A3C60超导体的研究

 1991年5月,Bell实验室的A.F.Hebard等人首次在掺钾的C₆₀中观察到高达18K的超导电性.于是,这项研究结果在全世界掀起广泛的研究热潮,成为近年高温超导研究领域中又一热点.本文扼要介绍C₆₀掺杂实验的物性测量结果,引起C₆₀超导的几种可能机制及对C₆₀掺杂研究的意义.一、A_xC₆₀的掺杂实验及物性测量结果Hebard等人发现掺钾的C₆₀高达18K的超导电性是在9GHz频率下,用微波损耗测量样品的电阻率随温度的变化时观察到的.当样品冷却到约20K时,电阻率略有增加,后又降低,直到5K时电阻率降为零.超导转变宽度为4.6K,4K时临界电流密度为40A/cm².用SQUID磁强计测量证明了它的超导转变为18K.     

硫掺杂C60薄膜电学性质研究

对硫掺杂C₆₀薄膜样品在433K进行真空退火，并测量了其电导率随温度的变化关系．发现硫掺杂后C₆₀薄膜的电导激活能减小，电导率显著增大．电导率随温度的变化曲线在368K到388K的范围内，存在一个电导率与温度的关系不严格遵循指数规律的过渡区，在过渡区的两侧硫掺杂的C₆₀薄膜则表现出明显的半导体特性，这是由于在不同温度范围内样品中硫分子的结构相变所引起的 关键词：     

MgB2超导薄膜的变温拉曼光谱研究

MgB₂作为迄今为止超导转变温度最高的合金超导体,由于其具有结构简单、相干长度长、晶界间不存在弱连接、上临界场很高、电-声散射时间短等特点,MgB₂超导薄膜在电子学领域有着广阔的的应用前景。拉曼光谱是研究电-声子相互作用和超导能带的一种有效方法,且已广泛用于分析MgB₂材料的电子、声子特征以及超导体能带结构,研究表明,样品质量、晶粒尺寸以及测试条件对MgB₂拉曼峰的峰位和峰形影响很大,其中拉曼光谱随温度的变化也是一个研究重点,但目前关于MgB₂变温拉曼光谱的研究,测试的温度范围相对较小,局限在83 K到室温区域或是转变温度附近。研究了大范围温度区间内MgB₂薄膜的拉曼光谱变化,采用混合物理化学沉积法在(0001)SiC衬底上制备了MgB₂多晶薄膜,薄膜的晶粒尺寸约为300 nm,超导转变温度为39.3 K,对其在10～293 K之间的拉曼光谱进行了测试,测量的波数范围为20～1 200 cm-1。变温拉曼光谱的测试结果显示,在高频620 cm-1附近以及低频80和110 cm-1附近存在MgB₂的拉曼峰。经分析,低频区域出现的两个拉曼峰的频率与超导能隙宽度相对应,表明MgB₂的双能隙特性。考虑到MgB₂中四种声子模式的拉曼活性,高频620 cm-1附近的拉曼峰应是由E_2g振动模所贡献的,且随着测试温度的降低,该拉曼峰的峰位未发生明显的偏移,但半高宽显著变小,从293 K时的380.7 cm-1减小到10 K时的155 .7 cm-1,分析表明E_2g声子与电子系统的非线性耦合所引起的非简谐效应可能是拉曼峰半高宽线性变小的主要原因。     

紫外激光和压力共同作用下C60-peapod的聚合相变研究

采用气相扩散方法将C₆₀分子填充到单壁碳纳米管(SWNTs)中，制备出高填充比率的豆荚形纳米材料C₆₀@SWNT，又称为peapod.用金刚石对顶砧(DAC)装置获得高压，在高压下同时利用紫外激光处理样品，通过激光和压力的共同作用研究了C₆₀分子在碳管内的聚合相变.在21.5GPa高压下，同时紫外激光(325nm)照射30min后，拉曼光谱表明C₆₀分子在碳管内发生了聚合，形成一维链状O相聚合结构，且该相变是不可逆的. 关键词： 60 peapod')" href="#">C₆₀ peapod 紫外激光 高压 拉曼光谱     

