  We propose a model to account for the observed ESR-like signal at the Larmor
frequency in the current noise STM experiments identifying spin centers on
various substrates. The theoretical understanding of this phenomenon, which
allows for single spin detection on surfaces at room temperature, is not
settled for the experimentally relevant case that the tip and substrate are not
spin polarized. Our model is based on a direct tip-substrate tunneling in
parallel with a current flowing via the spin states. We find a sharp signal at
the Larmor frequency even at high temperatures, in good agreement with
experimental data. We also evaluate the noise in presence of an ac field near
resonance and predict splitting of the signal by the Rabi frequency.
