La_(2-x)Sr_xCuO_4的高氧压处理与XPS研究

本文采用高氧压对 La_(2-x)Sr_xCuO_4进行处理.高氧压处理(2.3GPa,～800℃)不但对x=0组分及附近的样品提供过量的氧,使之成为超导体,而且可以使无氧缺位的状态保持到x=0.3.但电导率测量表明,高温超导相范围并没有扩展,因此排除了氧缺位的出现使超导相存在范围降低的看法.XPS 测量表明表面完全污染的 La_(2-x)Sr_xCuO_4样品与半导体 La_2CuO_4或 CuO 中 Cu~(2+) 谱一致,而未完全污染的样品表现出很宽的峰,明显包含局域态的 Cu~(2+)及LaCuO_3中的 Cu_(3+)成份.论文首次引入 La3d XPS 谱的伴线来研究超导体的电子态.对于“Clean”La_(1.85)Sr_(0.15)CuO_4及 La_2CuO_4,La3d XPS 谱具有完全不同的结构,前者伴线为 Shakedown,表明较低的费米能级位置,意味着强氧化态.后者为 Shake up,意味着较高的费米能级位置.在超导样品中,无法利用 O_(15)谱来区分污染样品表面还是处在深度氧化态的表面时,可用 La3d 谱来鉴别样品的表面.     

La2—xSrxCuO4—δ多晶样品的差分比热

系统地研究了La_(2-x)Sr_xCuO_(4-δ)多晶样品的差分比热.La_(2-x)Sr_xCuO_(4-δ)超导转变的比热跳跃△C 和超导转变温度 T_c 随 Sr 含量而有规律地变化.在 x=0.15处,△C/T_c 和Sommerfeld 常数△γ=C_(es)/T-C_(en)/T 均为极大值,当 Sr 含量偏离 X=0.15时,比热反常峰变圆滑,峰值和 Sommerfeld 常数 △γ 减小,T_c 降低.约化比热反常△C/△γ(0)T_c 随样品烧结条件改变出现不同的变化趋势.上述现象表示,最均匀的超导电性只出现在 Sr 含量x=0.15附近很窄的范围内.     

关于La_2CuO_(4-y)反铁磁性的研究

本文测量了三个不同制备条件的 La_2CuO_(4-y)样品以及原材料 CuO 样品在液氮温度以上的比热,实验发现四个样品在200K 附近都有明显的比热反常峰,分析表明 La_2CuO_(4-y)的比热反常并非 CuO 杂质相引起,而是样品发生反铁磁有序相变所致.在 Nèel 温度 T_N 附近,磁比热按 C_M∝1n|T-T_N|规律变化,与二维伊辛模型基本吻合,很小的熵变则表明大的零点量子自旋涨落的存在.其磁化率变化服从居里-外斯定律,并计算出 Cu^(2+)的有效磁矩为～0.6μ_B.低温下电阻按 InR∝(T_c/T)^(1/3)规律变化,表现为二维空穴导电机制.     

Ba-Y-Cu-O系统的高温超导电性和金属-半导体相变

对不同组分及烧结条件下的Ba_xY_(1-x)CuO_(3-y)多相和Ba_2YCu_3O_(9-δ)单相超导氧化物分别作了电阻、交流磁化率和比热随温度的变化,以及X射线衍射、X射线光电子谱的测量。结果支持样品中存在大块超导电性的观点,其中对超导起主要作用的是Ba_2YCu_3O(9-δ)相。并且对于较高Tc的样品具有较大的Fermi面态密度。但含Ba量仅为0.5％(x= 0.005)的样品仍可导致非界面的高温超导电性。因此不能排除Ba以杂质原子身份影响电子结构,从而导致超导电性的可能性。另外,在x=0处观察到可能由电子推动的金属-半导体相变。     

用热天平方法研究Y1Ba2Cu3O7—x高温超导体的氧吸收过程

用热天平分析方法,探讨了Y-1Ba_2CuO_(7-x)高温超导体在四角结构组态下的吸氧过程动力学。实验结果表明,在氧气氛下,具有四角晶体结构的Y_1Ba_2Cu_3O_(7-x)高温超导体,在不同的等温条件下,通过吸取气体氧,使其氧含量提高并转变为正交晶体结构。此过程分两阶段,氧分子首先被超导体吸附,然后通过吸附层向晶体内部扩散,其后一过程的扩散激活能约为19600cal/mol。     

YBa_(2-x)Pr_xCu_3O_(7-δ)体系中的Pr替代效应

报道了YBa_(2-x)Pr_xCu_3O_(7-δ)(0≤x≤0.15)体系的超导电性和正常态输运性质的测量。实验结果表明随着Pr在Ba晶位替代量的增加,超导转变临界温度T_c和由Hall系数推算的荷电载流子浓度P_H均单调下降。通过与Y_(2-x)Pr_xBa_2Cu_3O_(7-δ)和Nd_(1 x)Ba_(2-x)Cu_3O_(7-δ)体系的比较研究,作者发现上述三个体系的T_c变化均与载流子浓度的变化相关联。作者进而提出,在YBa_(2-x)Pr_xCu_3O_(7-δ)体系中Pr的空穴填充效应导致可迁移空穴浓度下降,从而压制T_c;而在Y_(2-x)Pr_xBa_2Cu_3O_(7-δ)体系中载流子浓度的下降则是主要来源于Pr替代对空穴的局域作用,T_c同样被压制。Pr的拆对效应不明显。     

