A new quantum fluid at high magnetic fields in the marginal charge-density-wave system $\alpha$-(BEDT-TTF)$_2M$Hg(SCN)$_4$ (where $M=$~K and Rb)

TL;DR

This study uncovers a novel quantum fluid in high magnetic fields within a charge-density-wave system, revealing two distinct screening currents and proposing a model that reconciles previous conflicting experimental interpretations.

Contribution

It introduces a new quantum fluid model involving coexisting CDW and Fermi-surface pockets to explain unique screening currents in high-field phases.

Findings

Identification of two types of screening currents in high-field CDW phases

Long decay time of free surface currents compared to resistivity expectations

A proposed quantum fluid model that explains previous experimental contradictions

Abstract

Single crystals of the organic charge-transfer salts $\alpha$-(BEDT-TTF)$_2M$Hg(SCN)$_4$ have been studied using Hall-potential measurements ($M=$K) and magnetization experiments ($M$ = K, Rb). The data show that two types of screening currents occur within the high-field, low-temperature CDW$_x$ phases of these salts in response to time-dependent magnetic fields. The first, which gives rise to the induced Hall potential, is a free current (${\bf j}_{\rm free}$), present at the surface of the sample. The time constant for the decay of these currents is much longer than that expected from the sample resistivity. The second component of the current appears to be magnetic (${\bf j}_{\rm mag}$), in that it is a microscopic, quasi-orbital effect; it is evenly distributed within the bulk of the sample upon saturation. To explain these data, we propose a simple model invoking a new type of quantum fluid comprising a CDW coexisting with a two-dimensional Fermi-surface pocket which describes the two types of current. The model and data are able to account for the body of previous experimental data which had generated apparently contradictory interpretations in terms of the quantum Hall effect or superconductivity.