We report the observation of quantum oscillations in the underdoped cuprate superconductor YBa2Cu4O8 using a tunnel-diode oscillator technique in pulsed magnetic fields up to 85 T. There is a clear signal, periodic in inverse field, with frequency 660+/-15 T and possible evidence for the presence of two components of slightly different frequency. The quasiparticle mass is m(*)=3.0+/-0.3m(e). In conjunction with the results of Doiron-Leyraud et al. for YBa2Cu3O6.5, the present measurements suggest that Fermi surface pockets are a general feature of underdoped copper oxide planes and provide information about the doping dependence of the Fermi surface.

We report observations of quantum oscillations in single crystals of the high temperature superconductor MgB2. Three de Haas-van Alphen frequencies are clearly resolved. Comparison with band structure calculations strongly suggests that two of these come from a single warped Fermi surface tube along the c direction, and that the third arises from cylindrical sections of an in-plane honeycomb network. The measured values of the effective mass range from (0.44-0.68)m(e). By comparing these to calculated band masses, we find that the electron-phonon coupling strength lambda is a factor of approximately 3 larger for the c-axis tube orbits than for the in-plane network orbit, in accord with recent microscopic calculations.

In this paper we explore whether the quantum oscillation signals recently observed in ortho-II YBa2Cu3O6.5 (OII-Y123) may be explained by conventional density functional band-structure theory. Our calculations show that the Fermi surface of OII-Y123 is extremely sensitive to small shifts in the relative positions of the bands. With rigid band shifts of around ±30 meV small tubular pockets of Fermi surface develop around the Y point in the Brillouin zone. The cross-sectional areas and band masses of the quantum oscillatory orbits on these pockets are close to those observed. The differences between the band-structure of OII-Y123 and YBa2Cu4O8 (Y124) are discussed with reference to the very recent observation of quantum oscillations in Y124.The nature of the normal state of the high temperature cuprate superconductors (HTC) has been a topic of intense discussion ever since their discovery. The unusual temperature dependence shown by the resistivity and Hall coefficient for example, and how these evolve as a function of doping, has led to a wide range of exotic theories of the nature of the normal state.1 Many of these theories depart substantially from the standard Fermi liquid model of a metal, particularly in the underdoped region of the phase diagram.The recent report 2 of the observation of Shubnikovde Haas (SdH) oscillations in the Hall and longitudinal resistivities of underdoped ortho-II YBa 2 Cu 3 O 6.5 (OII-Y123) was a surprising and potentially extremely important result. These oscillations suggest that even in the underdoped region some well-defined pockets of Fermi surface exist. The observed orbits have a frequency of ∼ 530 ± 20 T and a mass, m * = 1.9 ± 0.1m e (m e is the free electron mass). A single pocket with this dHvA frequency corresponds to a small (∼ 2%) fraction of the Brillouin zone. Identifying the origin of this orbit is clearly very important for its interpretation. In this paper, we discuss whether the SdH signals could come from small pockets of Fermi surface predicted by conventional density functional band-structure calculations.Our understanding of the electronic structure of the cuprates has, to date, mostly been led by angle resolved photoemission spectroscopy (ARPES). The resolution of this technique has advanced rapidly over the past decade and had provided many key insights into the physics of HTC.3 More recently, the strong angle dependent magnetoresistance of strongly overdoped Tl 2 Ba 2 CuO 6+δ has been used to extract information about the Fermi surface and scattering rate.4,5 The shape and size of the Fermi surfaces measured by both these techniques are generally in very good agreement with conventional density functional theory (DFT) band-structure calculations. 3,6Quantum oscillatory effects, e.g. the de Haas-van Alphen (dHvA) and Shubnikov-de Haas effects, are very powerful probes of the Fermi surface properties of a metal. Unlike ARPES, they probe the bulk of the material (and so are not sensitive to surface defects) and are a true three dimensional probe of the quasip...

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We report measurements of the de Haas-van Alphen (dHvA) effect for single crystals of MgB2, in magnetic fields up to 32 T. In contrast to our earlier work, dHvA orbits from all four sheets of the Fermi surface were detected. Our results are in good overall agreement with calculations of the electronic structure and the electron-phonon mass enhancements of the various orbits, but there are some small quantitative discrepancies. In particular, systematic differences in the relative volumes of the Fermi-surface sheets and the magnitudes of the electron-phonon coupling constants could be large enough to affect detailed calculations of T(c) and other superconducting properties.

We show that the superconductivity observed recently in the weak itinerant ferromagnet ZrZn2 [C. Pfleiderer et al., Nature (London) 412, 58 (2001)] is due to remnants of a superconducting layer induced by spark erosion. Results of resistivity, susceptibility, specific heat and surface analysis measurements on high-quality ZrZn2 crystals show that cutting by spark erosion leaves a superconducting surface layer. The resistive superconducting transition is destroyed by chemically etching a layer of 5 µm from the sample. No signature of superconductivity is observed in ρ(T ) of etched samples at the lowest current density measured, J = 675 Am −2 , and at T ≥ 45 mK. EDX analysis shows that spark-eroded surfaces are strongly Zn depleted. The simplest explanation of our results is that the superconductivity results from an alloy with higher Zr content than ZrZn2.

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