Strontium ruthenate

Strontium ruthenate (SRO) is an oxide of strontium and ruthenium with the chemical formula Sr₂RuO₄. It was the first reported perovskite superconductor that did not contain copper.[1][2] Strontium ruthenate is structurally very similar to the high-temperature cuprate superconductors,[3] and in particular, is almost identical to the lanthanum doped superconductor (La, Sr)₂CuO₄.[4] However, the transition temperature for the superconducting phase transition is 0.93 K (about 1.5 K for the best sample), which is much lower than the corresponding value for cuprates.[1]

Strontium ruthenate

The unit cell of the layered perovskite structure of strontium ruthenate. Ruthenium ions are red, strontium ions are blue, and oxygen ions are green.
Identifiers
Properties
Chemical formula	Sr2RuO4
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Infobox references

High-quality crystals of strontium ruthenate are synthesized using a floating zone method in a controlled atmosphere with ruthenium as flux. The perovskite structure can be deduced based on powder x-ray diffraction measurements. Strontium ruthenate behaves as a conventional Fermi liquid at temperatures below 25 K.[2]

Superconductivity

Superconductivity in SRO was first observed by Yoshiteru Maeno and his group in 1994 when they were looking for high temperature superconductors with structures similar to the cuprates. Unlike the cuprates, SRO displays superconductivity even in the absence of doping.[3] The superconducting order parameter in SRO has been shown to exhibit signatures of time-reversal symmetry breaking,[5] and hence, it can be classified as an unconventional superconductor.

Sr₂RuO₄ is believed to be a fairly two-dimensional system, with superconductivity occurring primarily on the Ru-O plane. The electronic structure of Sr₂RuO₄ is characterized by three bands derived from the Ru t_2g 4d orbitals, namely, α, β and γ bands, of which the first is hole-like while the other two are electron-like. Among them, the γ band arises mainly from the d_xy orbital, while the α and β bands emerge from the hybridization of d_xz and _yz orbitals. Due to the two-dimensionality of Sr₂RuO₄, its Fermi surface consists of three nearly two-dimensional sheets with little dispersion along the crystalline c-axis and that the compound is nearly magnetic.[6]

Early proposals have suggested that superconductivity is dominant in the γ band. In particular, the candidate chiral p-wave order parameter in the momentum space exhibits k-dependence phase winding which is characteristic of time-reversal symmetry breaking. This peculiar single-band superconducting order is expected to give rise to appreciable spontaneous supercurrent at the edge of the sample. Such an effect is closely associated with the topology of the Hamiltonian describing Sr₂RuO₄ in the superconducting state, which is characterized by a nonzero Chern number. However, scanning probes have so far failed to detect expected time-reversal symmetry breaking fields generated by the supercurrent, off by orders of magnitude.[7] This has led some to speculate that superconductivity arises dominantly from the α and β bands instead.[8] Such a two-band superconductor, although having k-dependence phase winding in its order parameters on the two relevant bands, is topologically trivial with the two bands featuring opposite Chern numbers. Therefore, it could possibly give a much reduced if not completely cancelled supercurrent at the edge. However, this naive reasoning was later found not to be entirely correct: the magnitude of edge current is not directly related to the topological property of the chiral state.[9] In particular, although the non-trivial topology is expected to give rise to protected chiral edge states, due to U(1) symmetry-breaking the edge current is not a protected quantity. In fact, it has been shown that the edge current vanishes identically for any higher angular momentum chiral pairing states which feature even larger Chern numbers, such as chiral d-, f-wave etc.[10][11]

T_c seems to increase under uniaxial compression.[12]

