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By examining the reflectance spectra of polycrystalline La1.85Sr0.15CuO4-y and La2NiO4-y in relation to the results of ir studies on single-crystal La2NiO4-y, we have determined the normal-state (room temperature) optical anisotropy of La1.85Sr0.15CuO4-y in the frequency range 40 cm-1 to 4800 cm-1. Vibrational and electronic properties corresponding to this anisotropy are examined, and a method is presented for extracting the optical properties of the superconducting state of La1.85Sr0.15CuO4-y.

Raman microprobe spectra of SbCl5, Br2, and FeCl3 graphite intercalation compounds have been obtained with a spatial resolution of 2 m. Depletion of the intercalate was observed within 10 m of the sample edge in the SbCl5-graphite samples, and this observation is found to be consistent with a model in which the intercalate contracts thermally as the sample is cooled after the intercalation reaction is completed. No depletion was observed in the Br2 and FeCl3 graphite intercalation compounds, consistent with the low intercalation temperature of the former and the small thermal expansion coefficient of the latter.

We report results from a Raman scattering study of stage 2 Graphite-Rb and Graphite-K in the low frequency region (ω < 150 cm-1) as a function of temperature (80K < T < 300K). Four features are seen in the 80K spectra and are interpreted in terms of a Born– von Kármán lattice dynamics model. The temperature-dependence of the Raman spectra is discussed in connection with reported phase transitions in stage 2 alkalimetal graphite compounds.

Magnetic phase transitions have been studied in both donor and acceptor-type magnetic GICs using AC magnetic susceptibility χ and magnetization M measurements.1 However, C6Eu is the only GIC for which a magnetic phase diagram has been successfully determined using magnetoresistance (transport) measurements.2 The strong coupling between graphite layers and Eu layers in C6Eu results in strong scattering effects on the graphite conduction π-electrons by the magnetic excitations of the Eu ions. Therefore dramatic changes in the magnetoresistance of C6Eu are observed as the system undergoes magnetic phase transitions. In contrast, acceptor-type magnetic GICs have a much weaker magnetic coupling between intercalate and graphite layers because of the relatively larger spacing between the graphite and the magnetic species.3,4,5,6 Therefore the change in magnetoresistance due to changes in magnetic ordering is expected to be smaller than for the case of C6Eu. In this work we show that transport-property measurements (magnetoresistance and temperature-dependent zero-field resistivity) can yield measurable effects associated with magnetic phase transitions for the acceptor-type magnetic stage-1 and stage-2 CoC12-GICs. Thus studies on the transport properties of acceptor magnetic GICs provide a complementary way to investigate the magnetic phase diagrams.1 In addition, such measurements provide information on the π-d electron coupling constants (J π−d) and the interplane exchange coupling constants (J′).7

Graphite intercalation compounds (GICs) are chemically formed superlattices which can show extremely small interlayer interaction and correlation. These compounds provide good experimental evidence for quasi-two dimensional structural and magnetic phase transitions. An example of a commensurate-incommensurate structural phase transition is presented. A review is given of the evidence for 2D magnetism in graphite intercalation compounds (GICs) with particular reference to the possible 2D-XY phase transition in CoC12-GICs.

Magnetooptical experiments are extremely important for the determination of the electronic (or lattice mode) structures of materials. For a number of these solids, the application of magnetooptical experiments is based on the use of the zone folding technique. This approach is well known for good metals where the free-electron dispersion relation (\( E\left( {\vec k} \right)\) = ħ2k2/2m*) is folded into a Brillouin zone appropriate to the crystal symmetry. The effect of the periodic potential is then treated in perturbation theory in the context of the zone-folded representation.

Dramatic implantation-induced changes in the Raman and infrared spectra of polycrystalline samples of La2CuO4-y are reported for implantation of Sr in the fluence range 0.5 to 1.5 × 1016/cm2. The Raman and ir-reflectance spectra of the Sr implanted material reveal disorder-induced lines corresponding to breakdown of selection rules and to non-zone-center phonon modes. Whereas the main features in the Raman spectra of the implanted sample are broad disorder-induced lines centered at ∼ 560 ± 5 cm−1, reststrahlen lineshapes are observed above ∼ 800 cm−1 in the ir-reflectance spectra. After thermal annealing (550°C), the multiphonon lines in the ir spectra diminish, and the disorder-induced lines in the Raman spectra split to form well-resolved, sharper lines at 530 and 580 cm−1, identified with maxima in the phonon density of states. The sensitivity and complementarity of Raman and infrared spectroscopy to implantation-induced disorder and subsequent partial recovery by thermal annealing suggest that these techniques can be used to characterize sensitively disorder in ion implanted La2CuO4-y high Tc, materials.

