Origin of the anomalous spin resonance in a strongly correlated electron system

We study the origin of the anomalous spin resonance detected electrically in a strongly correlated twodimensional electron system. Such resonance reveals itself around nominally nonmagnetic even filling factors nu of the integer quantum Hall effect and is a consequence of strong electron-electron interactions. We directly demonstrate that while the spin resonance at odd nu manifests itself as an increase in the longitudinal resistance of the two-dimensional channel induced by the usual heating due to the radiation absorption, the anomalous resonance around even fillings is detected as a drop in the resistance, as if the electron system is cooled. In contrast, if the magnetic field is tilted so that the ground state of the system becomes ferromagnetic at even nu, the spin resonance around even fillings turns to a more conventional "heating"-like behavior. Analysis of both the evolution of the spin resonance with increasing tilt angle and the measured temperature dependencies allowed us to put forth a possible mechanism of anomalous spin resonance around even fillings that qualitatively explains all of the puzzling experimental findings.