高温超导体La_(2-x)Ba_xCuO_4的结构不稳定性

本文从二维电声子相互作用紧束缚模型哈密顿量出发,研究高T_c超导体La_(2-x)Ba_xCuO_4的结构不稳定性。结果给出,La_2CuO_4结构表现出半导体的行为,在费米面上约有0.6eV的能隙,氧原子位移约为0.07A,铜格点之间存在△Ω=0.26e的电荷转移,进一步研究了二价元素Ba或Sr掺杂抑制Peierls相变的过程。     

零电阻温度96.3K的Y_(0.4)Ba_(0.6)CuO_(3-δ)超导体及其不稳定性

已获得零电阻温度为96.3K,中点转变温度为97.6K的Y_(0.4)Ba_(0.6)CuO_(3-δ)超导体,并发现这个超导体经过数次冷热循环和表面结霜以后,超导电性逐渐退化的现象。     

Ba_xY_(1-x)CuO_(3-y)系列的高Tc超导电性

本文研究了钡-钇-铜氧化物的高Tc超导电性,其名义组份为Ba_xY_(1-x)CuO_(3-y)(0.05≤x≤0.95)。结果表明,x<0.50时,超导转变温度TC_(1/2)随组份x单调上升;在较宽的组份范围0.60相似文献   

Pb掺杂对REOBiS_2(R_E=La,Yb)电磁性能的影响

利用固相反应法合成了一系列的La_(1-x_Pb_xOBiS_2和LaOBi_(1-x)Pb_xS_2(x=0,0.01,0.02,0.03,0.05,0.1)样品,并研究了Pb掺杂对LaOBiS_2的晶体结构及电磁性能的影响.XRD测试结果表明,所有样品均具有CeOBiS_2型晶体结构,而且Pb的掺杂导致LaOBiS2的晶格发生明显的变化.磁性测量结果表明,所有样品和LaOBiS_2的磁性相似,在低温下均为顺磁性特征.尽管Pb在LaOBiS_2中掺杂不能导致超导电性的发生,但对其电阻率产生了明显的影响,Pb在La位掺杂抑制了LaOBiS_2的金属-半导体转变,使其电阻率在室温以下表现为半导体的特征.同样Pb在YbOBiS_2中的掺杂也没观察到超导转变.     

高压对高温超导体La1.85Sr0.15CuO4电子结构的影响

 采用Recursion方法计算了常压和1 GPa、4 GPa、8 GPa高压下La_1.85Sr_0.15CuO₄的电子结构，给出了费米能、费米能处态密度以及各晶位原子价的定量变化。结果表明：高压下T_c的变化可能源于CuO₂面上氧空穴和铜空穴数目的改变，且氧空穴数目的增加有利于超导性。     

Combinative energy, oxygen deficiency and superconductivity in LnBa2Cu3O7-x （Ln=Nd, Er, Sm）

The correlation among the combinative energy, superconductivity, oxygen content, the position of holes in different planes, and the position of holes in the Cu(2)-O plane in LnBa2Cu3O7-x (Ln=Nd, Er, and Sin) has been investigated on the basis of a block model. The results indicate that the combinative energy decreases with increasing Tc in all of these compounds. And also, the combinative energies are obviously different with holes at different positions in the Cu(2)-O plane when the oxygen deficiency is low. However, this difference becomes less with increasing the oxygen deficiency.The effect of the holes in different positions on the combinative energy supplies some clue to the understanding of an unresolved problem, i.e. whether the distribution of carriers in the CuO2 plane is uniform or inhomogeneous. The results not only show that the structural characters, the combinative energy between two structural blocks and the superconductivity are closely interrelated in this class of compounds, but also reveal some differences among these systems. The relationship between the combinative energy and the Tc value in NdBa2Cu3O7-x shows some features different from the systems containing Er or Sm.     

Enhancement of the superconducting transition temperature of La2-xSrxCuO4 bilayers: role of pairing and phase stiffness

The superconducting transition temperature T(c) of bilayers comprising underdoped La(2-x)Sr(x)CuO(4) films capped by a thin heavily overdoped metallic La(1.65)Sr(0.35)CuO(4) layer, is found to increase with respect to T(c) of the bare underdoped films. The highest T(c) is achieved for x=0.12, close to the "anomalous" 1/8 doping level, and exceeds that of the optimally doped bare film. Our data suggest that the enhanced superconductivity is confined to the interface between the layers. We attribute the effect to a combination of the high pairing scale in the underdoped layer with an enhanced phase stiffness induced by the overdoped film.     

