Superconductivity induced by doping Ru in SrFe2−xRuxAs2

Using one-step solid state reaction method, we have successfully synthesized the superconductor SrFe_1−xRu_xAs. X-ray diffraction indicates that the material has formed the ThCr₂Si₂-type structure with a space group I4/mmm. The systematic evolution of the lattice constants demonstrates that the Fe ions are successfully replaced by the Ru. By increasing the doping content of Ru, the spin-density-wave (SDW) transition in the parent compound is suppressed and superconductivity emerges. The maximum superconducting transition temperature is found at 13.5 K with the doping level of x = 0.7. The temperature dependence of DC magnetization confirms superconducting transitions at around 12 K. Our results indicate that similar to non-isoelectronic substitution, isoelectronic substitution contributes to changes in both the carrier concentration and internal pressure, and superconductivity could be induced by isoelectronic substitution.     

Superconductivity in Bi2.2Ca2.8−x Sr x Cu2O y system and effect of rare earth substitution

The oxide system Bi_2.2Ca_2.8−x Sr _x Cu₂O _y has been investigated for superconductivity as a function of Ca/Sr ratio withx=0.7, 1.0, 1.4 and 1.8. All these compositions are found to be superconducting with onset temperature in the range of 80–90 K implying a large homogeneity range between Sr and Ca for the superconductivity to occur in this system. Effect of partial replacement of Bi by rare earths has been studied and it is observed that 20% replacement of Bi by Nd and Eu does not significantly affect the superconducting transition temperature.     

Elastic moduli and step-like elastic anomalies in Ce-substituted Tl0.9Bi0.1Sr2−xCexCa0.9Y0.1Cu1.99Fe0.01O7−δ (x = 0–0.20) superconductors

Ultrasonic velocity measurements have been performed on polycrystalline Tl_0.9Bi_0.1Sr_2?xCe_xCa_0.9Y_0.1Cu_1.99Fe_0.01O_7?δ superconductor to study the influence of Ce substitution on elastic properties of the samples. Ce was observed to influence elastic moduli at 80 K which showed the largest value obtained at x = 0.10 where, coincidentally, the highest superconducting temperature T_c among samples was also shown. A longitudinal velocity anomaly was observed at around 260 K for the unsubstituted sample (x = 0). Ce substitution caused the temperature of the elastic anomaly to shift to 250 K (x = 0.1) and 262 K (x = 0.2). The existence of the step-like elastic anomaly was suggested to be due to oxygen ordering taking place in Tl–O planes. The analysis of elastic behavior in the vicinity of the elastic anomalies using the Landau free-energy model suggests that the anomaly is due to a phase transition which involves oxygen ordering.     

Positron annihilation measurements across the superconducting transition in Y1Ba2Cu3O7−x

Positron lifetime and Doppler broadened annihilation radiation lineshape measurements have been carried out in Y₁Ba₂Cu₃O_7−x as a function of temperature in the range of 300 K to 58 K. The positron lifetime and the peak parameter of the annihilation radiation lineshape are observed to decrease on lowering the temperature without showing any discontinuous change across the superconducting transition temperature of 90 K as determined by susceptibility measurements. The variation of positron annihilation parameters with temperature in the superconducting state is significantly larger than that in the normal state. This is qualitatively explained in terms of the dimerization of oxygen ions in the superconducting state of Y₁Ba₂Cu₃O_7−x .     

Scanning tunneling microscopy/spectroscopy on multi-layered cuprate superconductor Ba2Ca5Cu6O12 (O1−x Fx)2

Scanning tunneling microscopy/spectroscopy (STM/STS) measurements on multi-layered cuprate superconductor Ba₂Ca₅Cu₆O₁₂ (O_1−x F_x)₂ are carried out. STM topographies show randomly distributed bright spot structures with a typical spot size of 0.8 nm. These bright spots are occupied about 28% per one unit cell of c-plane, which is comparable to the regular amount of apical oxygen of 20% obtained from element analysis. Tunneling spectra simultaneously show both the small and the large gap structures. These gap sizes at 4.9 K are about Δ 15 meV and 90 meV, respectively. The small gap structure disappears at the temperature close to T_C, while the large gap persists up to 200 K. Therefore, these features correspond to the superconducting gap and pseudogap, respectively. These facts give evidence for some ordered state with large energy scale even in the superconducting state. For the superconducting gap, the ratio of 2Δ/K_BT_C = 4.9 is obtained with T_C = 70 K, which is determined from temperature dependence of the tunneling spectra.     

