Superconductivity observations

Figure 5. Concentrational sections M gdr Rh, x)4B4 phase boundaries (schematic) of different structure types. Open circles indicate the occurrence of superconductivity filled symbols mean T = 1.2 K (no superconductivity observed above 1.2 K). Circles denote the existence of a CeCo4B4-type phase the thiek solid line represents the (metastable) phase boundary of the CeCo4B4-type structure the dashed line encloses the superconductivity region. ...

Graphite was known from the 60 s to become metallic under suitable doping with potassium. It was then normal to try and dope with alkali metals the fullerenes Qq molecules discovered in 1985 by H.W. Kroto and R.E. Smalley, and to extend this treatment to balloons or tubes-like C molecules later developed in great quantities. The superconductivities observed here, often at fairly high temperatures, seem of a normal BCS type, with notable electron-phonon coupling and also with electron-electron repulsions weakened by the electrical polarisability of these big and easily excited molecules. Here again, more remains to be done on the detailed characteristics of conduction and superconductivity. 

Therefore, superconductivity observed in doped C can be explained in terms of the generalized BCS theory and electron-phonon interaction. The theoretical estimations using a tight-binding method have shown that with an increase in the lattice constant also the density of electronic states around the Fermi level grows. Therefore, the temperature of transition into a superconductive state is increased [60]. These fects are also evidence of the benefit of the traditional BCS mechanism of superconductivity. 

The superconductivity observed below 0.8 K is strongly anisotropic with respect to the direction of the applied magnetic field. The initial slope — /x0 dHc2/dT as r-> Tc is the same (4 T/K) in both directions. However, as T is reduced the slope decreases when measured parallel to the c-axis as usual, but it increases strongly reaching a value of 14 T/K when measured parallel to the a-axis. 

But the most fascinating feature is the coexistence of magnetism and superconductivity observed in RTr2B2C (R = Dy, Ho, Er, Tm). These compounds display an intricate interplay between magnetic ordering and superconductivity and the study of this has deepened understanding of superconductivity itself and vortex formation, for examp le. ... 

The simple scenario described above seems to be able to roughly explain the superconductivity observed in alkali-metal C o fullerides. However, if we recall that metallic properties can only be found in the fullerides with the stoichiometry A3C60 and so is superconductivity, the real picture of the band structure of alkali-metal Ceo fullerides will be more complex. This is discussed later in this section. 

Of course, condensed phases also exliibit interesting physical properties such as electronic, magnetic, and mechanical phenomena that are not observed in the gas or liquid phase. Conductivity issues are generally not studied in isolated molecular species, but are actively examined in solids. Recent work in solids has focused on dramatic conductivity changes in superconducting solids. Superconducting solids have resistivities that are identically zero below some transition temperature [1, 9, 10]. These systems caimot be characterized by interactions over a few atomic species. Rather, the phenomenon involves a collective mode characterized by a phase representative of the entire solid. 

Phase transitions are involved in critical temperature thermistors. Vanadium, VO2, and vanadium trioxide [1314-34-7] V2O3, have semiconductors—metal transitions in which the conductivity decreases by several orders of magnitude on cooling. Electronic phase transitions are also observed in superconducting ceramics like YBa2Cu30y but here the conductivity increases sharply on cooling through the phase transition. 

Uranium metal is weaMy paramagnetic, with a magnetic susceptibility of 1.740 X 10 A/g at 20°C, and 1.804 x 10 A/g (A = 10 emu) at 350°C (51). Uranium is a relatively poor electrical conductor. Superconductivity has been observed in a-uranium, with the value of the superconducting temperature, being pressure-dependent. This was shown to be a result of the fact that there are actually three transformations within a-uranium (37,52). 

Oxide superconductors have been known since the 1960s. Compounds such as niobium oxide [12034-57-0] NbO, TiO, SrTi02, and AWO, where A is an alkah or alkaline earth cation, were found to be superconducting at 6 K or below. The highest T observed in oxides before 1986 was 13 Kin the perovskite compound BaPb Bi O, x = 0.27. Then in 1986 possible superconductivity at 35 K in the La—Ba—Cu—O compound was discovered (21). The compound composition was later determined to be La 85 A the Y—Ba—Cu—O system was pubUshed in 1987 and reported a transition... 

Superconductivity The physical state in which all resistance to the flow of direct-current electricity disappears is defined as superconductivity. The Bardeen-Cooper-Schriefer (BCS) theoiy has been reasonably successful in accounting for most of the basic features observed of the superconducting state for low-temperature superconductors (LTS) operating below 23 K. The advent of the ceramic high-temperature superconductors (HTS) by Bednorz and Miller (Z. Phys. B64, 189, 1989) has called for modifications to existing theories which have not been finahzed to date. The massive interest in the new superconductors that can be cooled with liquid nitrogen is just now beginning to make its way into new applications. 

One of the most exciting and perhaps unexpected discoveries in science within the last decade has been the observation of superconductivity (the complete absence of resistivity to electric current) in metal oxides at temperature < 90 K. This tempera-... 

Cyclitol Spectra at 220 MHz with the Superconducting Solenoid. In 1964, Nelson and Weaver (34) at Varian Associates constructed a superconducting solenoid with which proton spectra can be observed at 51.7 kilogauss (220 MHz.) or even higher fields. Other nuclei have been observed at suitable field/frequency combinations. 

Not all observations are summarized by laws. There are many properties of matter (such as superconductivity, the ability of a few cold solids to conduct electricity without any resistance) that are currently at the forefront of research but are not described by grand laws that embrace hundreds of different compounds. A major current puzzle, which might be resolved in the future either hy finding the appropriate law or by detailed individual computation, is what determines the shapes of big protein molecules. Formulating a law is just one way, not the only way, of summarizing data. 

Superconductivity is the loss of all electrical resistance when a substance is cooled below a certain characteristic transition temperature (Ts). It is thought that the low temperatures are required to reduce the effect of the vibrations of the atoms in their crystalline lattice. Superconductivity was first observed in 1911 in mercury, for which Ts = 4 K. Over the years, many other metallic superconductors were identified, some having transition temperatures as high as 23 K. However, low-temperature superconductors need to be cooled with liquid helium, which is very expensive. To use superconducting devices on a large scale, higher transition temperatures would be required. 

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