Insulating States in the Integer Quantum Hall Regime*

Quantum Hall measurements are performed at temperatures 20-300 mK in high quality $p$-type GaAs quantum well systems having mobility $\mu=4\times10^{6}$ cm$^2$/V$\cdot$s for density $5\times10^{10}$ cm$^{-2}$. We report a series of insulating phases appearing at or near integer filling factors $\nu \geq 1$. The DC resistance demonstrates a maximum of 25M$\Omega$, much larger than the quantum resistance $h/e^2$, with threshold transport behavior at low currents around 10 pA at low temperatures. The threshold diminishes upon heating up to 200 mK, consistent with a finite temperature melting of bubble phases or Wigner crystal. Additionally, these peaks have a complex electrical impedance for AC signals, with large phase shifts down to 1Hz. In this regime, the ac impedance of the two chiral edges show distinct correlated characteristics.