Evidence for intermediate temperature superconductivity as a bulk effect

Foundations of heavy-fermion superconductivity: lattice Kondo effect and Mott physics

“…The latter compounds not only contain well localized 4 f-electrons, but also lack the afore-mentioned metallurgical difficulties. Intensive studies on the canonical Kondo alloy (La,Ce)Al 2 [9] in the early 1970s revealed a plethora of phenomena, such as reentrant superconductivity [10][11][12] and a giant Seebeck effect [13]. An increase of the Ce-concentration in this quasi-binary alloy system from less than 1 at% to 100 at% allows for a detailed study of the interplay between AF correlations and Kondo effect, all the way up to the AF Kondo-lattice system CeAl 2 .…”

“…Here, the depression of the superconducting critical temperature T c as a function of the content of magnetic impurities is in perfect agreement with the de-pairing effect due to the spin-exchange scattering of conduction electrons off the local 4 f-shell, the explanation of which, in Born approximation, was pioneered by Abrikosov and Gor'kov [36]. If instead of Gd 3+ , Ce 3+ is chosen as dopant, the ( ) T x c dependence of (La 1−x Ce x )Al 2 becomes reentrant close to its critical concentration x 0.6 cr at% [10][11][12]. Such an unusual superconducting-to-normal phase boundary had been predicted by Müller-Hartmann and Zittartz [37] by taking into account the pronounced temperature-dependent pair breaking of Kondo impurities which becomes strongest at T T K .…”

Heavy-fermion superconductivity in the Kondo-lattice system CeCu2Si2

“…They showed that the pair breaking in a Kondo superconductor is temperature-dependent and becomes strongest near T K (<< T c , the transition temperature of the host metal), which may cause reentrant superconductivity, i.e., its disappearance at low temperatures. Shortly after publication of the theoretical prediction [55], this was discovered for (La 1-y Ce y )Al 2 , y ≤ y c ≈ 0.6at%, independently by groups at the University of Cologne and the University of California, San Diego [56][57][58]. For a certain part of the parameter space, the results of [55] predicted even three subsequent values of T c , which was indeed observed by K. Winzer for the dilute alloy system (La 0.80 Y 0.20 ) 1-x Ce x [59].…”