Random‐field experiments in dilute antiferromagnets

Experiments in d = 2 and d = 3 random-field Ising systems are reviewed. Random fields destroy the phase transition for the d = 2 system Rb₂Co _x Mg_1-xF₄. The d = 3 Fe _x Zn_1-xF₄ system shows a new phase transition with unusual dynamics. Extreme critical slowing down plays an essential role in the d = 3 measurements. Experiments on the related d = 3 systems Mn _x Zn_1-xF₂ and Fe _x Mg_1-xC1₂ are also discussed.     

High temperature superconductors under pressure: Experimental review and test on the RVB model

Abstract

An update is given of all published data on δT_c/δp, extending our previous review. Implications of new data for the RVB model are discussed. Under particular assumptions a striking discrepancy is found.     

Shock wave working of high-T c materials

Abstract

The properties of high T_c-ceramics (Y-123, Bi-1112, Bi-4334, Bi-4457, (Bi-Pb)-4457) under shock loading and subsequent heat treatment were investigated. The peculiarities of their structure, tecture creation, changes of superconductivity parameters depending on loading conditions at P～ 10GPa Were revealed, and regimes of heat treatment were determined. The possibilities of obtaining crack-free cylindrical superconducting shields as well as strong metal high-T^cceramic joints were demonstrated.     

Effect of chemical substitution and oxide additives on the Tc behaviour of the oxide superconductor,Bi4Sr3Ca3Cu4O16

Studies on the superconductivity behaviour of pure and doped (Pb,TI) Bi₄Sr₃Ca₃Cu₄O₁₆ phases are reported. Effect of oxide additives PbO, BaPbO₃ and BaBiO₃ has also been examined. Results show that an oxygen treatment at 600°C for 12 h during the final stages of the synthesis has the beneficial effect of sharpening the T_c ^zero of the 4334 Bi-composition. Pb and TI substitution has only a marginally beneficial effect on the T_c , though it is not definitely known whether substitution is actually occurring in the system. PbO as an additive raises the T_c ^zero to 89 K, whereas other additives do not basically affect the properties of the 4334 system.     

Local and non-destructive diagnostics of high-Tc superconductor layers by millimeter waves

For the first time millimeter waves are used for non-destructive diagnostics of superconducting layers and their quality control. The quality of the whole layer depending on T_C and T_C is determined by local transmission of millimeter waves.     

Effect of vapors of the simplest alcohols on the structure and properties of YBa2Cu3Oy compounds

With low-temperature treatments (200–300°C), the effect of alcohol vapors on the structural and electrophysical properties of YBa₂Cu₃O_y (123) having different oxygen content has been studied. It has been established that CuO and YBa₂Cu₃O_y are catalysts for the dehydration of alcohols to form water and aldehydes. Similar to the action of water vapors, the treatment in vapors of the simplest alcohols leads to hydration of 123 and its transformation into the pseudo-124 phase. In this case, planar stacking faults are formed to be centres of pinning and can improve the critical characteristics of high-temperature superconductors (HTSС) in magnetic fields. The influence of the alcohol vapors is more ‘mild’ as compared to the treatment in water vapors. At t?=?300°C, the alcohols are reducing agents of copper, followed by the decomposition of 123 into simple compounds.     

On the pairing interaction in highT c superconductors

It is argued, from the dependence of the critical current density and ofT _c on applied magnetic field, that the pairing interaction in highT _c superconductors must be strongly field-dependent.     

Fe掺杂YBCO体系结构变化与团簇效应的正电子实验研究

用固态反应法制备了YBa₂Cu_3-xFe_xO_y(x=0—0.5)一系列样品,反应在空气中进行.运用正电子湮没技术、扫描电子显微镜和X射线衍射进行了研究,并测定了氧含量.正电子湮没结果发现,x=0.12时短寿命分量τ₁和长寿命分量τ₂都存在一异常变化.X射线衍射和扫描电子显微镜研究分别表明,材料在x=0.12—0.15区间内发生正交四方相变,晶粒尺寸突然由小变大.通过对上述实验结果的分析可以得到,在该类材料中,正电子对结构相变十分敏感;另外,当Fe掺杂量增大到一定程度时,Fe原子由随机分布变为成簇分布 关键词： Fe掺杂YBCO 高温超导电性 正电子湮没 结构相变     

Raman studies of the high pressure effects in high Tc superconductors

Abstract

The effect of a hydrostatic pressure (up to 6GPa) on the Raman spectra of YBa₂Cu₃O_x(x = 6.5 and overdoped) single crystals is investigated in the temperature range 77-300 K in connection with all available data in the literature for the other oxygen concentrations. All five strong modes (4A_g+1B_1g) are examined for both oxygen doping and compared with previously published results. From the systematic investigation of the phonon spectral characteristics with the application of pressure, the formation or disappearance of the phases is examined. The results provide a spectroscopic support for the changes induced by the hydrostatic pressure on the chain ordering and the corresponding phases.     

Influence of defects on structural transitions

Experiments related to the influence of defects on ordinary structural transitions or on Incommensurate phases are briefly reviewed. The reference theoretical and experimental framework of magnetic systems with frozen random field defects is also recalled.     

