Negative differential conductivity induced current instability in two-dimensional electron gas system in high magnetic fields

High mobility two-dimensional electron gas (2DEG) formed in the interface of a GaAs/AlGaAs hetero-structure in high magnetic field ($B$) exhibits interring nonlinear response either under microwave radiation or to a dc electric field ($E$). It is general believed that this kind nonlinear behavior is closely related to the occurrence of negative-differential conductance (NDC) in the presence of strong $B$ and $E$. We observe a new type NDC state driven by a direct current above a threshold value ({\boldmath $I_{th}$}) applied to a 2DEG as a function of $B$ at relatively high temperatures ($T$). A current instability is observed in 2DEG system at high \textit{B}$\sim$6-8 T and at high $T$ $\sim$ 20- 30 K while the applied current is over {\boldmath $I_{th}$}. The longitudinal voltage $V_{xx}$ shows sub-linear behavior with the increase of \textbf{\textit{I}}. As the current exceed {\boldmath $I_{th}$}, $V_{xx}$ suddenly drops a $\Delta V_{xx}$ and becomes irregular associated with the appearance of hysteresis with sweeping \textbf{\textit{I}}. We find that {\boldmath $I_{th}$} increases with the increase of $B$ and of $T$; meanwhile, $\Delta V_{xx}$ is larger at higher $B$ but lower $T$. Data analysis suggest that the onset of voltage fluctuation can be described by a NDC model proposed by Kurosawa \textit{et al. } in 1976. The general behaviors of $T$ and $B$ dependence of current instability are analog to those recently reported at lower both $T$ and $B$. This consistence suggests the same genuine mechanism of NDC phenomena observed in 2DEG system.