Superconductivity alloy systems exhibiting

In the present section, we review the properties of the stoichiometric Ce compounds exhibiting competition between heavy-fermion superconductivity and long-range magnetic order. We shall not discuss the highly complex phase diagrams obtained in alloy systems. In addition, we summarize recent attempts at a theoretical description of the complex low-frequency dynamics resulting from the competition between Kondo effect and RKKY interaction. 

Superconductivity has been known since 1911, and superconducting systems based on various metal alloys (e.g., NbTi and Nb3Sn) are currently used as magnets and in electronics. These materials exhibit superconductivity only at temperatures below 23 K and require cooling by liquid helium. The discovery of ceramics that exhibit superconductivity at temperatures up to 120 K, the so-called high-temperature superconductors, has sparked a tremendous amount of scientific activity and commercial interest around the world. 

After the initial attempt to prepare alloy and interstitial superconductors, several ceramists, chemists, and materials scientists joined the group of physicists and metallurgists in search of other superconducting materials. These scientists turned to ternary compounds and to more complex systems. From the mid-60 s to the mid-70 s, several new "inorganic materials" were found to exhibit the superconducting phenomenon. 

In matrix-impurity systems in which the matrix is a superconductor and Tg is sufficiently low compared to the critical temperature of the pure host, the temperature dependent scattering of conduction electrons by impurity spins may even lead to the striking phenomenon of re-entrant superconductivity (where alloys within a certain impurity concentration range exhibit a transition to the superconducting state at a critical temperature Tg, which is followed by a return to the normal state at a second lower critical temperature as well as pronounced deviations of the specific heat jump from the BCS law of corresponding states. 

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. 

Many of the properties of superconductors have been successfully accounted for by the Bardeen-Cooper-Schrieffer (BCS) microscopic theory of superconductivity. However, the theory cannot predict the occurrence of superconductivity in materials, i.e., it does not tell us what atomic constituents should be put together and what crystal system is necessary in order to obtain materials exhibiting high critical temperatures 7. However, prior to the advent of the BCS theory a large body of information regarding the occurrence of the superconductive state in elements, alloys, and intermetallic compounds was accummulated, notably by Matthias and his... 

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