  In this theoretical study we qualitatively and quantitatively investigate the
electric dipole spin resonance (EDSR) in a single Si/SiGe quantum dot in the
presence of a magnetic field gradient, e.g., produced by a ferromagnet. We
model a situation in which the control of electron spin states is achieved by
applying an oscillatory electric field, inducing real-space oscillations of the
electron inside the quantum dot. One of the goals of our study is to present a
microscopic theory of valley dependent $g$-factors in Si/SiGe quantum dots and
investigate how valley relaxation combined with a valley dependent $g$-factor
leads to a novel electron spin dephasing mechanism. Furthermore, we discuss the
interplay of spin and valley relaxations in Si/SiGe quantum dots. Our findings
suggest that the electron spin dephases due to valley relaxation, and are in
agreement with recent experimental studies [Nature Nanotechnology 9, 666-670
(2014)].
