| The formation of spin-reversed pairs of electrons at low magnetic field (B) has been suggested as a possible mechanism for the even-denominator quantization in the fractional quantum Hall (FQH) regime of the two-dimensional electron system (2DES). We examined the possibilities of the even-denominator quantization at filling factors {dollar}nu{dollar} = 1/2, 3/4, 5/8, 3/2, and 5/2, in the very-low-density regime down to the lowest ever reported density of {dollar}4.0times10sp9{dollar} cm{dollar}sp{lcub}-2{rcub}{dollar}. The previously reported quantization at {dollar}nu{dollar} = 5/2 is observed at a density as low as {dollar}1.3times10sp{lcub}11{rcub}{dollar} cm{dollar}sp{lcub}-2{rcub}{dollar}. During the course of this investigation, we also observed the development of new FQH states at {dollar}nu{dollar} = 8/11 and 8/13.; The FQH liquid series is expected to terminate at sufficiently small {dollar}nu{dollar}, and the 2DES is expected to form a Wigner crystal (WC) in the absence of disorder at low temperature (T). We have systematically studied the generic transport characteristics of the newly discovered reentrant insulating phase (IP) around {dollar}nu{dollar} = 1/5.; We present the first observation of quantized Hall plateaus {dollar}rhosb{lcub}rm xy{rcub}{dollar} = 5h/e{dollar}sp2{dollar} down to 22mK at {dollar}nu{dollar} = 1/5, and {dollar}rhosb{lcub}rm xy{rcub}{dollar} = 9h/2e{dollar}sp2{dollar} down to 40 mK at {dollar}nu{dollar} = 2/9. We demonstrate, for the first time, that the Hall resistance is normal ({dollar}rhosb{lcub}rm xy{rcub} approx{dollar} B/ne) in the reentrant IP for {dollar}nu{dollar} not much smaller than 1/5. Our findings directly imply that {dollar}sigmasb{lcub}rm xx{rcub} to 0{dollar} and {dollar}sigmasb{lcub}rm xy{rcub}{dollar} = e{dollar}sp2{dollar}/5h at {dollar}nu{dollar} = 1/5, and that {dollar}sigmasb{lcub}rm xx{rcub} to 0{dollar} and {dollar}sigmasb{lcub}rm xy{rcub} to 0{dollar} in the reentrant IP as T {dollar}to{dollar} 0. Our results rule out the possibility of single-particle magnetic freeze-out as the mechanism behind the formation of the reentrant IP for {dollar}nu{dollar} not much smaller than 1/5. The data are consistent with the possible existence of WC or Hall insulator, but rule out the possibility of Hall crystal or partial Hall crystal as the energetically favorable ground states.; We studied the generic overall features and finer details of the (B, {dollar}nu{dollar}, T) evolution of the Ohmic {dollar}rhosb{lcub}rm xx{rcub},{dollar} and examined the presence of strikingly diverse (smooth, sharp, and step) nonlinear 4-terminal I-V characteristics in the reentrant IP. The various distinct characteristic T's deduced from the Ohmic {dollar}rhosb{lcub}rm xx{rcub},{dollar} I-V, and noise measurements, as well as the implications of such findings are discussed. The systematic (B, {dollar}nu{dollar}, T) evolution of the sharp I-V characteristics, and the salient features of the closely-correlated low-frequency noise data are analyzed. The results can be interpreted in terms of a weakly-pinned 2DWC reentrant around the 1/5 FQH liquid for B {dollar}{dollar} 12.70T. At present, we cannot conclusively identify the observed transport characteristics as direct manifestation of the existence of 2DWC. The diverse generic charge transport characteristics could be attributed to the difference in the distribution of the disorder potential fluctuations for different samples with different cooldown and illumination procedures. |
