The phase of the macroscopic electron-pair wavefunction in a superconductor can vary only by multiples of 2pi when going around a closed contour. This results in quantization of magnetic flux, one of the most striking demonstrations of quantum phase coherence in superconductors. By using superconductors with unconventional pairing symmetry, or by incorporating pi-Josephson junctions, a phase shift of pi can be introduced in such loops. Under appropriate conditions, this phase shift results in doubly degenerate time-reversed ground states, which are characterized by the spontaneous generation of half quanta of magnetic flux, with magnitude 1/2 Phi(0)(Phi(0) = h/2e = 2.07 x 10(-15) Wb) (ref. 7). Until now, it has only been possible to generate individual half flux quanta. Here we report the realization of large-scale coupled pi-loop arrays based on YBa2Cu3O7-Au-Nb Josephson contacts. Scanning SQUID (superconducting quantum interference device) microscopy has been used to study the ordering of half flux quanta in these structures. The possibility of manipulating the polarities of individual half flux quanta is also demonstrated. These pi-loop arrays are of interest as model systems for studying magnetic phenomena--including frustration effects--in Ising antiferromagnets. Furthermore, studies of coupled pi-loops can be useful for designing quantum computers based on flux-qubits with viable quantum error correction capabilities.

A thin interlayer is incorporated in ramp-type Josephson junctions to obtain an increased transparency. The interlayer restores the surface damaged by ion milling and has the advantage of an all in situ barrier deposition between two superconductors, leading to clean and well-defined interfaces. The method has been applied to Josephson junctions between high (YBa2Cu3O7−δ) and low temperature (Nb) superconductors, separated by a Au barrier. Transmission electron microscopy images of these junctions reveal crystalline YBa2Cu3O7−δ up to the interface with the Au barrier. The junctions have improved critical current density values exceeding 20 kA/cm2, normal state resistances of 3×10−8 Ω cm2 and IcRn products of 0.7 mV at 4.2 K. Furthermore, the junction properties can be controlled by varying the Au barrier thickness.

Superconducting quantum interference devices (SQUIDs) have been realized with a built-in π∕2 phase shift of the superconducting wave function. Its design is inspired by the phase-shifter concept, first proposed by Ioffe et al. [Nature 398, 679 (1999)] as the key element in a solid-state qubit configuration. It exploits the dx2−y2 order parameter symmetry in the high-Tc cuprates in multilayer structures based on YBa2Cu3O7-Nb ramp-type Josephson contacts. By reversing the sign of the persistent circulating current in the phase-shifting element, the π∕2 SQUID can be controllably switched between two energetically degenerate ground states, with complementary magnetic flux dependencies.