Anomalous spin resonance around even fillings in the quantum Hall regime

The electron spin resonance (ESR) of two-dimensional electrons with large effective mass was studied experimentally near even filling factors of the integer quantum Hall effect. Surprisingly, the electron spin resonance did not vanish at the exact even fillings even in the case of large electron densities, where the ground state of the system was previously reported to be paramagnetic. Furthermore, the ESR amplitude was comparable between even and odd fillings. Such anomalous behavior suggests a substantial degree of spin polarization of the even fillings and was observed in two different material systems, namely AlAs quantum wells and ZnO/MgZnO heterojunctions. In AlAs quantum wells, spin resonance tended to split into two well-resolved lines near even fillings whereas it remained a single peak near odd ones. The evolution of ESR with varying filling factor around even fillings is studied in detail. The reported nontrivial findings suggest the modification of the ground state around even filling factors with the aid of strong e-e interaction and may be viewed as a precursor for the Stoner instability.