Chang C. Tsuei

Ordering and manipulation of the magnetic moments in large-scale superconducting pi-loop arrays

The phase of the macroscopic electron-pair wavefunction in a superconductor can vary only by multiples of 2π 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 π-Josephson junctions, a phase shift of π 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 Φ0(Φ0 = h/2e = 2.07 × 10-15u2009Wb) (ref. 7). Until now, it has only been possible to generate individual half flux quanta. Here we report the realization of large-scale coupled π-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 π-loop arrays are of interest as model systems for studying magnetic phenomena—including frustration effects—in Ising antiferromagnets. Furthermore, studies of coupled π-loops can be useful for designing quantum computers based on flux-qubits with viable quantum error correction capabilities.

Limits of the critical current density of polycrystalline high-temperature superconductors based on the current transport properties of single grain boundaries

A semiquantitative technique for estimating the limits of the critical current density in polycrystalline high-temperature superconductors is presented. This technique is based on the transport properties of single grain boundaries. Various factors essential in deducing the critical current density of polycrystalline high-Tc materials from the characteristics of single grain boundaries are discussed. The results for tectured, polycrystalline YBa2Cu3O7 fibers are reported, and the influence of the microstructure on their critical current density is discussed.

Design and applications of a scanning SQUID microscope

The scanning SQUID (Superconducting Quantum Interference Device) microscope is an extremely sensitive instrument for imaging local magnetic fields. The authors describe one such instrument which combines a novel pivoting lever mechanism for coarse-scale imaging with a piezoelectric tube scanner for fine-scale scans. The magnetic field sensor is an integrated miniature SQUID magnetometer. This instrument has a demonstrated magnetic field sensitivity of <10{sup {minus}6} gauss/{radical}Hz at a spatial resolution of {approximately}10 {micro}m. The design and operation of this scanning SQUID microscope are described, and several illustrations of the capabilities of this technique are presented. The absolute calibration of this instrument with an ideal point source, a single vortex trapped in a superconducting film, is shown. The use of this instrument for the first observation of half-integer flux quanta, in tricrystal thin-film rings of YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}}, is described. The half-integer flux quantum effect is a general test of the symmetry of the superconducting order parameter. One such test rules out symmetry-independent mechanisms for the half-integer flux quantum effect, and proves that the order parameter in YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}} has lobes and nodes consistent with d-wave symmetry.

Current transport across grain boundary networks in high-Tc superconductors

Electric transport measurements across single grain boundaries in YBa2Cu3O7 films are summarized. These experiments demonstrate that YBa2Cu3O7 grain boundaries behave like SNS-type Josephson junctions. The implications of this weak link behavior for the critical currents of polycrystalline high-Tc superconductors are discussed in conjunction with spatially resolved measurements of the current transport in polycrystalline YBa2Cu3O7.

Spatial variation of the critical current density in Y1Ba2Cu3O7 films

Abstract The critical current density of single-, bi-, and polycrystalline Y 1 Ba 2 Cu 3 O 7 films has been imaged by low Temperature Scanning Electron Microscopy (LTSEM) with a spatial resolution of 1/μ. Our experiments directly show that grain boundaries limit the critical current density in polycrystalline films. Substantial variatoons of the critical current density have also been found in high quality, c-axis oriented epitaxial films, which have critical current densities above 1×10 6 A/cm 2 at 77K.