Microwave resonance of the reentrant insulating quantum Hall phases in the first excited Landau level

We present measurements of the real diagonal microwave conductivity of the reentrant insulating quantum Hall phases in the first excited Landau level at temperatures below 50 mK. A resonance is detected around filling factor nu=2.58 and weaker frequency dependence is seen at nu=2.42 and 2.28. These measurements are consistent with the formation of a bubble phase crystal centered around these v at very low temperatures.