Condensed Matter Physics Seminar
Thursday, October 21, 1999, 2 p.m.
Plant Sciences Building, Room 1130
Magnetism and Superconductivity in Ruthenates
Igor Mazin
(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
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