Experiments Map the Magnetic Phases of High‐$T_{\mathrm{c}}$ Superconductors

One of the early promises of high‐$T_c$ superconductors was their potential to superconduct at relatively high magnetic fields. As type II‐superconductors, they allow the magnetic field to penetrate in quantized flux lines while still remaining superconducting except in the cores of the flux lines. In fact, the oxide materials should have vanishing resistivity at magnetic fields much higher than those that destroy superconductivity in conventional superconductors—if you ignore thermal fluctuations. But thermal fluctuations do play a large role in the oxide materials, and experimenters soon discovered that in magnetic fields, the resistance of the new materials remains quite high down to temperatures that are a fraction of the zero‐field superconducting transition temperature $T_{\mathrm{c}}.$ At 5 tesla, for example, ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_8,$ with a $T_{\mathrm{c}}$ of 90 K, doesn’t beat out copper until the thermometer drops below about 30 K.