Quantum oscillations in the underdoped cuprate YBa2Cu4O8.

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.

Magnetic excitations in an itinerant ferromagnet near quantum criticality.

We determine the initial temperature dependence of the exchange splitting Delta(T) in the weak itinerant ferromagnet ZrZn2 (T{C}=28 K) using the de Haas-van Alphen effect. There is a large decrease in Delta with temperature in the range 0.5< or =T< or =4 K. A comparison of Delta(T) with the magnetization M(T) shows that the dominant process responsible for the reduction of M is not the thermal excitation of spin waves, but a repopulation of the spin- upward arrow and spin- downward arrow Fermi surfaces. This contrasts with the behavior in Fe where there is no observable change in Delta and the thermal excitation of spin waves is the only observable spin-flipping process at low temperatures.

Determination of the Fermi surface of MgB2 by the de Haas-van Alphen effect.

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.

de Haas-van Alphen effect in single crystal MgB2.

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.