Janice C. Wynn

Scanning superconducting quantum interference device susceptometry

We report a scanning superconducting quantum interference device (SQUID) microsusceptometer with a spatial resolution of 8 μm, tested by measuring the susceptibility of individual 3 μm 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 1×105 μB/Hz while scanning.

Manipulation of single vortices in YBa2Cu3O6.354 with a locally applied magnetic field

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 (SQUID). 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 vortex–antivortex pairs. We calculate the force applied on a rigid vortex and find that ∼0.5 pN is necessary to move vortices in underdoped single crystals of YBa2Cu3O6.354 with Tc∼ 12 K.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 (SQUID). 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 vortex–antivortex pairs. We calculate the force applied on a rigid vortex and find that ∼0.5 pN is necessary to move vortices in underdoped single crystals of YBa2Cu3O6.354 with Tc∼ 12 K.

A limit on spin-charge separation in high-Tc superconductors from the absence of a vortex-memory effect

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.

Preparation and characterization of homogeneous YBCO single crystals with doping level near the SC-AFM boundary

Abstract 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.

Vortex–antivortex annihilation in underdoped YBa2Cu3O6.354

Abstract Locally applied magnetic fields can be used to create vortex–antivortex pairs in superconducting films and thin crystals. These pairs typically annihilate on some timescale which depends on temperature. We use a 21 μm diameter field coil integrated onto a scanning superconducting quantum interference device to create and observe vortex–antivortex pairs. We present measurements of the distribution of annihilation times as a function of temperature, which should allow us to determine the pinning forces for vortices in these highly underdoped samples of YBa 2 Cu 3 O 6.354 .