London penetration depth

London penetration depth is a measure of how deeply magnetic fields can enter a superconductor, a special material that conducts electricity perfectly without resistance. When magnetic fields are applied to a superconductor, they don't penetrate freely; instead, they decay exponentially inside the material over a tiny distance called the London penetration depth. This depth depends on the properties of the superconductor and determines how it interacts with magnetic fields. Think of it as the "reach" of the magnetic field into the superconductor, usually only a few nanometers to micrometers thick, which is essential for understanding and designing superconducting applications.