Resistance Oscillations in Two-Dimensional Electron Systems Induced by Both ac and dc Fields

We report on magnetotransport measurements in a high-mobility two-dimensional electron system subject simultaneously to ac (microwave) and dc (Hall) fields. We find that dc excitation affects microwave photoresistance in a nontrivial way. Photoresistance maxima (minima) evolve into minima (maxima) and back, reflecting strong coupling and interplay of ac- and dc-induced effects. Most of our observations can be explained in terms of indirect electron transitions using a new, combined resonant condition. Observed quenching of microwave-induced zero resistance by a dc field cannot be unambiguously linked to a domain model, at least before a systematic theory treating both excitation types within a single framework is developed.

Figure 1

Microwave magnetoresistance $r/r_0$ under dc excitations: $I=0\mu \mathrm{A}$ (black curve) in (a)–(c); $6\mu \mathrm{A}$ [light (blue)] and $9\mu \mathrm{A}$ [dark (red)] in (a); $12\mu \mathrm{A}$ [light (blue)] and $21\mu \mathrm{A}$ [dark (red) ] in (b); $33\mu \mathrm{A}$ [light (blue)] and $51\mu \mathrm{A}$ [dark (red)] in (c).

Figure 2

(a) [(b)] Microwave magnetoresistance $r/r_0$ about ${ϵ}_{1\pm }^{\mathrm{ac}}$ [${ϵ}_{2\pm }^{\mathrm{ac}}$] at $I$ from 1 [2] to $7\left[14\right]\mu \mathrm{A}$, in $1\left[2\right]\mu \mathrm{A}$ steps. (c) [(d)] CVCs at ${ϵ}_{1+}^{\mathrm{ac}}$ [${ϵ}_{2+}^{\mathrm{ac}}$] and ${ϵ}_{1-}^{\mathrm{ac}}$ [${ϵ}_{2-}^{\mathrm{ac}}$]. Dashed lines: CVC at $B=0$.

Figure 3

(a) False-color intensity plot of $r$ in the $(B,I)$ plane. White contours separate the maxima ($+$) and the minima ($-$) (see text). Crossings of vertical (${ϵ}^{\mathrm{ac}}=1,2$) and inclined (${ϵ}^{\mathrm{dc}}=1,2$) dashed lines are marked by $({ϵ}^{\mathrm{ac}},{ϵ}^{\mathrm{dc}})$. Solid line: ${ϵ}^{\mathrm{dc}}=3/4$. (b) [(c)] The lower portion of (a) in the MIRO [ZRS] regime, with $B$ changed to ${ϵ}^{\mathrm{ac}}$.

Figure 4

(a) Positions of the $r$ maxima (circles) and minima (squares) in the $(I,{ϵ}^{\mathrm{ac}})$ plane. Solid curve: ${ϵ}^{\mathrm{dc}}=1$. (b) Positions of the $r$ maxima in the $({ϵ}^{\mathrm{ac}},{ϵ}^{\mathrm{dc}})$ plane. Solid lines: ${ϵ}^{\mathrm{ac}}+{ϵ}^{\mathrm{dc}}=k$, $k=1,2,\dots$. Inset: LLs at $({ϵ}^{\mathrm{ac}},{ϵ}^{\mathrm{dc}})=(3,3)$. Horizontal, vertical, and inclined arrows mark electron transitions due to dc, ac, and ac/dc excitations, respectively.