Critical state in a low-dimensional metal induced by strong magnetic fields

We present the results of magnetotransport and magnetic torque measurements on the alpha-(BEDT-TTF)(2)KHg(SCN)(4) charge-transfer salt within its high -magnetic-field phase, in magnetic fields extending to 33 T and temperature s as low as 27 mK. While the experimentally determined phase diagram closel y resembles that predicted by theoretical models for charge-density waves i n strong magnetic fields, the phase that occurs at fields above similar to 23 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 resisti vity undergoes a dramatic drop below similar to3 K within this phase, falli ng in an approximately exponential fashion at low temperatures, while the m agnetic torque undergoes extensive hysteresis. This hysteresis, which occur s over a broad range of fields and gives rise to a large negative different ial susceptibility partial derivativeM/partial derivativeB on reversing the direction of sweep of the magnetic field, is strongly temperature dependen t and also has several of the physical characteristics predicted by critica l-state models normally used to describe the pinning of vortices in type II superconductors. Such a behavior appears therefore only to be explained co nsistently in terms of persistent currents within the high-magnetic-held ph ase of alpha-(BEDT-TTF)(2)KHg(SCN)(4), although the origin of these current s remains an open question.