and assigned to the in-plane boron stretching E 2g mode. It exhibits a large Grüneisen parameter indicating that the vibration is highly anharmonic. The pressure dependencies of the E 2g mode and T c reveal anomalies at 15–22 GPa (isotope dependent). The anharmonic character of the E 2g phonon mode, its anomalous pressure dependence, and also that for T c are interpreted as a result of a phonon-assisted Lifshitz electronic topological transition.

Luminescence and Raman spectroscopy on MgB 2

From a comparison of luminescence and Raman measurements on MgB 2-ceramics, we ascribe the broad and intense background in the Raman spectra to a luminescence from MgB 2. We also find that the frequency and linewidth of the E 2g mode at 600 cm −1 exhibits a considerable temperature dependence which we discuss in terms of free-carrier induced effects and electron–phonon coupling.

Raman spectroscopic studies on bismuth nanoparticles prepared by laser ablation technique

Bi nanoparticles are prepared by means of laser ablation in Ar atmosphere (0.2–10 Torr) with KrF (248 nm) excimer laser of the power 200 mJ. The size of the Bi particles estimated by TEM measurements is in the range 3–10 nm. Raman active E g mode shifts to a higher frequency and becomes broader for a sample prepared in a lower pressure of Ar atmosphere. However, the peak frequency and the bandwidth of A 1g mode show almost no change with the change of the particle size. These experimental results can be well explained by a phonon confinement model of Campbell and Fauchet by taking the phonon dispersion properties that the E g mode of the crystal has a large dependence on the wave numbers near the Γ point, but the A 1g mode is rather independent of the phonon wave numbers.

Coherent phonons in bismuth under high-density excitation

We investigate the dynamics of coherent phonons in bismuth under high-density excitation with a pump–probe technique. As the pump power density is increased, the E g mode appears in addition to the A 1g mode, and the A 1g mode in the Fourier-transformed spectra is asymmetrically broadened. Analysis based on a time-partitioning Fourier transformation reveals that the frequency of the coherent A 1g phonon is instantaneously downshifted upon the excitation and that the line shape of the A 1g mode changes from an asymmetric to symmetric one as the coherent oscillation decays. These behaviors of the coherent phonons are attributed to anharmonicity of the lattice vibrations.

A systematic optical study of phonon properties in optimally doped Bi 2Sr 2CaCu 2O 8+ δ single crystals

A systematic optical study of infrared-active phonons is carried out for optimally doped Bi 2Sr 2CaCu 2O 8+ δ (Bi2212) single crystals with E-vector along c-axis as well as in the ab-plane at 295 K. Strong phonon anisotropy has been observed in our ab-plane conductivity measurements. Ten infrared-active phonons have been observed along c-axis. Many of these c-axis infrared-active phonons have energies that are very close to the Raman-active A 1g modes. The group theory analysis based on the tetragonal group (D 4h) is inadequate to explain our experimental observations.

Raman spectra of magnesium chloride thin films

We have observed lattice vibrations of MgCl 2 thin films for the first time by surface enhanced Raman scattering induced by a silver underlayer. Two Raman active bands of the MgCl 2 crystal, E g and A 1g, were observed at ∼160 and 240 cm −1. The band width of A 1g mode was about 20 cm −1. The Raman spectra indicate that the lattice structure of the evaporated MgCl 2 film is well ordered like that of a polycrystal. We have also measured Raman intensity of A 1g mode, varying the film thickness from 0.6 to 7 nm. This film thickness dependence indicates that the Raman enhancement occurs within a very short distance from the substrate.

Expanded model for anharmonic vibrational excitation of oxygen in silicon

To consistently describe the infrared absorptions by the oxygen in silicon discovered previously (30-, 1100-,1200-, and 1700-cm −1 bands) and recently (600- and 1800-cm −1 bands), our original model for the two-dimensional low-energy excitation of oxygen coupled to the A 2u local mode has been expanded so as to include the coupling to the A 1g local mode. In addition to the four unknown parameters contained in our original model, the expanded model contains two more parameters: the A 1g mode frequency and the A 1g-mode coupling constant⋅ Employing the set of appropriate basis functions of sufficient size, we calculated the Hamiltonian matrix and then diagonalized it numerically. We have found the parameter values that well reproduce the energy levels we previously determined up to one-A 2u-phonon plus one-A 1g-phonon states.

