Oxygen drips upwards from superconductors

Abstract

In a famous experiment, a magnet is levitated above a superconductor that has been cooled to 77 K in liquid nitrogen. Here we show that in this set-up, liquid oxygen drops form on the superconductor, drip upwards, hit the magnet and then boil.

Main

In this arrangement, oxygen (which boils at 92 K) condenses out of the air into the liquid nitrogen. Liquid oxygen is sufficiently paramagnetic that it can be pulled into the high-field region between the magnet and the superconductor to form a bridge (Fig. 1).

Figure 1: A bridge of liquid oxygen between a levitating neodymium–iron–boron magnet and a YBa₂Cu₃O₇ superconductor.

The oxygen drop develops on the surface of the superconductor, drips upwards and boils on the magnet; as the old drop boils, a new drop forms on the superconductor. Scale bar, 10 mm.

When a strong neodymium–iron–boron magnet is levitated above a YBa₂Cu₃O₇ superconductor, the bulk and surface-screening supercurrents provide both buoyancy and a restoring force on the magnet. The bulk supercurrents are produced by the penetration of the magnetic field into the superconductor in the form of quantized flux lines that are pinned by defects and impurities in the material.

Large drops of liquid oxygen can bridge the gap between the magnet and the superconductor. Small drops can instigate a continuous process in which a new drop begins to form on the superconductor after an old one has been pulled across the gap onto the magnet and boiled away.

An advantageous feature of this demonstration is that it does not require the handling of liquid oxygen. For further details and a demonstration of the superconductivity, magnetism and thermodynamics in action, see http://www.dur.ac.uk/d.p.hampshire/drop.html.