Variable electron-phonon coupling in isolated metallic carbon nanotubes observed by Raman scattering

We report the existence of broad and weakly asymmetric features in the high-energy (G) Raman modes of freely suspended metallic carbon nanotubes of defined chiral index. A significant variation in peak width (from 12 cm(-1) to 110 cm(-1)) is observed as a function of the nanotube's chiral structure. When the nanotubes are electrostatically gated, the peak widths decrease. The broadness of the Raman features is understood as the consequence of coupling of the phonon to electron-hole pairs, the strength of which varies with the nanotube chiral index and the position of the Fermi energy.     

Chemical surface analysis by Raman spectroscopy utilizing dimer vibrations

We report on the investigation of reconstructed semiconductor surfaces by Raman spectroscopy from dimer vibrations. Localized modes of Te dimers on the (100) surface of the zinc blende II-VI semiconductor BeTe allow the analysis of Te- and Be-rich surface structures, as well as oxidation effects. The Te-rich surface exhibits one dimer-vibration mode at 165 cm(-1), while two modes appear at the Be-rich surface (157 and 188 cm(-1)). The mode assignment as dimer vibrations is underscored by their symmetry properties and by frozen phonon calculations, yielding mode frequencies and eigenvectors. This approach opens up a new field of surface chemistry analysis by dimer-vibration spectroscopy.     

Y_（1－x）Nd_xSr_2Cu_（2．7）Mo_（0．3）O_（7－δ）体系的拉曼和红外光谱研究

利用拉曼散射和红外吸收光谱研究了Ｙ１－ｘＮｄｘＳｒ２Ｃｕ２．７Ｍｏ０．３Ｏ７－δ（ｘ＝０，０．２，０．５，０．８，１．０）系列样品的声子振动性质。实验结果表明，对ｘ＝０的样品，在拉曼光谱中主要出现３２３，４４３，５２２和５７８ｃｍ－１几个特征峰，在中红外吸收光谱中出现５２２，５８０和６４６ｃｍ－１特征峰。随着样品中Ｎｄ组分的增加，３２３ｃｍ－１峰向低波数发生位移，而５２２和６４６ｃｍ－１峰则向高波数发生位移。本文对这些振动模进行了指认，并对其随不同样品组分的变化行为进行了讨论     

碳纳米管及其H_2吸附体系的Raman光谱

利用紫外Raman光谱技术 ,对分别以CO和CH4为碳源、由化学催化法制备的两种多壁碳纳米管以及它们的H2 吸附体系进行Raman光谱表征 ,观测到可分别归属于类石墨结构的基频模G (1 5 80cm- 1 )和D (1 41 6cm- 1 ,缺陷诱导 ) ,以及它们的二阶和三阶组合频 2D(2 83 2cm- 1 ) ,D +G (2 996cm- 1 ) ,2G (3 1 6 0cm- 1 )和 2D +G (441 2cm- 1 )的Raman峰 ;H2在这些多壁碳纳米管上吸附有两种形式 :非解离吸附分子氢H2 (a)和解离吸附生成含氢表面物种CHX(x =3 ,2 ,1 ) ,所观测在 2 85 0 ,2 96 7和 3 95 0cm- 1 处的Raman谱峰可分别归属于表面CH2 基的对称C -H伸缩模 ,CH3基的不对称C -H伸缩模 ,以及吸附态分子氢H2 (a)的H -H伸缩模     

Exchange interaction effects on the optical properties of LuMnO3

We have measured the optical conductivity of single crystal LuMnO3 from 10 to 45000 cm(-1) at temperatures between 4 and 300 K. A symmetry allowed on-site Mn d-d transition near 1.7 eV is observed to blueshift ( approximately 0.1 eV) in the antiferromagnetic state due to Mn-Mn superexchange interactions. Similar anomalies are observed in the temperature dependence of the TO phonon frequencies which arise from spin-phonon interaction. We find that the known anomaly in the temperature dependence of the quasistatic dielectric constant epsilon(0) below T(N) approximately 90 K is overwhelmingly dominated by the phonon contributions.     

Pressure dependence of zone-boundary phonons in AlSb

Abstract

We have investigated the effect of hydrostatic pressure on zone-boundary and other critical-point phonon frequencies of AlSb by second-order Raman scattering. A softening of the TA(X), TA(L) and (L/T)A([Sgrave]) modes has been observed for pressures up to the first phase transition at 7.7 GPa. The LA(L) as well as the optical TO at X-, L-, and LO at [Sgrave]-, X-points harden with increasing pressure. Mode Griineisen parameters of all the resolved modes were calculated. Reflectivity measurements indicate that the high pressure phase above 7.7 GPa is metallic.     