稀土钇、镧掺杂TiO2薄膜的拉曼谱分析

采用溶胶-凝胶法在石英玻璃衬底上用旋涂法制备了未掺杂、掺杂钇和掺杂镧的TiO₂薄膜样品,对样品在700—1100 ℃范围内进行退火处理,并对样品的拉曼光谱进行了分析.分析表明:随着退火温度的升高,未掺杂TiO₂薄膜发生了从锐钛矿相经混相最终向金红石相的转换,掺杂钇和掺杂镧对TiO₂薄膜的晶相转换起阻碍作用,掺杂镧的阻碍作用更强;稀土掺杂能使TiO₂薄膜晶粒细化,并使晶粒内部应力增大从而阻碍晶格振动,掺杂镧比掺杂钇的效果 关键词： 2薄膜')" href="#">TiO₂薄膜 稀土掺杂 拉曼光谱 溶胶-凝胶     

烧结温度对Eu3+在羟基磷灰石中发光性能的影响

采用化学沉淀法制备了一系列Eu³⁺掺杂的羟基磷灰石样品,并在不同温度下对样品进行了烧结.使用X射线衍射、红外光谱以及荧光光谱等对样品的结构及发光特性进行了研究.分析表明,烧结对Eu掺杂羟基磷灰石的结构及结晶度产生了影响.在394 nm激发下,样品出现Eu的特征发射,掺杂摩尔分数一定时,随烧结温度增加,样品的荧光发光强度先增大后减小,在500 ℃达到峰值.其荧光寿命随烧结温度的升高而延长.此外,样品中电偶极跃迁与磁偶极跃迁强度之比(I_R:I_O)也随烧结温度的增加先增大后减小.分析表明,烧结温度的改变通过晶体结构对样品的荧光特性以及掺杂取代位置产生了影响.     

Co掺杂Bi5Ti3FeO15多铁陶瓷的磁电性能

采用了传统的固相烧结工艺制备出Bi₅Fe_1-xCo _{x Ti3O15(BFCT-x,x=0.0—0.6)多铁陶瓷样品,研究了Co掺杂对Bi5FeTi3O15(BFTO)微观结构、铁电和磁性能的影响.X射线衍射谱显示样品均已形成四层铋系层状钙钛矿相,且随着掺杂量的增加发生了结构变化.拉曼光谱进一步证实掺入的Co占据了< 关键词： 固相烧结 多铁陶瓷 剩余磁化 剩余极化}     

块体金属玻璃Zr46.75Ti8.25Cu7.5Ni10Be27.5的超导与负电阻温度系数

测量了块体金属玻璃Zr_46.75Ti_8.25Cu_7.5Ni₁₀Be_27.5在退火前后其电阻值随温度的变化,测量的温度范围为1.5—300K.样品在退火前后都发现有超导现象.零磁场下其超导转变温度T_c分别为1.84和3.76K.在5—300K温度范围内,原始样品具有负的电阻温度系数.如果取Zr, Ti, Cu, Ni及Be分别贡献出1.5, 1.5, 0.5, 0.5及两个传导 关键词： 块体金属玻璃 超导 电阻温度系数     

强自旋轨道耦合化合物Sr2-xLaxIrO4的掺杂和拉曼谱学

利用固相反应法成功制备了Sr_2-xLa_xIrO₄系列掺杂样品, 并详细研究了样品晶体结构随掺杂的演变. 拉曼散射峰向高频移动和X射线衍射谱的结构精修数据发现随着掺杂量的增加, c轴晶格常数减小, 顶角Ir–O1键键长随之减小, 表明掺杂导致晶格收缩, 而且IrO₆八面体畸变程度减弱. 变温拉曼散射谱显示随着温度降低也出现蓝移现象, 且与顶角氧相关的拉曼振动模式的蓝移在110 K附近出现明显跳变, 表明在该温度附近出现了结构变化和磁性质转变.     

一种制备氧化物超导材料的新方法—热压烧结

本文报道用热压烧结方法首次制备出 Y 含量不同的 YBaCuO 系超导陶瓷和化学掺杂的ErYBa_2Cu_3O_x 以及 YBa_2Cu_3(OF)_(x1)超导陶瓷.电阻与交流磁化率测量表明上述样品具有液氮温区以上的超导电性,经掺杂后的样品其零电阻转变温度(T_(co))均高于80K.与常压烧结制备的同类样品相比较,热压烧结的样品更为致密,但其超导转变较宽.若经空气或氧气流中高于1023K 温度重新加热处理后,超导电性可得到明显改善。     

Structural transitions of SWNT filled with C_(60) under high pressure

Raman spectra of C_(60) filled single-walled carbon nanotubes(C_(60)@SWNTs) with diameters of 1.3–1.5 nm have been studied under high pressure. A plateau in the pressure dependence of the G-band frequency at around 10 GPa was observed in both experiments with 514 nm and 830 nm excitation lasers, which is similar to the high pressure behaviors of pristine SWNTs. This structural transition has been assigned to the transformation into a peanut-like structure of the nanotubes. At pressure below 2 GPa, no obvious Raman signature related to the structural transition of nanotubes was observed, unlike what has been reported for C_(70) filled nanotubes. We discussed this point in terms of the arrangement differences of C_(60) and C_(70)molecules inside the nanotubes. At higher pressure up to 15 GPa, a graphite-like pressure evolution was observed in our C_(60)@SWNTs.     

