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Arrays of nanometre-thick Bi2212-intrinsic Josephson junctions (IJJ's) are studied in various geometries. The samples with only a few IJJ's allow for the intrinsic-tunnelling spectroscopy with minimum of Joule heating. The reproducible low-voltage peaks of the spectra probably stem from a superconducting gap which is half the usual size. We estimate the internal temperature in the IJJ stacks and analyze the importance of the self-heating for the macroscopic-quantum-tunnel ling experiments involving IJJ's.

We describe experiments which are only possible through an ultimate control of sample shape and dimensions down to nanometer scale whereby transport measurements can be done in various restricted geometries. We use photolithography patterning together with a flip-chip technique to isolate very thin (d similar to 100 nm) pieces of BiSr2CaCu2O8+delta (BSCCO) single crystals. Ar-ion milling allows us to further thin these crystals down to a few nanometers in a controlled way. With decreasing thickness below two to three unit cells, the superconducting transition temperature gradually decreases to zero and the in-plane resistivity increases to large values indicating the existence of a superconductor-insulator transition in these ultrathin single crystals. In a refined technique, a precise control of the etching depth from both sides of the crystal makes it possible to form stacks of intrinsic Josephson junctions (IJJs) inside the ultrathin single crystals. The stacks can be tailor-made to any microscopic height (0-9 nm < d, i.e. enclosing a specific number of IJJs (0-6). In certain geometries, by feeding current into the topmost CU2O4-layer of a mesa on the surface of a BSCCO single crystal, we measured the critical value of this current by detecting a sharp upturn or break in the current-voltage characteristics. From this, we estimate the sheet critical current density of a single CU2O4 plane to be similar to 0.3-0.7 A/cm at 4.5 K, corresponding to a bulk current density of similar to 2-5 MA/cm(2). These values are among the largest ever reported for BSCCO single crystals, thin-films and tapes.

We measured the c -axis magnetoresistance R of ≥200Aheigh mesas patterned on the surfaces of Bi:2212 single crystals. For the magnetic field H | c , we observed a large peak in R(T) at a temperature T * below the zero-field transition temperature T co : R /( T *)/ R (120) ≈ 9atμ 0 H ≈ 7 T . The full current-voltage characteristics were also measured at each H and T . The data clearly demonstrate that the c -axis transport of the Bi:2212 single crystals is similar to that of a series array of Josephson tunnel junctions.

We have investigated the acoustic attenuation X, sound velocity ν and the thermal conductivity κ. The experiments were performed both on the ceramic and crystal samples. The ultrasonic measurements show no anomaly at the temperatures close to the critical temperature Tc for the superconducting samples. The κ measurements of the Y Ba Cu O system with different oxygen content permit us to estimate the phonon-electron relaxation time to be about 10 −12 s. 1