Observation of inter-grain Josephson effects in Y-Ba-Cu-O samples

Josephson effects have been observed in bulk samples of Y-Ba-Cu-O. The magnitude of the zero-voltage current is found to change systematically with externally applied small magnetic fields of a few mG. It is also found to vary when samples are irradiated with microwaves. These observations suggest the presence of inter-grain Josephson junctions.     

Effect of heat treatment on Ag/Y-Ba-Cu-O contact resistance

The contact resistance (R _c) of the metal/YBCO interface has been studied in pressed indium, painted colloidal silver and thermally-evaporated silver contact pads. Indium contacts always show the highest resistance amongst these three systems. In thermally-evaporated Ag contacts, post-deposition thermal treatments show favourable effects on the reduction ofR _c. Heat treatment in oxygen atmosphere in two steps is essential to reduce theR _c values. Significant improvement in obtaining low resistivity contacts has been attributed to the diffusion of silver atoms to grain boundaries at the surface of YBCO and to the enrichment of oxygen-deficient layer at the interface during thermal treatment.     

Anomalous change in the photoacoustic phase near superconducting transition in YBa2Cu3O7

Photoacoustic amplitude and phase measurements have been carried out near the superconducting transition temperature in the highT _csuperconductor YBa₂Cu₃O₇. In both the samples investigated, the photoacoustic phase undergoes an anomalous step-like decrease aroundT _c≈ 96 K and increases at lower temperatures. A clear temperature hysteresis for the phase change has been noticed. No anomalous variation has been observed in the amplitude values. Neither phase nor amplitude shows any anomaly in a non-superconducting sample of the same composition.     

Phase dependent superconductivity in the Y-Ba-Cu-O system

Studies of stable phases containing copper have been made when mixtures of Y₂O₃, BaCO₃ and CuO are reacted. It is found that out of four phases there is only one phase which is superconducting and the volume ratio of superconducting to non-superconducting phase decreases rapidly as we move away from the stoichiometry of the superconducting phase having composition YBa₂Cu₃O_8−y.     

On identical nature of the superconductivity mechanisms in 30 K and 90 K superconductors

A systematic study of Zn substitution in both lanthanum and yttrium-based oxide superconductors withT _cof 30 K and 90 K has been carried out simultaneously under identical conditions. Contrary to the general belief, the results indicate that the mechanism responsible for superconductivity in 30 K compounds is unlikely to be radically different from that of the 90 K compounds.     

Normal-phase transport properties of highT c Y-Ba-Cu-O compounds

The normal state resistivity of highT _c Y-Ba-Cu-O compounds has been investigated and it is shown that the room temperature resistivity depends exponentially on the volume fraction of the non-superconducting phase. It is further shown that the dependence of normal state resistivity on temperature upto 300 K can be explained on the basis of a short-range co-operative electron-phonon coupling. This model also explains the observed positive Seebeck coefficient for the pure phase superconducting sample.     

Observation of inverse a.c. Josephson effect in bulk Y-Ba-Cu-O possessing highT c

Microwave-induced d.c. voltage due to inverse a.c. Josephson effect has been observed across bulk samples of Y-Ba-Cu-O. Variation of the d.c. voltage with small external magnetic field and temperature has been investigated. Our results indicate that weakly coupled superconducting grains exist up to 230 K.     

Theory of high temperature superconductivity in YBa2Cu3O7_δ

A theory of high temperature superconductivity in YBa₂Cu₃O₇__δ compound has been developed on the basis of the momentum pairing of electrons through the relativistic Darwin interaction. The transport behaviour of electrons is explained in terms of a mechanism of correlated electron transfer arising from the electron-phonon coupling. A model Hamiltonian has been developed to describe the superconducting properties of the system. This gives an energy gap which is higher than the BCS value. Attempts have been made to explain the absence of isotope effect, the linear dependence of specific heat, the presence of larger temperature-independent paramagnetism in the normal phase and the softening of some of the optic phonon modes observed in this system.     

Performance of 2-hole YBCO rf SQUID in flux-locked-loop mode at 77 K

A 2-hole rf SQUID has been fabricated using naturally present grain boundary Josephson weaklinks in a microbridge of bulk Y-Ba-Cu-O superconductor. Periodic oscillations in voltage-flux characteristics have been observed up to 80.5 K. The spectral density of flux noise of the SQUID is 8 × 10⁻⁴ Φ₀/√Hz at 100 Hz and 77 K for open-loop mode. The SQUID has been successfully operated in flux-locked-loop mode at 77 K, demonstrating the feasiblity of the device for practical applications. In the flux-locked-loop mode the stability as well as flux noise of the SQUID has been found to improve considerably as compared to that in the open-loop mode. Various SQUID parameters have also been estimated and reported here.     

