Unusual narrowing of the ESR line width in ordered structures with linear chains of Ge/Si quantum dots

Electron states in ordered Ge/Si heterostructures with linear chains of quantum dots (QDs) were studied by the electron spin resonance (ESR) method. A new ESR signal with principal $g$-factor values $g_{zz}=1.9993\pm0.0001$, $g_{xx}=g_{yy}=1.9990\pm0.0001$ was detected. Unlike non-ordered QD structures, where ESR line broadening is usually observed (evidence of Dyakonov–Perel mechanism efficiency), the structures under study demonstrate the narrowing of ESR line when the external magnetic field deviates from the growth direction. The ESR line width is $\Delta H=1.2$ Oe for perpendicular magnetic field (along the growth direction) and $\Delta H=0.8$ Oe for in-plane magnetic field. The narrowing of ESR line can be explained by combination of two mechanisms. The first one is suppression of Dyakonov–Perel spin relaxation due to a settled direction of electron motion and finiteness of QD chains. The second one is cancelation of the wave function shrinking effect with decreasing the perpendicular component of the magnetic field.