Hc2

Hc2, the upper critical magnetic field, is the magnetic field strength at which a superconductor ceases to remain superconducting as the field is increased at a fixed temperature below the critical temperature. In type-II superconductors, Hc2 marks the boundary between the mixed state, in which superconducting vortices penetrate the material, and the normal conducting state.

In contrast to type-I superconductors, which have a single critical field Hc, type-II materials exhibit two characteristic fields: the lower critical field Hc1, where vortices begin to enter, and the upper critical field Hc2, where superconductivity is fully destroyed. Between Hc1 and Hc2 the material remains superconducting but supports a vortex lattice, with properties that depend on temperature, crystallographic direction, and impurity content.

Hc2 depends on temperature, typical to go to zero as the temperature approaches Tc. The slope of Hc2 versus temperature near Tc is commonly used to estimate the zero-temperature value Hc2(0); in the Werthamer–Helfand–Hohenberg framework for dirty superconductors, Hc2(0) ≈ -0.69 Tc (dHc2/dT)|Tc. In anisotropic or layered superconductors, Hc2 varies with field orientation, often denoted as Hc2^ab and Hc2^c, reflecting different coherence lengths along crystal axes. The Pauli paramagnetic limit and spin-orbit scattering can further influence Hc2, especially in materials with strong spin effects.

Experimentally, Hc2 is determined from measurements of resistivity, magnetization, or specific heat as a function of field and temperature, typically using a chosen criterion to define the transition. Materials with very high Hc2 values—often exceeding tens of tesla at low temperatures—are of particular interest for high-field applications, including superconducting magnets and advanced technologies.