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Why do we need high magnetic fields to study high-Tc superconductors?

Magnetic fields allow to suppress superconductivity and thus to access the underlying electronic properties called the normal state. When superconductivity is suppressed, it is possible to probe the Fermi surface of the system with transport and magnetisation measurements. We are hence able to access the ground state upon which superconductivity grows. Magnetic field suppresses superconductivty by unbinding the two electrons involved in a Cooper pair. The higher the binding energy of the Cooper pair, the greater the intensity of the magnetic field required to break it apart. The binding energy of a Cooper pair is proportional to the critical temperature Tc. Hence, for low Tc superconductors (such as tin or lead), a commercially available magnet is usually more than enough to suppress superconductivity. However, in the case of high-Tc cuprates, world-class high magnetic fields are required to access the normal state. Those high magnetic fields are produced in special facilities such as the LNCMI here in Toulouse.