Metamorphosis of the Quantum Hall Ferromagnet at $\nu =2/5$

We report on the dramatic evolution of the quantum Hall ferromagnet in the fractional quantum Hall regime at $\nu =2/5$ filling. A large enhancement in the characteristic time scale gives rise to a dynamical transition into a novel quantized Hall state. The observed Hall state is determined to be a zero-temperature phase distinct from the spin-polarized and spin-unpolarized $\nu =2/5$ fractional quantum Hall states. It is characterized by a strong temperature dependence and puzzling correlation between temperature and time.

Figure 1

History dependent transport of the quantum Hall ferromagnet at $\nu =2/5$. The data were obtained under 13.5 kbar of pressure. (a) Hysteresis in magnetotransport associated with the quantum Hall ferromagnet at $\nu =2/5$. (b) Comparison of the magnetotransport before and after annealing of $1.5\times {10}^5\mathrm{s}$. The arrows indicate the changes in magnetoresistance due to annealing. Only the down field sweeps are shown for clarity. (c) Time dependence of magnetoresistance at $\nu =2/5$ from annealing at temperature of 30 mK.

Figure 2

Magnetoresistance at $\nu =2/5$ as a function of temperature and time. (a) Time dependence of magnetoresistance for annealing at different temperatures. From top to bottom, the traces correspond to 50, 47, 43, 40, 35, 34, and 30 mK. (b) Scaling analysis of the change in magnetoresistance above $2\times {10}^4\mathrm{s}$ from (a). The change in magnetoresistance, $\Delta R_{xx}$, for different temperatures can be collapsed onto a universal curve of the form $b(T)\Delta R_{xx}\sim (T_0-T)/t$. $b(T)$ is the size of $\Delta R_{xx}$ and $T_0$ is the characteristic temperature of $\sim 50\mathrm{m}\mathrm{K}$. (c) Magnetoresistance in the lowest Landau level obtained after a different duration of annealing. From top to bottom, the annealing duration corresponds to 0, 36 700, 65 700, 94 300, 132 700, and 162 900 s. (d) Magnetoresistance obtained after $\sim 1.5\times {10}^5\mathrm{s}$ for temperatures of 50, 47, 43, 40, 34, 30 mK.

Figure 3

Angular dependence of the $\nu =2/5$ fractional quantum Hall state at a temperature of 40 mK. The sample was annealed for $\sim 48\mathrm{h}$ prior to the field sweeps. The resistance minimum at $\nu =2/5$ rises slightly with increasing in-plane magnetic field.