Magnetization of high-T c superconductors

We have tried to understand the field dependence of magnetization of high temperature superconductors in the light of phenomenological theory. Especially, the field dependence of dM/d lnB of polycrystalline Bi(2212) is understood by incorporating the overlap of vortices in the London theory.     

Bilayer exchange coupling and neel temperature of YBa2Cu3O6.2

The present paper attempts to study the Neel temperature of bilayer antiferromagnetic cuprate YBa₂Cu₃O_6.2 within anisotropic Heisenberg model. The double time Green’s function formalism within random phase approximation (RPA) has been used to obtain various correlation functions. The magnetization and the Neel temperature (T _N) are evaluated. It is observed that the ratio of intrabilayer to inplane exchange coupling (r=J⊥/J‖) plays an important role in the magnetic dynamics of bilayer systems. The recent experimental data of bilayer system YBa₂Cu₃O_6.2 have been used to estimate the ratio r from the expression for Neel temperature. The estimated values of spin gap and the ratio of hopping matrix elements t⊥/t‖ are found to be in fairly good agreement with the existing experimental results.     

高温超导体中约瑟夫森涡旋流阻的振荡效应

通过数值计算耦合sine-Gordon方程组研究高温超导体中约瑟夫森涡旋的运动,得到约瑟夫森涡旋电压和流阻随平面磁场和驱动电流的变化规律.固定驱动电流,约瑟夫森涡旋电压和流阻随着磁场的增大出现周期性的振荡行为,振荡周期与每层约瑟夫森结中进入一个磁通量子相对应.分析和阐明了产生这种周期性振荡的原因. 关键词： 约瑟夫森涡旋 涡旋格子 高温超导     

MBE growth and properties of T′-La2CuO4 thin films

We prepared thin films of T′-La₂CuO₄, which usually crystallizes in the T structure, by MBE, and investigated their properties while systematically changing the post-reduction conditions with a view towards obtaining superconductivity along the lines of the parent compound superconductors we have recently reported (O. Matsumoto et al., Phys. Rev. B 79 (2009) 100508(R)). The results indicate that the optimal reduction window is very narrow, near which metallic conductivity is obtained down to 50 K. The resistivity of the T′-La₂CuO₄ films is in the range of 10⁻²–10⁻³ Ω cm, which is several orders of magnitude lower than that of the counterpart T-La₂CuO₄, the implication of which is briefly discussed from the viewpoint of the difference in electronic structure induced by different oxygen coordination.     

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.     

Far infrared reflectance of a mixed phase Y-Ba-Cu-O superconductor

We have studied the far infrared reflectance of the mixed phase superconductor Y_1.2Ba_0.8CuO_4–y. The reflectance spectrum shows sharp phonon resonances and an electronic contribution that we attribute to reflection at a mesh-like structure of the conducting phase embedded in nonconducting material; in comparison, the reflectance of single phase superconducting material is mainly determined by the dynamical electronic conductivity.     

Shock wave working of high-TC superconductors

Abstract

The results of investigating the processes of explosive compaction of Y-Ba-Cu-O and Bi-Sr-Ca-Cu-O powdered ceramic high temperature superconducting materials and the shock-induced variation of their properties are presented. The possibility for obtaining metal-ceramic cylinder and spiral-shaped articles is illustrated. The shock wave loading of sintered ceramics is shown to result in degrading its superconducting properties up on R(T), which are subsequently restored by low-temperature annealing. The loss of superconducting properties is demonstrated to correlate with the X-ray lines width. In case of Bi-4334 ceramic, we have observed the improving SP-properties on susceptibility just after shocking it. The effect of the compaction pressure, initial particle size, atmosphere (air, O₂, N₂), initial temperature on material properties is studied.     

Do we really need a superconducting glass model?

Muller and coworkers have introduced the concept of a superconducting glass to explain some magnetization data in highT _csuperconductors. We explain this data using conventional ideas of type-II superconductivity.     

Observation of rf SQUID characteristics in YBCO bulk at 77 K

RF SQUID behaviour due to grain boundary weak links in a bulk YBCO is observed at 77 K using modified commercial rf electronics. Porous samples with lowI _c are found to show this characteristic whereas dense samples with higherI _c do not show SQUID behaviour.V-B modulation characteristic is found to be better when the rf pumping frequency is kept slightly higher than the resonance frequency of the tank circuit. Designing of coil for tank circuit with appropriateQ has been found to be very crucial for seeing the SQUID behaviour. Estimation of parameters such as coupling constant, mutual inductance, inductance and radius of the SQUID loop, have been made and their significance is discussed. Flux noise spectrum of the bulk rf SQUID in flux locked mode is also reported.     

EuSr2Ru1-xTaxCu2O8中Ta替代效应

通过对EuSr₂Ru_1-xTa_xCu₂O₈ (x=0.0, 0.1, 0.2, 0.5和1.0)体系的结构、电阻和磁化强度的观测，发现EuSr₂RuCu₂O₈(x=0.0)样品在130.2K以下呈现铁磁有序，在35K时发生了超导转变，并呈现典型的欠掺杂高温超导体特征；随着Ta对Ru替代浓度x值的增加，铁磁相变温度和超导临界温度均下降 关键词： 高温超导电性 铁磁有序 Ru-Cu氧化物