Bi-Sr-Ca-Cu-O2212相超导单晶的定向生长

用定向凝固法已成功地生长出Bi-Sr-Ca-O2212相的超导大单晶,最大的单晶尺寸达到19×3×2mm³。定向生长的单晶(001)面平行于生长方向,[100]方向为单晶的定向生长方向。劳厄像表明大晶体确为单晶,其质量比其它方法生长的单晶要高。氧气中退火后的单晶电阻测量和交流磁化率测量表明,零电阻温度T_c(0)=81.5k。 关键词：     

铁基超导单晶NaFe1-xCoxAs的空气敏感性探究

NaFe1-xCoxAs单晶因含有碱金属Na元素,暴露在空气中时其表面会剧烈地产生气泡,这为该类单晶的生长和测量带来了难度.为此探究了无水乙醇对降低NaFe1-xCoxAs单晶空气敏感性的作用效果,因乙醇分子渗入单晶的Na+层,经无水乙醇浸泡的单晶样品,长时间(超30 d)暴露于空气中,其表面形貌及磁化率等物性几乎稳定...     

铁基K0.8Fe2-ySe2体系的单晶生长与结构研究

本文报道了采用自助溶剂法对不同组分K0.8Fe2-ySe2单晶生长和结构研究的实验结果.X-射线衍射XRD(Cu靶和同步辐射)结构分析表明样品中存在两套衍射峰(分别对应相1和相2),相1能够用ThCr2Si2结构,空间群I4/mmm(No.139)指标化,对应超导相.而相2能够用单一米勒指数(0 0l)标定,可能对应一个新的物相.相2的存在能够在不同组分的KxFe2-ySe2单晶样品中很好的重复.利用高分辨透射电子显微镜(TEM)研究了K0.8Fe2Se2样品的"结构相分离"特征,结果表明样品中存在明显的位错缺陷,可能与两相结构的竞争有关.对K0.8Fe2Se2单晶样品直流磁化测量表明相2可能为弱铁磁性,从而引起高温Tp～125K的ZFC与FC磁性反常.     

铁基超导线带材研究现状及展望

铁基超导体具有极高的上临界磁场、较小的各向异性、简单的制备工艺等优点,在高场超导磁体、核磁共振谱仪、可控核聚变装置、高能粒子加速器等领域具有重要的应用前景。为满足上述高场强电应用需求,必须制备出高性能超导线带材。文章详细介绍了多种体系铁基超导线带材的研究进展,分析当前线带材研究中存在的问题,提出改善线材传输性能的途径,并对铁基超导线材的发展趋势进行展望。     

铊系超导单昌的低温生长

采用蠕爬型助熔剂ＣａＯ－ＣｕＯ和蒸发型复合助熔剂ＣａＯ－ＣｕＯ－ＮａＣｌ来制备Ｔｌ－Ｂａ－Ｃａ－Ｃｕ－Ｏ超导单晶，生长炉最高温度８９０℃和８５０℃，因而实现了铊系单晶的低温生工。生长出的单晶分别是典型尺寸为２．０×１．５×０．２ｍｍ＾３，Ｔｃ在１００Ｋ与１１９Ｋ之间的Ｔｌ－２２１２相单晶和尺寸为１．２×１．０×０．１ｍｍ＾３。Ｔｃ约为１１５Ｋ的Ｔｌ－２２２３相超导单晶。     

铁基超导中拓扑量子态研究进展

铁基超导体和拓扑量子材料是近年来凝聚态物理两个重要的前沿研究方向.铁基超导体中是否能衍生出非平庸的拓扑现象是一个非常有意义的问题.本文从晶体对称性、布里渊区高对称点附近的有效模型以及自旋轨道耦合相互作用三个方面具体分析了铁基超导的电子结构的基本特点.在此基础上,重点阐述铁基超导的正常态、临近超导的长程有序态以及超导态中非平庸的拓扑量子态是如何衍生的;具体介绍了相关的理论模型以及结果,回顾了相关的实验进展,展望了该领域的发展前景.     

Pb、La掺杂Bi2201相高质量单晶的生长和表征

自助熔法从富Ｂｉ２Ｏ３ 的Ｂｉ１ ．８ ＋ ｘＰｂ０ ．２Ｓｒ１ ．６５Ｌａ０ ．３５ＣｕＯｙ（ｘ＝ ０ ．１ ,０ ．２,０ ．３ ,０．４ ,０ ．５） 熔体中生长出了高质量的Ｐｂ、Ｌａ 掺杂的Ｂｉ２２０１ 相大单晶,最大尺寸达８ ×４ ×０ ．０５ｍｍ３ ．在生长单晶时不同程度过量的Ｂｉ２Ｏ３ 用作助熔剂,最佳助熔剂含量以及最佳温度控制过程被确定．通过单晶的（００ｌ） ＸＲａｙ 衍射和摇摆曲线以及电子衍射对一块尺寸为４ ×１ ．６ ×０ ．０３ｍｍ３ 的高质量单晶做了结构表征．由试验发现从富Ｂｉ２Ｏ３ 的熔体中生长高质量Ｂｉ２２０１ 相大单晶的关键是合适的助熔剂含量和温度控制过程     

