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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. ...

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We show that the pressure-temperature phase diagram of the Mott insulator Ca2RuO4 features a metal-insulator transition at 0.5GPa: at 300K from paramagnetic insulator to paramagnetic quasi-two-dimensional metal; at T ≤ 12K from antiferromagnetic insulator to ferromagnetic, highly anisotropic, three-dimensional metal. We compare the metallic state to that of the structurally related p-wave superconductor Sr2RuO4, and discuss the importance of structural distortions, which are expected to couple strongly to pressure. PACS numbers: 71.30+h, 75.30Kz, 74.70Pq, and 74.62Fj

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Inelastic light-scattering spectra of underdoped La2-xSrxCuO4 single crystals are presented which provide direct evidence of the formation of quasi-one-dimensional charged structures in the two-dimensional CuO2 planes. The stripes manifest themselves in a Drude-like peak at low energies and temperatures. The selection rules allow us to determine the orientation to be along the diagonals at x=0.02 and along the principal axes at x=0.10. The electron-lattice interaction determines the correlation length which turns out to be larger in compound classes with lower superconducting transition temperatures. Temperature is the only scale of the response at different doping levels demonstrating the importance of quantum critical behavior.