C+注入a-SiNx：H薄膜的微结构及光发射的研究

常温下对低压化学气相沉积制备的纳米硅镶嵌结构的a-SiN_x:H薄膜进行低能量高剂量的C⁺注入后,在800~1200℃高温进行常规退火处理。X射线光电子能谱(XPS)及X射线光电子衍射(XRD)等实验结果表明,当退火温度由800℃升高到1200℃后,薄膜部分结构由SiC_xN_y转变成SiN_x和SiC的混合结构。低温下利用真空紫外光激发,获得分别来自于SiN_x、SiC_xN_y、SiC的,位于2.95,2.58,2.29 eV的光致发光光谱。随着退火温度的升高,薄膜的结构发生了变化,发光光谱也有相应的改变。     

JAHN-TELLER EFFECT AND SUPERCONDUCTING MECHANISM OF THE HOLE-DOPED C60 SOLID

A possible superconducting mechanism of the hole-doped C₆₀ solid is discussed. Under the assumption that the superconductivity results from the strong coupling between electrons at Fermi surface and intramolecular vibrations, a simple expression for the electron-phonon coupling parameter λ is derived. The related transition temperature is estimated and can be much greater than that of the alkali-metal-doped C₆₀ solid.     

Structural evolution of low-pressure polymerised C60 with polymerisation conditions

We show in this paper that characteristic features in the Raman spectra, especially the frequency of the pentagonal pinch mode, can give information about the polymeric structure of pressure polymerised C₆₀. High-pressure treatment at 1 GPa below 510 K for 3 h results in the formation of a low fraction of dimers only, while treatment at the same pressure and time above 540 K affords a fully polymerised material. In the latter case, different relative fractions of dimers and polymer chains are obtained depending on whether the final reaction conditions were reached by isobaric or isothermal path. We suggest that this difference results from different reaction dynamics in the two cases. The polymerisation rate depends on T and p and on the rotational and orientational states of the molecules. At 1 GPa no polymerisation is observed in sc C₆₀, while in “hexagon” oriented sc C₆₀ at 1.7 GPa dimers are already formed 175 K below the fcc–sc transition and a fully polymerised material is obtained just below the transition to the fcc phase.     

ELECTRONIC STRUCTURE STUDIES OF THE C60 FILM CONDENSED ON 2H-MoS2(0001) SURFACE

The electronic structure and vibrational spectrum of the C₆₀ film condensed on a 2H- MoS₂(0001) surface have been investigated by X-ray photoelectron spectroscopy (XPS), ul-traviolet photoelectron spectroscopy (UPS), Auger electron spectroscopy (AES) and infrared high-resolution electron-energy-loss spectroscopy (HREELS). AES analysis showed that at low energy side of the main transition, C₆₀ contains a total of three peaks just like that of graphite. However, the energy position of the KLL main Auger transition of C₆₀ looks like that of diamond, indicating that the hybridization of the carbon atoms in C₆₀ is not strictly in sp²- bonded state but that the curvature of the molecular surface introduces some sp²pz- bonded character into the molecular orbitals. XPS showed that the C 1s binding energy in C₆₀ was 285.0eV, and its main line was very symmetric and offered no indication of more than a single carbon species. In UPS measurement the valence band spectrum of C₆₀ within 10eV below the Fermi level (E_F) shows a very distinct five-band structure that character-izes the electronic structure of the C₆₀ molecule. HREEL results showed that the spectrum obtained from the C₆₀ film has very rich vibrational structure. At least, four distinct main loss peaks can be identified below 200 meV. The most intense loss was recorded at 66 meV, and relatively less intense losses were recorded at 95, 164 and 197meV at a primary energy of electron beam E_P = 2.0eV. The other energy-loss peaks at 46, 136, 157 and 186meV in HREEL spectrum are rather weak. These results have been compared to infrared spectrum data of the crystalline solid C₆₀ taken from recent literatures.     

