Fermi surface pockets in ortho-II YBa$_2$Cu$_3$O$_{6.5}$: the origin of quantum oscillations?

In this paper we explore whether the quantum oscillation signals recently observed in ortho-II YBa$_2$Cu$_3$O$_{6.5}$ may be explained by conventional density functional band-structure theory. Our calculations show that the Fermi surface of YBa$_2$Cu$_3$O$_{6.5}$ 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 difference between the bandstructure of YBa$_2$Cu$_3$O$_{6.5}$ and YBa$_2$Cu$_4$O$_{8}$ are discussed.

de Haas-van Alphen effect investigations of the electronic structure of pure and aluminum-doped MgB_2

Understanding the superconducting properties of MgB_2 is based strongly on knowledge of its electronic structure. In this paper we review experimental measurements of the Fermi surface parameters of pure and Al-doped MgB_2 using the de Haas-van Alphen (dHvA) effect. In general, the measurements are in excellent agreement with the theoretical predictions of the electronic structure, including the strength of the electron-phonon coupling on each Fermi surface sheet. For the Al doped samples, we are able to measure how the band structure changes with doping and again these are in excellent agreement with calculations based on the virtual crystal approximation. We also review work on the dHvA effect in the superconducting state.

de Haas-van Alphen effect in single crystal MgB_2

We report observations of quantum oscillations in single crystals of the high temperature superconductor MgB_2. 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 with band masses calculated recently by three groups, we find that the electron-phonon coupling strength lambda, is a factor ~3 larger for the c-axis tube orbits than for the in-plane network orbit, in accord with recent microscopic calculations.