We present the results of magnetotransport and magnetic torque measurements on the $\alpha -(\mathrm{BEDT}-\mathrm{TTF}{)}_2\mathrm{KHg}(\mathrm{SCN}{)}_4$ charge-transfer salt within its high-magnetic-field phase, in magnetic fields extending to 33 T and temperatures as low as 27 mK. While the experimentally determined phase diagram closely resembles that predicted by theoretical models for charge-density waves in strong magnetic fields, the phase that occurs at fields above $\sim 23\hspace{0.5em}\mathrm{T},$ which is expected to be either a modulated charge-density wave phase or a charge/spin-density wave hybrid, exhibits unusual physical properties that are most atypical of a density wave ground state. Notably, the resistivity undergoes a dramatic drop below $\sim 3\hspace{0.5em}\mathrm{K}$ within this phase, falling in an approximately exponential fashion at low temperatures, while the magnetic torque undergoes extensive hysteresis. This hysteresis, which occurs over a broad range of fields and gives rise to a large negative differential susceptibility $\partial M/\partial B$ on reversing the direction of sweep of the magnetic field, is strongly temperature dependent and also has several of the physical characteristics predicted by critical-state models normally used to describe the pinning of vortices in type II superconductors. Such a behavior appears therefore only to be explained consistently in terms of persistent currents within the high-magnetic-field phase of $\alpha -(\mathrm{BEDT}-\mathrm{TTF}{)}_2\mathrm{KHg}(\mathrm{SCN}{)}_4,$ although the origin of these currents remains an open question.

• Received 22 February 2000

DOI:https://doi.org/10.1103/PhysRevB.62.14212