Reflectivity measurements on graphite-CoCl2 intercalation compounds between 0.06 eV and 2.0 eV for stages 3 and 5 show sharp Drude edges and superimposed interband structure. Stage 5 shows structure near 0.4 eV and 0.55 eV while stage 5 shows structure between 0.12 eV and 0.55 eV. The observed number of interband transitions is equal to the stage index which is consistent with a recently proposed kz-axis zone-folding model of the electronic energy bands. In addition to the electronic optical transitions, features associated with lattice vibrations are observed in the spectra. In particular, the graphite-like IR mode near 1580 cm–1 is clearly resolved in the intercalation compounds. A reflectivity minimum and splitting of this vibrational mode is observed for both stages 3 and 5 of graphite-CoCl2.

We have calculated the inter band absorption of potassium and sodium by means of a KKR-Z pseudopotential1,2 obtained from a fit to the experimental Fermi surface data. Previous calculations of the optical properties of the alkali metals have relied either on a local pseudopotential3 or else introduced non-locality as arising solely from the corrections due to the atomic core.4 However, it has been demonstrated that the local pseudopotential formulation5 is inadequate to account for the measured Fermi surface of K. It has also been shown6 that the pseudopotential that appears in the optical matrix element will incorporate the effects of many-body correlations only if it is consistent with the experimental data of Fermi surface measurements (such as dHvA results). The method used here was applied with this requirement being satisfied. Recent theoretical results6 and the successful application of a phenomenological model for the optical properties of Al7 suggest that a one-electron potential which has been fitted to the experimental Fermi surface should implicitly contain electron-electron effects in the optical absorption. The measurement of the inter-band absorption of the alkalis, although difficult to carry out, is nevertheless of some importance as the absorption depends critically on the pseudopotential.

The study of thermoelectric materials has recently been revived as an active research field, in part due to the recent demonstration of enhancement in the thermoelectric figure of merit of a two-dimensional (2D) PbTe quantum well system, relative to its three-dimensional (3D) bulk counterpart. Calculations suggest that the thermoelectric performance of any 3D material should show an enhanced thermoelectric figure of merit, when prepared as a 2D multi-quantum well superlattice, utilizing the enhanced density of states at the onset of each electronic subband, and the increased scattering of vibrational waves at the boundary between the quantum well and the adjacent barrier of the superlattice. In principle, low dimensionality also allows certain materials such as bismuth, which are poor thermoelectrics in 3D, to become good thermoelectrics. Thus, the successful fabrication of 1D bismuth nanowires offers new possibilities for the study of 1D systems for possible thermoelectric applications.

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.

A review of double resonance Raman spectroscopy is presented. Non-zone centre phonon modes in solids can be observed in the double resonance Raman spectra, in which weak Raman signals appear in a wide frequency region and their combination or overtone modes can be assigned. By changing the excitation laser energy, we can derive the phonon dispersion relations of a single nanotube.

The use of recent advances in resonance Raman spectroscopy studies on isolated carbon nanotubes and the scientific knowledge achieved so far from these studies is discussed in the context of advancing carbon nanotube-based technology. Changes in the Raman spectra can be used to probe and monitor structural modifications of the nanotube sidewalls that come from the introduction of defects and the attachment of different chemical species. The former effect can be probed through the analysis of the disorder-induced Raman modes and the latter through the upshifts/downshifts observed in the various Raman modes due to charge transfer effects.

The use of Raman spectroscopy as a characterization tool for individual single wall carbon nanotubes is briefly reviewed. New physical phenomena occurring at the single nanotube level are discussed, with special emphasis given to the use of resonance Raman scattering for the structural determination of (n, m) for individual nanotubes, based on diameter and chirality dependent phenomena associated with the radial breathing mode, the G-band and the G[prime]-band features. Examples are given to show how single nanotube spectroscopy provides insight into the use of Raman spectroscopy for the characterization of nanotube bundles and for the study of new physical phenomena occurring at the single nanotube level.

Resonant Raman scattering (RRS) measurements made with a tunable laser provide a highly reliable technique to study the shape of the joint density of electronic states (JDOS) of isolated single-wall carbon nanotubes (SWNTs). RRS can be used to determine the energy value for the one-dimensional (1D) van Hove singularities of a SWNT with a precision better than 5 meV, thereby providing important information that could be used for subsequent measurements on this same SWNT. With RRS, the measured width of the JDOS is on the order of $\ensuremath{\sim}0.1\ensuremath{-}1.0\mathrm{meV},$ further demonstrating that SWNTs really provide a remarkably good model for 1D mesoscopic systems.

