Shunji Sugai

Raman and infrared reflection spectroscopy in black phosphorus

The symmetry and energies of all optically active phonon modes in black phosphorous are determined by polarized Raman scattering and infrared reflection spectroscopy at room temperature. The obtained energies are; 365 and 470 cm-1 for Ag modes, 197 for Blg, 442 for B2g, 223 and 440 for B3g, 136 (TO) and 138 (LO) for Blu, and 468 (TO) and 470 (LO) for B2u, respectively. The small TO-LO splitting is related to the charge transfer between phosphorus atoms induced by the atomic displacement.

Pressure Dependence of the Lattice Vibration in the Orthorhombic and Rhombohedral Structures of Black Phosphorus

The lattice vibration of black phosphorus was investigated under hydrostatic high pressure by polarized Raman scattering. On applying pressure the crystal structure changes to the rhombohedral A7-structure from the layered orthorhombic structure at 52 kbar. At atmospheric pressure four phonon modes are observed at 362 cm -1 with A g -symmetry, 467 cm -1 A g , 194 cm -1 B 1 g and 439 cm -1 B 2 g . The Gruneisen parameters are 0.74 for the 362 cm -1 A g -mode, 0.10 for the 467 cm -1 A g -mode and 0.22 for the 439 cm -1 B 2 g -mode. The energies of the latter two modes change little with pressure, since in these modes the atoms displace to deform the rigid zigzag chain along the a -axis. In the A7-structure the A 1 g -mode was observed at 466 cm -1 and the E g -mode at 392 cm -1 . The comparison with the phonon modes in other group V materials was made.

Infrared Phonons in Semiconducting Phase of BaPb1-xBixO3

Infrared reflectivity spectra of BaPb 1- x Bi x O 3 alloys in their semiconducting phase ( x ≥0.4) are presented. Four optical phonon bands are clearly observed in the reflectivity spectrum of BaBiO 3 . It is found that the oscillator strength of one phonon mode remarkably decreases with increase of the Pb composition. This provides a strong evidence for the charge disproportionation which leads to the formation of a superlattice of alternating inequivalent Bi sites in BaBiO 3 . It is suggested that the macroscopic formation of such Bi superlattice is also responsible for the semiconducting properties of alloys. Based on the experimental observations the nature of the metal-semiconductor transition of this system is discussed.

Carrier density dependence of soft TO-phonon in SnTe by Raman scattering

Abstract The frequency of soft TO-phonon in SnTe was observed by Raman scattering below the transition temperature, for the specimens of various carrier concentrations. The result was analyzed in terms of the interband electron to TO-phonon interaction.

Optical reflectivity spectra measured on cleaved surfaces of La1-xSrxMnO3 : Evidence against extremely small Drude weight

Optical reflectivity spectra were measured on cleaved surfaces of La 1- x Sr x MnO 3 single crystals (0≤ x ≤0.30). The optical conductivity σ(ω) shows, keeping single-component nature, an incoheren...

Resonant Raman scattering in organic conductors α and β-(BEDT-TTF)2X (X=I3 and IBr2)

Abstract Molecular vibrations in the crystals of organic conductors α and β-(BEDT-TTF) 2 X, (X=I 3 , IBr 2 ), a neutral donor BEDT-TTF, and acceptors n-Bu 4 N·I 3 and n-Bu 4 N·IBr 2 are investigated by Raman scattering. Strong Resonant effects are observed in the I 3 compounds, when the incident laser wavelength approaches the optical transition energy of the I 3 anion. More than ten overtones of the symmetric stretching mode of anions appear. For the modes related to the C=C vibrations the decrease of peak energies and their broadening are observed in the charge-transfer salts.

Symmetry breaking on the phonon Raman spectra only at the superconductor compositions in La2−xSrxCuO4

Abstract Phonons in La2−xSrxCuO4 were systematically investigated by Raman and infrared spectroscopy covering from the insulator, superconductor, to normal metal compositions. In the Raman spectra, modes which appear only at the superconductor compositions were found. Many of those modes were assigned to the infrared active transverse optical modes with the polarization vectors parallel to the CuO2 plane. The Raman and infrared activity is mutually exclusive in the crystal with inversion symmetry as in the present crystal structure. The appearance of the infrared active modes in the Raman spectra implies that the doped carriers take the states which break the inversion symmetry of phonons only at the superconductor compositions. The larger scattering intensity of those modes than of the natively active modes indicates that the deformation-potential-type electron-phonon interactions are very large. The experimental fact that the longitudinal optical modes do not appear in the Raman spectra indicates that the Frohlich-type electron-phonon interactions induced by the macroscopic electric field of the longitudinal optical modes do not work.

Raman Study of High Tc Superconductors (La1-xSrx)2CuO4

Lattice vibrations in high Tc superconductors (La1-xSrx)2CuO4 are investigated by Raman scattering using single crystals. In La2CuO4 the 664 and 153 cm-1 peaks are resonantly enhanced at low temperatures. The enhancement is caused by the electronic transition spanning the quasi-square Fermi surface to the nesting directions. The 664 cm-1 mode is assigned to the planar breathing mode which strongly interacts with electrons at the Fermi level and the 796 cm-1 mode in (La0.925Sr0.075)2CuO4 is tentatively assigned to the axial breathing mode.

Interaction of the TO-phonon with the Acoustic Phonons near the Phase Transition Temperature in Pb1-xGexTe

The phase transition in Pb 1- x Ge x Te with x =0.03 ∼0.1 was studied by Raman scattering and ultrasonic velocity measurements. Below T c the frequency of the A 1 -mode is 1.99 ×10 23 ( T c - T ) rad 2 /sec 2 and the E -mode 5.32 ×10 23 ( T c - T ) rad 2 /sec 2 for x =0.05. The ultrasonic velocity shows a step-like behaviors at T c . Just above T c the normal elastic moduli are C 11 =11.0 ×10 11 , C 12 =0.02 ×10 11 and C 44 =1.35 ×10 11 in dyn/cm 2 . The steps are Δ C 11 =1.52 ×10 11 , Δ C 12 =-0.20 ×10 11 and Δ C 44 =0.26 ×10 11 in dyn/cm 2 . The parameters of the free energy expansion were calculated by using the above results together with the data of the shear strain. The order parameter, the spontaneous strain and the TO-phonon frequency above T c which were estimated from thus obtained parameters are consistent with the results experimentally obtained satisfactorily.

Observation of soft TO-phonon in SnTe by Raman scattering

Abstract The soft TO-phonon in SnTe ( p = 1.1 × 10 20 cm −3 ) has been observed by Raman scattering below the phase transition temperature ( T c = 105 K ). The temperature dependence of the phonon frequency is given by ω TO 2 = γ ( T c − T ) cm −2 . Several temperature independent peaks and a center peak which is temperature dependent are also observed.