Observation and identification of the molecular triplet in C 60 thin films

The excited state properties of C 60 thin films have been probed at 77 K using Raman spectroscopy. The change in the Raman, A 2g mode of C 60, whose position is largely independent of temperature was monitored as a function of the excitation intensity at 514.5 nm. This mode, normally positioned at 1469 cm −1, was seen to shift to a lower Raman frequency with increasing laser intensity. Two excited state species have been identified. The first, at 1466 cm −1, has been associated with the molecular triplet of C 60, while the second species, at 1463 cm −1, has been speculated to be an excited state co-operative in the solid.

Vibronic interaction in a copper oxide cluster

The orbital vibronic interaction in a D 4h copper oxide cluster Cu 4O 12H 8, which is one of the smallest cluster models for the CuO 2 sheet of copper oxide compounds, is investigated to increase our knowledge of high- T c superconductors. Only the B 1g and B 2g vibrational modes can couple to the degenerate e u HOMO of Cu 4O 12H 8. The orbital vibronic coupling constants of the B 1g and B 2g modes to the e u HOMO are calculated and analyzed with the hybrid UB3LYP method that can reasonably describe the antiferromagnetic interactions between neighboring copper ions. The B 1g mode of 191 cm −1 and the B 2g modes of 51 and 505 cm −1 give very large coupling constants. The lowest frequency B 2g mode of 51 cm −1 most strongly couples to the e u HOMO. The HOMO is localized diagonally in the broken-symmetry antiferromagnetic state, and therefore it is reasonable that this diamond-shaped distortion mode significantly couples to the HOMO.

Phase stability of doped carbon nanotubes

This report presents spectroscopic probes of intercalation processes in single walled carbon nanotubes. Before saturation doping of alkali metals (KC x , x∼9), we found a rather stable intermediate phase KC 27 . where the Raman active E 2g mode shows an anomalous hardening. Considering the close correlation with the graphite intercalation compounds, we postulate that in KC 27 phase, alkali ions are inserted between two tubes rather than the hollow site surrounded by three tubes.

Generation of coherent THz phonons in GeTe ferroelectrics

We have examined a possibility of light-induced phase transition in GeTe ferroelectrics by using amplified femtosecond laser pulses. The frequency of the A 1g mode corresponding to the soft mode is largely red-shifted and the amplitude of the A 1g mode increases linearly with increasing pumping power densities. The results of double-pulse excitation experiment indicate that the red-shift of the soft mode induced by the second pulse excitation is caused by the electronic weakening of crystal bonding due to the excitation of valence electrons.

Raman spectroscopic studies on decomposition of spinel-type cobalt chromium indium sulfide solid solutions

Raman spectra of spinel-type cobalt chromium indium sulfide solid solutions are presented in order to establish the decomposition of these solid solutions to chromium-rich and indium-rich sulfides on annealing at 873 and 1073 K. The results suggest that in the case of the undecomposed solid solutions there is a random distribution of the trivalent metal ions even in a vibrational spectroscopic size-scale. In the case of the at ambient temperature metastable solid solutions, the three F 2g modes display two-mode behavior, whereas the A 1g and E g modes exhibit one-mode behavior. The phonon mode of species E g of inverse spinel-type CoIn 2S 4 is reassigned to being close to 250 cm −1 instead of 180 cm −1 as established by Raman single-crystal experiments.

Polarized raman study of aligned multiwalled carbon nanotubes

Polarized Raman spectra of high purity aligned arrays of multiwalled carbon nanotubes, prepared on silica substrates from the thermal decomposition of a ferrocene-xylene mixture, show a strong dependence of the graphitelike G band and the disorder-induced D band on the polarization geometry employed in the experiments. The experimental G-band intensity exhibits a minimum at straight theta(m) = 55 degrees in the VV configuration, in good agreement with theoretical predictions of a characteristic minimum at 54.7 degrees for A(1g) modes in single wall nanotubes, where straight theta(m) denotes the angle between the polarization direction and the nanotube axis.

Quantum dynamics of intramolecular vibrational energy redistribution for initally excited CC ring modes in 30-mode benzene

Energy flow from four CC ring-mode bright states (A 1g , B 2u , E 1u , and E 2g ) was examined for the CC( v=2) overtone. The A 1g normal mode exhibits completely reversible intramolecular vibrational energy redistribution (IVR) over the course of the dynamics (9.6 ps); energy flow out of the initially excited mode is less complete but less reversible for the B 2u and E 2g modes, however. IVR pathways from the E 1u and E 2g states are quite different and show the E 1u state losing more than 50% of the initial energy while the E 2g state retains more than 90% of this quantity. In all cases, the in-plane CCC bending modes and out-of-plane modes do not significantly contribute to IVR.