We present a detailed photoluminescence study of a (6, 5) enriched DNA-wrapped single wall carbon nanotube (DNA-CNT) solid sample and an as-produced DNA-CNT solution. Multiple strong PL peaks were observed at excitation energies that do not correspond to the interband electronic transitions that are strongly enhanced by electronic van Hove singularities. These strong PL peaks are assigned to different mechanisms of excitation and relaxation, including one phonon, two phonon, hot luminescence processes, as well as radiative and non-radiative energy transferring mechanisms between neighboring nanotubes. These processes are assigned to different channels of phonon-assisted electron relaxation. The study shows that the electronic relaxation processes observed in PL can be used as a means to probe different physical interactions between photons, electrons, and phonons that are not separately identified in bulk semiconducting materials.

Micro-Raman spectroscopy of an isolated single wall carbon nanotube is overviewed in connection with the recent observation of (1) the chirality-dependent G-band Raman intensity, (2) BWF line analysis and (3) D-band feature of an individual single wall carbon nanotube. The localized phonons appeared around a point defect of carbon nanotube might be relevant to D-band intensity.

The concept of using nanostructured composite materials to enhance the dimensionless thermoelectric figure of merit ZT relative to that for their counterpart homogeneous alloyed bulk crystalline materials of similar chemical composition is presented in general terms. Specific applications are made to the Si-Ge and Bi2-−xSbxTe3 systems for use in high-temperature power generation and cooling applications. The scientific advantages of the nanocomposite approach for the simultaneous increase in the power factor and decrease in the thermal conductivity are emphasized insofar as their simultaneous occurrence is enabled by the independent control of these physical properties through the special properties of their nanostructures. Also emphasized are the practical advantages of using such bulk samples both for thermoelectric property measurements and for providing a straightforward path to scaling up the materials synthesis and integration of such nanostructured materials into practical thermoelectric powergeneration and cooling modules and devices.

Department of Physics, Tohoku Universityand CREST, JST, Sendai 980-8578, Japan~Received 10 May 2003; revised manuscript received 29 September 2003; published 26 March 2004!By measuring the anti-Stokes~AS! and Stokes ~S! Raman spectra on the same isolated single-wall carbonnanotube ~SWNT!, we here determine the electronic transition energies E

The double resonance Raman spectra of the disorder-induced D-band and some other non-zone-center phonon modes are discussed for two-dimensional (2D) graphite and to some extent for single wall carbon nanotubes (SWNTs). The phonon dispersion relations of graphite can be determined using Raman spectroscopy b y measuring the non-zone center Raman phonon frequencies in combination with theoretically determined phonon q-vectors. We report a t of the phonon dispersion relations to experimental Raman spectra which were previously observed but have not y et been assigned to speci c phonon branches. We found that the D-band and the G0-band of 2D graphite consist of, respectively, two and one Raman Lorentzian peaks, while 3D graphite shows two G0-band Lorentzian peaks. The appearance of two G0 peaks in the resonance Raman spectra of SWNTs may come from resonances of one laser line with two di erent van Hov e singularities.

We calculate the second order Raman spectra in graphite and compare it to recent measurements on highly oriented pyrolytic graphite (HOPG). Electron‐phonon interaction is calculated as the inner product of the amplitude of vibration with the deformation potential vector. The deformation potential vector is calculated by atomic basis functions and the potential fitted by Gaussians. When we include the electron‐phonon matrix element, the peak positions and the shapes agree well with the experimental results. The characteristic double peak structure of the G′ band in HOPG is reproduced well.

Resonance Raman studies on single wall carbon nanotubes (SWNTs) show that resonance with cross polarized light, i.e., with the E(mu,mu+/-1) van Hove singularities in the joint density of states needs to be taken into account when analyzing the Raman and optical absorption spectra from isolated SWNTs. This study is performed by analyzing the polarization, laser energy, and diameter dependence of two Raman features, the tangential modes (G band) and a second-order mode (G' band), at the isolated SWNT level.

Resonance Raman spectroscopy of small diameter (below 1.2 nm) single‐wall carbon nanotubes (SWCNT) is presented. The diameter and chirality dependent many‐body corrections to the tight binding based Kataura plot are discussed. The radial breathing modes also show small chirality dependence, giving evidence for the deviations of the small nanotube diameters from the ideal folded graphene structure. The use of spectroscopy for the characterization of small environmental effects and the (n,m) population on HiPco and CoMoCAT SWNT samples is pointed out. The richness of the intermediate frequency modes is highlighted.