Controllable preparation and disorder-dependent photoluminescence of morphologically different C60 microcrystals

Different C₆₀ crystals were synthesized by precipitation from a mixture of the good solvent m-xylene and the poor solvent isopropyl alcohol. The samples were characterized by scanning electron microscopy (SEM), Raman spectroscopy, thermogravimetric analysis, and high resolution transmission electron microscope (HRTEM). We found that the morphologies and sizes of the samples could be controlled by adjusting the volume ratio between the good and poor solvents. Especially, an unexpected short flower column-like crystal was synthesized at low ratios (from 1:6 to 1:12). Room temperature photoluminescence (PL) and HRTEM studies of the C₆₀ crystal samples reveal that the PL efficiency of the crystals decreases with increasing crystalline order and that the disordered C₆₀ crystals synthesized at the ratio of 1:2 show 10 times higher PL efficiency than that of pristine C₆₀. The mechanism of the growth process of these C₆₀ crystals was also studied by replacing the good solvents m-xylene with toluene and mesitylene.     

Defect induced surface chemistry: A comparison of the adsorption and thermal decomposition of C2H4 on Rh{111} and Rh{331}

The adsorption and thermal decomposition of C₂H₂ on Rh{111} is compared to the atomically stepped Rh{331} surface over a temperature range of 300 to 800 K. Using X-ray photoelectron spectroscopy (XPS) we find that the C 1s spectra as a function of C₂H₄ exposure exhibit a shift in binding energy (E_b) from 283.5 eV at 1 L C₂H₄ exposure on both surfaces to 283.8 eV on Rh{33 and to 284.1 eV on Rh{111} at saturation coverage (4 L). Careful analysis of the C 1s E_b value and full width at half maximum as a function of surface temperature after a 10 L exposure of C₂H₄ at 300 K reveals that a species consistent with a C₂H adsorbate composition is formed between 400 and 450 K on Rh{111}. This species is also observed on Rh{331} although at the lower temperature of 375 K. Computer peak deconvolution of the C 1s spectra between 500 and 700 K suggests that a CH_ads or C_ads surface fragment is formed and increases in concentration at the expense of the C₂H species as the surface temperature increases. Above 750 K a graphite overlayer is formed on both surfaces. This overlayer, however, exhibits a low degree of carbon π-character bonding on Rh{331}. The adsorption and decomposition mechanisms suggest that the 300 K C₂H₄ adsorbate on Rh{331} is ethylidyne and that the stepped surface is more thermally reactive than the flat Rh{111} surface.     

采用快速升温烧结方法生长Tl-1223超导薄膜的研究

报道了在铝酸镧(00l)衬底上生长Tl-1223超导薄膜的快速升温烧结方法以及铊(Tl)源陪烧靶的配比对Tl-1223薄膜晶体结构的影响.扫描电子显微镜观测表明,采用快速升温烧结方法生长的Tl-1223超导薄膜具有致密的晶体结构.X-射线衍射等测试表明,采用合适配比的陪烧靶在氩气环境下可以制备出纯c轴取向的Tl-1223超导薄膜,充氧退火后的薄膜具有较好的电学性能,其临界转变温度T_(c onset)达到116K,临界电流密度达到1.5MA/cm~2(77 K,0 T).实验结果表明,采用这一新的烧结方法制备Tl系超导薄膜具有升降温时间和恒温时间短、生产成本低等特点.     

Large-scale synthesis of polyynes with commercial laser marking technology

The space-confined synthesis method has been an efficient way for the preparation of linear carbon chains. However, the large-scale preparation of linear carbon chains still faces many challenges due to the lack of methods for the large-scale synthesis of precursors, such as short carbon chains (polyynes), and regulation technology for the transport of reactants in one-dimensional space. Here, we report a facile method for the rapid preparation of polyynes in large quantities using a commercial laser marking machine. Spectroscopic characterizations show that a large number of polyynes, such as C₈H₂, C₁₀H₂, C₁₂H₂, and C₁₄H₂, can be produced by ablating the graphite plate immersed in the organic liquid using a laser marking machine. The results of in situ Raman spectroscopy investigation of C_2nH₂-filled single-walled carbon nanotubes further confirm that a variety of polyyne molecules are synthesized. Meanwhile, in situ Raman spectroscopy also shows that the local heating treatment can accelerate the filling process of C_2nH₂ into one-dimensional channels. This work provides new insights into the study of linear carbon chains and space-confined synthesis methods.