Anomalous Insulating States in Landau Levels $N \geq 1$*

Quantum Hall measurements are performed for a rectangular two-dimensional (2D) hole system confined to a 20 nm quantum well in $\langle 100 \rangle$ GaAs. Quantum oscillations reveal a density of $4.3 \cdot 10^{10}$ cm$^{-2}$ with mobility $\mu = 1.9\cdot10^{6}$ cm$^2$/V$\cdot$s. For temperatures less than $\sim$150 mK, anomalous insulating peaks are observed near integer fillings 1,2, and 3 for which both in-phase and out-of-phase signals rise substantially to be near or well above the quantum resistance. They differ from usual re-entrant insulating phases such as that observed before $\nu = 1/3$ where the out-of-phase signal remains less than 3\% of the in-phase signal. The relationship between in-phase and out-of-phase signals of the magnetoresistances resembles that of the orthogonal components $\rho_{xx}$ and $\rho_{yy}$ previously observed for collective anisotropic states in $\langle 100 \rangle$ and $\langle 311 \rangle$ GaAs 2D systems. These non-monotonic phase shifts will be discussed in relation to possible stripe phases.