The nature of superconductivity

In a normal metal, resistivity is the result of interactions of the current-carrying electrons with the crystal structure. This clearly does not happen in the 

Type-I superconductors are well explained by the Bardeen-Cooper-Schrieffer (BCS) theory. In this, the superconducting state is characterised by having the mobile electrons coupled in pairs. Each pair consists of two electrons with opposite spins, called Cooper pairs. At normal temperatures, electrons strongly repel one another. As the temperature falls and the lattice vibrations diminish, a weak attractive force between pairs of electrons becomes significant. In Type-I superconductors the glue between the Cooper pairs are phonons (lattice vibrations). 

In the newly discovered high-temperature superconductors (described in Section 13.4.5) it has been shown that the electrons are also paired. Unfortunately, the BCS theory is not able to account for the much stronger coupling that must occur in these solids, and no satisfactory theory has yet been suggested. 

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These instruments can address both fundamental and applied research problems. Whether one is trying to solve a device development riddle, understand the complicated vapor-phase thin-film growth process, or delve into the fundamental issues of the nature of superconductivity there is a configuration that can suit that problem. Materials science or physics, whatever the issue, structure or property or their relationship to each other, the SPM capabilities, high resolution, and ability to simultaneously measure structure and property make these instruments valuable additions to the electron microscope family. 

Thus we see a wide diversity of behaviors in these compounds and studies on such materials will enable us to better understand the natures of superconductivity and magnetism and the competition between them. More details can be found in the two-volume series by Maple and Fischer (1982), and the reviews by Fischer (1978) and Shelton (1982). 

HTS materials, because of their ceramic nature, are quite brittle. This has introduced problems relative to the winding of superconducting magnets. One solution is to first wind the magnet with the powder-in-tube wire before the ceramic powder has been bonded and then heat treat the desired configuration to form the final product. Another solution is to form the superconductor into such fine fila-... 

For all these reasons, the stability of the superconducting state and ways to control it are questions of prime importance. Many studies have addressed the degradation of the properties of HTSC under the influence of a variety of factors. They included more particularly the corrosion resistance of HTSC materials exposed to aqueous and nonaqueous electrolyte solutions as well as to water vapor and the vapors of other solvents. It was seen that the corrosion resistance depends strongly both on the nature (chemical composition, structure, etc.) of the HTSC materials themselves and on the nature of the aggressive medium. 

Hehum is used for low-temperature research (—272.2°C or 34°F). It has become important as a coolant for superconducting electrical systems that, when cooled, oiler httle resistance to the electrons passing through a conductor (wire or magnet). When the electrons are stripped from the hehum atom, a positive He ion results. The positive hehum ions (nuclei) occur in both natural and man-made radioactive emissions and are referred to as alpha particles. Hehum ions (alpha particles) are used in high-energy physics to study the nature of matter. 

Cgo doped with K T < 8.1 K) [322] and Rb T < 23 K) [323] exhibit superconductivity on LB films, which was detected by the AC complex magnetic susceptibility or low magnetic field microwave absorption measurements. However, both the stmctural disorder inherent to the LB films and the low-dimensional nature of the thin-layer structure severely prohibit the observation of superconductivity by resistivity measurements. 

The electron microscopy studies of the superconductive cuprates show that the different families differ from each other by the nature of their defect chemistry, in spite of their great structural similarities. For example, the La2Cu04-type oxides and the bismuth cuprates rarely exhibit extended defects, contrary to YBa2Cu307 and to the thallium cuprates. The latter compounds are characterized by quite different phenomena. 

Many layer-perovskites with Cu as the B cation are superconductors at relatively high temperatures ( 100 K). Although the mechanism of superconductivity is not well understood, a necessary condition is that the oxidation state of the copper ion be around +2.2. In many compounds, such as the well-known YBa2Cu307 (63324) this is achieved naturally through the relaxation of... 

To gain some insight into this problem we focus here on the analysis of the Berry phase [1] in a weakly dissipative system. It is particularly timely to address this issue now given the recent experimental activities in realization of controlled quantum two-level systems (qubits), and in particular, the interest in observing a Berry phase (BP) (see, e.g., [5]). For instance, the superconducting qubits have a coupling to their environment, which is weak but not negligible [10, 15, 4], and thus it is important to find both the conditions under which the Berry phase can be observed and the nature of that Berry phase. 

The neodymium based plumbides NdCuPb, NdAgPb, and NdAuPb (Oner et al., 1999) show antiferromagnetic ordering at Tn = 14.6,12.6, and 18.9 K, respectively. These samples revealed an additional transition to a superconducting state at transition temperatures of 7.25, 7.00, and 6.85 K, respectively. The nature of these transitions is still not clear. Due to hystereses behavior of the magnetization of NdAgPb at 5 K, the authors claimed that NdAgPb is a type II superconductor. 

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