Quantum Oscillations in the Layered Perovskite Superconductor Sr2RuO4

Abstract

We report a comprehensive study of magneto-oscillatory phenomena in the normal state of Sr 2 RuO 4 , the first layered perovskite superconductor ͑T c Х 1 K͒ not based on copper. The form of the quasiparticle spectrum observed may be interpreted in terms of an almost two-dimensional Fermi liquid model which is consistent with Luttinger's theorem and successfully predicts bulk thermodynamic and transport properties at low temperatures. A study of the spectra and transport along the c axis provides insights into the different normal state and superconducting behavior of Sr 2 RuO 4 and the cuprates.[S0031-9007(96)00174-3] PACS numbers: 71.18.+y, 71.27.+a, 74.25.Jb A decade of intensive research on the cuprate superconductors and related systems has raised fundamental challenges to our understanding of the metallic state. A surprising development is the realization that not only the superconducting but also the normal phases can exhibit properties which are difficult to reconcile with the standard (Fermi liquid) description. A number of mechanisms for the breakdown of at least some of the predictions of the usual Fermi liquid model have been proposed, but their applicability to the cuprates remains controversial. Some of the proposals stress the unique chemistry of the planar Cu-O bond [1] while a common theme in many of the others is the importance of reduced dimensionality. The latter favors long range effective interactions between the quasiparticles which can lead to an instability of the standard Fermi liquid state or at least to temperature ͑T ͒ dependences of physical properties at variance with those normally associated with this state. Perhaps the most novel of these proposals is that of Anderson [2] in which a singular quasiparticle pseudopotential arises quite generally in a two-dimensional system at not too low a density due to the reduced phase space available for recoil in collisions, an effect which in higher dimensions tends to stabilize the normal Fermi liquid.Experimental constraints on these models come not only from studies of the cuprates, but also of other related layered perovskites which share with them a quasi-twodimensional structure, but differ in other details. Of particular interest is the recently discovered superconductor Sr 2 RuO 4 [3] which has a similar crystal structure to the parent compound, La 2 CuO 4 , of one of the best studied families of the cuprate superconductors, La 22x Sr x CuO 4 , but has four valence electrons (for Ru 41 ) instead of one hole per formula unit.In stoichiometric La 2 CuO 4 the holes in a starting half filled d͑x 2 2 y 2 ͒-p s band undergo a transition to a Mott insulating state with spin 1͞2 per formula unit, and finite conductivity is achieved only upon doping. For a corresponding description of Sr 2 RuO 4 , it is convenient to begin with the isostructural and isoelectronic relative Sr 2 FeO 4 in which the four valence electrons that in a starting model occupy three d͑xy, xz, yz͒-p p orbitals undergo a Mott transition to an insulator with a high spin. ...

Time-reversal symmetry-breaking superconductivity in Sr2RuO4

“…Despite its low transition temperature, Sr 2 RuO 4 is of great interest as there is growing evidence for an unconventional superconducting state. In this system, strong correlation effects enhance the effective mass seen in quantum oscillation [8] and Pauli spin susceptibility measurements, in the same way as in 3 He [9]. Combining this feature with Sr 2 RuO 4 's expected tendency to display ferromagnetic spin fluctuations, Rice and Sigrist [10], and later Baskaran [11] argued that the pairing in Sr 2 RuO 4 could be of odd parity (spin triplet) type.…”

Cyclotron effective masses in layered metals

“…1 Although transport properties of these materials show unconventional behavior with temperature at high temperatures, at low temperatures ͑below about 20 K in the organics͒ the resistivity is quadratic with temperature, the thermopower is linear in temperature, and a Drude peak is present in the optical conductivity. 2 Furthermore, magnetic oscillations such as the de Haas-van Alphen effect is observed 3,4 suggesting the presence of a well-defined Fermi surface and quasiparticle excitations described by Fermi-liquid theory. In order to understand the role of electron-electron interactions in these materials it is then necessary to quantify the strength of electron correlations and test how robust the Fermi liquid description is.…”

Quasiclassical determination of the in-plane magnetic field phase diagram of superconductingSr2RuO4

“…In particular, the states near the Fermi surface are derived from the Ru t 2g orbitals. The degeneracy of these orbitals are split by the tetragonal crystal field into a xy orbital and the degenerate {xz, yz} orbitals [24][25][26][27] . These two sets of orbitals have a different parity under a mirror reflection through the basal plane.…”