Superconductor electron-type

Metals and semiconductors are electronic conductors in which an electric current is carried by delocalized electrons. A metallic conductor is an electronic conductor in which the electrical conductivity decreases as the temperature is raised. A semiconductor is an electronic conductor in which the electrical conductivity increases as the temperature is raised. In most cases, a metallic conductor has a much higher electrical conductivity than a semiconductor, but it is the temperature dependence of the conductivity that distinguishes the two types of conductors. An insulator does not conduct electricity. A superconductor is a solid that has zero resistance to an electric current. Some metals become superconductors at very low temperatures, at about 20 K or less, and some compounds also show superconductivity (see Box 5.2). High-temperature superconductors have enormous technological potential because they offer the prospect of more efficient power transmission and the generation of high magnetic fields for use in transport systems (Fig. 3.42). 

It is clear that the decrease of the rate of the electron transfer operated by the temperature makes the oxidation of ferrocene become quasi-reversible for both the electrode materials. Moreover, it is noted that for both types of electrode the faradaic current increases with temperature. For both the electrodes the oxidation process is governed by diffusion, since in both cases the plot of log(/p) vs. 1/T is linear. Furthermore, one should note in particular that, contrary to the naive expectation, for the superconducting electrode one does not observe any abrupt change in the response upon crossing the barrier from superconductor (that should exchange pairs of electrons) to simple conductor (that should exchange single electrons). 

Tcr cuprates), whose superconductivity depends on subtle, phonon-free coupling between electrons. It is interesting that the highest known temperature Type-I superconductor, MgB2, shows a much larger isotope effect than does mercury (TCr (MgnB2) = 39.2 K, TCr (Mg10B2) = 40.2 K). 

Here again certain trends were observed, and the most influential factor was the crystal structure which the superconducting material adopted. The most fruitful system was the NaCl-type structure (also referred to as the B1 structure by metallurgists). Many of the important superconductors in this ceramic class are based on this common structure, or one derived from it. Other crystal structures of importance for these ceramic materials include the Pu2C3 and MoB2 (or ThSi2) prototypes. A plot of transition temperature versus the number of valence electrons for binary and ternary carbides shows a broad maximum at 5 electrons per atom, with a Tc maximum at 13 K. 

Plots of Tc vs in-plane rCu 0 for the series of p-type cuprate superconductors are grouped into three classes distinguished by the size of the 9-coordinate site cations (that is, La-, Sr- and Ba-classes) because of the combined electronic and nonelectronic effects. Every class of the Tc vs in-plane rCu 0 plot shows a maximum, so that every class of the p-type cuprate superconductors possesses an optimum hole density for which the Tc is maximum (40). 

There is still no consensus for a mechanism for the high Tc in the cuprate superconductors. Nonetheless, we have learned much about the electronic structure of such materials. Some proposed theories may now be discarded. The discovery of the n-type cuprate superconductors was the clinching evidence needed to discard theories based on some unique feature of an oxygen 2p band, a x band, or overlapping bands. The central question now for the cuprate... 

Until 1988, all the high temperature superconductors that had been found were p-type, and it was assumed by many that this would be a feature of high temperature superconductors. However, some n-type superconductors have also been discovered, where the charge carriers are electrons the first to be found was based on the compound Nd2Cu04 with small amounts of the three-valent neodymium substituted by four-valent cerium—Nd2-/le/lu04-j where v 0.17 (samarium, europium, or praeseodymium can... 

In this edition, we have incorporated new material in all the chapters and updated references to the literature. New sections dealing with porous solids, fullerenes and related materials, metal nitrides, metal tellurides, molecular magnets and other organic materials have been added. Under preparative strategies, we have included new types of synthesis reported in the literature, specially those based on soft chemistry routes. We have a new section covering typical results from empirical theory and electron spectroscopy. There is a major section dealing with high-temperature oxide superconductors. We hope that this edition of the book will prove to be a useful text and reference work for all those interested in solid state chemistry and materials science. 

The superconducting state can coexist with magnetic moments of localized electrons (e.g. of 4f type). It was experimentally found by Matthias et al. (1958a) that for rare-earth impurities substituted into a superconductor Tc rapidly decreases with increasing impurity concentration and that superconductivity is completely destroyed beyond a... 

Here we will summarize, from the previous subsections as well as from literature, some typical properties and representative parameters (see table 6) of the superconducting state of YNi2B2C and LuNi2B2C where completeness is not attempted. These materials are usually clean-limit type II superconductors. However by substitutional disorder on the rare earth site in (Y,Lu)Ni2B2C or on the transition-metal site in Lu(Ni,Co)2B2C the residual resistance ratio RRR = p(300 K)/p(Tc), where p(T) is the normal state resistivity, and the mean free path / of the electrons in the normal state can be considerably reduced... 

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