Coexistence of superconductivity and spin density wave orderings in the organic superconductor (TMTSF) 2 PF 6

The phase diagram of the organic superconductor (TMTSF)₂PF₆has been revisited using transport measurements with an improved control of the applied pressure. We have found a 0.8 kbar wide pressure domain below the critical point (9.43 kbar, 1.2 K) for the stabilisation of the superconducting ground state featuring a coexistence regime between spin density wave (SDW) and superconductivity (SC). The inhomogeneous character of the said pressure domain is supported by the analysis of the resistivity between T _SDW and T _SC and the superconducting critical current. The onset temperature T _SC is practically constant ( 1.20±0.01 K) in this region where only the SC/SDW domain proportion below T _SC is increasing under pressure. An homogeneous superconducting state is recovered above the critical pressure with T _SC falling at increasing pressure. We propose a model comparing the free energy of a phase exhibiting a segregation between SDW and SC domains and the free energy of homogeneous phases which explains fairly well our experimental findings. Received 3 September 2001 and Received in final form 9 November 2001     

Study of the magnetic phase diagram of Y1-x Cax Ba2 Cu3 O6

We have studied the magnetic phase diagram for the bilayer compound Y_1-x Ca_x Ba₂ Cu₃ O₆ in the regime of low doping (hole concentration within a CuO₂ plane, p_sh < 0.1). For p_sh < 0.03, the data demonstrate the freezing of the spin degrees of freedom associated with the doped holes into a spin‐glass‐like state which is superimposed on the preexisting long range order of the Cu²⁺ spins. Only a single spin‐glass like transition at a temperature T_g is observed for samples in the hole concentration range 0.03 < p_sh < 0.1. While the threshold for the occurrence of superconductivity is almost the same (p_sh > 0.06) for the Y_1-x Ca_x Ba₂ Cu₃ O₆ and the La_2-x Sr_x CuO₄ systems, the magnetic correlations that coexist with superconductivity are considerably stronger in the bilayer Y_1-x Ca_x Ba₂ Cu₃ O₆ system as indicated by significantly increased transition temperatures T_g. This revised version was published online in August 2006 with corrections to the Cover Date.     

Dependence of T<Subscript>c</Subscript> on hydrostatic pressure in a 123 superconductor

Hydrostatic pressure studies on the tetragonal 123 superconductor (La_1-xCa_x) (Ba_1.75-xLa_0.25+x)Cu₃O_y for x =0.1 and variable oxygen content y show that increases rapidly under pressure (+5 K/GPa) for underdoped, optimally doped and overdoped samples. This points to a common cause for the pressure-induced changes in the superconductivity at all levels of doping, with negligible effects from charge transfer. Weak relaxation behavior in is observed only for the most underdoped sample. Received 26 August 1999     

Local Pairs in High-Temperature Superconductors: Proof of Existence and Manifestation in Heat Transfer

The comparison of the temperature anomalies of the charge carrier density in high-temperature cuprate superconductor Bi_1.6(Pb_0.4)Sr₂Ca₂Cu₃O_{10 + x} and features of the parameters of the crystal lattice has demonstrated that the concept of pairing of single-particle hole excitations on the Fermi surface into singlet pairs is realistic. Anomalies of the thermal conductivity correlated with the presence of paired quasiparticle states have been revealed. Both types of anomalies indicate the existence of three stages of the temperature evolution of a pseudogap.     

Investigation of the high-temperature superconductor Tl−Ca−Ba−Cu−Fe−O in a wide range of temperature and iron concentration

A series of the composition Tl₂Ca₂Ba₂Cu_3?x Fe _x O_δ with 0≦x≦0.1 were prepared by the usual ceramic method. The superconducting behaviour was investigated by electrical conductivity and Mössbauer effect. X-ray diffraction on the samples was also used to study the different phases. The study revealed a superconducting transition temperature about 100 K for samples withx=0.00 and above 80 K forx=0.02 and 0.04. From the Mössbauer effect spectra measured for different compositions and at different temperatures, it is evident that the Fe ions substituted Cu into different sites. A clear evidence of vibration anisotropy is observed, together with a low spin state for each position. These results are discussed on the basis of the structure and electronic state of the compounds.     

