Anharmonic phonon lifetimes in carbon nanotubes: evidence for a one-dimensional phonon decay bottleneck

High-resolution Raman spectroscopy is applied to suspended single-walled carbon nanotubes (SWNTs) to elucidate the puzzling differences in the lifetime of the radial breathing mode (RBM) obtained from different experimental techniques. Whereas recent tunneling experiments suggest a room temperature RBM lifetime as long as 10 ns, previous Raman experiments yield lifetimes shorter than 2 ps. The lifetimes obtained in this study are longer than 5 ps-a significant step in the direction of the tunneling results. We argue that the remaining discrepancy is due to the existence of phonon decay bottlenecks caused by the one-dimensional nature of nanotubes. Numerical simulations of the RBM decay show that it is possible to reconcile the short lifetimes measured spectroscopically with the long lifetimes obtained in tunneling experiments.     

Temperature dependences of Raman scattering in different types of GaN epilayers

First-order Raman scatterings of hexagonal GaN layers deposited by the hydride vapour phase epitaxy and by metal-organic chemical vapour deposition on SiC and sapphire substrates are studied in a temperature range between 303 K and 503 K. The temperature dependences of two GaN Raman modes (A₁ (LO) and E₂ (high)) are obtained. We focus our attention on the temperature dependence of E₂ (high) mode and find that for different types of GaN epilayers their temperature dependences are somewhat different. We compare their differences and give them an explanation. The simplified formulas we obtained are in good accordance with experiment data. The results can be used to determine the temperature of a GaN sample.     

QUANTUM EFFICIENCY AND MULTIPHONON NONRADIATIVE TRANSITION OF Er3+ IONS IN FLUORIDE GLASS

Optical absorption, emission and excitation spectra, lifetimes of ⁴S_3/2 state and ⁴F_9/2 state from 10K to 500K, and Raman spectra were measured for Er³⁺ ions in fluoride glass. The radiative transition probabilities were calculated on the basis of Judd-Ofelt theory. The nonradiative transition probabilities and the quantum efficiencies were determined by calculating the difference between the measured lifetimes and the calculated radiative transition probabilities. The temperature dependence of nonradiative transition provavility was investigated using the Huang-Rhys theory of multiphonon relaxation , in which two kinds of phonons as well as the parameter s were taken into consideration. A fairly good agreement of the theoretical calculation with the experimental results has been obtained. The value of s is estimated and the effect of s is discussed.     

Langevin description of Markov master equations II: Noise correlations

We present experimental results of dielectric and birefringence measurements on 3.5% and 8% Li∶KTaO₃ as a function of temperature. These results indicate that the 8% Li∶KTaO₃ sample undergoes a ferroelectric phase transition at 97 K. The 3.5% sample seems to undergo a distortive transition at ～75 K. Raman experiments show that the splitting of the TO₁ mode of the 8% Li sample extrapolates to zero at ～97 K supporting the conclusion that this sample undergoes a phase transition at this temperature. Raman spectra of both samples at temperatures above the phase transition or freezing temperatures exhibit in addition to second order Raman features first order Raman lines. These lines have the properties of fluctuation induced first order Raman scattering. Measurement of the integrated intensity of these lines is used to obtain the temperature dependence of the fluctuation intensity. The results are compared to the Halperin and Varma theory. The data obtained for the 3.5% Li sample partialy fits the theory. However the results for the 8% Li sample do not fit the theory. The misfit is attributed to the overlap of defect zones.     

