磁性离子Fe和Ni替代YBCO体系的结构特征和载流子局域化

为阐明磁性离子在不同替代位置对YBCO体系超导电性的影响机制，利用正电子湮没及相关实验手段结合数值模拟，系统研究了Fe和Ni掺杂的YBa₂Cu₃O_7-δ体系. 结果表明，Fe和Ni离子在替代过程中均以离子团簇的形式进入晶格. 当离子进入CuO₂面时，由于团簇改变了周围的电子结构，造成电子的局域化，并直接影响了电子对的配对和输运，因而强烈抑制了体系的超导电性.而当掺杂离子进入Cu-O链区时，它们同样通过团簇的形式改变周围 关键词： YBCO超导体 磁性离子替代 正电子湮没 数值模拟     

Y1-xCaxBa2Cu3-xMxO7-δ (M=Fe,Ni)体系的超导电性

本文报道,通过对Y_1-xCa_xBa₂Cu_3-xMxO_7-δ(M=Fe,Ni)体系样品的晶体结构、氧含量、正常态电阻率与温度的关系,以及超导转变温度等测量,并与YBa₂Cu_3-xMxO_7-δ(M=Fe,Ni)体系进行比较,发现Y_1-xCa_xBa₂Cu_3-xFe_xO_7-δ体系的T_c显著地高于相应x值的YBa₂Cu_3-xFe_xO_7-δ体系,而Y_1-xCa_xBa₂Cu_3-xNi_xO_7-δ体系则相反,T_c低于仅Ni替代的体系,表明Ca和Fe同时替代时两者引起的载流子浓度(n_H)变化相互补偿,抑制了仅Fe替代时引起的n_H和T_c急剧下降;而作Ca和Ni同时替代时主要的不是两者引起载流子浓度变化的相互补偿,Ca和Ni替代效应之间的关联较弱。作者认为,对Y_1-xCa_xBa₂Cu_3-xFe_xO_7-δ体系属于CuO₂平面外的元素替代,这时载流子浓度是决定Tc的主要因素;而对Y_1-xCa_xBa₂Cu_3-xNi_xO_7-δ体系,由于Ni²⁺离子主要占据Cu(Ⅱ)位,它导致磁拆对效应,Ni²⁺离子的拆对效应是引起T_c下降的直接原因。 关键词：     

从对Cu的取代看YBa2Cu3O7-y中Cu-O平面和Cu—O链的作用

通过测量YBa₂Cu_3-xM_xO_7-y(M=Zn,Ni,x=0,0.025,0.05,0.075,0.1,0.15,0.2,0.3)系列样品的晶体结构、正常态电子输运性质、超导电性以及O含量,给出了Zn择优取代Cu(2)的更直接证据;同时,观察到掺Zn系统中的由正交相Ⅰ到正交相Ⅱ的结构相变以及Ni取代Cu带来的电子局域化效应。研究结果表明,具有正二价态的Zn对Cu(2)具有择优取代性,而具有正二价和正三价的Ni并不具有明显的择优取代。对Cu(2)的取代引起超导临界温度的显著变化,但对正常态电子输运性质的影响不明显;对Cu(1)的取代更显著地影响了晶体结构和正常态电子输运性质,同时对T_c产生抑制,我们认为,YBa₂Cu₃0₇中的高温超导电性以及正常态输运性质是由CU-O平面和Cu-0链共同承担,而Cu-O平面和Cu-0链之间的耦合强度决定着该系统的超导电性的强弱。 关键词：     

Gd替代YBa2Cu3O7－δ超导体的相结构与局域电子结构的研究

利用正电子湮没实验，结合x射线衍射(XRD)结构分析，研究了具有混合稀土特征的(Y_{1 -x}Gd_x)Ba₂Cu₃O_7-δ系列样品. XRD 实验结果表明，半径较大的Gd离子Y位替代使得样品晶胞参数和晶胞体积增大，但所有样品 仍保持与YBa₂Cu₃O_7-δ(YBCO)样品相同的单相正交结 构. 正电子湮没实验表明，正电子各寿命参数表现出很强的Gd替代依赖关系. 从正电子实验 结果出发，计算了Cu-O链区局域电子密度n_e的变化. 结果表明，局域电子密度n _e随Gd含量x的增加而减小，而超导转变温度T_c随局域电子密度ne的减小而增加，这种局域电子密度n_e与超导电性的关联是与铜位替代 完全不同的，且可能是近年来人们关于混合稀土铜氧化物体系具有较高临界电流密度的原因 之一. 该研究结果为铜氧化物超导体的应用和机理研究提供了相应的正电子实验资料. 关键词： 超导电性 正电子湮没 相结构 局域电子密度     

Y1-xCaxBa2Cu3O6系统中空穴掺杂诱导的绝缘体-金属转变和超导电性

本文报道利用Ca部分取代Y的方法,将空穴引入绝缘的YBa₂Cu₃O₆系统中,使其发生由绝缘体到超导体的转变,并通过研究其电子输运性质、磁性质,以及晶体结构的变化,揭示了具有适当载流子浓度的CuO面是维持高T_c超导电性的重要条件。 关键词：     