Bi2222相单晶生长的相关问题研究

从名义组份Ｂｉ２．４Ｓｒ２ＰｒＣｕ２Ｏｙ生长出一种新的Ｂｉ２２２２相单晶，其实际组份测定为Ｂｉ２Ｓｒ１．５Ｐｒ２．５Ｃｕ２Ｏｙ．用Ｘ射线衍射对单晶的结构进行了表征，并且研究了该单晶生长中Ｂｉ２２０１和Ｂｉ２２１２相的交生等相关问题．另外，如果从名义组份为Ｂｉ２．４Ｓｒ１．５Ｐｒ２．５Ｃｕ２Ｏｙ出发，得到的却是Ｐｒ２ＣｕＯ４单晶．     

反常输运性质Bi2Sr2CaCu2O8单晶的生长

使用自助熔剂融化法成功的制备了Bi2Sr2CaCu2O8超导单晶。在对所得单晶进行输运性质的研究时,我们发现一种反常扔物理特征：在超导涨落区观察到反常的电阻峰,从Bi2Sr2CaCu2O8单晶的颗粒超导电性方面我们给出了合理的解释。     

铁基超导体的微结构和结构相变研究

铁基超导体表现出丰富的结构和物理性质,在多个典型体系中存在着结构相变和多重有序态之间的关联与竞争. 例如,母相化合物LaFeAsO在150K附近发生从四方相(P4/nmm)到正交相(Cmma)的结构相变,同时其电输运性质和磁化率也表现出明显的反常行为. 微结构分析发现,在CaFe2As2 样品中,存在准周期调制结构,而且在低温区,122体系存在丰富的孪晶畴结构,这种孪晶结构是结构相变的直接结果. 另外,在超导材料KxFe2-ySe2 （0.7≤x≤0.8, 0.2≤y≤0.4）中,Fe空位有序态和结构相分离是理解其结构,也是理解其磁性和输运性质的关键问题,特别是沿［130］晶体带轴方向的5倍超结构与系统的超导电性存在密切关系.     

Growth and transport properties of CaFeAsF_(1-x) single crystals

Using self-flux method,we have successfully grown the parent phase of the single crystals of CaFeAsF1-x.The X-ray di?raction indicates good crystallinity.In-plane resistivity shows a bad metallic behavior with a sharp drop of resistivity at about T SDW=119K.This anomaly is associated with the possible spin density wave(SDW)order.Interestingly near T SDW,the resistivity exhibits a cusp-like feature,which may be understood as the strong coupling effect between the electrons and the antiferromagnetic(AF)spin fluctuations.A reduction of fluorine or application of a high pressure will suppress the SDW feature and induce superconductivity.Hall effect measurements reveal a positive Hall coefficient below T SDW indicating a dominant role of the hole-like charge carriers in the parent phase.Strong magnetoresistance has been observed below T SDW suggesting multiple conduction channels of the charge carriers.     

112型铁基化合物EuFeAs2的单晶生长与表征

铁基超导体中含有一类特殊的112型结构化合物,其层状结构中含有一层锯齿形的As链构型.本文报道了用CsCl助熔剂法生长新型铁基112型EuFeAs₂母体单晶的具体方法,以及对该单晶的结构和物性的详细表征.通过能量色散X射线能谱扫描对单晶样品进行的化学成分分析,以及单晶X射线衍射的结构解析,确定该单晶样品属于EuFeAs₂相,结构精修得到EuFeAs₂具有空间群为Imm2（No.44）的正交晶体结构,晶格常数分别是a=21.285（9）Å,b=3.9082（10）Å,c=3.9752（9）Å.通过低温电阻测量,发现在110 K附近和46 K附近存在两个异常电阻跳变.进一步分析表明,110 K附近存在两个邻近的相变,这两个相变与铁基母体材料中常见的结构相变和Fe²⁺的反铁磁相变相符合.结合磁化率测量分析,可知46 K附近的相变属于Eu²⁺的反铁磁相变.     

Single crystal growth,structural and transport properties of bad metal RhSb_2

We have successfully grown an arsenopyrite marcasite type RhSb2 single crystal, and systematically investigated its crystal structure, electrical transport, magnetic susceptibility, heat capacity, and thermodynamic properties. We found that the temperature-dependent resistivity exhibits a bad metal behavior with a board peak around 200 K. The magnetic susceptibility of RhSb2 shows diamagnetism from 300 K to 2 K. The low-temperature specific heat shows a metallic behavior with a quite small electronic specific-heat coefficient. No phase transition is observed in both specific heat and magnetic susceptibility data. The Hall resistivity measurements show that the conduction carriers are dominated by electrons with ne = 8.62 × 10¹⁸ cm^-3 at 2 K, and the electron carrier density increases rapidly above 200 K without change sign. Combining with ab-initio band structure calculations, we showed that the unusual peak around 200 K in resistivity is related to the distinct electronic structure of RhSb_2. In addition, a large thermopower S(T) about -140 μV/K is observed around 200 K, which might be useful for future thermoelectric applications.     

