The Raman spectra of the parent compound Na x CoO 2 (x=0.75) and the superconducting were collected in back-scattering geometry at room temperature using a Jobin-Yvon T64000 triple spectrometer equipped with a cooled change-couple device. In the spectrometer an objective of 100X-magnification was used to focus the laser beam on the sample surface and to collect the scattered light. Two excitation wavelengths at 488.0nm and 514.5 nm of an Ar + ion laser were used in our experiments. The laser power at the focus spot of 2-3 µm in diameter was kept below 1 mW to prevent laser-induce damage to the samples. parallel to the hexagon surfaces. All diffraction peaks in the corresponding XRD pattern can be indexed by a hexagonal cell with lattice parameters a =2.834Å, and c=10.94Å. The a-axis parameter agrees with the data for the ceramic samples as reported in our previous paper [3], while the c-axis parameter is slightly longer.The Raman spectra are very sensitive to properties of the sample surface. Fig. 2(b) shows a typical result obtained from the as-grown sample without pre-cleaning. It is noted 3 that this spectrum shows up certain similarities with those reported in Ref. 16. The EDAX analysis (see the inset of Fig. 2(b)) suggests that the surface of the as-made sample is covered by impurity phases mainly identified as Na 2 CO 3 . The formation of Na 2 CO 3 on the sample surface can be explained as following; when the Na x CoO 2 compound is stored under ambient condition, it reacts with water in air, decomposes into Na 2 O, and then further react with the CO 2 to form a more stable phase Na 2 CO 3 . On the other hand, the migration of Na + ions in this kind of materials also contributes to this phenomenon. As Ronald et al discussed in Ref.17, the structure of Na x CoO 2 consists of CoO 6 sheets and Na + ions intercalated within a trigonal prismatic site between the CoO 6 sheets. The distance between the faces of oxygen ions above and below the sodium is 0.81Å, which is wide enough to allow the sodium ion moving freely through this material, as there is a tunnel available for motion between oxygen sheets. Hence, it is indeed necessary to clean the surface of Na x CoO 2 samples thoroughly before Raman scattering measurements to ensure precise data to be obtained.The hexagonal crystal structure of Na x CoO 2 consisting of CoO 2 and Na layers parallel to the ab planes belongs to space group D 6h ( P6 3 /mmc, Z=2 ). Its vibrations at a wave vector of are classified following the irreducible representation of the factor group. There are five Raman active phonon modes: A 0 q ≅ 1g + E 1g + 3E 2g , which can be identified unambiguously by specific polarization configurations. Their second-order susceptibilities are restricted by the symmetric properties:where the absence of an entry in the matrix position ij implies a zero component, the letters Polarized Raman spectra were measured from ab and ac single crystal surfaces of Na x CoO 2 . In order to insure that the incident and scattered polarizations are parallel to the cry...

The morphology, stress, and composition distributions of the crosshatch pattern on a SiGe film grown on a Si͑001͒ substrate using a low-temperature Si buffer are studied by atomic force and Raman microscopies. Crosshatching is not related to composition fluctuation regardless of the stress undulation associated with strain relaxation in the SiGe film. The crosshatch morphology arises from vertical lattice relaxation induced by piled-up misfit dislocations in the Si buffer layer and substrate. A model for crosshatch formation is proposed.

The fast electron flux driven by Lower Hybrid Wave (LHW) in the scrape-off layer (SOL) in EAST is analyzed both theoretically and experimentally. The five bright belts flowing along the magnetic field lines in the SOL and hot spots at LHW guard limiters observed by charge coupled device and infrared cameras are attributed to the fast electron flux, which is directly measured by retarding field analyzers (RFA). The current carried by the fast electron flux, ranging from 400 to 6000 A/m 2 and in the direction opposite to the plasma current, is scanned along the radial direction from the limiter surface to the position about 25 mm beyond the limiter. The measured fast electron flux is attributed to the high parallel wave refractive index n jj components of LHW. According to the antenna structure and the LHW power absorbed by plasma, a broad parallel electric field spectrum of incident wave from the antennas is estimated. The radial distribution of LHW-driven current density is analyzed in SOL based on Landau damping of the LHW. The analytical results support the RFA measurements, showing a certain level of consistency. In addition, the deposition profile of the LHW power density in SOL is also calculated utilizing this simple model. This study provides some fundamental insight into the heating and current drive effects induced by LHW in SOL, and should also help to interpret the observations and related numerical analyses of the behaviors of bright belts and hot spots induced by LHW. V C 2015 AIP Publishing LLC.

The poloidal distribution of type-III edge localized modes in the Mega-Ampère spherical tokamak (MAST)