Superconductors in strong static or quasi-static electric fields

The electric field induced superconducting ball formation has been found to be general for all powdered superconductors, including low temperature superconductors (LTSC), both type I and type II, and high temperature superconductors (HTSC), in either a static or a quasi-static electric field. Both LTSC and HTSC in either static or low frequency ac fields form balls consisting of up to million particles but only for fields E between two critical fields Ec1, and Ec2. The balls formed from LTSC powders are generally weaker than the HTSC balls and easy to break. In contrast to the behavior in a static electric field. Both the LTSC and the HTSC particles first form chains along the field direction in a low frequency ac field if the electric field is below the critical value Ec1. As soon as the electric field exceeds Ec1, the chains are broken and the particles form balls. For both static and low frequency cases the ball size decreases with further increase of the electric field. Ec1 as a function of frequency, first drops from the value for the static field and reaches a minimum at a very low frequency and then increases monotonically with the frequency. The ball formation has also been observed in a strong static or quasi-static electric field with MgB2 powder, which has Tc around 39K. The effect of temperature and magnetic field on the ball formation shows surprising features. The ball size of MgB2 is proportional to 1-TTc from 39 K down to about 20 K. Below 20 K the ball size becomes almost constant. If MgB2 particles are in a strong electric field and a moderate magnetic field, the electric field induced balls align in the magnetic field direction to form ball chains.; This phenomenon comes from the interaction between Cooper pairs and the applied electric field. A positive surface energy associated with the induced surface charge distribution on superconductors provides an explanation of the phenomenon. The critical fields and ball size derived from theoretical calculations seem to agree with the experimental results reasonably well.