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, which is much lower than the corresponding value for cuprates.^[1]

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 Maenu 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.

Early proposals have suggested that superconductivity is dominant in the γ band. In particular, the 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.^[6] This has led some to speculate that superconductivity arises dominantly from the α and β bands instead.^[7] 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.

References