Superconductivity in a new NdBa2Ca3Sr4Cu5Ox system

A new compound, NdBa₂Ca₃Sr₄Cu₅O_x, has been synthesised using the conventional solid state reaction technique. The material was characterised by powder XRD, electrical resistivity, ac susceptibility and dc magnetisation studies. The results of powder X-ray diffraction show that the structure is pseudo orthorhombic, having unit cell dimensions a = 5.47 Å, b = 5.46 Å and c = 14.58 Å. Magnetisation studies on a SQUID magnetometer showed a superconducting transition at 52 K. This was confirmed by the measurements of electrical resistance and ac susceptibility of this sample, which also showed superconductivity at 52 K. Details of preparation and characterisation are discussed.     

Specific features of praseodymium-doping induced changes in the critical temperature and energy spectrum parameters of YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript> in the presence of calcium ions in the lattice

The results of the experimental study of the specific features of changes in the temperature and concentration dependences of the thermopower coefficient and the critical temperature in the Y_{0.85 − x} Ca_0.15Pr _x Ba₂Cu₃O _y system as the praseodymium content increases are presented. The results obtained have been analyzed based on the narrow band model, the energy spectrum and charge carrier system parameters have been determined, and their behavior with an increase in the doping level has been analyzed. It has been found that both superconducting properties and parameters of the normal state of Y_{0.85 − x} Ca_0.15Pr _x Ba₂Cu₃O _y vary differently in various doping ranges. Based on a comparison of the results obtained with the available data for the case of single doping of YBa₂Cu₃O _y with praseodymium, conclusions have been drawn regarding the mechanism of the energy spectrum modification in the studied compound. The Fermi-level pinning effect has been revealed in the region of a local peak of the density-of-states function, and the energy position of this peak has been determined. It has been shown that the consideration of the Fermi level dynamics caused by the specific features of the structure and transformation of the Y_{0.85 − x} Ca_0.15Pr _x Ba₂Cu₃O _y energy spectrum upon doping makes it possible to explain the observed dependence of the critical temperature on the praseodymium content.     

Neutron-spin resonance in the optimally electron-doped superconductor Nd1.85Ce0.15CuO4-delta

We use inelastic neutron scattering to probe magnetic excitations of an optimally electron-doped superconductor Nd1.85Ce0.15CuO4-delta above and below its superconducting transition temperature Tc=25 K. In addition to gradually opening a spin pseudogap at the antiferromagnetic ordering wave vector Q=(1/2,1/2,0), the effect of superconductivity is to form a resonance centered also at Q=(1/2,1/2,0) but at energies above the spin pseudogap. The intensity of the resonance develops like a superconducting order parameter, similar to those for hole-doped superconductors and electron-doped Pr0.88LaCe0.12CuO4. The resonance is therefore a general phenomenon of cuprate superconductors, and must be fundamental to the mechanism of high-Tc superconductivity.     

Conservation of the particle-hole symmetry in the pseudogap state in optimally-doped Bi2Sr2CuO6+δ superconductor

The pseudogap state is one of the most enigmatic characteristics in the anomalous normal state properties of the high temperature cuprate superconductors. A central issue is to reveal whether there is a symmetry breaking and which symmetries are broken across the pseudogap transition. By performing high resolution laser-based angle-resolved photoemission measurements on the optimally-doped Bi₂Sr_1.6La_0.4CuO_6+δ superconductor, we report the observations of the particle-hole symmetry conservation in both the superconducting state and the pseudogap state along the entire Fermi surface. These results provide key insights in understanding the nature of the pseudogap and its relation with high temperature superconductivity.     

High temperature superconductivity in Bi-Sr-Ca-Cu-O oxide system

Stable, reproducible high temperature superconductivity with onset temperatures of 90 K and above is confirmed in the oxide system with nominal metal ion composition BiSrCaCu₂O _x . The onset is nearly the same, but zero-resistance temperatures depend on the heat treatment. Sample quenched from 850 C in air (close to melting) to liquid nitrogen temperature is non-superconducting. BiSr_0.5Ca_1.5Cu₂O _x is also found to be superconducting in which a two-step drop in resistance is observed, one starting around 118 K and the other around 80 K.     

