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：     

用局域密度泛函线性丸盒轨道原子球近似超元胞法计算(Ba1-xKx)BiO3的Hopfield常数与Tc随组分x的变化

用局域密度泛函线性丸盒轨道原子球近似(LDF-LMTO-ASA)超元胞法计算(Ba_1-xK_x)BiO₃特定组分x的电子结构,其中x=0.0,0.5,1.0三种组分由扩大1倍的元胞计算;x=0.25和0.75两种组分由扩大4倍的元胞计算,所得能带参数总态密度TDOS(E_F)、分波态密度PDOS(E_F)以及自洽晶体势V^t(r),结合由实验测定的Θ_D≈210K,按McMillan强耦合公式以及Gaspari-Gyorffy近似,分别计算各原子的Hopfield常数η_t,电声子耦合常数λ以及超导转变温度T_c随组分x的变化。计算结果λ≈1而T_c最大约10K,且随x变化缓慢。与实验结果对比似乎暗示,在(Ba_1-xK_x)BiO₃中除电声子机制外,随组分变化的复杂结构相变亦将起重要的作用。 关键词：     

Model spectral function and superconductivity in BaxK1−xBiO3 (x=0.5-0.7)

The Eliashberg spectral density function is constructed for the superconducting compound Ba_xK_1−xBiO₃ with x=0.5, 0.6, 0.7. The functions, besides yielding some prominent structures, show widely spreading spectra that inhibit lattice instability by yielding not too large values of λ(<1.32). The resulting spectral functions show similar shape as other results, where available, and also successfully reproduce the observed critical temperature, isotope exponent, gap ratio and several other parameters.     

Structural instability and superconductivity in B{a_{1 - x}}{K_x}Bi{o_3}

We report results of the ultrasonic investigation of Ba_1-xK_xBiO₃ superconducting (SC) single crystals for two potassium concentrations and in a wide temperature range including the normal and the SC states. An instability of the crystal lattice that develops above the superconducting phase transition leads to a softening of both the transverse c₄₄ and the longitudinal c₁₁ modes at temperatures between 200 K and 50 K. In the case of Ba_0.65K_0.35BiO₃ a pronounced hysteresis was discovered. Low temperature X-ray powder diffraction analysis does not reveal any change of the cubic structure in the samples within a resolution of our X-ray technique. The softening of the elastic moduli, the hysteresis, the maximum in the attenuation of sound along with the possible short- (or long-) range structural distortion can be explained qualitatively in a simple model by assuming a coupling of the acoustic modes with the anharmonic oscillations of BiO₆ octahedra. Some weak anomalies are discovered in the velocity of the longitudinal sound in the vicinity of the superconducting phase transition. Received 25 June 1999 and Received in final form 14 February 2000     

Superconductivity and normal state resistivity of Ba0.6K0.4BiO3: an optical phonon approach

We discuss the nature of the pairing mechanism and the physical properties associated with the normal as well as the superconducting state of cubic perovskites Ba_0.6K_0.4BiO₃using the strong coupling theory. An interaction potential which includes the Coulomb, electron–optical phonon and electron–plasmon interactions is developed to elucidate the superconducting state. A model dielectric function is constructed with these interactions fulfilling thef-sum rule. The screening parameter (μ^* = 0.26) infers the poor screening of charge carriers. The electron–optical phonon strength (λ) estimated as 0.98 is consistent with an attractive electron–electron interaction and supports the moderate to strong coupling theory. The superconducting transition temperature of Ba_0.6K_0.4BiO₃is then estimated as 32 K. Ziman's formula of resistivity is employed to analyse and compare this with the temperature-dependent resistivity of a single crystal. The estimated contribution from the electron–optical phonon together with the residual resistivity clearly infers a difference when a comparison is made with experimental data. The subtracted data infer a quadratic temperature dependence in the temperature domain (30 ≤ T ≤ 200 K). The quadratic temperature dependence of ρ [ = ρ_exp − (ρ₀ + ρ_e–ph)] is understood in terms of 3D electron–electron inelastic scattering. The presence of these el–el and el–ph interactions allows a coherent interpretation of the physical properties. Analysis reveals that a moderate to strong coupling exists in the Ba_0.6K_0.4BiO₃system and the coupling of electrons with the high-energy optical phonons of the oxygen breathing mode will be a reason for superconductivity. The implications of the above analysis are discussed.     

Self-consistent spatially inhomogeneous state in HTSC Ba1-x KxBiO3 single crystals

A spatially inhomogeneous state in HTSC Ba_1-x K_xBiO₃ single crystals (T _c ≈ 30 K, x = 0.4) has been studied at T < ? ≈ 17 K, where ? is the temperature of separation into superconducting and dielectric phases. This separation has earlier been detected in Ba_1-x K_xBiO₃ polycrystals. In the single crystals, the main results are obtained from hysteretic magnetization reversal curves, whereas the character of separation in the polycrystals is clearly visible from I–V curves. The results obtained support the theoretical model of HTSC separation proposed in [1].     