Co,Zn元素对YBa2Cu3O7-δ的不同的掺杂效应

对单相掺杂样品YBa₂Cu_3-xCo_xO_y (x=0.00,0.025,0.05,0.075,0.10,0.125,0.15,0.20,0.25,O.275,0.30,0.325,0.35,0.375,0.40)和YBa₂Cu_3-xZn_xO_y(x=O.025,0.05,0.075,0.10,0.15,0.20,0.30)作了室温下的 关键词：     

YBa2Cu3Oy中氧缺位的磁通钉扎效应

制备了一组不同氧含量(T_cmid>90K)的YBa₂Cu₃O_y单相多晶样品。对其结构、超导电性和磁通钉扎行为的观测结果表明:氧缺位导致临界电流密度J_c和钉扎力密度F_p同时增加,存在一最佳的氧缺位浓度,可使J_c和F_p有最大程度提高。与氧含量y=6.96和6.83时的情况不同,对y=6.94和6.86的样品,其J_c 关键词：     

新块材超导体Pr1-xCaxBa2Cu3O7-δ(0.4≤x≤0.6)的高压合成与超导电性

采用高温高压合成方法，合成出了Pr_1-xCa_xBa₂Cu₃O_7-δ(0.4≤x≤0.6) 系列块材超导体，在缺氧的Pr_0.5Ca_0.5Ba₂Cu₃O_7-δ四方123结构样品中得到了T_c为98K.实验结果表明Pr在123结构中的价态为大于3+的混合价态，因而空穴填充和杂化导致的载流子局域化是Pr抑制123相超导电性的关键因素. 关键词：     

YBa2Cu1-xCox(Cu1-yZny)2Oz体系的Co,Zn共同掺杂效应

本文报道通过对YBa₂Cu_1-xCo_x(Cu_1-yZn_y)₂O_z(0≤x,y≤0.1)体系晶体结构、氧含量、正常态电阻-温度关系、Hall效应以及超导临界温度等的综合测量,发现随着Co和Zn含量的增加,体系经历了从正交结构的超导金属向四方结构的非超导半导体的转变,超导临界温度T_c和载流子浓度n_h均迅速下降,Co 关键词：     

YBa2Cu3-xFexOy的离子配位与结构转变

本文用穆斯堡尔效应、X射线衍射等手段,系统研究了掺铁的YBa₂Cu_3-xFe_xO_y(0.01≤x≤0.2)超导氧化物经过不同温度真空退火后超导转变温度T_c、相结构、氧含量与氧配位之间的关系,结果表明,铁主要占Cu(1)位,铁含量的增加和高温真空退火都引起正交相向四方相转变,T_c逐渐减小至零,但这两种转变过程的铁的配位结构和氧含量却不相同,对前者,相变是与出现相当数量的六配位F 关键词：     

Structure and superconductivity in FexCu1−xBa2YCu2O7+y superconductors synthesized by high pressure

Comprehensive studies of X-ray diffraction, oxygen content, superconductivity and Mössbauer effect have been made on Fe_xCu_1−xBa₂YCu₂O_7+y superconductors (0.00≤x≤0.70) synthesized by ambient (AM) and high pressure (HP). Results indicate that all the HP-samples have tetragonal structure, smaller lattice parameter c and unit-cell volume than the AM-samples. The studies of oxygen content, and Mössbauer spectroscopy indicate that the HP-samples have higher oxygen content, carrier concentration and average valence of Fe than the AM-samples. Moreover, for the HP-samples more Fe atoms located in CuO_x chains have fivefold-oxygen coordination. These are important reasons for the enhancement of T_c in the HP-samples.     

The oxygen content and its ordering in the chain planes in fine-grained HTSC YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript>

A comparative analysis of the results of the X-ray and Mösbauer studies of the high-temperature superconductor (HTSC) YBa₂Cu₃O _y and YBa₂Cu_{3 ? x} ⁵⁷Fe _x O _y (x = 0.015, T _c ≈ 91.5 K) samples with different average grain sizes <D> in the micron and submicron ranges has been performed. The regularities in the change in the lattice parameter c and in the degree of occupation of different oxygen sites in the CuO_δ chain planes taking place at the decrease in <D> have been studied. The quantitative interrelation between the parameter c and the oxygen content δ in the CuO_δ planes exceeding the amount of the mobile oxygen due to the interplane oxygen redistribution is established.     

The influence of interplane oxygen redistribution on the magnetization of fine-grained HTSC YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript>

The magnetization of the fine-grained high-temperature superconductor (HTSC) YBa₂Cu₃O _y is experimentally investigated at T < T _c . A distinctive feature of this material is the increased oxygen content in CuO_δ planes. The magnetization decrease with an increase in δ is revealed. This correlation indicates that during interplane oxygen redistribution, which is characteristic of fine-grained samples, the oxygen content in the chain planes increases due to its reduction in the superconducting CuO₂ planes.     

