Observation of Fractional Microwave-Induced Resistance Oscillations using Co-Planar Waveguide on High-Mobility 2DES

The microwave-induced resistance oscillations (MIRO) are commonly observed in high-mobility GaAs 2D electron systems (2DES) irradiated by microwaves. Usually this is accomplished using an antenna or waveguide, where the electromagnetic components (E$_{\omega }$ and H$_{\omega })$ coincide with the 2DES plane. We explore MIRO in a co-planar waveguide (CPW) geometry, in which E$_{\omega }$ is the dominant excitation component in the 2DES plane. Our samples are Hall bars of high-mobility, $\mu $= (6 - 12) $\times $10$^{6}$ cm$^{2}$/Vs, GaAs/Al$_{x}$Ga$_{1-x}$As quantum wells with electron densities ranging from 3 to 5 $\times $10$^{11}$cm$^{-2}$. Microwaves from a tunable source (2 - 40 GHz) were fed in, via a semi-rigid coax cable, to an impedance-matched CPW across the length of the Hall bar, and brought out via a similar semi-rigid coax to a power sensor. Using this CPW geometry, we are able to simultaneously measure the photoconductivity and the microwave transmission across the sample. In a temperature range of 2.0 K - 5.0 K, we observed fractional MIRO associated with $\varepsilon $ =1/2, 1/3, 1/4, and 1/5, where $\varepsilon =\omega $/$\omega _{c}$, and $\omega _{c}$ is the cyclotron frequency. Experimental data as well as a brief discussion will be presented. The work at Rice was funded by NSF DMR-0706634.