X-ray diffraction study of spontaneous strain in Fe-pnictide superconductor, NdFeAsO0.89F0.11

The lattice parameters of a Nd 1111 Fe-pnictide superconductor, NdFeAsO_0.89F_0.11, were accurately measured using X-ray diffraction between 20 K and 280 K. A very small change in the lattice parameter could be detected in the superconducting phase. This change can be attributed to a spontaneous strain generated in the superconducting phase by the coupling between a superconducting order parameter and the strain. The present results are compared with the thermal expansion coefficients of Ba (Fe_1−x Co _x )₂As₂, in addition to the previous lattice parameter measurements of YBa₂Cu₃O_6.5, MgB₂, La_1.85Sr_0.15CuO₄, and Ba_0.6K_0.4BiO₃.     

Superconductivity in Ba1−x KxBiO3: Possible scenario of spatially separated Fermi-Bose mixture

We propose a new scenario for the metal-insulator phase transition and superconductivity in the perovskite-like bismuthates Ba_1?x K_xBiO₃ (BKBO) based on our EXAFS studies. We show that two types of charge carriers, the local pairs (real-space bosons) and the itinerant electrons, exist in the metallic compound Ba_{1? x} K_xBiO₃ (x≥0.37). The real-space bosons are responsible for the charge transport in semiconducting BaBiO₃ and for superconductivity in the metallic BKBO. The appearance of the Fermi liquid state as the percolation threshold is overcome (x≥0.37) explains the observed metal-insulator phase transition. Because bosons and fermions occupy different types of the octahedral BiO₆ complexes, they are separated in real space, and therefore, the spatially separated Fermi-Bose mixture of a new type is likely to be realized in the bismuthates. The nature of superconductivity is consistently explained in the framework of this scenario. A new superconducting oxide Ba_1?x La_xPbO₃ has been successfully synthesized to check our conclusions.     

BaSn1-xSbxO3-δ和Ba1-yLaySnO3-δ的导电机制

测量了ＢａＳｎ_1-xＳｂ_xＯ_3-δ和Ｂａ_1-yＬａ_yＳｎＯ_3-δ样品的低温电阻率和磁化率．实验结果表明，在较低温度区域，两类样品的导电机制均是传导电子的范围可变跳跃。而在较高温度区域，ＢａＳｎ_1-xＳｂ_xＯ_3-δ样品的电导主要是电子从定域态到扩展态的跃迁所贡献，Ｂａ_1-yＬａ_yＳｎＯ_3-δ样品的电导则可能来源于电子的最近邻跳跃。 关键词：     

YbxY1-xBa2Cu3O7-y的超导载流特性

本文系统地研究了Yb_xY_1-xBa₂Cu₃O_7-y系统的微结构、超导临界温度以及超导载流特性。发现:随着Yb含量的增大,样品的超导临界温度没有很大变化,但晶粒度逐渐减小,超导临界电流密度系统地下降。作者认为:在这种弱连接颗粒超导体中,细化晶粒无助于提高其临界电流密度。 关键词：     

Resonant Raman scattering in superconducting Ba<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>K<Subscript>x</Subscript>BiO<Subscript>3</Subscript>

The effect of the photon energy of the exciting laser radiation on the Raman spectra of Ba_1?xK_xBiO₃ with x=0.25, 0.40, and 0.50 is studied. An increase in the laser wavelength from 488 to 750 nm scarcely affects the amplitudes and frequencies of the spectral lines in the Raman spectra of the nonsuperconducting compound with x=0.25. For the optimally doped (x=0.40) and overdoped (x=0.50) superconducting compounds, a substantial increase in the line intensity and a considerable shift of the characteristic frequencies are observed. This result suggests that, in the whole range of superconducting compositions 0.37≤x≤0.50, the local symmetry of the Ba_1?xK_xBiO₃ crystal lattice differs from the perfect cubic symmetry, which should take place according to the literature data. The fact that resonance phenomena are observed when the laser photon energy is shifted toward the optical gap testifies to the presence of local electron pairs in the whole range of superconducting compositions 0.37≤x≤0.50 and is evidence in favor of the superconductivity mechanism proposed for Ba_1?xK_xBiO₃ on the basis of the X-ray absorption studies in our previous paper.     

On the bistable behavior of a low-frequency Raman-active phonon line in superconducting oxide Ba<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>K<Subscript>x</Subscript>BiO<Subscript>3</Subscript>

The two-loop temperature hysteresis of integrated intensity of Raman scattering of light is explained theoretically for a line of frequency 50 cm^?1 in a superconducting oxide Ba_1?xK_xBiO₃ single crystal.     

