Distrontium ruthenate: Difference between revisions

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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.<ref>{{cite journal | last=Mazin | first=I. I. | last2=Singh | first2=David J. | title=Ferromagnetic Spin Fluctuation Induced Superconductivity in Sr₂RuO₄ | journal=Physical Review Letters | publisher=American Physical Society (APS) | volume=79 | issue=4 | date=1997-07-28 | issn=0031-9007 | doi=10.1103/physrevlett.79.733 | pages=733–736| arxiv=cond-mat/9703068 }}</ref>

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.<ref name=Hicks2010>{{cite journal|last=Hicks|first=Clifford W.|title=Limits on superconductivity-related magnetization in Sr2RuO4 and PrOs4Sb12 from scanning SQUID microscopy|journal=Physical Review B|year=2010|volume=81|issue=21|pages=214501|doi=10.1103/PhysRevB.81.214501|arxiv = 1003.2189 |bibcode = 2010PhRvB..81u4501H |display-authors=etal}}</ref> This has led some to speculate that superconductivity arises dominantly from the α and β bands instead.<ref name=Raghu2010>{{cite journal|last1=Raghu|first1=S.|last2=Marini|first2=Aharon|last3=Pankratov|first3=Steve|title=Hidden Quasi-One-Dimensional Superconductivity in Sr2RuO4|url=http://prl.aps.org/abstract/PRL/v105/i13/e136401|journal= Physical Review Letters|volume=105|issue=13| page=136401|year=2010|arxiv = 1003.3927 |bibcode = 2010PhRvL.105b6401B |doi = 10.1103/PhysRevLett.105.026401|pmid=20867720|last4=Rubio|first4=Angel }}</ref> 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 latterlater found not to be entirely correct: the magnitude of edge current is not directly related to the topological property of the chiral state.<ref>{{cite journal | last=Huang | first=Wen | last2=Lederer | first2=Samuel | last3=Taylor | first3=Edward | last4=Kallin | first4=Catherine | title=Nontopological nature of the edge current in a chiralp-wave superconductor | journal=Physical Review B | publisher=American Physical Society (APS) | volume=91 | issue=9 | date=2015-03-12 | issn=1098-0121 | doi=10.1103/physrevb.91.094507 | page=094507| doi-access=free }}</ref> 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.<ref>{{cite journal | last=Huang | first=Wen | last2=Taylor | first2=Edward | last3=Kallin | first3=Catherine | title=Vanishing edge currents in non-p-wave topological chiral superconductors | journal=Physical Review B | publisher=American Physical Society (APS) | volume=90 | issue=22 | date=2014-12-19 | issn=1098-0121 | doi=10.1103/physrevb.90.224519 | page=224519| arxiv=1410.0377 }}</ref><ref>{{cite journal | last=Tada | first=Yasuhiro | last2=Nie | first2=Wenxing | last3=Oshikawa | first3=Masaki | title=Orbital Angular Momentum and Spectral Flow in Two-Dimensional Chiral Superfluids | journal=Physical Review Letters | publisher=American Physical Society (APS) | volume=114 | issue=19 | date=2015-05-13 | issn=0031-9007 | doi=10.1103/physrevlett.114.195301 | page=195301| pmid=26024177 | arxiv=1409.7459 }}</ref>

T_c seems to increase under uniaxial compression.<ref>{{cite journal | last=Steppke | first=Alexander | last2=Zhao | first2=Lishan | last3=Barber | first3=Mark E. | last4=Scaffidi | first4=Thomas | last5=Jerzembeck | first5=Fabian | last6=Rosner | first6=Helge | last7=Gibbs | first7=Alexandra S. | last8=Maeno | first8=Yoshiteru | last9=Simon | first9=Steven H. | last10=Mackenzie | first10=Andrew P. | last11=Hicks | first11=Clifford W. | title=Strong peak in T_c of Sr₂RuO₄ under uniaxial pressure | journal=Science | publisher=American Association for the Advancement of Science (AAAS) | volume=355 | issue=6321 | date=2017-01-12 | issn=0036-8075 | doi=10.1126/science.aaf9398 | page=eaaf9398| pmid=28082534 | hdl=10023/10113 | hdl-access=free }}</ref>