轨道交通用高温超导直线感应电机电磁分析

设计研究了一种用于轨道交通的新型高温超导直线感应电机模型。并结合轨道交通用直线感应电机的实际运行状况,利用ANSYS软件对超导直线感应电机模型进行电磁分析。获得电机内部的磁场分布规律;并研究了电磁气隙、次级材料及次级感应板厚度对电机电磁推力的影响。最后在相同运行条件下,将传统铜绕组电机与高温超导电机做了比较,超导直线感应电机在电磁推力上有优势。     

Estimation of the influence of ionic dielectricity on the dynamic superconducting order parameter

We consider the influence of an ω-dependent ionic dielectric constant ?(ω) on the properties of a superconductor. Assuming that the pairing interaction is proportional to ?² we have solved the Eliashberg equations for this case, both for imaginary and real frequencies. The interaction potential depends on a coupling constant λ and on a longitudinal phonon frequency Ω. The dielectric constant is assumed to be independent of wavevector q, and to depend on frequency through the expression: ?(ω) = (ω² - ω²_long)/(ω² - ω²_trans), where ω_long, ω_trans are the frequencies of optical phonons of the dielectric. We find that along the imaginary frequency axis (but not for real frequencies) the weighted phonon propagator can be modeled by an appropriate choice of a cutoff frequency and an effective coupling constant. The influence of ?(ω) on T_c, the gap δ(ω), and the renormalization function Z(ω) are studied and it is found that these quantities increase significantly with the dielectric constant.     

Vortex core states in the presence of a staggered flux phase

We investigate the vortex properties of a system where superconductivity competes with a staggered flux state. Such a competition has been proposed to be at the origin of the pseudogap phenomenon observed in the cuprates. We show that the presence of a staggered flux state is responsible for a substantial modification of the vortex structure and reversion of the vortex charge. We demonstrate that this scenario may explain some experimental results obtained with the help of scanning tunneling microscopy.     

Indications of superconductivity in the 200–300K range in the Bi-Ca-Sr-Cu-O system

Some of the Bi-Ca-Sr-Cu-O compositions show indications of onset of superconductivity in the 200–300 K region, possibly due to the intergrowth of different layered sequences. Contribution No. 529 from the Solid State and Structural Chemistry Unit.     

Bi-2223/Ag超导带材的失超传播速度研究

文中对超导带材的失超产生机理进行了详细分析,建立了其失超传播的计算模型。在此基础上,对Bi-2223/Ag高温超导带材失超传播进行了实验研究,用四引线法测得不同条件下其失超传播的U-t曲线,由此计算出Bi-2223/Ag超导带材的失超传播速度为0.4cm/s～1.98cm/s。实验结果表明:带材的失超传播速度与载流大小有关,与触发失超的能量无关,带材载流越大,失超传播得越快。     

超导材料的发展与研究现状

新型超导材料一直是人类追求的目标。该文主要从超导材料的探索与发现、制备技术、基础研究面临的挑战等几个方面来探讨超导材料的发展与研究现状。     

Low concentration of the superconducting phase and thermoelectric power in 90 K superconductors

The thermoelectrical power (TEP) and the electrical resistivity behaviour of three 90 K superconductors viz, Y₁Ba₂Cu₃O_7−x , Sm₁Ba₂Cu₃O_7−x and Gd₁Ba₂Cu₃O_7−x , after the specimens were quenched from the sintering temperature (920°C) to 77 K, are reported. Interestingly the Y₁₂₃ specimen, which has the presence of trace amount of the orthorhombic phase in an otherwise tetragonal phase and does not show a superconducting transition down to 77 K shows zero TEP around 82 K, theT _c for the well oxygenated specimen. The Sm and Gd specimens on the other hand show completely tetragonal structure, semiconducting behaviour in resistivity and no zero TEP up to 77 K. It is argued that the critical concentration of the superconducting phase necessary to make the TEP zero is much smaller than that required for zero resistivity.     

无绝缘高温超导双饼线圈的瞬态电热特性分析

无绝缘高温超导线圈具有良好的电热稳定性和机械紧凑性,但其充电过程中却有明显的磁场延迟现象。为详细了解无绝缘高温超导线圈励磁过程的瞬态特性,建立了无绝缘高温超导线圈的同轴圆环等效电路模型。通过绕制一个670匝的无绝缘高温超导双饼线圈,在液氮温度下进行不同充电速率的励磁实验,初步验证了等效电路模型的正确性。基于该模型,针对线圈励磁过程的充电和恒流阶段,仿真得到了线圈各匝的径向电流分布规律和电热损耗特性。     

Identification of the high-temperature superconducting phase in the Y-Ba-Cu-O system as the perovskite YBa2Cu3O7±δ

The oxide responsible for high-temperature superconductivity (onset ∼100 K, zero resistance above liquid N₂ temperature) is found to be YBa₂Cu₃O₇±δ. Contribution No. 432 from the Solid State and Structural Chemistry Unit     

Low temperature heat capacity of YBa2Cu3O7

We have measured the heat capacity of superconducting, single phase YBa₂Cu₃O₇ in the temperature range 2 to 18 K. An extrapolation of the data between 4 and 9 K gives aC/T (T → 0) of ∼ 25 mJ/mole K². The Debye temperature obtained from the high temperature linear portion ofC/T vsT ² plot is 325 K.