Ensemble spin relaxation of shallow donor qubits in ZnO

TL;DR

This study combines experimental measurements and theoretical modeling to analyze the electron spin relaxation times of shallow donors in ZnO, revealing a dominant spin-orbit mechanism and extremely long relaxation times.

Contribution

The paper provides the first combined experimental and theoretical analysis of $T_1$ in ZnO donors, identifying the admixture spin-orbit mechanism as the main relaxation process.

Findings

$T_1$ follows an inverse power law with magnetic field, $T_1 o B^{-n}$, with $4 \\leq n \\leq 5$.

The longest measured $T_1$ is 480 ms at 1.75 T.

Excellent agreement between experiment and theory confirms the dominance of the spin-orbit admixture mechanism.

Abstract

We present an experimental and theoretical study of the longitudinal electron spin relaxation ($T_1$) of shallow donors in the direct band-gap semiconductor ZnO. $T_1$ is measured via resonant excitation of the Ga donor-bound exciton. $T_1$ exhibits an inverse-power dependence on magnetic field $T_1\propto B^{-n}$, with $4\leq n\leq 5$, over a field range of 1.75 T to 7 T. We derive an analytic expression for the donor spin-relaxation rate due to spin-orbit (admixture mechanism) and electron-phonon (piezoelectric) coupling for the wurtzite crystal symmetry. Excellent quantitative agreement is found between experiment and theory suggesting the admixture spin-orbit mechanism is the dominant contribution to $T_1$ in the measured magnetic field range. Temperature and excitation-energy dependent measurements indicate a donor density dependent interaction may contribute to small deviations between experiment and theory. The longest $T_1$ measured is 480 ms at 1.75 T with increasing $T_1$ at smaller fields theoretically expected. This work highlights the extremely long longitudinal spin-relaxation time for ZnO donors due to their small spin-orbit coupling.