We study the sensitivity of $T_c$ and the pseudogap onset temperature, $T^{*},$ to low fields, H, using a BCS-based approach extended to arbitrary coupling. We find that $T^{*}$ and $T_c,$ which are of the same superconducting origin, have very different H dependences. This is due to the pseudogap, ${\Delta }_{\mathrm{pg}},$ which is present at the latter, but not former temperature. Our results for the coherence length $\xi$ fit well with existing experiments. We predict that very near the insulator $\xi$ will rapidly increase.

• Received 29 March 2001

DOI:https://doi.org/10.1103/PhysRevB.64.140505