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...

A residual linear term is observed in the thermal conductivity of optimally-doped Bi2Sr2CaCu2O8 at very low temperatures whose magnitude is in excellent agreement with the value expected from Fermi-liquid theory and the d-wave energy spectrum measured by photoemission spectroscopy, with no adjustable parameters. This solid basis allows us to make a quantitative analysis of thermodynamic properties at low temperature and establish that thermally-excited quasiparticles are a significant, perhaps even the dominant mechanism in suppressing the superfluid density in cuprate superconductors Bi2Sr2CaCu2O8 and YBa2Cu3O7.

The effect of vortices on quasiparticle transport in cuprate superconductors was investigated by measuring the low temperature thermal conductivity of YBa 2 Cu 3 O 6.9 in magnetic fields up to 8 T. The residual linear term (as T ! 0) is found to increase with field, directly reflecting the occupation of extended quasiparticle states. A study for different Zn impurity concentrations reveals good agreement with recent calculations for a d-wave superconductor. It also provides a quantitative measure of the gap near the nodes. [S0031-9007 (99)08763-3] PACS numbers: 74.25.Fy, 74.72.Bk In unconventional superconductors, where the gap structure has nodes along certain directions, it was argued by Volovik [1] that the dominant effect of a magnetic field should be the Doppler shift of extended quasiparticle states due to the presence of a superfluid flow around each vortex. Reports of a p H field dependence in the heat capacity of YBa 2 Cu 3 O 6.9 (YBCO) have generally been accepted as strong evidence for this effect [2]. However, a similar field dependence is also observed in an s-wave superconductor such as NbSe 2 [3], where it can only arise from localized states bound to the vortex core.In this respect, it is interesting to look at heat conduction, to which only delocalized states contribute. So far, in all measurements performed above 5 K or so, the thermal conductivity k is found to drop with field [4-6], much as it does in a clean type-II superconductor such as Nb [7], and eventually levels off to a roughly constant plateau [8]. In Bi 2 Sr 2 CaCu 2 O 8 (BSCCO), the crossover from drop to plateau occurs abruptly, at a field whose magnitude increases with temperature [6]. Franz has recently proposed that the plateau is due to a compensation between the increasing occupation of extended quasiparticle states à la Volovik and a parallel increase in the scattering of quasiparticles by vortices [9]. While this may well be part of the explanation, it can hardly serve as a direct confirmation of the "Volovik effect." Neither can it explain the abruptness of the onset of the plateau. The situation remains quite puzzling, in part because of the large phonon background at these temperatures.In this Letter, we present a study of heat transport which provides a solid experimental basis for a description of quasiparticle properties in terms of the field-induced Doppler shift of a d-wave spectrum due to the superflow around vortices. Furthermore, the good agreement found with recent calculations by Kübert and Hirschfeld [10] allows us to conclude that, in YBCO at low temperature, impurity scattering is close to the unitarity limit and vortex scattering is weak. Moreover, it provides a measure of the two parameters that govern the Dirac-like spectrum near the nodes, responsible for all low-energy properties.We have measured the thermal conductivity of three optimally doped untwinned single crystals of YBa 2 ͑Cu 12x -Zn x ͒ 3 O 6.9 down to 0.07 K, with a current along the a axis of the basal plane and a magnetic field along the c ax...