Spin relaxation and dephasing in a ²⁸SiGe QD with nanomagnet

The isotopical purification of Si has significantly improved coherent control of a single electron spin in electrostatically defined ²⁸Si/SiGe quantum dots ^[1]. The question to what extent the spin coherence can be improved by further purification is currently connected to the open question whether electrical noise in the device, in combination with the stray field of a micromagnet dominantly limits the spin dephasing time.
Here, we study the spin relaxation and dephasing in an MBE-grown ²⁸Si/SiGe device with a residual ²⁹Si concentration of less than 60 ppm. In our devices, instead of using a micromagnet, one of the electrostatic gates is replaced by a cobalt nanomagnet to generate the magnetic field gradient required for electrical dipole spin resonance. We find a maximum T₁ = 480 ms and present a detailed study of the spin relaxation time T₁ as a function of magnetic field. We observe a spin relaxation hotspot due to enhanced spin-valley mixing at a Zeeman energy of 207 µeV and find this large valley splitting to be tunable with the gate voltages. Finally, we discuss the long spin dephasing times T₂^* and T₂^echo obtained in this device.

[1] J. Yoneda et al., Nat. Nanotechnology 13, 102 (2018).