用线性丸盒轨道原子球近似超元胞法计算(Ba1-xKx)BiO3立方超导相(x=0.25,0.5,0.75)的电子结构

用线性丸盒轨道原子球近似(LMTO-ASA)超元胞法计算(Ba_1-xKx)BiO₃立方超导相x=0.25与x=0.5的电子结构,作为对照同时计算立方结构x=0.75的电子结构。计算结果表明,用超元胞法可以得到比“刚带近似”较精确的电子结构参数。给出Fermi面上的能带参数随组分的变化趋势。讨论了不等价的氧原子与周围金属原子价轨道的杂化特征。 关键词：     

K3Ba3C60在不同温度下的Raman光谱研究

研究了K₃Ba₃C₆₀在不同温度下的Raman光谱.发现Raman光谱随温度的变化发生有规律的变化.随着温度的降低,所有模的线宽和强度的变化情况与纯的C₆₀的情况明显不同.径向Ag(2)模的频率在20K到室温之间有一个反常大的向高频漂移(9cm^-1),这表明在C₆₀分子和掺杂离子之间存在着轨道杂化.另外Raman光谱中Ag(1)模两个分量的相对强度随着温度的降低发生有规律的变化. 关键词： 高频漂移 轨道杂化     

掺Ba对Bi系2212相超导电性的影响

研究了掺Ｂａ对Bi₂Sr_2-xBa_xCaCu₂O_y（０≤ｘ≤０．１５，０．３）单晶和多晶样品超导电性的影响，结果表明，有少量Ｂａ^２＋离子进入了超导相，且有固溶度极限．对于２２１２相单晶，ｃ轴参数和Ｔ_c均随Ｂａ含量增加而增加；对于慢冷多晶样品，掺Ｂａ可明显提高Ｔ_c；然而对于淬大多晶样品，Ｔ_c没有明显变化，用掺Ｂａ 关键词：     

Structural phase transition in superconducting BaPb0.75Bi0.25O3

The room temperature crystallographic properties of distorted perovskite-type system BaPb_1-xBi_xO₃ were examined by using X-ray diffraction analysis. In addition, structural temperature changes were investigated for superconducting BaPb_0.75Bi_0.25O₃ crystals with T_c=11.4K. It was found that the room temperature structure of BaPb_1-xBi_xO₃ has an orthorhombic symmetry in the range of O≤×?0.9 and a monoclinic one in 0.9?×≤1, and that superconducting BaPb_0.75Bi_0.25O₃ shows a structural transition from an orthorhombic to a monoclinic phase at about 160K.     

Pr0.2Yb0.8-xLaxBa2Cu3O7-δ体系的超导电性和磁性

本文研究了Pr_0.2Yb_0.8-xLa_xBa₂Cu₃O_7-δ体系的晶体结构,超导电性和磁性。结果表明,在Pr含量不变的条件下,超导转变温度随稀土离子半径增大而降低,当x≈0.65时,T_c=0;电子比热和费密能级上的态密度也随稀土离子半径增大而降低。并讨论了稀土离子半径与杂化的关系。 关键词：     

BaF2-InF3系统中各个相的离子电导研究

在BaF₂-InF₃系统中存在一个萤石型固溶体Ba_1-xIn_xF_2+x(03(InF₆)₂和BaInF₅。测定了系统中各个相的电导率并和BaF₂-BiF₃系统进行比较。讨论了阳离子极化性和晶体共价性对电性能的影响。 关键词：     

Effect of pressure on the electrical resistivity and thermoelectric power of the compounds Ba(Pb1-xBi x )O3 and K y Ba1-yPb0.5Cu0.5O3-x

High pressure studies on the electrical resistivity and thermoelectric power of BaPb_1-xBi _x O₃ and K _y Ba_1-yPb_0.5Cu_0.5O_3-x show that many compounds in these systems exhibit qualitatively the same type of behaviour as observed in the superconducting oxides (La, Sr)CuO₄ and YBa₂Cu₃O_7-x. The compounds K _y Ba_1-yPb_0.5Cu_0.5O_3-x show temperature-independent resistivity over a wide range of temperatures, but are not found to be superconducting down to 77 K.     

Y1-xPrxBa2Cu3O7+δ的红外反射谱

测量了温度范围为4.2—300K的Y_1-xPr_xBa₂Cu₃O_7+δ(x=0,0.1,0.2,0.4,0.5,0.61.0)的红外反射谱,发现在低温下Ba模有双模行为,随着x值增大双模强度反转,在YBa₂Cu₃O_7+δ中已被判定为Y模的位于194cm^-1的反射峰的位置与Pr的含量无关,即用Pr部份或全部替代Y时此峰不发生频移。建议Pr在Y_1-xPr_xBa₂Cu₃O_7+δ,中是正4价,Pr的替代使原来可移动的空穴被定域在Pr的周围,使超导电性受到压制。 关键词：