Our paper reports the discovery of heavy fermion superconductivity in the filled skutterudite compound PrOs₄Sb₁₂. This is the first example of a compound of the rare earth element praseodymium (Pr) that has been found to exhibit heavy fermion superconductivity. All of the other heavy fermion superconductors, which number about 20, are compounds of the rare earth element cerium (Ce) or the actinide element uranium (U). Of particular interest is the possibility that the heavy fermion state and the superconductivity in PrOs₄Sb₁₂ are both associated with the interaction between Pr electric quadrupole moments and the charges of the conduction electrons. In the Ce- and U-based heavy fermion superconductors, the heavy fermion state and the superconductivity are both believed to be due to the interaction between the localized magnetic dipole moments of the Ce or U ions and the spins of the conduction electrons. Our paper has stimulated experimental and theoretical efforts to characterize, and develop an understanding of, the heavy fermion state and the unconventional superconductivity in this extraordinary compound.

Our paper describes a new discovery that may open up a new area of research on heavy fermion superconductivity in Pr-based compounds.

Our paper describes a new discovery that may open up a new area of research on heavy fermion superconductivity in Pr-based compounds. This will involve the search for other Pr-based heavy fermion superconductors, the characterization of the physical properties of Pr-based heavy fermion compounds in the normal and superconducting states, and the development of an understanding of the quadrupolar mechanism that may be responsible for the heavy fermion state and the unconventional superconductivity in Pr-based heavy fermion compounds. Studies of the unconventional superconductivity in PrOs₄Sb₁₂ and other Pr-based heavy fermion compounds could provide valuable insights into the nature of superconductivity that could lead to the development of strategies for finding new exotic superconducting materials.

The significance of our paper is twofold: (1) it reports heavy fermion superconductivity in the filled skutterudite compound PrOs₄Sb₁₂, the first example of a Pr-based heavy fermion superconductor (all of the other heavy fermion superconductors are compounds of Ce or U); and (2) the heavy fermion state and superconductivity may arise from a new mechanism that is different from that of Ce- and U-based heavy fermion superconductors.

In a metal that exhibits heavy fermion behavior, the conduction electrons have effective masses that can be as large as several hundred times the mass of the free electron. In the Ce- and U-based heavy fermion compounds, the heavy fermion state is believed to be associated with the Kondo effect wherein the magnetic dipole moments of the Ce- or U-ions are compensated by oppositely oriented spins of the conduction electrons, rendering the compound nonmagnetic at low temperatures. In PrOs₄Sb₁₂, the heavy fermion state appears to be due to an analogous nonmagnetic mechanism involving the interaction between the electric quadrupole moments (which correspond to aspherical charge distributions) of the Pr ions and the charges of the conduction electrons.

In conventional superconductors, the superconducting state is comprised of electron pairs, called Cooper pairs, which consist of two electrons with opposite spins and momenta. The attractive force that binds the two electrons in a Cooper pair together is mediated by the positively charged ions that form the crystal lattice and is called the electron-phonon interaction (phonons are the quantized vibrations of the crystal lattice). The attractive electron-phonon interaction can be visualized in the following way: a negatively charged electron passing through the crystal attracts positively charged ions towards it, leaving a region in the crystal with net positive charge which, in turn, attracts a second electron traveling in its vicinity. The Cooper pairs form a macroscopic quantum state in which they move freely and carry an electric current with no resistance, whatsoever, so that the compound behaves as a perfect conductor (a "superconductor").

In contrast, it is widely believed that the attractive interaction between two electrons in a Cooper pair in Ce- and U-based heavy fermion compounds is mediated by magnetic excitations of the crystal lattice. Here, the spin of an electron traveling through the crystal lattice induces a magnetic dipole moment on the ions in the crystal lattice that, in turn, interacts with the spin of a second electron passing in its vicinity. This can lead to the formation of Cooper pairs consisting of two electrons with their spins aligned antiparallel to one another ("singlet spin pairs") or parallel to one another ("triplet spin pairs").

In the new PrOs₄Sb₁₂ heavy fermion superconductor, the superconductivity is also unconventional, but is different from that displayed by the Ce- and U-based heavy fermion superconductors. In this case, the attractive interaction between the two electrons in a Cooper pair may be mediated by electric quadrupole excitations in the crystal lattice, in analogy to the magnetic dipole excitations in the Ce- and U-based heavy fermion superconductors. Experiments are currently underway in various laboratories throughout the world to characterize the superconducting properties of this extraordinary new material.

We became involved in this research while we were engaged in a systematic investigation of the physical properties of the class of filled skutterudite compounds, of which PrOs₄Sb₁₂ is a member, that have been found to exhibit a variety of interesting strongly correlated electron phenomena. We have a long history of studying heavy fermion superconductivity in Ce and U compounds and were aware of the reports of heavy fermion behavior (but not superconductivity!) in the Pr compounds PrInAg₂ and PrFe₄P₁₂.