# Condensed Matter Physics Seminar

Thursday, October 21, 1999, 2 p.m.
Plant Sciences Building, Room 1130

## (Naval Research Laboratory)

Abstract:  Ru based perovskites are known to have varied magnetic properties.  Both 3D and 2D subfamilies include ferromagnetic (FM), antiferromagnetic (AFM), and nonmagnetic compounds (NM), some of them being metallic and insulating. Superconductivity was found in  one member of the 2D family, Sr2RuO4. It is believed to be triplet, and induced by FM spin fluctuations.  A popular interpretation of the magnetism in ruthenates uses the same concept as in cuprates, namely Hubbard-type strong Coulomb correlations.  In this concept, the ground state of a ruthenate is defined by the competition between the hopping t and the Hubbard U. This interpretation, however, has difficulty explaining ferromagnetism in such materials as SrRuO3, and the fact that an extremely 2D Sr2RuO4 appears to be one of the least correlated.  A litmus paper for the strongly correlations always was failure of the mean-field LDA calculations to provide accurate descriptions of the corresponding material, as for instance in cuprates.  However, for ruthenates, including AFM and insulators, LDA seems to work about as well as for typical conventional material.  In particular, it does yield AFM solutions for the two AFM ruthenates where the crystal structure is sufficiently well known. The reason for magnetism appears to be typical Stoner.  Tendency to FM is due to the role of oxygen, and is stronger in 3D than in 2D, and the tendency to AFM appears due to a Fermi surface nesting.  Numerical estimates give for Sr2RuO4 AFM spin fluctuation at $q\approx (2\pi/3,2\pi/3)$ (recently confirmed by neutron scattering) comparable to, or even stronger than, the FM ones. This fact has important implications for superconductivity: there are two competing superconducting states, p-wave, and d-wave. The former, being gapped everywhere, enjoys a larger range of stability in the parameter space than the latter, and current experiments do indeed point to a p-wave state.

Host: Victor Yakovenko