How can a cold superconductor support a magnet above it?

A magnet can be supported above a suitably cold superconducting material without a visible solid support. This remarkable magnetic response accompanies a distinct material state. Below a material-dependent critical temperature, a superconductor can carry direct current without electrical resistance under suitable conditions. It also exhibits the Meissner effect: sufficiently weak applied magnetic fields are expelled from most of its interior as it enters the superconducting state.

Screening currents near the surface produce a magnetic response that opposes the applied field within the bulk. A thin surface region still allows field penetration, and the surrounding space still contains magnetic field. The right-hand sketch shows this bulk exclusion schematically. The left-hand sketch represents possible upward magnetic support, not a calculation of a stable levitation height.

Complete bulk exclusion is not the only superconducting magnetic state. Between appropriate field thresholds, a type-II superconductor can remain superconducting while admitting magnetic flux in quantized vortices. Defects can hinder vortex motion, a process called flux pinning. Such pinning can help retain a magnet’s relative position and is distinct from perfectly excluding all flux.

Temperature, magnetic field, and current each have limits beyond which the superconducting response changes or is lost. Material type, geometry, cooling history, and pinning also affect magnetic support and its stability. The three illustrated regimes separate normal penetration, weak-field Meissner screening, and a pinned mixed state; they are not a claim that every cold material or every superconducting arrangement will levitate a magnet.