Raman spectroscopy in graphene

https://www.bc.edu/content/dam/files/schools/cas_sites/physics/pdf/Ren/p85.pdf

Advances in the science and technology of carbon nanotubes and their composites: a review

Multiferroic magnetoelectrics are materials that are both ferromagnetic and ferroelectric in the same phase. As a result, they have a spontaneous magnetization that can be switched by an applied magnetic field, a spontaneous polarization that can be switched by an applied electric field, and often some coupling between the two. Very few exist in nature or have been synthesized in the laboratory. In this paper, we explore the fundamental physics behind the scarcity of ferromagnetic ferroelectric coexistence. In addition, we examine the properties of some known magnetically ordered ferroelectric materials. We find that, in general, the transition metal d electrons, which are essential for magnetism, reduce the tendency for off-center ferroelectric distortion. Consequently, an additional electronic or structural driving force must be present for ferromagnetism and ferroelectricity to occur simultaneously.

Why Are There so Few Magnetic Ferroelectrics

http://sces.phys.utk.edu/mostcited/mc1_1114.pdf

Colossal Magnetoresistant Materials: The Key Role of Phase Separation

Thermal conductivity of carbon nanotubes and their polymer nanocomposites: A review

Carbon nanotubes are unique tubular structures of nanometer diameter and large length/diameter ratio. The nanotubes may consist of one up to tens and hundreds of concentric shells of carbons with adjacent shells separation of ∼0.34 nm. The carbon network of the shells is closely related to the honeycomb arrangement of the carbon atoms in the graphite sheets. The amazing mechanical and electronic properties of the nanotubes stem in their quasi-one-dimensional (1D) structure and the graphite-like arrangement of the carbon atoms in the shells. Thus, the nanotubes have high Young’s modulus and tensile strength, which makes them preferable for composite materials with improved mechanical properties. The nanotubes can be metallic or semiconducting depending on their structural parameters. This opens the ways for application of the nanotubes as central elements in electronic devices including field-effect transistors (FET), single-electron transistors and rectifying diodes. Possibilities for using of the nanotubes as high-capacity hydrogen storage media were also considered. This report is intended to summarize some of the major achievements in the field of the carbon nanotube research both experimental and theoretical in connection with the possible industrial applications of the nanotubes.

Carbon nanotubes: properties and application

The Raman-active phonons in orthorhombic perovskitelike ${\mathrm{YMnO}}_{3}$ and ${\mathrm{LaMnO}}_{3}$ were studied by measuring Raman spectra in various scattering configurations. The experimental Raman line wave numbers and the expected shapes for the phonon modes were compared to those reported for other perovskitelike compounds with the $\mathrm{Pnma}$ structure and to the results of lattice dynamical calculations. The observed Raman lines in the spectra of ${\mathrm{YMnO}}_{3}$ and ${\mathrm{LaMnO}}_{3}$ were assigned to definite atomic motions. The much larger linewidths and strong variation of the Raman spectra with increasing laser power in the case of ${\mathrm{LaMnO}}_{3}$ provide evidence for a structural instability that may result in a phase transition under laser annealing.

Raman spectroscopy of orthorhombic perovskitelike YMnO 3 and LaMnO 3

Analytical expressions for the velocities of the longitudinal and the torsional sound waves in single-walled carbon nanotubes are derived using Born's perturbation technique within a lattice-dynamical model. These expressions are compared to the formulas for the velocities of the sound waves in an elastic hollow cylinder from the theory of elasticity to obtain analytical expressions for the Young's and shear moduli of nanotubes. The calculated elastic moduli for different chiral and achiral ~armchair and zigzag! nanotubes using force constants of the valence force field type are compared to the existing experimental and theoretical data. vibration amplitude of several isolated MWNT's was ana- lyzed in a transmission electron microscope to eventually obtain 1.8 TPa for the average Young's modulus. Later on, this technique was applied to measure Young's modulus of isolated SWNT's in the diameter range 1.0 21.5 nm and an average value ^Y&51.2520.35/10.45 TPa was derived. 7 In another experimental approach 8 the MWNT's were pinned to a substrate by conventional lithography and the force was measured at different distances from the pinned point by atomic force microscope ~ATM!. The average Young's modulus for different MWNT's with diameters from 26 to 76 nm was found to be 1.2860.59 TPa. Recently, Young's and shear moduli of ropes of SWNT's were measured by sus- pending the ropes over the pores of a membrane and using ATM to determine directly the resulting deflection of the rope. 9 The theoretical estimation of the elastic moduli was accomplished exclusively by numerical second derivatives of the energy of the strained nanotubes. In the calculation of the elastic moduli of various SWNT's within a simple force- constant model 10 it was found that the moduli were insensi- tive to tube size and helicity and had the average values of ^Y&50.97 TPa and ^G&50.45 TPa. In several works, molecular-dynamics ~MD! simulation algorithms using the Tersoff-Brenner potential for the carbon-carbon interactions were implemented to relax the strained nanotubes and calcu- late their energy. 11-13 For tubes of diameter of 1 nm values for Y of 5.5 TPa ~Ref. 12! and 0.8 TPa ~Ref. 13! were ob- tained. A non-orthogonal tight-binding ~TB! scheme was ap- plied to calculate Young's modulus of several chiral and achiral SWNT's yielding an average value of 1.24 TPa. 14 Recently, the second derivative of the strain energy with re- spect to the axial strain, calculated with a pseudopotential density-functional-theory ~DFT! model for a number of SWNT's, 15 was found to vary slightly with the tube type and to have the average value of 56 eV. In this paper, we choose a different approach to the cal- culation of the elastic properties of SWNT's. Namely, we derive analytical expressions for the elastic ~Young's and shear! moduli of SWNT's using a perturbation technique due to Born 16 within a lattice-dynamical model for nanotubes. 17 This scheme has the advantage that the elastic moduli are consistent with the lattice dynamics of the nanotubes and that each of these moduli is obtained in one calculational step only. The essential features of a model of the lattice dynamics of SWNT's based on the explicit accounting for the helical symmetry of the tubes 17 are summarized in Sec. II A. This model is applied to study of the long-wavelength vibrations in nanotubes using Born's perturbation technique 16 and to obtain analytical expressions for the velocities of the longi- tudinal and the torsional sound waves in SWNT's ~see Sec. II B!. The comparison of these expressions with the formulas from the theory of elasticity for the velocities of these waves in an elastic hollow cylinder allows one to determine the Young's and shear moduli of the nanotubes. The calculated phonon dispersion of a (10,10) nanotube and elastic moduli for various chiral and achiral ~armchair and zigzag! nano- tubes using force constants of the valence force field ~VFF! type 18 are presented in Sec. III and discussed in comparison with the existing experimental and theoretical data.

