4 it is shown that the QH state at ν = 5 / 3 is absent, while the state at ν = 1 / 3 is present. We point out that in our AlAs samples, near ϵ = 0, we see a distinct resistance minimum at ν = 5 / 3 only in highest quality samples and at very low temperatures. Notably, in an 11 nm-wide AlAs quantum well [12] at 50 mK, the resistance mini- mum at ν = 5 / 3 is almost absent when the valleys are degenerate, suggesting a very small gap. We attribute this to the lower sample quality where the 2DES suf- fers more from interface roughness scattering due to the smaller well-width. We thank the NSF for financial support, and J.K. Jain, S.L. Sondhi, S.A. Parameswaran, and D. Abanin for il- luminating discussions. We also thank N.C. Bishop, G. Jones, S. A. Maier, T. Murphy, E. Palm, J.H. Park and Y.P. Shkolnikov for assistance with the experiment. A portion of this work was performed at the National High Magnetic Field Laboratory, which is supported by NSF Cooperative Agreement No. DMR-0654118, by the State of Florida, and by the DOE. the formation of ferromagnetic domains of opposite po- larity. Charged excitations with energy gaps less than that of the 2D bulk skyrmions have been predicted at these domain walls [23]. Our results find a natural interpretation in light of the composite fermion (CF) picture [20, 22, 24, 25] where the fractional QHE of electrons is interpreted as the integer QHE of CFs. Every electronic fractional filling factor ν has an integer CF counterpart p . The fan diagram in Fig. 1(b) can in fact be applied to single-spin, two- valley CFs with the modification that E c denotes the cyclotron energy for CFs. Note that ν = 1 / 3 of electrons maps to p = 1 of CFs, consistent with the qualitative similarities seen in the gap data for ν = 1 and 1/3 (Fig. 2). In a two-spin system, at η = 0, interaction between the CFs is expected to lead to a ferromagnetic QH state and skyrmionic excitations at ν = 1 / 3 [20, 25]. This is qualitatively consistent with our observations . Note that according to the fan diagram of Fig. 1(b), the saturation gap for p = 1 is given by the CF cyclotron energy ( ≃ 0 . 1 V C for an ideal 2DES) [22, 25]. The results of our gap measurements at ν = 5 / 3 (Figs. 2(c) and 4(b)) are qualitatively similar to the ν = 1 / 3 data except that the gaps are even smaller. The slope of gap vs. η in Fig. 2(c) is also smaller than that in Fig. 2(b). Within the theoretical framework, ideally, there is no distinction between the FQH states at ν = 1 / 3 and 5/3 since they are related by particle-hole symme- try. However, in real systems this symmetry is broken by factors such as LL mixing which is particularly signif- icant in our system because of the large effective mass. We would like to add that in our AlAs 2DES, at much higher strains ( | ϵ | > ∼ 2 . 3 × 10 − 4 for Fig. 1(c)), we ob- serve a crossing of the electron valley LLs as a result of which the sample resistance near ν = 5 / 3 dramatically increases thereby destroying the FQH state [26]. We now compare our results to those of a previous study of the role of spin splitting on the ν = 1 / 3 QHE in GaAs [7]. Leadley et al. measured the ν = 1 / 3 energy gap as they tuned the g -factor (and thus η ) through zero via the application of hydrostatic pressure. The magnetore- sistance trace taken at η = 0 did not show a resistance minimum at ν = 1 / 3 but, extrapolating the gaps mea- sured at finite η , they concluded a finite gap ( ≃ 0 . 01 V C ) at η = 0. In contrast, our data shows a deep resistance minimum with a strong T -dependence at η = 0. Also, in Ref. [7], an initial slope of 3 was deduced from the plot of gap vs. η , about a factor of two larger than ours. 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