  Electron paramagnetic resonance of ensembles of phosphorus donors in silicon
has been detected electrically with externally applied magnetic fields lower
than 200 G. Because the spin Hamiltonian was dominated by the contact hyperfine
term rather than by the Zeeman terms at such low magnetic fields, superposition
states $ \alpha{}| \uparrow \downarrow >+\beta{}| \downarrow \uparrow >$ and
$-\beta{}| \uparrow \downarrow > + \alpha{}| \downarrow \uparrow >$ were formed
between phosphorus electron and nuclear spins, and electron paramagnetic
resonance transitions between these superposition states and $| \uparrow
\uparrow >$ or $| \downarrow \downarrow >$ states are observed clearly. A
continuous change of $\alpha{}$ and $\beta{}$ with the magnetic field was
observed with a behavior fully consistent with theory of phosphorus donors in
silicon.