Phonon softening in individual metallic carbon nanotubes due to the Kohn Anomaly

We have studied the line shape and frequency of the G band Raman modes in individual metallic single walled carbon nanotubes (M-SWNTs) as a function of Fermi level (epsilonF) position, by tuning a polymer electrolyte gate. Our study focuses on the data from M-SWNTs where explicit assignment of the G- and G+ peaks can be made. The frequency and line shape of the G- peak in the Raman spectrum of M-SWNTs is very sensitive to the position of the Fermi level. Within +/- variant Planck's over 2piomega/2 (where variant Planck's over 2piomega is the phonon energy) around the band crossing point, the G- mode is softened and broadened. In contrast, as the Fermi level is tuned away from the band crossing point, a semiconductinglike G band line shape is recovered both in terms of frequency and linewidth. Our results confirm the predicted softening of the A-symmetry LO phonon mode frequency due to a Kohn anomaly in M-SWNTs.     

Structural ( n, m) determination of isolated single-wall carbon nanotubes by resonant Raman scattering

We show that the Raman scattering technique can give complete structural information for one-dimensional systems, such as carbon nanotubes. Resonant confocal micro-Raman spectroscopy of an (n,m) individual single-wall nanotube makes it possible to assign its chirality uniquely by measuring one radial breathing mode frequency omega(RBM) and using the theory of resonant transitions. A unique chirality assignment can be made for both metallic and semiconducting nanotubes of diameter d(t), using the parameters gamma(0) = 2.9 eV and omega(RBM) = 248/d(t). For example, the strong RBM intensity observed at 156 cm(-1) for 785 nm laser excitation is assigned to the (13,10) metallic chiral nanotube on a Si/SiO2 surface.     

新型Hg系高Tc超导体(Hg_(0.8)Mo_(0.2))Sr_2(Y_(1-x)Ca_x)Cu_2O_(6+δ)的拉曼光谱

新型Ｈｇ系高Ｔｃ超导体（Ｈｇ０．８Ｍｏ０．２）Ｓｒ２（Ｙ１－ｘＣａｘ）Ｃｕ２Ｏ６＋δ的拉曼光谱许存义左健冯建平（中国科学技术大学结构分析开放实验室合肥２３００２６）吴若（江西师范大学物理系南昌３３００２７）ＲａｍａｎＳｐｅｃｔｒａｏｆｔｈｅＮｅｗＳｅ...     

Phonon freedom and confinement in GaAsAlxGa1−xAs superlattices

Phonon modes in GaAsAl_xGa_1?xAs superlattices simplify when the phonon wavevector q is perpendicular to the plane of the layers. We have studied such modes using a Raman back-scattering technique on SL's grown by MBE. The results are consistent with simple ideas of LA phonon freedom and LO phonon confinement suggested by one-dimensional lattice dynamical calculations. The longitudinal acoustic (LA) modes show zone folding due to mini-zone formation. Their frequencies occur in doublets linearly dependent on q and show little mini-gap formation. This is consistent with a picture of approximately free plane wave propagating through the interfaces with Raman coupling due to SL layering of the photoelastic coefficient. By contrast, Raman data on LO modes in small period GaAsAlAs SL's suggest that these modes are standing waves strongly confined in either GaAs or AlAs.     

Phonon dispersion relations and soft modes of 4? carbon nanotubes

The phonon dispersion relations of three kinds of 4~? carbon nanotubes are calculated by using the density functional perturbation theory. It is found that the frequencies of some phonon modes are very sensitive to the smearing width used in the calculations, and eventually become negative at low electronic temperature. Moreover, two kinds of soft modes are identified for the (5,0) tube which are quite different from those reported previously. Our results suggest that the (5,0) tube remains metallic at very low temperature, instead of the metallic-semiconducting transition claimed before.     

纳米氮化镓的拉曼光谱研究

纳米氮化镓的拉曼光谱研究罗薇邹广田崔启良赵永年崔田李红东韩杰（吉林大学超硬材料国家重点实验室长春１３００２３）ＳｔｕｄｙｏｆＲａｍａｎＳｐｅｃｔｒａｏｎＧａＮＮａｎｏｃｒｙｓｔａｌＬｕｏＷｅｉ，ＺｏｕＧｕａｎｇｔｉａｎ，ＣｕｉＱｉｌｉａｎｇ，Ｚｈａｏ...     

Measuring the uniaxial strain of individual single-wall carbon nanotubes: resonance Raman spectra of atomic-force-microscope modified single-wall nanotubes

Raman spectroscopy is used to measure the strain in individual single-wall carbon nanotubes, strained by manipulation with an atomic-force-microscope tip. Under strains varying from 0.06%-1.65%, the in-plane vibrational mode frequencies are lowered by as much as 1.5% (40 cm(-1)), while the radial breathing mode (RBM) remains unchanged. The RBM Stokes/anti-Stokes intensity ratio remains unchanged under strain. The elasticity of these strain deformations is demonstrated as the down-shifted Raman modes resume their prestrain frequencies after a nanotube is broken under excessive strain.     

