The concept of quantum criticality is proving to be central to attempts to understand the physics of strongly correlated electrons. Here, we argue that observations on the itinerant metamagnet Sr3Ru2O7 represent good evidence for a new class of quantum critical point, arising when the critical end point terminating a line of first-order transitions is depressed toward zero temperature. This is of interest both in its own right and because of the convenience of having a quantum critical point for which the tuning parameter is the magnetic field. The relationship between the resultant critical fluctuations and novel behavior very near the critical field is discussed.

We report the results of low temperature transport, specific heat and magnetisation measurements on high quality single crystals of the bilayer perovskite Sr3Ru2O7, which is a close relative of the unconventional superconductor Sr2RuO4. Metamagnetism is observed, and transport and thermodynamic evidence for associated critical fluctuations is presented. These relatively unusual fluctuations might be pictured as variations in the Fermi surface topography itself. No equivalent behaviour has been observed in the metallic state of Sr2RuO4.PACS 71.27.+a, 75.30.Kz Research over the past decade has shown the potential of perovskite ruthenate metals to play a pivotal role in our understanding of the behaviour of strongly correlated electrons. The position of the Fermi level in bands resulting from the hybridisation of oxygen 2p and ruthenium 4d levels leads to ground state behaviour covering a wider range than that seen in almost any other transition metal oxide series. Pseudocubic SrRuO 3 is a rare example of an itinerant ferromagnet based on 4d electrons, and has a good lattice match to the cuprates [1,2]. Sr 2 RuO 4 has the layered perovskite structure with a single RuO 2 plane per formula unit. It is strongly two-dimensional, and shows a Pauli-like paramagnetic susceptibility [3]. It is best known for its unconventional superconductivity [3], which is thought to involve spin triplet pairing [4]. Structural distortions in Sr-based ruthenates are either small or absent, but substituting Ca for Sr introduces larger rotations of the Ru-O octahedra, causing bandwidth narrowing and changes to the crystal field splitting. Thus, although Ca and Sr are both divalent cations, the properties of the Ca-based materials are markedly different. CaRuO 3 is a paramagnetic metal with a large mass enhancement [5], while Ca 2 RuO 4 is an antiferromagnetic insulator [6]. This diversity shows that the ruthenates are characterised by a series of competing, nearly degenerate instabilities, giving a clear motivation for the careful investigation of all the compounds in the series. An even more important feature of the ruthenates is that, in contrast to 3d oxides such as most manganites and many cuprates, no explicit chemical doping is required to produce metallic conduction. This gives a unique opportunity to probe a wide range of correlated electron physics in the low disorder limit, leading to considerable advances in understanding. The superconductivity of Sr 2 RuO 4 , for example, is strongly disorder-dependent [7], and further examples of unconventional superconductivity may be expected in other ruthenates if they can be grown with mean free paths as long as those of Sr 2 RuO 4 . Of particular interest is the subject of this study, Sr 3 Ru 2 O 7 , which has a Ru-O bilayer per formula unit, and hence an effective dimensionality which is intermediate between those of Sr 2 RuO 4 and SrRuO 3 .The synthesis of Sr 3 Ru 2 O 7 in polycrystalline form has been reported by several groups over the past three decades [8][9][10], but investigati...

We show that single-crystalline Sr 3 Ru 2 O 7 grown by a floating-zone technique is an isotropic paramagnet and a quasi-two-dimensional metal, as spin-triplet superconducting Sr 2 RuO 4 is. The ground state is a Fermi liquid with very low residual resistivity (Ϸ3 ⍀ cm for in-plane currents͒ and a nearly ferromagnetic metal with the largest Wilson ratio R W у10 among paramagnets so far. This contrasts with the ferromagnetic order at T c ϭ104 K reported on single crystals grown by a flux method ͓Cao et al., Phys. Rev. B 55, R672 ͑1997͔͒. However, we have found a dramatic changeover from paramagnetism to ferromagnetism under applied pressure. This suggests the existence of a substantial ferromagnetic instability in the Fermi-liquid state. RAPID COMMUNICATIONS R6090PRB 62 IKEDA, MAENO, NAKATSUJI, KOSAKA, AND UWATOKO

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