/pdf/elastic-properties-of-single-walled-carbon-nanotubes-4u466rpfw4.pdf

Elastic properties of single-walled carbon nanotubes

The Raman and infrared phonons of isostructural rhombohedral ${\mathrm{LaMnO}}_{3}$ and ${\mathrm{LaAlO}}_{3}$ are studied at room temperature. The experimental spectra are compared with the prediction of lattice-dynamical calculations and the lines observed are assigned to definite atomic vibrations. It is shown that the Raman mode of ${A}_{1g}$ symmetry in ${\mathrm{LaAlO}}_{3}$ and ${\mathrm{LaMnO}}_{3}$ (at $123 {\mathrm{cm}}^{\ensuremath{-}1}$ and $236 {\mathrm{cm}}^{\ensuremath{-}1},$ respectively) involves atomic motions that cause the rhombohedral distortion, i.e., it is a ``soft'' mode, and its position could be used as a measure of the degree of the distortion. It is also argued that the broad Raman bands in the high-frequency range of ${\mathrm{LaMnO}}_{3}$ are not proper modes of the rhombohedral $R3\ifmmode\bar\else\textasciimacron\fi{}c$ structure, but are rather induced by the dynamic Jahn-Teller effect.

/pdf/comparative-study-of-optical-phonons-in-the-rhombohedrally-3qg7y095ad.pdf

Comparative study of optical phonons in the rhombohedrally distorted perovskites LaAlO 3 and LaMnO 3

The effects of curvature on the structure, electronic and optical properties of isolated single-walled carbon nanotubes are studied within a symmetry-adapted non-orthogonal tight-binding model using 2s and 2p electrons of carbon. The symmetry-adapted scheme allows reducing the matrix eigenvalue problem for the electrons to diagonalization of 8×8 matrices for any nanotube type. Due to this simplification, the electronic band structure of nanotubes with a very large number of atoms in the unit cell can be calculated. Using this model, the structure of 187 small- and moderate-radius nanotubes is optimized. It is found that the deviations of the optimized structure from the non-optimized one are large for tube radii smaller than 5 A. The band structure and the dielectric function of 101 small- and moderate-radius nanotubes are calculated. The optical transition energies for these nanotubes are derived from the dielectric function and plotted versus tube radius. It is shown that the structural optimization introduces small changes to the transition energies obtained within the non-orthogonal tight-binding model. The transition energies for the optimized structure within this model agree well with the available ab initio data for a few nanotube types. On the other hand, the results for the former deviate widely from those used for nanotube characterization in π-band tight-binding model especially for small-radius tubes. The derived transition energies can be used for the assignment of nanotube absorption spectra and for the selection of nanotube types for which the Raman scattering is resonant.

https://iopscience.iop.org/article/10.1088/1367-2630/6/1/017/pdf

Curvature effects on the structural, electronic and optical properties of isolated single-walled carbon nanotubes within a symmetry-adapted non-orthogonal tight-binding model

Valentin N. Popov

Papers

Carbon nanotubes: properties and application

Raman spectroscopy of orthorhombic perovskitelike YMnO 3 and LaMnO 3

Elastic properties of single-walled carbon nanotubes

Comparative study of optical phonons in the rhombohedrally distorted perovskites LaAlO 3 and LaMnO 3

Curvature effects on the structural, electronic and optical properties of isolated single-walled carbon nanotubes within a symmetry-adapted non-orthogonal tight-binding model