Reentrant Field-Induced Spin-Density-Wave Transitions

We have used magnetotransport measurements to determine the phase diagram of ${\mathrm{}\left(\mathrm{TMTSF}\right)\mathrm{}}_2$Cl${\mathrm{O}}_4$ from 0.5 to 10 K in magnetic fields to 30 T. This organic metal becomes a spin-density-wave semimetal as temperature is reduced in a magnetic field greater than a threshold field $H_c\mathrm{}(T=0\mathrm{})\mathrm{}\sim 3$ T. The critical temperature $T_c\mathrm{}\left(H\right)\mathrm{}$ increases with field in agreement with present theories, and reaches a maximum of 5.6 K at 18 T. Above this field, however, $T_c\mathrm{}\left(H\right)\mathrm{}$ decreases, extrapolating to zero temperature at 17 T. This reentrant phase diagram is not present in any theoretical model.