No abstract

In a dilute, low-disorder, two-dimensional hole system at the GaAs/AlGaAs heterointerface, we observe a reentrant insulating behavior around the v= y fractional quantum Hall liquid at B -5 T, strikingly similar to recent observations in low-disorder 2D electron systems near v-j. We interpret this behavior as manifesting a weakly pinned hole Wigner crystal around v= j, and suggest that its observation at such large v is a result of Landau-level mixing which, in the case of much heavier holes, significantly modifies the ground-state energies of the fractional quantum Hall and Wigner crystal states of the system. PACS numbers: 73.40. Kp, 7I.55.Jv, 72.20.Ht One of the most exciting aspects of the physics of twodimensional electron systems (2DES) concerns the termination of the fractional quantum Hall (FQH) effect [1] at low Landau-level fillings, v. It is intuitively clear that strong disorder will terminate the FQH effect by magnetic freeze-out. However, in a pure system, transition to a Wigner crystal (WC) is expected to occur at sufficiently low v (=1/6.5) and low temperature [2][3][4]. Thanks to the availability of very low-disorder dilute 2DES, research on this subject has intensified in the last three years and has been fueled by new experimental results as well as controversy [5][6][7][8][9][10][11][12][13][14][15]. Magnetotransport experiments on 2DES in GaAs/AlGaAs heterostructures, which have been the subject of most of these studies [7-10,12-15], have established that at v=j the ground state is FQH liquid. This is evidenced by the vanishing of the diagonal resistance R xx at v = j and the quantization of the Hall resistance R xv at 5h/e 2 . At v slightly above and below y, however, R xx diverges as T-* 0, indicating an insulating ground state with a strongly nonlinear I-V characteristic. Although there is still no direct and conclusive evidence for the transition to a WC, the results have been generally interpreted as consistent with the formation of a reentrant, weakly pinned electron WC near In this Letter we report magnetotransport data for a low-disorder 2D hole system (2DHS) at the GaAs/AlGa-As heterointerface. The areal density in this sample, /? = 4xl0 10 cm -2 , is comparable to the density of some of the 2DES in which the formation of an electron WC near v-j has been widely discussed [9,[13][14][15]. The magnetotransport data for this 2DHS are strikingly similar to those for the 2DES, with the notable exception that the reentrant insulating phase is observed around v^j rather than v***j. The observation of such similar behavior at a markedly higher filling factor is most surprising and unexpected. We attribute this difference to the profound effect of Landau-level (LL) mixing on the ground-state energies of the FQH liquids and the WC [16], and interpret the results as further evidence that the reentrant insulating phase is a weakly pinned WC. Such LL mixing is expected to be much more substantial for holes, whose heavier mass reduces the LL separation by a factor of 5 compared to the separation fo...

No abstract