Observation of Microwave-Induced Zero-Conductance State in Corbino Rings of a Two-Dimensional Electron System

Using Corbino samples we have observed oscillatory dc conductance in a high-mobility two-dimensional electron system when it is subjected to crossed microwave and magnetic fields. At the strongest of the oscillation minima the conductance is found to be vanishingly small, indicating a macroscopic insulating state associated with this minimum. With increasing voltage bias, a crossover from Ohmic to electron-heating regime is observed.

Figure 1

(a) Magnetoconductance of a Corbino sample (without MW irradiation) is shown to exhibit sharp SdH oscillations at low magnetic field and vanishing IQHE minima at high magnetic field. The inset depicts the geometry of the sample and the measurement circuit. (b) The conductance oscillations observed in the Corbino sample with microwave irradiation. A vanishing-conductance state at the first minimum is observed. A trace without MW irradiation (dotted line) is also presented for comparison.

Figure 2

The conductance at the center of the first minimum for $f=57\mathrm{G}\mathrm{H}\mathrm{z}$ with $P\approx 10\mu \mathrm{W}$ (left panel) and $P\approx 100\mu \mathrm{W}$ (right panel), plotted against $1/T$. The insets show the conductance traces around the minimum at selected temperatures. Solid lines represent fits to $G_{xx}(T)\propto \exp (-T_0/T)$.

Figure 3

The measured $g_{xx}$ (normalized conductance, as defined in the text) of a Corbino sample is shown together with a $g_{xx}^{\prime }$ (dotted line) inverted via Eq. (1) from $R_{xx}$ measured on a square sample. An excellent agreement between the measured $g_{xx}$ and $g_{xx}^{\prime }$ is found.

Figure 4

Current, $I$ (left axis) and the deduced conductance, $G$ (right axis) at the first minimum as a function of applied bias voltage $V$. Microwave power is approximately $10\mu \mathrm{W}$. Top axis shows an estimate for maximum (close to the inner perimeter of the Corbino sample) electric field, $E_x$. The inset shows conductance traces at selected bias voltages.

Figure 5

The conductance oscillations of the Corbino sample for selected MW frequencies with roughly the same $P\approx 10\mu \mathrm{W}$ and $T\approx 0.65\mathrm{K}$ (for clarity, traces are vertically shifted in steps of 0.25 mS), plotted against $\epsilon =\omega /{\omega }_c$ with $m^{*}=0.068m_e$. For all frequencies, strong oscillations and the vanishing-conductance state are observed.