Correlation effects of quantum rotors in Ge crystals

We address the accurate treatment of dipolar interaction effects between oxygen defects in Ge crystals on the low-temperature properties of bulk systems. On the basis of a quantum rotor model, we reveal that the interaction between adjacent oxygen defects generates nontrivial low-lying excitations that result in power-law specific heats below 0.1K. In addition, a peculiar hump is observed for the dielectric susceptibilities at approximately 1K. Further, we contend that the predicted power-law specific heats are well described by the two-level tunneling theory, which is based on the random distribution of interacting oxygen defects in Ge samples.