In the Bardeen-Cooper-Schrieffer theory of superconductivity, electrons form (Cooper) pairs through an interaction mediated by vibrations in the underlying crystal structure. Like lattice vibrations, antiferromagnetic fluctuations can also produce an attractive interaction creating Cooper pairs, though with spin and angular momentum properties different from those of conventional superconductors. Such interactions have been implicated for two disparate classes of materials--the copper oxides and a set of Ce- and U-based compounds. But because their transition temperatures differ by nearly two orders of magnitude, this raises the question of whether a common pairing mechanism applies. PuCoGa5 has a transition temperature intermediate between those classes and therefore may bridge these extremes. Here we report measurements of the nuclear spin-lattice relaxation rate and Knight shift in PuCoGa5, which demonstrate that it is an unconventional superconductor with properties as expected for antiferromagnetically mediated superconductivity. Scaling of the relaxation rates among all of these materials (a feature not exhibited by their Knight shifts) establishes antiferromagnetic fluctuations as a likely mechanism for their unconventional superconductivity and suggests that related classes of exotic superconductors may yet be discovered.

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Unconventional superconductivity in PuCoGa5.

Theory of superconductivity in strongly correlated electron systems

We report $^{11}\mathrm{B}$ and $^{195}\mathrm{Pt}$ NMR measurements in noncentrosymmetric superconductor ${\mathrm{Li}}_{2}{\mathrm{Pt}}_{3}\mathrm{B}$. We find that the spin susceptibility measured by the Knight shift remains unchanged across the superconducting transition temperature ${T}_{c}$. With decreasing temperature ($T$) below ${T}_{c}$, the spin-lattice relaxation rate $1/{T}_{1}$ decreases with no coherence peak and is in proportion to ${T}^{3}$. These results indicate that the Cooper pair is in the spin-triplet state and that there exist line nodes in the superconducting gap function. They are in sharp contrast to those in the isostructural ${\mathrm{Li}}_{2}{\mathrm{Pd}}_{3}\mathrm{B}$ which is a spin-singlet, $s$-wave superconductor, and are ascribed to the enhanced spin-orbit coupling due to the lack of spatial inversion symmetry. Our finding points to a new paradigm where exotic superconductivity arises in the absence of electron-electron correlations.

Spin Triplet Superconducting State due to Broken Inversion Symmetry in Li(2)Pt(3)B.

The transport and thermal properties of heavy-fermion superconductors are explained in terms of (i) an anisotropic order parameter with a line or lines of nodes and (ii) an effective mean free path which, except for the lowest temperatures, is approximately temperature independent and of a similar magnitude as in the normal state. Such a mean free path is shown to arise in a consistent treatment of impurity scattering close to the unitarity limit.

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Transport and Thermal Properties of Heavy-Fermion Superconductors: A Unified Picture

The proton spin lattice relaxation rate (1 / T 1 ) in the superconducting state of(TMTSF) 2 ClO 4 has been measured at zero magnetic field using the field cycling technique. It was found that the enhancement of 1 / T 1 just below T c , commonly observed in typical BCS superconductors, was absent and 1 / T 1 varied as T 3 . These results indicate that the superconductivity in this system is associated with the anisotropic order parameter vanishing along lines on the Fermi surface.

Proton Spin Relaxation in the Superconducting State of (TMTSF)2ClO4

Nuclear magnetic relaxation in the heavy electron superconductor CeCu2Si2

Unusual behavior of nuclear relaxation in CeCu2Si2 “possible evidence for triplet superconductivity”

We present a study of 29 Si and 63 Cu Knight shifts in the superconducting CeCu 2 Si 2 . The shifts K ∥ , parallel and K ⊥ perpendicular to the tetragonal c -axis were obtained from the anisotropic 29 Si NMR sectrum. K ⊥ shows a rapid decrease below T c ( H )=0.66 K, while the change of K ∥ is not well resolved. K ∥ at 0.2 K decreases to 40%of the value at T c ( H ). Furthermore, the distinct decrease of K ∥ and K ⊥ has also been observed for 63 Cu below T c . The strong reduction of the shift, which is independent of the direction of the crystallographic axis, supports the model of even-parity pairing without significant spin-orbit scattering by impurities, although the energy gap has an anisotropic nature, as is demonstrated by the nuclear relaxation study.

29 Si Knight Shift in the Heavy-Fermion Superconductor CeCu 2 Si 2

In the normal dense Kondo state of CeCu 2 Si 2 , the nuclear magnetic relaxation rate 1/ T 1 of 63 Cu divided by T , 1/ T 1 T is almost constant below 2.5 K with a slight increase near the superconducting transition temperature. Together with the results of other normal Kondo lattice compounds, it is concluded that the temperature dependence of 1/ T 1 T at low temperatures does not scale to that of the specific heat divided by T , C ( T )/ T in Kondo lattice compounds in contrast with the case of dilute Kondo impurities. In the superconducting state, 1/ T 1 T varies as T 2 , suggesting the gap zeros along lines, while the reported data of C s ( T )/ T show no simple power law dependence. It is claimed that this difference below T c may be correlated with the difference between 1/ T 1 T and C ( T )/ T in the normal dense Kondo state.

Koh-ichi Ueda

Papers

Nuclear magnetic relaxation in the heavy electron superconductor CeCu2Si2

Unusual behavior of nuclear relaxation in CeCu2Si2 “possible evidence for triplet superconductivity”

29 Si Knight Shift in the Heavy-Fermion Superconductor CeCu 2 Si 2

NMR investigation of superconductivity and Kondo-coherency in CeCu2Si2

NQR and NMR studies of Cu in the superconducting La2CuO4+δ