Muonium as a probe of point defects in type-Ib diamond

TL;DR

This study demonstrates how muonium can be used as a sensitive probe to detect and analyze point defects like nitrogen and NV centers in type-Ib diamond through spin relaxation measurements.

Contribution

The paper introduces a modeling and simulation approach to deconvolute muonium states and extract defect interaction rates in diamond.

Findings

Muonium interacts with nitrogen centers via electron spin exchange.

Muonium converts to diamagnetic centers upon interaction with NV centers.

A global fitting method can extract transition rates from muon spin data.

Abstract

Muonium (Mu), a bound state of a positively charged muon and an electron, can diffuse through crystal lattices and interact with defect centers in insulators and semiconductors. We demonstrate that this Mu's diffusive property can be used to probe defects in a diamond crystal lattice; specifically, substitutional nitrogen atoms (N$_\text{s}^0$) and nitrogen-vacancy (NV) centers in type-Ib diamond. Upon interaction with these defects, Mu can exchange its electron's spin or change its charge state, which result in muon spin relaxation. However, muons in diamond (and semiconductors in general) can be in a few distinctive muonium states, with each state contributing to the muon signal. In addition, these states can undergo site and charge exchange interaction, forming a dynamic network. Hence, to study the Mu interaction with point defects, the muon data have to be deconvoluted to isolate signals from the diffusing species. To achieve this goal, we have modeled the Mu state exchange dynamics and numerically simulated the time evolution of muon spin polarization by the density matrix method. With a global curve fit to a set of longitudinal field scan data, one can extract the Mu transition rates that involve interaction with the defect centers. The diffusing tetrahedral interstitial Mu was found to interact with the paramagnetic N$_\text{s}^0$ center via electron spin exchange. In contrast, they are converted to form a diamagnetic center upon interaction with the negatively charged NV center.