Cyclotron mass of two-dimensional holes in (100) oriented GaAs/AlGaAs heterostructures

We have measured the microwave cyclotron resonance of low-density two-dimensional hole gases in (100) oriented heterostructures, including single interface heterostructures (SIHs) and 20nm-wide quantum wells(QWs). It was found that in the case of the SIHs, the cyclotron mass is relatively insensitive to the hole density, ranging from $0.25\times10^{10}$/cm$^2$ to $13.1\times10^{10}$/cm$^2$, and has a value of approximately $0.5m_{e}$. On the other hand, for holes in the QWs, the cyclotron mass decreases from $0.48m_{e}$ at density $p=6.9\times10^{10}$/cm$^2$ to $0.29m_{e}$ at $p=0.8\times10^{10}$/cm$^2$. In this talk, the absorption spectra will be shown, and a qualitative explanation will be presented.