Evidence of local structural transition during oxidation process of RBCO epitaxial films using time resolved reflexafs measurements

We report results of a time resolved X-ray Absorption Spectroscopy (XAS) experiment on the oxidation process of epitaxial Y₁(Nd_0.05Ba_1.95)Cu₃O_x superconducting thin films. For the first time Cu K-edge XAS technique has been used to explore local structural changes around the Cu ions during the oxidation process of a high critical temperature superconducting film. The results show that during the oxygenation of a tetragonal Y₁(Nd_0.05Ba_1.95)Cu₃O_x additional local transitions appear in the orthorhombic I phase, which are not linked to long range order transformations as shown by in situ X-ray diffraction experiment. New questions concerning the dynamic microscopic process leading to the oxygenation and eventually to superconductivity of the complex R_1+xBa_2-xCu₃O_x (R = Y or rare earth) compounds arose from these results. Received 21 December 2001 Published online 25 June 2002     

Kramers-Kronig analysis of the reflectance spectra of Pb-doped Bi-4334 type glass and the corresponding glass-ceramic superconductor

The room temperature reflectance spectra in UV-VIS-NIR region (energy range of 0.6 to 6.2 eV) for glassy, partially crystalline and its fully crystalline superconducting ceramic phases of Bi_3.9Pb_0.1Sr₃Ca₃Cu₄O _x have been studied by Kramers-Kronig (KK) analysis. A comparative study of the energy loss function [− Im (1/ε)] and the absorption coefficient [α(E)] has been done. Excitions in the superconducting phase hitherto evidenced by the authors are located in the polarizable layers of the superconducting cuprate and their implications for superconductivity have been pointed out. An estimate of the optical band gap energy (E _g) has also been made from the linear fit ofα ² vs.E curve for the superconducting phase. Jezierski’s method ofR-extrapolations in the higher energy has been used to show that both methods yield results that agree quantitatively and can be relied upon.     

Intrinsic tunneling study of underdoped Bi2Sr2-xLaxCuO6+δ superconductors

We report on a tunneling study of underdoped submicron Bi₂Sr_2-xLa_xCuO_6+δ (La-Bi2201) intrinsic Josephson junctions (IJJs), whose self-heating is sufficiently suppressed. The tunneling spectra are measured from 4.2 K up to the pseudogap opening temperature of T^* = 260 K. The gap value found from the spectral peak position is about 35 meV and has a weak temperature dependence both below and above the superconducting transition temperature of T_c = 29 K. Since the superconducting gap should have a value of 10-15 meV, our results indicate that the pseudogap (～35 meV) plays an important role in the underdoped La-Bi2201 intrinsic tunneling spectroscopy down to the lowest temperature of 4.2 K. However, the contribution of the superconducting gap can be separated by normalizing the spectra to the one near and above T_c, which shows that the IJJs can be a useful tool for the study of the electronic properties of the La-Bi2201 cuprate superconductors.     

A new 1223-type high Tc cuprate (Tl,V) Sr2Ca2Cu3O9

A new 1223-type (Tl, V)-based superconducting layered cuprate (Tl_1–x V _x ) Sr₂Ca₂Cu₃O₉ withx=0.25 and 0.50 has been successfully synthesized in the nearly-pure form and identified by powder X-ray diffration analyses. An excessive quantity of Tl is necessary for the preparation of the (Tl, V)-based 1223-type compound. Resistance and ac susceptibility measurements showed that the new (Tl, V)-based 1223 cuprate exhibits T_c above 110 K.     

Gd1-xCaxBa2Cu3O7-y高温超导体压力效应的研究

研究了Ｇｄ_1-xＣａ_xＢａ₂Ｃｕ₃Ｏ_7-y（０．０≤Ｘ≤０．２０）高温超导体在常压和高压下的超导电性在１－３００Ｋ温度范围内，利用Ｂｒｉｄｇｍａｎ对顶砧获得压力达９．０ＧＰａ，测量了（Ｘ＝０．１０，０．１５，０．２０）样品的ｄＴ_c／ｄｐ分别为７．６８，７．８和４．４６Ｋ／ＧＰａ。发现Ｔ_c的压力导数随着ｃａ²⁺含量的增加而下降，分析了氧含量对Ｔ_c和ｄＴ_c／ｄＰ的影响．利用常压下晶格参数精修值和阳离子与氧离子间距随压力的改变，说明ＣｕＯ₂面在超导电性上的作用，用ＣｕＯ₂面之间耦合解释Ｔ_c（Ｐ）曲线的非线性关系。 关键词：