It has been recently found that a few galaxy clusters host diffuse radio halo emission with very steep synchrotron spectra ( \alpha > 1.6 ) , which may be classified as Ultra Steep Spectrum Radio Halos ( USSRHs ) .
USSRHs are expected in the turbulence re-acceleration model for the origin of cluster radio halos , and are best discovered and studied at low frequencies .
We performed GMRT follow up observations of three galaxy clusters at 150 MHz , selected from the GMRT radio halo survey , which are known to host an USSRH or candidate very steep spectrum diffuse emission .
This project is aimed to characterize the low frequency spectrum of USSRHs for a detailed study of their origin and connection with cluster mergers .
We present preliminary results at 150 MHz of the cluster A 697 .