|| S. H. Curnoe
Department of Physics and Physical Oceanography
Memorial University of Newfoundland
St. John's, Nfld.
Facsimile: (709) 864-8739
Office: Room C3013 Chemistry-Physics Building
Quantum spin configurations
Tb2Ti2O7 is a spin liquid which persists to very low temperatures, T< 50 mK. The rare earth (terbium) ions sit on the vertices of a corner-shared tetrahedral network. The ground state of the Tb spins is doubly degenerate, which can be visualised as classical spins that point into or out of the tetrahedra. However, these spins are not classical in any approximation. First, diffuse neutron scattering can only be explained if there are "quantum fluctuations" of the spins, that is, some probability that they can point in other directions. Second, the actual spin configuration is quite unlike the one shown. Rather, it is a linear combination of all similar-looking states such that the different spins states are entangled.
The materials RT4X12 are known as "filled skutterudites", where R is a rare earth element (La, Ce, Pr, Nd, Eu, Yb), T are transition elements (Fe, Ru or Os) and X are pnicogens (P, As or Sb). The pnicogens form the octahedra, with the transition element inside, and the spaces between octahedra are filled by the rare earth elements. The materials in this series exhibit a wide range of behaviour: superconductivity in the La-compounds, semiconductivity in the Ce-compounds, non-magnetic ordering and Kondo behaviour in the Pr-compounds, and ferromagnetism in the Nd- and Eu-compounds.
PrOs4Sb12 is the first Pr-based heavy fermion superconductor and the first among the family of filled skutterudite compounds. The superconducting order parameter is thought to be unconventional: it involves spin-triplet pairing of the electrons with broken time reversal symmetry. This leads to a very unusual superconducting gap function, with two different branches (centre). The lower branch of the gap function has four nodes which do not occur on lines of high symmetry (right).
MUN Physics & Physical Oceanography