The chirality-dependent G-band Raman intensity of an individual single wall carbon nanotube is presented both by a non-resonant theory for the Raman tensor and by confocal micro-Raman measurements. Theory predicts six or three intense Raman modes, respectively, for chiral or achiral nanotubes whose relative intensities depend on the chiral angle of the nanotube.

Recent progress on double resonance Raman spectroscopy and related optical properties of single wall carbon nanotubes (SWNTs) is presented. The electron‐photon and electron‐phonon interactions for graphite and SWNTs are calculated within the tight‐binding scheme. Optical absorption matrix element and phonon relaxation time calculations are consistent with experimental observations.

The basic concepts and characteristics of Raman spectra from carbon nanotubes (both isolated and bundled) are presented. The general characteristics of the radial breathing mode, tangential mode (G band), disorder-induced mode (D-band) and other Raman features are presented, with the focus directed toward their use for carbon nanotube characterization. Polarization analysis, surface enhanced Raman spectroscopy and complementary optical techniques are also discussed in terms of their advantages and limitations.

We discuss how resonance micro-Raman spectroscopy can determine the electronic and structural properties of individual isolated single-wall carbon nanotubes (SWNTs) that can be further used for potential nanodevices or studied by different experimental techniques. We show that it is possible to mark the surface of the Si/SiO2 substrate for localization of the isolated SWNTs by using a diamond tip. By timing the growth procedure, a low density of SWNTs on the substrate can be obtained so that the SWNT localization by atomic force microscopy (AFM) is trivial. We also characterize a SWNT by resonance Raman spectroscopy at the edge of a Si-SWNT-AFM tip. There results show that it is possible to make joint experiments on the same isolated SWNT.

Recently published thermoreflectance data for graphite in the photon energy range $0.6l\ensuremath{\hbar}\ensuremath{\omega}l1.0$ eV are shown to be consistent with the Slonczewski-Weiss-McClure band model using commonly accepted values for the band parameters of this model. These data show ${\ensuremath{\gamma}}_{5}$ to be positive and provide the first direct experimental determination of this parameter, yielding a value of ${\ensuremath{\gamma}}_{5}=0.025$ eV.

Electron microscopy can be used to study structures and phase transitions occurring in graphite intercalations compounds. The fundamental symmetry in graphite intercalation compounds is the staging periodicity whereby each intercalate layer is separated by n graphite layers, n denoting the stage index. The currently accepted model for intercalation proposed by Herold and Daumas assumes that the sample contains equal amounts of intercalant between any two graphite layers and staged regions are confined to domains. Specifically, in a stage 2 compound, the Herold-Daumas domain wall model predicts a pleated lattice plane structure.

This paper reports results of a neutron diffraction study on both the magnetic and the lattice dynamic structure of graphite-FeCl/sub 3/. The FeCl/sub 3/ system was selected for this study because of the interesting magnetic properties previously reported for this system. Of particular interest is the low temperature magnetic phase transition, which has been shown to be stage dependent. Special emphasis was given to the study of the low temperature magnetic state of the Fe/sup 3 +/ spins and to the longitudinal phonon modes for k/sub z/ axis acoustic branches. All experiments reported here were carried out on a stage 2 sample of graphite-FeCl/sub 3/. The sample was prepared by a two-zone vapor growth technique and was characterized by (00l) x-ray and neutron diffraction patterns to be > 95% stage 2, with only minor inclusions of other stages.

Room-temperature polarized reflectivity measurements are reported for single-crystal La2CuO4 for the (E ⊥ ĉ) and (E || ĉ) polarizations, where ĉ denotes the tetragonal axis. A Kramers–Kronig analysis is performed to obtain the frequency-dependent dielectric functions. Frequencies for the seven zone-center IR-active phonons are obtained and the longitudinal-optic–transverse-optic splittings are used to evaluate the dynamic charge on the oxygen atoms. The reflectivity spectrum for the (E ⊥ ĉ) polarization is decidedly metallic and is fitted accordingly with an overdamped Drude model and a plasma frequency of 95 meV. In the high-frequency region of the spectrum, selection rules are observed for allowed and forbidden electronic transitions. The importance of this study to the interpretation of optical measurements on high-Tc, polycrystalline pellets of La2CuO4 and related materials is also discussed.

The transient in the rate of Ni(CO) 4 formation has been studied as a function of the gas used in the initial heat treatment of the nickel samples. Quantitatively similar transient effects were seen whether the samples were heated in reducing (H 2 , D 2 ) or neutral (He) atmospheres. A mechanism is proposed in which the heat treatment activates the surface; in particular a finite number of low coordination nickel sites are activated. These sites are energetically favored for Ni(CO) 4 formation and are responsible for the transient behavior.