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 analyzed the transport of charge carriers across PrBa2Cu3−xGaxO7−δ (PBCGO) barriers as a function of barrier thickness, Ga-doping level, temperature, and bias voltage. It was found that by Ga doping, the normal state resistance (Rn) of the junctions was systematically increased, while the critical current (Ic) remained constant. We argue that pair transport takes place by direct tunneling, whereas the quasiparticles have access to channels formed by one or more localized states inside the barrier. By Ga doping the IcRn products were increased, up to 8 mV at 4.2 K for junctions with 8 nm thick PrBa2Cu2.6Ga0.4O7−δ barriers.

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No Mixing of Superconductivity and Antiferromagnetism in a High-TemperatureSuperconductor. -One-unit-cell-thick antiferromagnetic La 2 CuO 4 barrier layers remain highly insulating and completely block a supercurrent, indicating that the antiferromagnetic and superconducting phases do not mix. On the contrary, one-unit-cell-thick layers of La1.85Sr0.15CuO4 remain superconducting. The two observations are in conflict with most proposed models of high-temperature superconductivity, with a few notable exceptions that include postulated spin-charge separation. -(BOZOVIC*, I.; LOGVENOV, G.; VERHOEVEN, M. A. J.; CAPUTO, P.; GOLDOBIN, E.; GEBALLE, T. H.; Nature (London, UK) 422 (2003) 6934, 873-875;