Ultralow-temperature behavior of theν=52fractional quantum hall effect

The newly discovered even-denominator fractional quantum Hall effect at filling factor $\nu =\frac{5}{2}$ has been studied at ultralow temperatures. While ${\rho }_{\mathrm{xx}}$ is not found to vanish in the temperature range studied, the minimum in ${\rho }_{\mathrm{xx}}$ is seen to drop at the lowest temperatures. While this drop is insufficient to determine the energy gap, $\Delta$, it may be combined with the temperature dependence of the background resistivity to give a value of $\Delta \sim 26$ mK. Because of the high electron-phonon relaxation rate, ${\tau }_{\epsilon }^{-1\mathrm{}}=(2.9\mathrm{}\times {10}^3)T^3$ ${\sec }^{-1\mathrm{}}$ ${\mathrm{K}}^{-3\mathrm{}}$, a minimum electron temperature of 9 mK could be obtained with a residual heat leak of 8×${10}^{-14\mathrm{}}$ W. It appears likely that ${\rho }_{\mathrm{xx}}$ approaches zero as $T\to 0$.