Absorption and photoluminescence studies of the temperature dependence of exciton life time in lattice-matched and strained quantum well systems

We present systematic studies of the temperature dependence of linewidths and lifetimes of excitonic transitions in quantum wells grown by molecular beam epitaxy using both photoluminescence(PL) and optical absorption. The temperature ranged from 6K to room temperature. Samples under investigation were lattice-matched GaAs/AlGaAs and InGaAs/InAlAs, and strained InGaAs/GaAs and InGaAs/AlGaAs quantum wellssystems. In addition, the effects of well-size variations in GaAs/AlGaAs quantum wells were measured and analyzed. In all cases we were able to observe the excitonic transitions up to room temperature. By a careful fitting of the experimental data we separated the exciton transitions from band-to-band transitions. By deconvoluting the excitonic transitions we obtained the homogeneous and inhomogeneous linewidths. The homogeneous linewidths were used to calculate the exciton lifetimes as a function of temperature using the Heisenberg uncertainty principle. We found the lifetime decreases significantly with temperature and increases with increasing well size. These results are interpreted in terms of the exciton-phonon interaction and are expected to be very useful for the design of semiconductor optical devices operating at different temperatures.     

QUANTUM EFFICIENCY AND MULTIPHONON NONRADIATIVE TRANSITION OF Er3+ IONS IN FLUORIDE GLASS

Optical absorption, emission and excitation spectra, lifetimes of ⁴S_3/2 state and ⁴F_9/2 state from 10K to 500K, and Raman spectra were measured for Er³⁺ ions in fluoride glass. The radiative transition probabilities were calculated on the basis of Judd-Ofelt theory. The nonradiative transition probabilities and the quantum efficiencies were determined by calculating the difference between the measured lifetimes and the calculated radiative transition probabilities. The temperature dependence of nonradiative transition provavility was investigated using the Huang-Rhys theory of multiphonon relaxation , in which two kinds of phonons as well as the parameter s were taken into consideration. A fairly good agreement of the theoretical calculation with the experimental results has been obtained. The value of s is estimated and the effect of s is discussed.     

Electron spin lifetimes in Hg0.78Cd0.22 Te and InSb

We have made direct pump–probe measurements of spin lifetimes in long wavelength narrow-gap semiconductors at wavelengths between 4 and 10 μm and from 4 to 300 K. In particular, we measure remarkably long spin lifetimes, τ_s300 ps, even at 300 K for epilayers of degenerate n-type InSb. In this material the mobility is approximately constant between 77 and 300 K, and we find that τ_s is approximately constant in this temperature range. In order to determine the dominant spin relaxation mechanism we have investigated the temperature dependence of τ_s in non-degenerate lightly n-type Hg_0.78Cd_0.22Te of approximately the same band gap as InSb, and find that τ_s varies from 356 ps at 150 K to 24 ps at 300 K. Our results, both in magnitude and temperature dependence of τ_s, imply that the Elliott–Yafet model dominates in these materials.     

Time-resolved dynamics of thermal isomerization in cesium-halide cluster anions

We have performed time-resolved studies of the dynamics of thermal isomerization occurring in certain cesium-halide cluster anions. Using a pump-probe technique, we have observed the repopulation of a photodepleted isomer within an ensemble as a function of time by monitoring the photoelectron spectrum. The rates of isomerization increase and the isomer lifetimes decrease as functions of temperature. The clusters undergo a gradual phase transition from solid-like to liquid-like states with liquid-like behavior obtained at approximately 500 K, much lower than the bulk melting temperatures of approximately 900 K.     

Temperature dependent effects on luminescence polarization and recombination lifetime in serpentine superlattice quantum wire arrays

We have measured photoluminescence excitation (PLE) spectra and radiative lifetimes as functions of temperature for serpentine superlattice quantum-wire arrays. The (Al, Ga)As arrays have lateral periods near 10 nm, and lateral confining potentials of 120 meV in the conduction band. At low temperature the excitons are strongly localized within potential fluctuations along the wires. The radiative lifetime of these localized states is 340 ps at 2 K. The degree of exciton localization decreases with increasing temperature, from which we estimate the strongly-localizing potential fluctuations to be approximately 10 meV deep. Above 80 K the excitons have sufficient thermal energy for motion along the wires. The radiative lifetimes increase with temperature, to 20 ns for free carriers at 325 K. The lateral potential barriers inhibit diffusion to non-radiative recombination sites.     

