High-temperature superconductors — materials that conduct electricity without resistance when cooled to liquid-nitrogen temperature — find use in RF filters for mobile-telephone networks, MRI machines, and particle accelerators. One problem with the materials is how they respond to magnetic fields, as from electric motors. The fields create swirling tubes of electrical current (vortices) within the superconductor that dissipate energy and limit the amount of current it can carry without resistance.
Now, researchers at Argonne National Laboratory have used low-temperature scanningtunneling microscopy (STM) to image in great detail the interaction of magnetic vortices with nanoscale, engineered defects in a superconductor. Understanding this interaction could help scientists reduce the vortices' current-sapping effects and make possible superconducting, quantum-logic devices that use vortex manipulation.
STM brings an extremely sharp conducting probe to within a few atom diameters of a surface. This causes electrons to jump the gap or "tunnel" between the sample material and the stylus, producing an electrical signal. The stylus slowly scans the surface, raising and lowering to maintain a 0.01-° gap. Recording the stylus' vertical movement reveals the surface structure, atom by atom.
It turns out that vortices can be locked into position by "pinning" them to defects, each of which can hold up to six vortices. The vortices induced by a weak magnetic field attach themselves to the defects, as expected. Raising the magnetic field, however, makes vortices that can't find a home in a defect appear alongside in orderly lines or "chains."
Further increases to magnetic-field strength cause the vortex chains to become denser and split into two parallel chains. A peak in superconductor current density accompanies the transition, which is a measure of how well the material carries large electric currents.
The experiments are said to be the first to use STM to directly observe this phase transition. The method of sample preparation is also a first. Previously, heavy-particle accelerators such as Argonne's Atlas made superconductors with varying defect properties. In contrast, lithography introduces defects in the present samples so researchers have full control over the geometry and internal structure. Funding for the research comes from the DoE.