Evidence for a nematic phase of composite fermions

Electronic nematic phases are fascinating, strongly-correlated states characterized by spontaneous rotational symmetry breaking. They exhibit close connections to various fundamental phenomena such as high-T_C superconductivity and quantum Hall effect. In the quantum Hall regime, nematic phases typically emerge in half-filled, high (N ≧ 2) Landau levels (LLs) where short-range Coulomb interaction is softened by the nodes of electron wave functions. In the lowest (N = 0) LLs, nematic phases are not expected as short-range Coulomb interactions dominate. Instead, composite fermion (CF) liquids and fractional quantum Hall states, which are well explained in the picture of weakly interacting CF quasiparticles, are favored. Here we report the observation of an unexpected nematic phase in the lowest LL at v = 5/8 in an ultrahigh-quality GaAs two-dimensional hole system, evinced by a pronounced transport anisotropy. Remarkably, v = 5/8 can be mapped to a half-filled, high (N_CF = 2) CF LL, analogous to the N = 2 electron LL. The temperature and in-plane magnetic field dependence of this nematic phase indeed resemble those observed in high LLs. Our finding signals a novel nematic phase of CFs, rather than bare electrons, driven by the significant residual long-range interaction among these emergent quasiparticles. Evidently, the large hole effective mass and the ensuing severe LL mixing, which are believed to enhance CF interaction in the lowest LL, play a crucial role for stabilizing this exotic phase.