RbNiF3自旋波量子对的Raman散射谱理论分析

在温度由零度直到Néel温度的范围内讨论了RbNiF₃自旋波量子对的Raman散射谱。由一般的对称性原理及实验结果的分析导出磁性系统与辐射场相互作用哈密顿量的具体形式;应用推迟Green函数运动方程方法,得到散射截面的解析表达式;在自旋波量子对传播子中同时计入能量重正化和寿命的效应。最后,在适当的近似下用计算机作数值计算,在没有可调整参数的情况下得到了在Néel温度以下的Raman峰值位置和半宽度随温度的变化关系及各种温度下的Raman谱形,它们与实验结果符合得较好。 关键词：     

Molekül- und Gitterschwingungen im Natriumhyperoxid

We have measured the Raman spectrum of clear NaO₂ single crystals of mm size. In addition to the usual lattice modes we observe a purely molecular mode. The Raman line of 1156 cm^?1 at 300 K corresponds to the value which was proposed in the literature for the vibrational frequency of the O₂ ^? ion. This confirms the fact that the alkali hyperoxides are ionic crystals. The investigation of the Raman spectrum between 6 K and 300 K permits an accurate determination of the temperature dependence of the O₂ ^? stretching frequency. The results display structural phase transitions at 230 K and 201 K in agreement with the previous findings of specific heat measurements and X-ray analysis.     

Possible interplay between a two phonon mode and high energy magnetic excitations in BiFeO<Subscript>3</Subscript>

We have performed Raman measurements on high energy excitations in BiFeO₃ single crystals as a function of both temperature and laser excitation lines. A strong feature observed at 1250 cm^-1 in the Raman spectra has been previously assigned to two phonon overtone. This peak exhibits an unusual frequency shift with the laser lines and the temperature dependence of its Fano lineshape shows two singularities at 150 K and 200 K which can be related to magnetic excitations. In the same energy range, we have also identified the two-magnon excitation with a temperature dependence very similar to the one measured for the one-magnon modes.     

Raman scattering studies of the magnetic ordering in hexagonal HoMnO3 thin films

We present the results of the temperature dependence of the Raman spectra of hexagonal HoMnO₃ thin films in the 13–300 K temperature range. The films were grown on Pt(111)//Al₂O₃ (0001) substrates using the laser ablation method. In the HoMnO₃ thin films, we initially observedseveral broad Raman peaks at ∼510, 760, 955, 1120, and 1410 cm⁻¹. These broad Raman peaks display an anomalous behavior near the magnetic transition temperature, and the intensity difference of the Raman spectra at different temperatures shows several pairs of negative and positive peaks as the temperature is lowered below the Néel temperature. Our analyses indicate that all the broad peaks are correlated with magnetic ordering, and we have assigned the origin of all the broad peaks. Purely on the basis of the Raman analysis, we have deduced the Néel temperature and the spin exchange integrals of HoMnO₃ thin films. We also investigated the effects of the growth condition on the strongest broad peak at ∼760 cm⁻¹, which is related with pure magnetic ordering. This result indicates that the oxygen defect in the HoMnO₃ sample has negligible effect on magnetic ordering. Copyright © 2009 John Wiley & Sons, Ltd.     

Light scattering spectrum of disordered molecular crystals: KCN

We have investigated the effect of the low temperature order-disorder phase transitions on the Raman spectrum of KCN. In the disordered cubic room temperature phase we only find a broad central component in the spectrum which sharpens near the transition temperature at 168K. From the doubling of the librational mode of the CN^--molecules we conclude that an antipolar orientation of the CN^--molecules is probable for the low temperature phase below 83 K.     

