Strontium
Ruthenate
Rachel Wooten
Solid State II
Elbio Dagotto
April 24, 2008
University of Tennessee, Knoxville
Introduction
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Significance of strontium ruthenate
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Electronic configuration
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Structure of copper oxides
As compared to copper oxides
Cooper pairs and superconductivity
Structure of Strontium
ruthenate
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High-TC Lanthanum-doped Copper oxide
prompted search for other superconductors with
same structure.
Strontium Ruthenate was only one to exhibit
superconductivity, and only at a much lower
temperature.
Why?
Superficially
identical to
cuprates
•Perovskite structure
• Define the x-y plane
as Ruthenium oxide
plane,
•Z-axis perpendicular
•Like cuprates, highly
planar, distance
between planes very
large.
Electronic structure
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Ruthenium 4+ ion at center of RuO6 tetrahedron.
The 4d-orbitals are the active orbitals with 4
electrons between the five orbitals.
Oxygen with 2- formal valence= high electron
density at each oxygen site.
Degeneracy of d-orbitals split by oxygen.
•Lobes of the d-xz, d-yz, d-xy orbitals are off-axis, avoiding
electrons in the electron-rich oxygen p-orbitals on the axes.
•Energy of these three orbitals lowered compared to other 2
d-orbitals. Four electrons lie in these three.
eg
t2g
Labeled by symmetry group
Different from cuprates
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Contrast to cuprates, where d x2-y2 orbital
occupied by hole (introduced by doping).
Energy of that orbital lowered by attraction of
positively charged hole to oxygen’s electrons on
z-axis.
In addition, superconductivity in strontium
ruthenate doesn’t require doping, unlike the
cuprates.
Strontium Ruthenate’s unusual
Cooper pairs

Unlike many other superconductors,
superconducting pairs formed by electronelectron interaction rather than electron-phonon
interaction.
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Confirmed by effective mass enhancement, and T2
dependence of resistivity.
Much more like He-3 superfluid than like
cuprates.
Electron-electron interaction much stronger.
Cooper pairs
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Spin-triplet state with definite total angular
momentum l=1 (p-wave state).
Contrasting with d-wave spin-singlet state of
cuprates
Confirmation of l-state complicated, we’ll cover
spin state.
Knight Shift
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Under nuclear magnetic resonance, measure small change
in resonance energy due to weak spin polarization in
magnetic field.
In singlet state, all pairs are antiparallel, so applied field
does not change the resonance as the temperature
decreases to zero.
In triplet state, some triplet pairs lie in plane. Magnetic
field will change relative number of pairs with spin
parallel and antiparallel to field.

Knight shift remains unchanged for triplet states
as temperature drops.
Knight shift for Strontium ruthenate,
shown as dotted line.
Solid line shows prediction for
strontium ruthenate if its pairs were
spin singlets.
Strontium ruthenate Cooper pairs are
spin triplets.
Total antisymmetrization of electrons requires that for symmetric
spin function, antisymmetric spatial function, thus p-wave or fwave.
Conclusions

Strontium Ruthenate’s structure identical to
cuprates, but behavior completely different.
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Critical temperature much lower
Unusual superconductivity and cooper pairs,
promising for future study.
Cooper pairs behave like pairs in He-3.
May help in understanding of superconductivity.
Bibliography
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1.
Y. Maeno et al., Nature 372, 532 (1994)
2.
Y. Maeno, T. M. Rice, M. Sigrist. “The intriguing
superconductiviy of strontium ruthenate” Physics Today, (p 4247), Jan 2001.
3.
E. V. Kuz’min, S. G. Ovchinnikov, I. O. Baklanov.
“Comparison of superconductivity in Sr2RuO4 and copper
oxides.” Phys. Rev. B. 61, 22 (p15,392-15,397) Jun 2001
4.
M. B. Walker, M. F. Smith, and K. V. Samokhin.
“Electron-phonon interaction and ultrasonic attenuation in the
ruthenate and cuprate superconductors.” Phys. Rev. B. 65
014517. Dec 2001
5.
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