以Y/Ni为催化剂制备的单壁碳纳米管的拉曼光谱研究

采用电弧放电法以Y/Ni为催化制备了单壁碳纳米管（SWNTs），对样品进行了扫描电镜、透射电镜和拉曼光谱的研究。所制备的样品中单壁碳纳米管的含量较高。对单壁碳纳米管的共振拉曼散射增强效应进行了观察，随激光波长的不同，单壁碳纳米管的拉曼光谱也随之变化，尤其是低频区径向呼吸模的变化比较明显。利用布里渊区折叠法计算了单壁碳纳米管的电子态密度曲线，根据SWNTs电子态密度尖峰之间的能量差、管子的直径和呼吸模频率建立了一个图表，并对SWNTs的呼吸模进行了归属。分析结果表明：样品中单壁碳纳米管的直径分布在0.79-1.76nm范围，金属管和半导体管均存在，并且直径在1.45nm附近的碳管居多。     

Optical properties of 0.4-nm single-wall carbon nanotubes aligned in channels of AlPO<Subscript>4</Subscript>-5 single crystals

Single-walled carbon nanotubes (SWNTs) of 0.4 nm in diameter are produced inside channels of microporous zeolite single crystals. Three possible structures: (5,0), (4,2), and (3,3) contribute to three bands at 1.37, 2.1, and 3.1 eV in optical absorption spectra. The direct correspondence between chiralities and absorption bands is identified by density functional calculations. The resonant Raman spectrum shows the features that can be assigned to van Have singularities in calculated phonon density of states. In the low-frequency region, two peaks at 510 and 550 cm^-1 are attributed to the radial breathing modes (RBMs) of the (5,0) and (4,2) tubes. After removing the zeolite framework, the RBM frequencies downshift by 10 cm^-1. The charge-transfer behavior in lithium-doped 0.4-nm SWNTs is investigated. The Raman peak at 1558 cm^-1 couples with the electronic continuum, and shows a Breit–Wigner–Fano line shape. PACS 78.67.Ch; 73.22.-f; 82.75.Mj; 63.22.+m; 71.20.Tx     

GaN直纳米线的光学性能

对 Ga N直纳米线的拉曼光谱及光致发光光谱进行了研究。拉曼光谱表明 ,与计算值相比 ,E2 ( high)声子频率在 560 cm- 1有 -9cm- 1的移动 ,这种声子频率显示出向低能带频移及带变宽的特征 ,是由于纳米尺寸效应所引起的结果。体系的光致发光光谱在 3 44 .8nm附近的近带隙发光 ,与文献报道的 Ga N体材料的数值3 65nm相比有一蓝移 ,这是由于量子限制效应造成的     

Thermal expansion and lattice dynamics under pressure of ZnS,ZnSe and ZnTe

The thermal expansion against temperature of ZnS, ZnSe and ZnTe is studied theoretically using the experimental pressure dependence of elastic stiffness constants and phonon frequencies. The mode Grüneisen parameters obtained from the high pressure effect on the one- and two- phonon Raman spectra at the metallic transition pressure by Weinstein are used originally, but do not reproduce the experimental linear expansion coefficient at high temperatures. The contributions from optical modes with large phonon frequency are important to the thermal expansion at high temperatures, and a set of mode Grüneisen parameters, which bring good agreement with the observed linear expansion coefficient not only at low temperatures, but also at high temperatures, are obtained. Then, the phonon dispersion curves of ZnS, ZnSe and ZnTe at their metallic transition pressures are quantitatively shown.     

Raman scattering in intermediate valent SmB6

We have measured the Raman spectra of intermediate valent (IV) SmB₆ and compared it to LaB₆ and EuB₆. Beside the three high energy Raman active phonons we found additional features in the spectra. Most prominent is a peak at 172 cm^-1 for SmB₆, at 214 cm^-1 for LaB₆ and at ～220 cm^-1 for EuB₆. The spectra are analysed in terms of defect induced phonon excitations yielding a weighted phonon density of states. The softening of the line in IV SmB₆ compared to the other hexaborides is unusual since even in the trivalent MB₆ compounds the Raman active phonon modes normally are at lower frequencies than in semiconducting, divalent samples. This phonon softening is explained in analogy with the phonon anomalies in other IV compounds like Sm_1?xY_xS and TmSe.     

多壁碳纳米管的拉曼散射

本文报导了用直流碳弧放电方法制备的多层碳纳米管的拉曼光谱。和ＨＯＰＧ相比,由于碳纳米管的量子尺寸效应和碳纳米管直径的分布,在纯化和未纯化的碳纳米管中均存在Ｅ２ｇ模的软化,其红移范围分布在２～１３ｃｍ １范围内。