Superconductivity magnetic states near superconducting

Fig. 151. Temperature dependence of the nSR frequency in UPd2Al3 in an external field of 1T applied in two directions near and below the superconducting transition (i.e., always in the magnetically ordered state). The line) marks the frequency of a free muon. The dashed lines give the general Knight shift in the magnetic state. Note that changes in shift occur at the superconducting nansition temperature (Tj) whose sign depends on field orientation. From Feyerherm et al. (1994a).

We shall distinguish two fundamental attributes of superconductivity - the state of superconductivity and the effect of superconductivity - that lead to two complementary descriptions of superconductors. On one side the state of superconductivity is characterized by the state of a conducting material, which, after the Jahn-Teller condensation, becomes an insulator with several equivalent ground states. The state of superconductivity determines all statical properties of superconductors energy gap, its temperature dependence, specific heat, density of states near the Fermi surface etc. On the other side the effect of superconductivity determines all dynamical properties of superconductors supercurrent, Meissner effect, quantization of magnetic flux, etc. We shall devote in this section just to the problem of effect of superconductivity. 

NMR instrumentation consists of three chief components a magnet, a spectrometer console, and a probe. While in the past much solid state NMR research was conducted on home-built equipment, the current trend is toward the acquisition of commercial systems. The magnets used for solid state NMR applications generally are superconducting solenoids with a cylindrical bore of 89-mm diameter. The most common field strengths available, 4.7, 7.0, 9.4, and 11.7 Tesla, correspond to proton resonance frequencies near 200, 300, 400, and 500 MHz, respectively. 

Niobium alloyed with germanium becomes a superconductor of electricity that does not lose its superconductivity at 23.2° Kelvin as large amounts of electrical current are passed through it, as do some other superconductive alloys. In the pure metallic state, niobium wires are also superconductors when the temperatures are reduced to near absolute zero (—273°C). Niobium alloys are also used to make superconductive magnets as well as jewelry. 

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