Superconductor Types

As predicted by the Ginzburg-Landau theory and demonstrated later by experimental studies, superconducting materials can be classified according to their magnetic behavior into two distinct classes type I and type II superconductors. Only a few hundred superconducting materials (e.g., pure metals, alloys, ceramics, and, recently, organics compounds) are known today, and this leads one to consider superconductivity as a rare physical phenomenon. 

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Liquid crystals have found widespread application in optical display devices as well as in detection of temperature uniformity and impurities. These properties are related to the orientational order of molecules in the temperature region between and the melting point. The possible applications of ferroelectric liquid crystals are promising. Superconductors (type II) can be used to create high magnetic fields at low power the ability of type I superconductors to trap magnetic flux within the domains of the normal material may also have applications. 

Soft metallic elements such as Al, In, Pb, Hg, Sn, Zn, Tl, Ga, Cd, V and Nb are type I superconductors. Alloys and chemical compounds such as Nb3Sn, V3Ga, and lZa3In, and some transition elements, are type II superconductors. Type II substances generally have a higher Tc than do type I superconductors. The recently discovered transition metal oxide superconductors have generated intense interest because they are type II superconductors with very high transition temperatures. Table 13.1 summarizes Tc for selected superconductors. 

There are two types of superconductor, Type I and Type II, characterized by the way in which they respond to an applied magnetic field. 

From early on it has been recognized that low-field nonresonant microwave absorption can be used to characterize the high-temperature superconductors. Type-II superconductors can be penetrated by the magnetic field via flux vortices, and thermal excitations of these vortices are responsible for microwave absorption. Hence, this kind of nonresonant EPR spectroscopy can provide some information on the vortex dynamics and on the lower critical field Hc A number of EPR experiments focusing on the low-field signals have been reported on HTSC. The nonresonant absorption yields a broad peak in the field derivative of the absorption speetrum, sometimes an additional fine structure has been observed, and in all experiments strong hysteresis effects were noted. 

For alloy-t) e or impure systems (which are usually type-II superconductors), there are two critical fields, Hci and Hc2- Like type-I superconductors, type-II superconductors exhibit perfect diamagnetism up to Hci- The field starts to penetrate above Hci, but superconductivity remains imtil Hci is reached. Between Hqi and Hci, the system is in a mixed state. Lines of magnetic flux start to penetrate in regions that have become normal while the rest of the material remains superconducting. The bulk resistivity is still zero because the current is carried by the superconducting regions. 

There are two t)q3es of superconductors. Type-I superconductors are perfectly diamagnetic, meaning they reject the penetration of a magnetic field imtil they reach their critical magnetic field. He, where the superconductivity is destroyed. Type-II superconductors behave like t)q3e-I superconductors up to their first critical field, Hci. Then they become a... 

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). 

Buckminsterfullerene is an allotrope of carbon in which the carbon atoms form spheres of 60 atoms each (see Section 14.16). In the pure compound the spheres pack in a cubic close-packed array, (a) The length of a side of the face-centered cubic cell formed by buckminsterfullerene is 142 pm. Use this information to calculate the radius of the buckminsterfullerene molecule treated as a hard sphere, (b) The compound K3C60 is a superconductor at low temperatures. In this compound the K+ ions lie in holes in the C60 face-centered cubic lattice. Considering the radius of the K+ ion and assuming that the radius of Q,0 is the same as for the Cft0 molecule, predict in what type of holes the K ions lie (tetrahedral, octahedral, or both) and indicate what percentage of those holes are filled. 

Flexible superconducting tapes provide promise of uses for superconductors in motors, generators, and even electric transmission lines. Meanwhile, superconducting magnets cooled to the temperature of liquid helium already are in use. High-field nuclear magnetic resonance (NMR) spectrometers have become standard instruments in chemical research laboratories, and the same type of machine (called an MRI spectrometer) is used for medical diagnosis in hospitals worldwide. 

Among the high-temperature superconductors one finds various cuprates (i.e., ternary oxides of copper and barium) having a layered structure of the perovskite type, as well as more complicated oxides on the basis of copper oxide which also include oxides of yttrium, calcium, strontium, bismuth, thallium, and/or other metals. Today, all these oxide systems are studied closely by a variety of specialists, including physicists, chemists, physical chemists, and theoreticians attempting to elucidate the essence of this phenomenon. Studies of electrochemical aspects contribute markedly to progress in HTSCs. 

Like graphite, C60 can be transformed into diamond, but the process requires less stringent conditions. It has also been found that Cso becomes a superconductor at low temperature. Another interesting characteristic of Cso is that when it is prepared in the presence of certain metals, the Cso cage can enclose a metal atom. In some cases, other materials can be enclosed within the C60 cage in a "shrink wrapped" manner to form "complexes" that are described as endohedral. It has also been possible to prepare metal complexes of Cso that contain metal-carbon bonds. A compound of this type is (C6H5P)2PtC60. 

This review will include both types of studies, but will not discuss in any detail optically pumped NMR of semiconductors, which has been well-reviewed [5, 11, 12,14], or other unconventional techniques for detection of NMR signals. Physics-related NMR studies of more complicated semiconductor behavior such as Kondo insulators or semiconductors and other unusual semiconducting phases, and semiconducting phases of high-Tc superconductors, while very important in physics, will be neglected here. I have deemed it of some value to provide rather extensive citation of the older as well as of the more recent literature, since many of the key concepts and approaches relevant to current studies (e.g., of nanoparticle semiconductors) can be found in the older, often lesser-known, literature. My overall aim is to provide a necessarily individual perspective on experimental and theoretical approaches to the study of semiconductors by NMR techniques that will prove useful to chemists and other scientists. 

Among the M(dmit)2-based superconductors, a-(EDT-TTF)[Ni(dmit)2] is also of donor-acceptor type and has two outstanding features it is the only one to contain a 1 1 molar ratio of donor and acceptor units and to exhibit superconductivity at ambient pressure [32]. It was found to be superconductive below 1.3 K under ambient pressure (Fig. 3). 

The conductors, semiconductors, and superconductors that have been discussed are materials that can be prepared via some type of CVD process. In order to prepare each material, a precursor is required. The precursor chemistry of these materials is based heavily on organometallic and inorganic chemistry. Numerous ligand platforms have been investigated for use in the preparation of suitable CVD precursors. 

Apart from structures that are built of slabs, modular structures that can be constructed of columns in a jigsawlike assembly are well known. In the complex chemistry of the cuprate superconductors and related inorganic oxides, series of structures that are described as tubular, stairlike, and so on have been characterized. Alloy structures that are built of columns of intersecting structures are also well known. Structures built of linked columns, tunnels, and intersecting slabs are also found in minerals. Only one of these more complex structure types will be described, the niobium oxide block structures, chosen as they played a significant role in the history of nonstoichiometry. 

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