Using an advanced molecular beam epitaxy system, we have reproducibly synthesized atomically smooth films of high-temperature superconductors and uniform trilayer junctions with virtually perfect interfaces. We found that supercurrent runs through very thick barriers. We can rule out pinholes and microshorts; this "giant proximity effect" (GPE) is intrinsic. It defies the conventional explanation; it might originate in resonant tunneling through pair states in an almost-superconducting barrier. GPE may also be significant for superconducting electronics, since thick barriers are easier to fabricate.

There is still no universally accepted theory of high-temperature superconductivity. Most models assume that doping creates 'holes' in the valence band of an insulating, antiferromagnetic 'parent' compound, and that antiferromagnetism and high-temperature superconductivity are intimately related. If their respective energies are nearly equal, strong antiferromagnetic fluctuations (temporally and spatially restricted antiferromagnetic domains) would be expected in the superconductive phase, and superconducting fluctuations would be expected in the antiferromagnetic phase; the two states should 'mix' over an extended length scale. Here we report that one-unit-cell-thick antiferromagnetic La2CuO4 barrier layers remain highly insulating and completely block a supercurrent; the characteristic decay length is 1 A, indicating that the two phases do not mix. We likewise found that isolated one-unit-cell-thick layers of La1.85Sr0.15CuO4 remain superconducting. The latter further implies that, on doping, new electronic states are created near the middle of the bandgap. These two findings are in conflict with most proposed models, with a few notable exceptions that include postulated spin-charge separation.

We have developed an absolute magnetic field sensor using Superconducting Quantum Interference Filter (SQIF) made of high-Tc grain boundary Josephson junctions. The device shows the typical magnetic field dependent voltage response V (B), which is sharp delta-like dip in the vicinity of zero magnetic field. When the SQIF is cooled with magnetic shield, and then the shield is removed, the presence of the ambient magnetic field induces a shift of the dip position from B0 ≈ 0 to a value B ≈ B1, which is about the average value of the earth magnetic field, at our latitude. When the SQIF is cooled in the ambient field without shielding, the dip is first found at B ≈ B1, and the further shielding of the SQIF results in a shift of the dip towards B0 ≈ 0. The low hysteresis observed in the sequence of experiments (less than 5% of B1) makes SQIFs suitable for high precision measurements of the absolute magnetic field. The experimental results are discussed in view of potential applications of high-Tc SQIFs in magnetometry.

The surface roughness of sputtered Nb films was determined with high precision using x-ray specular reflectivity measurements in the 10 keV range. The roughness of Nb films increased from 0.9 nm for a 70-nm-thick film to 1.8 nm for a 210-nm-thick film. The roughness of the Nb surface strongly influences the tunnel barrier formation and the electrical properties of that barrier. For stacked tunnel junctions each thermal Al oxidation has to be adjusted as a function of the underlying Nb film thickness. Alternatively, one can use an Al or Al/AlOx underlayer in order to obtain stacks with small spread in critical currents.

Electromagnetic waves which propagate along a Josephson junction ladder are shown to manifest themselves by resonant steps in the current-voltage characteristics. We report on experimental observation of resonances in ladders of different geometries. The step voltages are mapped on the wave dispersion relation which we derive analytically for the general case of a ladder of arbitrary anisotropy. Using the developed model, current amplitudes of the resonances are also calculated and their dependence on magnetic field is found to be in good accord with experiment. ͓S0163-1829͑99͒06121-4͔