Phase transitions at filling ν = 5/2 in ZnO heterostructures

Half-filled Landau levels (LL) elicit competing phases which intimately depend on the nature of inter-particle interaction effects. The competition between emergent Fermi-surfaces, the fractional quantum Hall (FQH) effect and broken symmetry nematic ground states is now well documented. Here we report a complex cascade of phases which exist at filling ν = 5/2 in high mobility ZnO heterostructures and the ability to tune between them by selectively enhancing the Zeeman energy through sample rotation. We are able to induce a series of phase transitions between an incompressible FQH state, a compressible ground state, an anisotropic nematic state and an isotropic insulating state. These occur in the vicinity of the coincidence of the N = 1 and 0 LL as electrons are transferred between opposing spin branches. The even denominator FQH incompressibility only occurs when significant population of N = 1 exists, reinforcing the role of the orbital character of carriers for the development of this ground state. The anisotropic transport shows B-field sweep direction dependent hysteresis, suggesting the underlying nematic texture may be from magnetic domains of electrons of differing spin and orbital character close to the level crossing.