Superconductors spin-singlet

For the single chain (w = w2 = 0), there are seven two-particle correlation functions, four of which are divergent. The divergent ones are the CDW (charge-density wave), SDW (spin-density wave), SS (singlet superconductor), and TS (triplet superconductor) response functions and are shown in Fig. (3). The... 

The question of the nature of the superconducting state in the radical-ion salts can still not be unambiguously answered even after more than 20 years of intensive research. In the conventional superconductors, the phenomenon can be explained through the BCS theory by the formation of Cooper pairs. Here, the attractive interaction between two electrons is mediated by phonons. The total spin of the Cooper pairs is S = 0, and their total orbital angular momentum is I = 0. The pair wave-function is an s-function. The concept of the Cooper pairs as spin singlets can evidently also be applied to the radical-ion salts. This can for example be shown by measurements of the NMR spin-lattice relaxation time in the superconducting state [3]. 

In 1957, Bardeen, Cooper, and Schrieffer published their theory of superconductivity, known as the BCS theory. It predicts that under certain conditions, the attraction between two conduction electrons due to a succession of phonon interactions can slightly exceed the repulsion that they exert directly on one another due to the Coulomb interaction of their like charges. The two electrons are thus weakly bound together forming a so-called Cooper pair. It is these Cooper pairs that are responsible for superconductivity. In conventional superconductors, these electrons are paired so that their spin and orbital angular momenta cancel. They are described by a wave function, known as an order parameter. In this case the order parameter has symmetry similar to that of the wave function of s electrons and represents a singlet state. 

Thus, at least for UPta, there is a coexistence of superconductivity and spin fluctuations. This is contrary to the singlet-state, electron-electron coupling of the BCS model for superconductivity, but not for a triplet ground-state [65,67]. This type of fermion pairing is seen in superfluid He, but nowhere else in condensed-matter systems. The existence of spin fluctuations in the other heavy fermion superconductors is not confirmed. The strongest evidence for triplet-pairing occurs in UBeja and UPta [68]. 

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