Specific heat of a BCS superconductor

By expressing the free energy as an appropriate temperature integration over the square of the BCS energy gap function and by substituting simple but accurate analytic expressions for the latter, corresponding expressions are obtained for the specific heat in the superconducting state. In the upper portion of the temperature range, the ratio of the specific heats in the superconducting and normal states is 2.43 (1 + 0.936 in t), to better than one per cent accuracy, where t is the reduced temperature. Thus, the semilog plot of temperature versus the ratio of specific heat to temperature is a straight line over a large portion of one decade of specific heat. Corresponding expressions of similar simplicity and accuracy are obtained for the middle and low temperature ranges.     

On the addition of silver to highT c (Y1Ba2Cu3O7−x ) superconductor

The superconducting transition temperature of the highT _c compound Y₁Ba₂Cu₃O_7−x does not show degradation consequent upon the addition of metallic silver up to a concentration of 60 wt per cent. The X-ray diffraction data confirm that the oxygen deficient distorted orthorhombic perovskite structure of the pure compound stays intact as if silver does not enter the crystal lattice. SEM studies do show the presence of silver at the grain boundaries only. Silver is found to restrict the grain growth.     

The superconductor system of 2H-Tantalum disulfide intercalated with polyglycine

It was hypothesized that the formation of polypeptide bonds inside a lamellar superconducting compound such as 2H-TaS₂ could enhance the superconducting transition temperature. The preparation of tantalum disulfide intercalated with polyglycine is described. The intercalated systems were characterized by powder X-ray diffraction and thermogravimetric analysis. Superconducting transition temperatures were also measured. The results show that the system 2H-TaS₂/polyglycine does not have a more favourableT _cvalue than 2H-TaS₂/glycine.     

A universal trend in the structural correlation with oxygen content in RBa2Cu3O x systems

It is shown that the lattice parametersa, b andc in the superconducting perovskites RBa₂Cu₃O _x are inter-related and well correlated to the total oxygen content. The correlation shows a universal trend.     

高温超导微波器件在移动通信中的应用

高温超导微波器件在移动通信基站的应用，蕴涵了巨大的商机。由高温超导滤波器、低噪声前置放大器以及微型制冷机组成的高温超导移动通信基站接收机前端子系统，将给移动通信基站的性能带来了极大的提高。目前，全球已出现一批专业化商业公司。美国已有近2000套高温超导子系统产品在基站做商业运行；欧洲和日本也已完成样机研制。中国科学院物理研究所在2001年初完成了我国第一台高温超导移动通信基站原理性样机。     

Magnetization of Gd diffused YBa2Cu3O7-x superconductor：Experiment and theory

The magnetization of Gd diffused YBa2Cu3O7-x is measured by a vibrating sample magnetometer (VSM) at selected temperatures (5, 25, 50, 77 K). The experimental results for the magnetization are analyzed in the critical state framework involving Kim-Anderson field dependence Jc(H)=Jc0/(1+|H|/H0)n of critical current density and equilibrium magnetization M eq . It is found that the inclusion of the equilibrium magnetization becomes more important at higher temperatures. At 77 K, the shape of the isothermal M-H hysteresis curve is governed by the equilibrium magnetization. Some superconducting parameters are determined by fitting the calculated curves to the experimental data.     

Electronic phase diagram of NaFelxCoxAs investigated by scanning tunneling microscopy

Our recent scanning tunneling microscopy （STM） studies of the NaFelxCoxAs phase diagram over a wide range of dopings and temperatures are reviewed. Similar to the high-Tc cuprates, the iron-based superconductors lie in close proximity to a magnetically ordered phase. Therefore, it is widely believed that magnetic interactions or fluctuations play an important role in triggering their Cooper pairings. Among the key issues regarding the electronic phase diagram are the properties of the parent spin density wave （SDW） phase and the superconducting （SC） phase, as well as the interplay between them. The NaFe l-xCoxAs is an ideal system for resolving these issues due to its rich electronic phases and the charge-neutral cleaved surface. In our recent work, we directly observed the SDW gap in the parent state, and it exhibits unconventional features that are incompatible with the simple Fermi surface nesting picture. The optimally doped sample has a single SC gap, but in the underdoped regime we directly viewed the microscopic coexistence of the SDW and SC orders, which compete with each other. In the overdoped regime we observed a novel pseudogap-like feature that coexists with supercon- ductivity in the ground state, persists well into the normal state, and shows great spatial variations. The rich electronic structures across the phase diagram of NaFel_xCoxAs revealed here shed important new light for defining microscopic models of the iron-based superconductors. In particular, we argue that both the itinerant electrons and local moments should be considered on an equal footing in a realistic model.     

Superconductivity and pseudogap states studied by microwave conductivity measurements of λ-(BEDT-TSF)2GaCl4

We have investigated the microwave response at 45 GHz in an organic superconductor λ-(BEDT-TSF)₂GaCl₄ with T_c = 4.8 K. We determine the μ₀H_c2–T phase diagram from microwave loss and find that the superconducting state is in the pure limit (l/ξ_GL  10). Although the real part of the complex conductivity (=σ₁ + iσ₂) does not show a coherence peak just below T_c, the London penetration depth completely saturates at low temperatures down to T/T_c = 0.2, which may provide an evidence for a conventional s-wave pairing. In the metallic state below about 50 K, (parallel to the c-axis) deviates downward from , while σ₂, which should be zero in a conventional metal, increases exponentially toward T_c. In spite of the fact that the Hagen–Rubens limit is well satisfied as far as the dc conductivity is concerned, a Drude model is unable to explain the large positive σ₂. In order to explain such anomalies in the metallic state, we propose a possible existence of so-called a pseudogap near a Fermi level. The anomalous increase of the positive σ₂ may be attributed to an appearance of pre-formed electron pairs in the pseudogap state. This appearance can be regarded as a precursor to the superconducting transition. Such a precursory phenomenon has been observed also in the isostructural FeCl₄ salt with the anomalous metallic states, which shows a negative σ₂ in contrast to the GaCl₄ salt. Just the opposite of ground states in between the GaCl₄ and FeCl₄ salts may result in the contrasting anomalous metallic states with different precursory phenomena with opposite signs of σ₂.     

超导型复合氧化物在苯酚羟化反应中催化作用

采用柠檬酸络合法合成了一系列含铜复合氧化物(超导体)．用化学滴定方法和 O2-TPD等技术对所合成的复合氧化物的组成和表面性质进行了测定．通过探讨催化 剂结构与活性的关系，发现含铜复合氧化物中的非化学计量氧和铜离子的平均价态 与苯酚经化活性和过氧化氢分解活性密切相关．在实验中观察到酪类化合物在苯酚 经化反应循环中具有增强电子传递功能，在此基础上提出了酮类化合物参与催化循 环的机理．     

Thermal investigations of reactive powder mixtures as precursors for melt-processing of YBa2Cu3O7−x

DTA and XRD measurements were carried out with reactive precursors taken from the YO_1.5-BaO-CuO system in order to investigate mechanisms of YBa₂Cu₃O_7–x formation. The reactions taking place depend strongly on phase composition, phase purity, powder size and heating rate. Among the examined reactive precursors a mixture of 7Y₂O₃+22BaCuO₂+10CuO turned out as the most suited for melt-processing.