The anomalous superconducting phase diagram of (BEDO-TTF)2ReO4•H2O.

We have measured the critical magnetic fields in the quasi 2D superconductor (BEDO-TTF)ReO4.H2O as a function of temperature with the magnetic field perpendicular and parallel to the conducting planes. A sharp positive change in the curvature, or kink, in the H-T phase line, similar to one in the phase diagram of the superconductor lambda-(BETS)(2)GaCl4, occurs in the low temperature part of the phase diagram near T/T-c = 0.4. These kinks in the H-T phase line could be the signature of a change in the superconducting order parameter. We discuss other possibilities, such as magnetic transitions, and why they are less likely to exist in these salts. To support our claims we also present recent data showing part of the H-T phase diagram of lambda-(BETS)(2)GaCl4 with the applied magnetic field parallel to the conducting planes.