Temperature‐dependent Raman scattering study of multiferroic MnWO4

We have measured polarized Raman spectra of MnWO₄ single crystals at low temperatures, and studied the temperature dependence of the various phonon modes. From our Raman studies of the MnWO₄, a new transition temperature, ∼180 K, was found. We have completely assigned the symmetries of the 18 observed Raman modes of the MnWO₄, as expected from a group theoretical analysis. These Raman modes have been classified into three groups according to weak, intermediate and strong temperature dependence of the modes in each group. Six internal modes have been identified by their weak temperature dependence of the Raman wavenumbers. The temperature dependence of the wavenumbers of the B_g modes in Mg O bonds, modes of intermediate temperature dependence group, shows an anomalous behavior under 50 K. The phonon modes of strong temperature dependence show an anomalous change at ∼180 K in the linewidths. This is believed to be a new transition temperature which involves the changes in the inter‐WO₆ octahedra structure. Copyright © 2009 John Wiley & Sons, Ltd.     

Structural transformations in a single‐crystal Rb2NaYF6: Raman scattering study

This paper reports Raman spectroscopy investigation of phase transitions in Rb₂NaYF₆ crystal. The experimental spectra were compared with the calculated one. The spectra were obtained in temperature range from 8 to 300 K. The Raman spectra shows anomalous temperature‐dependent behavior at T₁ = 154 and T₂ = 122 K. Soft mode restoration has been found, which allows us to attribute first transition at 154 K to displacive type. Detailed analysis temperature dependencies of the line positions and widths have been performed. We found no effects of possible lattice disorder anywhere, except narrow (about 20 K) range above the T₁ temperature. The Raman spectra of Rb₂NaYF₆ crystal have been obtained and analyzed under hydrostatic pressure up to 4.33 GPa (at T = 295 K). The high pressure experiment up to 4.33 GPa did not disclose any effects associated with phase transitions. The lattice vibration spectra were calculated up to 10 GPa. The calculation has been demonstrated that the Rb₂NaYF₆ does not undergo high pressure phase transition. Copyright © 2013 John Wiley & Sons, Ltd.     

Polarized Raman Study On LiNbO3

Polarized Raman was used to study the lattice vibrations in LiNbO₃ single crystals which temperature range from 291K to 11K. The 13 vibrational modes expected from group-theoretical analysis were obtained. The nine internal modes and four external modes of NbO₆ octahedra have been assigned for the first time.     

Polarization effects on the line-strength calculations of Er3+-doped LiNbO3

The polarized optical absorption of Er³⁺-doped LiNbO₃ has been obtained and analyzed within the framework of the Judd-Ofelt theory including polarization dependences. These results are compared with those obtained under unpolarized conditions. Fluorescence lifetimes for different Er³⁺ concentrations have also been measured in the temperature range 10–300 K.     

LiTaO3的高温拉曼光谱研究

利用显微共聚焦拉曼光谱技术研究了LiTaO3晶体在298～948K温度范围内的拉曼光谱,通过对其高波数拉曼峰的分析,研究了LiTaO3的相变机制。研究发现,除466.7cm-1峰外,LiTaO3的其余拉曼峰随着温度的升高向低波数方向明显频移;所有拉曼峰的半高宽随着温度的升高逐渐增加,同时拉曼峰的强度逐渐减弱;在933K附近,LiTaO3的三个拉曼峰的消失,表明LiTaO3发生了从低温铁电相到高温顺电相的相变;与此同时,在该温度附近,359.5,385.0以及466.7cm-1峰的半高宽存在超线性增加。研究结果表明,LiTaO3从低温铁电相到高温顺电相的相变是一种混合型相变,其相变过程可逆。     

Temperature dependence of anomalous and ordinary Raman scattering in β-AgI

We report careful measurements of the temperature dependence of Raman scattering in β-AgI, with particular emphasis on the region 12°K to 150°K. Our main result is that the anomalous features in the Raman spectra of β-AgI are definitely first order. Furthermore from the temperature dependence of the anomalous scattering intensity we infer the possibility of a first partial disordering in the Ag⁺ sublattice at about 50°K.