Effect of disorder outside the CuO2 planes on Tc of copper oxide superconductors

The effect of disorder on the superconducting transition temperature T(c) of cuprate superconductors is examined. Disorder is introduced into the cation sites in the plane adjacent to the CuO2 planes of two single-layer systems, Bi(2.0)Sr(1.6)Ln(0.4)CuO(6+delta) and La(1.85-y)Nd(y)Sr0.15CuO4. Disorder is controlled by changing rare earth (Ln) ions with a different ionic radius in the former, and by varying the Nd content in the latter with the doped carrier density kept constant. We show that this type of disorder works as weak scatterers in contrast to the in-plane disorder produced by Zn, but remarkably reduces T(c), suggesting novel effects of disorder on high-T(c) superconductivity.     

快速测量单晶样品低温热电势的新方法

对传统的热电势倒相测量方法进行了改进,发展了一种快速的、针对毫米级样品的准连续的热电势测量方法,在不降低测量精度的情况下大幅度提高了热电势的测量速度.我们用高温超导材料Nd1.84Ce0.16CuO4单晶样品对该法进行了不同变温速率的测试,并与传统的倒相法进行比较,一定变温速率(<30K/hr)下和传统方法数据吻合很好,有很高的测量精度.由于此法为准连续测量法,变温速率可精确设定,所以对有明显热滞效应的材料如电子型超导体母体材料Nd2CuO4等的热电势测量有十分重要的意义.     

Magnetic properties of Nd0.5Pb0.5—xSrxMnO3 materials

The structure and magnetic properties of Nd_{0.5}Pb_{0.5-x}Sr_xMnO_3 (0≤x≤0.4) manganites were systematically investigated. Significant changes in Curie temperature and metal-insulator (MI) transition temperature of the samples were observed. All samples exhibited a transition from paramagnetic semiconducting to ferromagnetic metallic state. Curie temperature T_C and the MI transition temperature T_p increased with increasing Sr content. We attributed these behaviours to the enhancing of both the double exchange mechanism and the Jahn－Teller electron－phonon coupling.     

Isotope effect in the superfluid density of high-temperature superconducting cuprates: stripes, pseudogap, and impurities

Underdoped cuprates exhibit a normal-state pseudogap, and their spins and doped carriers tend to spatially separate into 1D or 2D stripes. Some view these as central to superconductivity and others as peripheral and merely competing. Using La(2-x)Sr(x)Cu(1-y)Zn(y)O4 we show that an oxygen isotope effect in Tc and in the superfluid density can be used to distinguish between the roles of stripes and pseudogap and also to detect the presence of impurity scattering. We conclude that stripes and pseudogap are distinct, and both compete and coexist with superconductivity.     

Revisiting the Electron-Doped SmFeAsO:Enhanced Superconductivity up to58.6K by Th and F Codoping

In the iron-based high-T_c bulk superconductors, T_c above 50 K was only observed in the electron-doped 1111-type compounds. Here we revisit the electron-doped SmFeAsO polycrystals to make a further investigation for the highest T_c in these materials. To introduce more electron carriers and less crystal lattice distortions, we study the Th and F codoping effects into the Sm-O layers with heavy electron doping. Dozens of Sm_(1-x)Th_x FeAsO_(1-y)F_y samples are synthesized through the solid state reaction method, and these samples are carefully characterized by the structural, resistive, and magnetic measurements. We find that the codoping of Th and F clearly enhances the superconducting T_c more than the Th or F single-doped samples, with the highest record T_c up to 58.6 K when x=0.2 and y=0.225. Further element doping causes more impurities and lattice distortions in the samples with a weakened superconductivity.     

非晶La_(1-x)Nd_xP_5O_(14)的拉曼散射研究

本文利用拉曼散射对非晶 La_(1-X)Nd_XP_5O_(14)及其晶化过程进行了研究。拉曼散射表明在非晶La_(1-X)Nd_XP_5O_(14)中仍保留着晶态 La_(1-X)Nd_XP_5O_(14)中具有的 PO_4~(3-)四面体结构单元。非晶La_(1-X)Nd_XP_5O_4在不同温度下晶化,其拉曼谱线的频率不同,这一现象可用应力模型给予解释。     

Competition between charge- and spin-density-wave order and superconductivity in La(1.875)Ba(0.125-x)Sr(x)CuO(4)

We have performed a series of elastic neutron scattering measurements on 1/8-hole doped La(1.875)-Ba(0.125-x)Sr(x)CuO(4) single crystals with x = 0.05, 0.06, 0.075, and 0.085. Both charge-density-wave (CDW) and spin-density-wave orders are found to develop simultaneously below the structural transition temperature between the low-temperature orthorhombic (LTO) and low-temperature tetragonal (LTT) or low-temperature less-orthorhombic (LTLO) phases. In the ground state the CDW order is observed only in the LTT/LTLO phase and drastically degrades towards the LTO boundary. The x dependence of T(c) strongly suggests a direct effect of the CDW order on the suppression of superconductivity.