References

1. Maeno, Yoshiteru; H. Hashimoto; et al. (1994). "Superconductivity in a layered perovskite without copper". Nature. 372 (6506): 532–534. Bibcode:1994Natur.372..532M. doi:10.1038/372532a0.
2. Yanoff, Brian (2000). Temperature dependence of the penetration depth in the unconventional superconductor Sr2RuO4 (PDF). University of Illinois at Urbana-Champaign.
3. Wooten, Rachel. "Strontium Ruthenate". University of Tennessee-Knoxville. Retrieved 16 April 2012.
4. Maeno, Yoshiteru; Maurice Rice; Manfred Sigrist (2001). "The intriguing superconductivity of Strontium Ruthenate" (PDF). Physics Today. 54 (1): 42. Bibcode:2001PhT....54a..42M. doi:10.1063/1.1349611. Retrieved 16 April 2012.
5. Kapitulnik, Aharon; Jing Xia; Elizabeth Schemm Alexander Palevski (May 2009). "Polar Kerr effect as probe for time-reversal symmetry breaking in unconventional superconductors". New Journal of Physics. 11 (5): 055060. arXiv:0906.2845. Bibcode:2009NJPh...11e5060K. doi:10.1088/1367-2630/11/5/055060.
6. Mazin, I. I.; Singh, David J. (1997-07-28). "Ferromagnetic Spin Fluctuation Induced Superconductivity in Sr₂RuO₄". Physical Review Letters. American Physical Society (APS). 79 (4): 733–736. arXiv:cond-mat/9703068. doi:10.1103/physrevlett.79.733. ISSN 0031-9007.
7. Hicks, Clifford W.; et al. (2010). "Limits on superconductivity-related magnetization in Sr2RuO4 and PrOs4Sb12 from scanning SQUID microscopy". Physical Review B. 81 (21): 214501. arXiv:1003.2189. Bibcode:2010PhRvB..81u4501H. doi:10.1103/PhysRevB.81.214501.
8. Raghu, S.; Marini, Aharon; Pankratov, Steve; Rubio, Angel (2010). "Hidden Quasi-One-Dimensional Superconductivity in Sr2RuO4". Physical Review Letters. 105 (13): 136401. arXiv:1003.3927. Bibcode:2010PhRvL.105b6401B. doi:10.1103/PhysRevLett.105.026401. PMID 20867720.
9. Huang, Wen; Lederer, Samuel; Taylor, Edward; Kallin, Catherine (2015-03-12). "Nontopological nature of the edge current in a chiralp-wave superconductor". Physical Review B. American Physical Society (APS). 91 (9): 094507. doi:10.1103/physrevb.91.094507. ISSN 1098-0121.
10. Huang, Wen; Taylor, Edward; Kallin, Catherine (2014-12-19). "Vanishing edge currents in non-p-wave topological chiral superconductors". Physical Review B. American Physical Society (APS). 90 (22): 224519. arXiv:1410.0377. doi:10.1103/physrevb.90.224519. ISSN 1098-0121.
11. Tada, Yasuhiro; Nie, Wenxing; Oshikawa, Masaki (2015-05-13). "Orbital Angular Momentum and Spectral Flow in Two-Dimensional Chiral Superfluids". Physical Review Letters. American Physical Society (APS). 114 (19): 195301. arXiv:1409.7459. doi:10.1103/physrevlett.114.195301. ISSN 0031-9007. PMID 26024177.
12. Steppke, Alexander; Zhao, Lishan; Barber, Mark E.; Scaffidi, Thomas; Jerzembeck, Fabian; Rosner, Helge; Gibbs, Alexandra S.; Maeno, Yoshiteru; Simon, Steven H.; Mackenzie, Andrew P.; Hicks, Clifford W. (2017-01-12). "Strong peak in T_c of Sr₂RuO₄ under uniaxial pressure". Science. American Association for the Advancement of Science (AAAS). 355 (6321): eaaf9398. doi:10.1126/science.aaf9398. hdl:10023/10113. ISSN 0036-8075. PMID 28082534.

Extra reading

• Armitage, N. Peter (9 December 2019). "Superconductivity mystery turns 25". Nature. 576 (7787): 386–387. doi:10.1038/d41586-019-03734-7. PMID 31844256.