  This paper reports on the observation and analysis of magnetotransport
phenomena in the nonlinear differential resistance $r_{xx}=dV_{xx}/dI$ of
high-mobility InGaAs/InP and GaAs/AlGaAs Hall bar samples driven by direct
current, $\Idc$. Specifically, it is observed that Shubnikov -de Haas (SdH)
oscillations at large filling factors invert their phase at sufficiently large
values of $\Idc$.
  This phase inversion is explained as being due to an electron heating effect.
In the quantum Hall effect regime the $r_{xx}$ oscillations transform into
diamond-shaped patterns with different slopes corresponding to odd and even
filling factors. The diamond-shaped features at odd filling factors can be used
as a probe to determine spin energy gaps.
  A Zero Current Anomaly (ZCA) which manifests itself as a narrow dip in the
$r_{xx}(\Idc)$ characteristics at zero current, is also observed. The ZCA
effect strongly depends upon temperature, vanishing above 1 K while the
transport diamonds persist to higher temperatures. The transport diamonds and
ZCA are fully reproduced in a higher mobility GaAs/AlGaAs Hall bar structure
confirming that these phenomena reflect intrinsic properties of two-dimensional
systems.
