Nested Fermi surface and electronic instability in Ca3Ru2O7

High-resolution angular resolved photoemission data reveal well-defined quasiparticle bands of unusually low weight, emerging in line with the metallic phase of Ca 3 Ru 2 O 7 below ∼ 30     K . At the bulk structural phase transition temperature of 48 K, we find clear evidence for an electronic instability, gapping large parts of the underlying Fermi surface that appears to be nested. Metallic pockets are found to survive in the small, non-nested sections, constituting a low-temperature Fermi surface with 2 orders of magnitude smaller volume than in all other metallic ruthenates. The Fermi velocities and volumes of these pockets are in agreement with the results of complementary quantum oscillation measurements on the same crystal batches

The effect of thickness on the magnetic properties of melt-processed YBCO thick films

Magnetic properties of melt-processed YBa2Cu3O 7-delta thick films have been measured and correlated with features in the microstructure at 4.2 and 77 K for film thicknesses between 50 and 140 mu m. A pronounced peak has been observed in both the measured volume magnetization and calculated length scale over which current flows at a film thickness of approximately 53 mu m and 4.2 K in fields of up to 10 T. An intra 'hub-spoke' (H-S) type grain current dominates the volume magnetization at this film thickness. Measurements at 77 K, on the other hand, exhibit a peak at 80 mu m, the magnitude of which varies significantly with applied field. This correlates well with observed increased connectivity between individual H-S grains and may be accounted for by the flow of inter H-S grain current. The H-S grains transform to a more granular microstructure for films greater than approximately 100 mu m thick which is characterized by the presence of smaller diameter current-carrying loops. This is observed as a decrease in the volume magnetization at 4.2 K and a levelling off of this parameter at 77 K with increasing film thickness. Further evidence for the presence of intra H-S and inter H-S grain critical current densities at 4.2 K in films up to a thickness of 80 mu m has been observed from length-scale analysis as a function of the difference between maximum and applied magnetic field. A qualitative model for the volume magnetization of the films at 4.2 K in terms of individual contributions from intra H-S grain, inter H-S grain and granular Jc components is proposed

Investigation of critical state in melt processed YBa<inf>2</inf>Cu<inf>3</inf>O<inf>7-δ</inf> thick films

The magnetic moment of square planar melt processed YBa2Cu3O7-δ thick films is observed to scale with the cube of the sample width at 4.2 K, suggesting that current flow on the length scale of the film determines its magnetization at this temperature. A well-defined discontinuity in slope in the scaling data at a sample width corresponding to the average grain size (≈2 mm) implies the coexistence of distinct intra- and inter-grain critical current densities of 1.1 × 105Acm-2 and 0.4 × 105Acm-2 at 1 T and 4.2 K. The presence of a critical state in the films at 4.2T is confirmed by removing the central section from a specimen. The observed change in magnetic moment is in excellent agreement with theory for fields greater than ≈2 T. A critical state is not observed at 77 K which suggests that the grains are only weakly coupled at the higher temperature. © 1994

Correlation of transport and magnetic critical currents in melt-processed YBa<inf>2</inf>Cu<inf>3</inf>O<inf>7-δ</inf> thick films

Transport critical current measurements have been carried out on melt-processed thick films of YBa2Cu3O7-δ on yttria-stabilized zirconia in fields of up to 8 T both within grains and across grain boundaries. These measurements yield Jc values of ∼3000 A cm-2 at 4.2 K and zero magnetic field and 400 A cm -2 at 77 K and zero magnetic field, taking the entire sample width as the definitive dimension. Optical and scanning electron microscopy reveals that the thick-film grains consist typically of a central "hub" region ∼50 μm in diameter, which is well connected to radial subgrains or "spokes" which extend ∼1 mm to define the complete grain structure. Attempts have been made to correlate the transport measurements of inter- and intra-hub-and-spoke (H-S) critical current with values of this parameter derived previously from magnetization measurements. Analysis of the transport measurements indicates that current flow through H-S grains is constrained to paths along the spokes via the grain hub. Taking the size of the hub as the definitive dimension yields an intra-H-S grain Jc of ∼60 000 A cm-2 at 4.2 K and 0 T, which is in reasonable agreement with the magnetization data. Experiments in which the hub is removed from individual grains confirm that this feature determines critically the J c of the film