SQUID measurements of magnetic vortices in very underdoped yttrium-barium-copper-oxide

This dissertation describes the first demonstration of a scanning Superconducting QUantum Interference Device (SQUID) susceptometer---a new tool for the study of local magnetic properties of materials---and applies it to a series of studies of vortices in the cuprate superconductor YBa2Cu 3O6+x.; The susceptometer is comprised of a scanning SQUID with an 8mum pickup loop and an integrated 21mum diameter field coil for applying a local magnetic field. We test the system by measuring the susceptibility of individual 3mum diameter tin disks. The spatial resolution of the device is 8mum, set by the size of the pickup loop, and the low-field spin sensitivity between 1.5 and 6K is 1 x 105muB/ Hz while scanning.; The susceptometer is applied to the controlled, reversible manipulation of individual vortices using a locally applied magnetic field, with the SQUID used to image the vortices before and after moving. We calculate the force applied on a rigid vortex and find that ∼0.5pN is necessary to move vortices in underdoped single crystals of YBa2Cu3O6+ x with Tc ∼ 12K.; Locally applied magnetic fields from the susceptometer can create vortex-antivortex pairs which typically annihilate on some timescale depending on temperature. Observations of annihilations at different locations in a sample of highly underdoped YBa2Cu3O6+x are used to infer the qualitative nature of the local pinning landscape. We find pinning sites have a wide range of energies, with deep pinning sites distributed on a length scale of microns. The distribution of annihilation times as a function of temperature did not fit a simple picture of thermal activation, suggesting that pinning is complicated.; We performed the spin charge separation experiment proposed by Senthil and Fisher, in which a topological excitation known as a vison was predicted to cause a vortex memory effect in a ring of cuprate superconductor. We detected no signature of the proposed vison and use this result to place a conservative upper limit on the vison energy of 190K·kB. The theory predicts a vison energy on the order of 500--700K· kB for our samples. As a result, spin-charge separation theories requiring visons seem unlikely as an explanation of cuprate superconductivity.; Finally, we report measurements of single vortex dynamics in a superconducting YBa2Cu3O6+x ring. We found robust intermediate states between the expected states of n and n + 1 vortices in the ring. The temperature dependence of transitions between states was consistent with thermal activation over a barrier much lower than would be expected for a rigid vortex moving across the ring wall. We conjecture that vortices in our ring behave as stacks of pancake vortices and that intermediate states were due to split stacks in with some pancake vortices remaining inside the ring, while some escape outside.