We report on the effects of strong electrical currents (~1 MA/cm ) applied at -300 K to YBa2Cu307 q thin-film microbridges that both do, and do not contain high-angle grain boundaries. In the grain-aligned microbridges, below a certain threshold level such currents promote a slow improvement of the normal and superconductive properties of the microbridge. This behavior is attributable to the current-driven increase in basal-plane oxygen order in a manner characteristic of dispersive transport. Above threshold, the effect of the electromigration current is to create rapidly regions of strong oxygen disorder, which increases the microbridge resistance and decreases the transition temperature and critical current. This disorder shows a tendency to segregate under electromigration bias and can be at least partially removed by reversal of the current. The threshold effect permits an estimate of the atomic force required to promote the creation of oxygen disorder in YBazCu307 z. The superconducting properties of the disordered regions have the characteristic of a composite superconductor, consisting of a network of superconducting filaments partially shunted by normal conducting paths. Comparison with the behavior of microbridges containing high-angle grain boundaries indicates that such boundaries consist of more or less random connections of these filaments.

We present the use of an alternating current (AC) signal as a means to monitor the conductance of an α-hemolysin (αHL) pore as a DNA hairpin with a polydeoxyadenosine tail is driven into and released from the pore. Specifically, a 12 base pair DNA hairpin attached to a 50-nucleotide poly-A tail (HP-A 50 ) is threaded into an αHL channel using a DC driving voltage. Once the HP-A 50 molecule is trapped within the αHL channel, the DC driving voltage is turned off and the conductance of the channel is monitored using an AC voltage. The escape time, defined as the time it takes the HP-A 50 molecule to transport out of the αHL channel, is then measured. This escape time has been monitored as a function of AC amplitude (20 to 250 mV ac ), AC frequency (60-200 kHz), DC drive voltage (0 to 100 mV dc ), and temperature (−10 to 20 °C), in order to determine their effect on the predominantly diffusive motion of the DNA through the nanopore. The applied AC voltage used to monitor the conductance of the nanopore has been found to play a significant role in the DNA/nanopore interaction. The experimental results are described by a onedimensional asymmetric periodic potential model that includes the influence of the AC voltage. An activation enthalpy barrier of 1.74 × 10 −19 J and a periodic potential asymmetry parameter of 0.575 are obtained for the diffusion at zero electrical bias of a single nucleotide through αHL.

The superconductive weak link properties of microbridges formed in c-axis normal YBa2Cu3O7−δ polycrystalline thin films containing a variable amount of large angle tilt boundaries have been studied. In the low critical current density limit these weak links have current-voltage (I-V) characteristics that are accurately modeled by the resistively shunted junction model. The I-V’s are found to accurately follow a simple scaling law with the product of the critical current and weak link resistance Rn varying linearly with the weak link conductance.

We discuss the results of a study of the effect of substrate preparation on the microstructure and superconductive properties of YBa2Cu3O7 thin films formed by laser ablation on (001) MgO substrates. Thermal annealing of the substrates is found to be highly effective in producing at fairly low growth temperatures (670 °C), epitaxial, c-axis normal films with good superconductive properties. Alternative surface treatments result in the formation of large angle tilt boundaries and inferior superconductive properties.