Theory of optical spin polarization of axial divacancy and nitrogen-vacancy defects in 4H-SiC

TL;DR

This paper provides a detailed theoretical analysis of the electron-phonon interactions and spin-dependent optical processes in divacancy and nitrogen-vacancy defects in 4H-SiC, crucial for quantum bit applications.

Contribution

It offers the first comprehensive first-principles investigation of the microscopic mechanisms behind optical spin polarization in these defects, including electron-phonon coupling and intersystem crossing.

Findings

Electronic level structures are quantitatively characterized.

Spin-orbit and spin-spin interactions are fully analyzed.

Electron-phonon coupling effects on spin polarization are elucidated.

Abstract

The neutral divacancy and the negatively charged nitrogen-vacancy defects in 4H-silicon carbide (SiC) are two of the most prominent candidates for functioning as room-temperature quantum bits (qubits) with telecommunication-wavelength emission. Nonetheless, the pivotal role of electron-phonon coupling in the spin polarization loop is still unrevealed. In this work, we theoretically investigate the microscopic magneto-optical properties and spin-dependent optical loops utilizing the first-principles calculations. First, we quantitatively demonstrate the electronic level structure, assisted by symmetry analysis. Moreover, the fine interactions, including spin-orbit coupling and spin-spin interaction, are fully characterized to provide versatile qubit functional parameters. Subsequently, we explore the electron-phonon coupling, encompassing dynamics- and pseudo-Jahn--Teller effects in the intersystem crossing transition. In addition, we analyze the photoluminescence PL lifetime based on the major transition rates in the optical spin polarization loop. We compare two promising qubits with similar electronic properties, but their respective rates differ substantially. Finally, we detail the threshold of ODMR contrast for further optimization of the qubit operation. This work not only reveals the mechanism underlying the optical spin polarization but also proposes productive avenues for optimizing quantum information processing tasks based on the ODMR protocol.