Magnetotransport studies of the insulating phase around =1/5 Landau-level filling

This paper presents a sequence of studies on electronic transport in high-mobility, two-dimensional electron systems in GaAs/AlxGa1-xAs heterostructures in high magnetic fields at low Landau-level filling factors where the formation of an electron solid has been postulated. It is an extension of our earlier work, summarizes our findings, and compares these findings with the results of other investigations. We observe a thermally activated insulating phase that surrounds the =1/5 quantum liquid. This reentrant behavior is now documented for several samples of varying electron density and mobility. This broad universality of the phenomenon suggests an intrinsic mechanism as the cause for the transport features such as electron-solid formation, rather than individual carrier freeze out. Current-voltage measurements in the insulating phase reveal several anomalies. In low-voltage measurements we find nonlinearities similar to earlier results. An analysis of these data, which have been taken as convincing evidence for electron-solid formation, reveals some inconsistencies in the adopted model of charge-density depinning. In high-voltage measurements we observe the strong threshold behavior that also has been attributed, by yet more reports, to depinning of an electron solid. In our analysis we find a strong resemblance of this high-voltage breakdown to the breakdown behavior in the integer quantum Hall regime caused by a temperature runaway. Such a breakdown does not require a many-particle explanation. On the basis of our results, their analysis, and an assessment of the data in the literature, we believe that at the present time, in spite of some circumstantial evidence, there does not exist an experimental study that can positively identify the sought-after electron solid.