Coexistence of superconductivity and magnetic order in YBa2Cu4O8

⁵⁷Fe Mössbauer spectroscopy, magnetic susceptibility and powder x-ray-diffraction measurements were used to study superconductivity and magnetic order in YBa₂(Cu_1?xFe_x)₄O_8+δ. T_c is decreasing with x, disappearing for x>x_c≈0.04. For xc iron substitutes Cu, predominantly in the Cu(1) site exhibiting a single quadrupole Mössbauer spectrum at 90 K. For x>x_c magnetic order is observed in the Cu(2) site, T_N=380 (5) K for x=0.1 and H_eff (Cu(2), 4.2 K)=510(2) kOe. However, the most surprising discovery is that for x=0.025, for which T_c=27(2) K, the Fe in the Cu(1) site orders magnetically at T_N=30(2) K and H_eff (Cu(1), 4.2 K)=461(2) kOe. The coexistence and competition between superconductivity and magnetic order in the Cu(1) and Cu(2) sites in YBa₂Cu₄O₈ are discussed in terms of the previously observed phase diagrams for Y_1?xPr_xBa₂(Cu_1?yFe_y)₃O_z.     

Stable tetragonal HTSC Fe-doped 1-2-3 phase with oxygen excess

Mössbauer study was performed on the YBa₂Cu_2.95Fe_0.05O _y compound prepared in three different states: orthorhombic phase with y7, tetragonal non-superconducting phase with oxygen deficiency, and a new tretragonal superconducting phase with oxygen excess (y>7). The new synthesis method was used for preparation of the Fe-doped YBa₂Cu₃O _y (named 1-2-3) compound. The comparison of Mössbauer spectra of three different states allowed us to identify the oxygen environments of the Fe atoms in the Cu1 positions.     

Mechanism of a strong rise of Tc due to the calcium doping in Y1−xCaxBa2Cu2.8Zn0.2Oy

The normal-state transport properties and the critical temperature, T_c, for three series of Y_1?xCa_xBa₂Cu_2.8Zn_0.2O_y (x = 0–0.2) samples with different oxygen content have been investigated and discussed in comparison with results for other calcium-doped YBa₂Cu₃O_y. We have observed the calcium doping to restore the superconductivity in the YBa₂Cu₃O_y system even in case of the preliminary T_c suppression by the direct destruction of the CuO₂ planes due to a preliminary doping by zinc. The T_c value rises strongly with increasing calcium content and this rise becomes faster for the oxygen-reduced series. Based on a narrow-band model, the parameters of the band spectrum structure and the charge-carrier system in the normal state were determined from the analysis of the thermopower temperature dependences in order to reveal the mechanism of the T_c rise under the Ca influence. It is shown that the T_c increase in Y_1?xCa_xBa₂Cu_2.8Zn_0.2O_y is related to the direct Ca ions effect. The mechanism of the calcium influence on the T_c value is discussed with respect to the Ca-induced conduction band modification. The calcium doping in Y_1?xCa_xBa₂Cu_2.8Zn_0.2O_y is shown to give a noticeable contribution to the density-of-states function leading to a strong rise in its value at the Fermi level that results in a restoration of the superconducting properties.     

利用氘粒子活化分析测定高温超导体中的氧含量

详细介绍测量高温超导体含氧量的实验方法。测定了YBa₂Cu₃O_y和Bi₂Sr₂CaCuO_y两类超导体内氧的含量。发现在YBa₂Cu_3(1-X)O_y和Bi_2(1-x)Sr₂CaCu₂O_y中随着Cu缺位的增大,氧的变化趋势。用X     

Synthesis and excess conductivity analysis of TlBa2(Ca3−y Be y )Cu4O12−δ superconductors

TlBa₂(Ca_3?y Be _y )Cu₄O_12?δ (y = 0, 0.5, 1.0, 1.5, 2) samples are synthesized at normal pressure and the influence of doped Be-atoms on the superconductivity parameters at the microscopic level is investigated by carrying out excess conductivity analyses of conductivity data. The samples have shown tetragonal structure and the unit cell volume decreases with increased Be-doping. The onset temperature of superconductivity [T _c(onset)] and zero resistivity critical temperature [T _c(R = 0)] decrease with increased Be, however, the magnitude of diamagnetism is enhanced with Be (except for Be-doping of y = 1.0). The apical oxygen mode of the type Tl–O_A–Cu(2) and CuO₂ planar oxygen mode are softened as observed in FTIR absorption measurements. The FIC analyses of conductivity data have shown an increase in the coherence length along the c-axis and inter-plane coupling. The values of B _c0(T), B _c1(T), J _c(0), τ _φ are improved with the doping of Be. These observations suggested that due to the proximity effect there is less suppression in the value of the order parameter of the Cooper pairs from |ψ|² = 1 value in the CuO₂ planes in Be-doped samples that maintains the density of carriers in the conducting CuO₂ planes [since the |ψ|² = n/2] which promotes enhancement in the magnitude of superconductivity.