We report a scanning superconducting quantum interference device ͑SQUID͒ microsusceptometer with a spatial resolution of 8 m, tested by measuring the susceptibility of individual 3 m diam tin disks. Images of the disks agree well with numerical modeling based on the known geometry of the SQUID microsusceptometers. The low-field spin sensitivity between 1.5 and 6 K is 1 ϫ10 5 B /ͱHz while scanning.

We demonstrate the controlled, reversible manipulation of individual vortices in a superconductor with a locally applied magnetic field. The local field is supplied by a field coil on a superconducting quantum interference device ͑SQUID͒. The SQUID is used to image the vortices before and after moving. This device can be used both to push individual vortices and to create individual vortexantivortex pairs. We calculate the force applied on a rigid vortex and find that ϳ0.5 pN is necessary to move vortices in underdoped single crystals of YBa 2 Cu 3 O 6.354 with T c ϳ 12 K.

There is a long-standing debate about whether spin-charge separation is the root cause of the peculiar normal-state properties and high superconducting transition temperatures of the high-Tc materials. In the proposed state of matter, the elementary excitations are not electron-like, as in conventional metals, but rather the electron 'fractionalizes' to give excitations that are chargeless spin-1/2 fermions (spinons) and charge +e bosons (chargons). Although spin-charge separation has been well established in one dimension, the theoretical situation for two dimensions is controversial and experimental evidence for it in the high-Tc materials is indirect. A model with sharp experimental tests for a particular type of separation in two dimensions has recently been proposed. Here we report the results of those experimental tests, placing a conservative upper limit of 190 K on the energy of the proposed topological defects known as visons. There is still debate about the extent to which this experiment can settle the issue of spin-charge separation in the high-Tc copper oxides, because some forms of the separation are able to avoid the need for visons. But at least one class of theories that all predict a vortex-memory effect now are unlikely models for the copper oxides.

Magnetic flux in superconductors is usually quantized in units of h/2e. Here we report scanning SQUID and scanning Hall probe studies of single fluxoids in high purity YBa2Cu3O6.35 crystals (T(c) less, similar 13 K), extending flux quantization studies to a region of the cuprate phase diagram where the superfluid density is sufficiently low that novel behavior has been predicted. Some scenarios in which superconductivity results from spin-charge separation predict h/e fluxoids in materials with low superfluid density. Our observations of only h/2e fluxoids set limits on these theories.

High-purity and homogeneous YBa 2 Cu 3 O y single crystals with carrier doping level near the AFM-SC boundary have been obtained in the oxygen content range between y = 6.340 and 6.370. The crystals are ortho-II phase at room temperature and undergo the orthorhombic to tetragonal transition at about 140°C. They show sharp superconducting transitions, with T c between 4 and 20 K. T c changes by 0.8 K when the oxygen content y is changed by 0.001, and is also sensitive to annealing conditions near room temperature, due to the dependence of doping on oxygen ordering correlation lengths. Crystals with oxygen content y lower than 6.345 are non-superconducting.

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