The Magnetic Phase Diagram of High Tc Superconductors

Among the many surprises in the held of high temperature superconductivity, new features discovered in the magnetic phase diagram are among the most exciting and controversial, generating many new physical concepts and impacting practical applications. This brief review complements several other recent reviews and refers mostly to the bulk crystal (not ceramic) Y1Ba2Cu3O7 and Bi2Sr2Ca1Cu2Ox materials, from now on denoted YBaCuO and BiSrCaCuO.The magnetic phase diagram of a conventional type II superconductor as a function of magnetic held H and temperature T is well known and understood in the mean-field Ginzburg-Landau and London theories. As shown in Figure 1a, a Meissner phase characterized by complete flux exclusion appears at low fields, delineated by a mean-field phase transition line called the lower critical field Hc1(T), which increases linearly with decreasing temperature below Tc and then saturates at low temperature. A second mean-field phase transition line, called the upper critical field Hc2(T), delineates the transition between the normal and superconducting states and shows a T-dependence similar to Hc1(T). In a strongly type II superconductor (in which the penetration depth λ is much larger than the coherence length ξ), the large region intervening between Hc1 and Hc2 is called the Abrikosov mixed phase. Here magnetic field penetrates the superconductor in the form of tubes of magnetic field called flux lines or vortices. In an ideal isotropic superconductor, these vortices self-organize into a hexagonal array. Defects disturb the hexagonal long-range order, causing the array to break up into a kind of glassy state with more or less short-range order.