Single crystal neutron diffraction study of triglycine sulphate revisited

In order to get the exact hydrogen-bonding scheme in triglycine sulphate (TGS), which is an important hydrogen bonded ferroelectric, a single crystal neutron diffraction study was undertaken. The structure was refined to an R-factor of R[F ²] = 0.034. Earlier neutron structure of TGS was reported with a very limited data set and large standard deviations. The differences between the present and the earlier reported neutron structure of TGS are discussed.      

Growth and characterization of superconducting Ca1-xNaxFe2As2 single crystals by NaAs-flux method

High-quality superconducting Ca$_{1-x}$Na$_x$Fe$_2$As$_2$ single crystals have been successfully grown by the NaAs-flux method, with sodium doping level $x = 0.4$-0.64. The typical sizes of these crystals are more than 10 mm in $ab$-plane and $\sim 0.1$ mm along $c$-axis in thickness. X-ray diffraction, resistance and magnetization measurements are carried out to characterize the quality of these crystals. While no signature of magnetic phase transitions is detected in the normal state, bulk superconductivity is found for these samples, with a sharp transition at $T_{\rm c}$ ranging from 19.8 K ($x = 0.4$) to 34.8 K ($x = 0.64$). The doping dependences of the $c$-axis parameter and $T_{\rm c}$ are consistent with previous reports, suggesting a possible connection between the lattice parameters and superconductivity.     

AlN晶体物理气相传输法生长坩埚的热场分析

采用物理气相传输法在钨制坩埚上制备AlN单晶.通过采用COMSOL软件中的固体传热和磁场模块,对AlN晶体生长的坩埚的热场进行仿真,同时针对不同的线圈直径以及不同的线圈位置对坩埚热场的影响进行模拟,提出了相应的处理方式.结果表明:当线圈直径增大,坩埚结晶区和升华区的温度在相同的加热时间下会增加,并且增加的温度存在峰值.当线圈的垂直位置发生变化的时候,结晶区和升华区的温度场也会发生变化,从上向下移动的过程中仍然存在温度的峰值,并且结晶区和升华区的温度关系会发生翻转,导致温度梯度阻碍晶体生长.在晶体生长过程中升华区和结晶区的温度关系依旧会发生翻转.但是通过线圈跟随籽晶表面生长层的变厚而同步移动,可以保持相对稳定的温度关系,维持晶体正常持续生长.     

相场晶体法模拟过冷熔体中的晶体生长

利用相场晶体(phase-field crystal)模型,采用有限差分法,模拟了过冷熔体中晶体生长过程,研究了不同相区、不同过冷度对晶体生长过程的影响.结果表明,在共存区中,随着演化的进行,晶体生长被抑制,液-晶边界层逐渐变厚;在单相区中,随着过冷度的增大,晶态区面积显著增加,液-晶边界层逐渐变薄.晶体生长速度和过冷度成近似线性的关系. 关键词： 相场晶体 自由能函数 过冷熔体 晶体生长     

FEM simulations and experimental studies of the temperature field in a large diamond crystal growth cell

We investigate the temperature field variation in the growth region of a diamond crystal in a sealed cell during the whole process of crystal growth by using the temperature gradient method (TGM) at high pressure and high temperature (HPHT). We employ both the finite element method (FEM) and in situ experiments. Simulation results show that the temperature in the center area of the growth cell continues to decrease during the process of large diamond crystal growth. These results are in good agreement with our experimental data, which demonstrates that the finite element model can successfully predict the temperature field variations in the growth cell. The FEM simulation will be useful to grow larger high-quality diamond crystal by using the TGM. Furthermore, this method will be helpful in designing better cells and improving the growth process of gem-quality diamond crystal.     

Faceting transitions in crystal growth and heteroepitaxial growth in the anisotropic phase-field crystal model

We modify the anisotropic phase-field crystal model(APFC),and present a semi-implicit spectral method to numerically solve the dynamic equation of the APFC model.The process results in the acceleration of computations by orders of magnitude relative to the conventional explicit finite-difference scheme,thereby,allowing us to work on a large system and for a long time.The faceting transitions introduced by the increasing anisotropy in crystal growth are then discussed.In particular,we investigate the morphological evolution in heteroepitaxial growth of our model.A new formation mechanism of misfit dislocations caused by vacancy trapping is found.The regular array of misfit dislocations produces a small-angle grain boundary under the right conditions,and it could significantly change the growth orientation of epitaxial layers.