Strong-coupling d-wave superconductivity in PuCoGa₅ probed by point-contact spectroscopy

According to the theory of superconductivity put forward by Bardeen, Cooper and Schrieffer (BCS), the interaction of valence electrons with vibrations of the ions in the crystal lattice can lead to formation of bound electron pairs. These so-called Cooper pairs then condense into a superconducting state at low temperatures. The BCS theory explains the properties of the so-called conventional superconductors. In the same time, it fails for unconventional superconductors that are technologically more interesting thanks to staying superconducting up to higher and hence more accessible temperatures. Better understanding of these high-temperature superconductors is thus very desirable.

The unconventional superconductivity is observed in a number of heavy-element compounds. One such superconductor is the intermetallic compound PuCoGa₅ whose properties were studied in collaboration with experimentalists from ITU in Karlsruhe and with theorists from the Polytechnic University of Turin. The spectroscopic measurements of the Andreev reflection at a point contact were used to unambiguously determine the symmetry of the order parameter in the superconducting state of this material. The obtained symmetry corresponds to Cooper pairs that are held together by magnetic interactions, which is a surprising finding since PuCoGa₅ does not display any macroscopic magnetism. This apparent contradiction can be understood in terms of a dynamical compensation of the microscopic magnetic momenta that is revealed by our first-principles quantum-mechanical calculations of the electronic structure of this compound.

Temperature dependence of the resistivity of the investigated compound exhibits the superconducting phase transition.

¹Dipartimento di Scienza Applicata e Tecnologia, Politecnico di Torino, Corso Duca degli Abruzzi 24, 10129 Torino, Italy
²European Commission, Joint Research Centre, Institute for Transuranium Elements, Postfach 2340, D-76125 Karlsruhe, Germany
³Institute of Physics, Academy of Sciences of the Czech Republic, Na Slovance 2, CZ-18221 Prague, Czech Republic
⁴University of Hamburg, Institute of Theoretical Physics, Jungiusstrasse 9, 20355 Hamburg, Germany