Compensation doping of the qubit host Ba2CaWO6-δ

Abstract

Emerging quantum information science (QIS) technologies require advances in controlling the type, number, and distribution of defects in complex crystalline matter. Building on recent reports of promising spin-lattice relaxation times in oxygen-vacancy-induced W⁵⁺ centers in the double perovskite Ba₂CaWO_6-δ, here we report on the viability of compensation doping with Zr⁴⁺ and Ge⁴⁺ to tune the number of active W⁵⁺ centers, a pre-requisite for mitigating spin bath effects and increasing spin-spin relaxation times. We prepared single crystals of nominal composition Ba₂CaW_1-xM_xO_6-δ (M = Zr⁴⁺, Ge⁴⁺) for x = 0–0.20. Electron paramagnetic resonance (EPR) and DC magnetic susceptibility measurements were used to understand the changes in spin-active defects as a function of substitution. We find that x = 0.01 (M = Zr⁴⁺) and x = 0.03 (M = Ge⁴⁺) are sufficient to quench the W⁵⁺ S = ½ EPR response (g = 2.00) within our limit of detection. Further substitution results in the appearance of a narrow S = ½ response (g = 1.98–2.00) that fades away at higher compositions. We conclude that compensation doping is an effective strategy for modulation of single ion centers in Ba₂CaWO_6-δ and identify future steps that are needed to bring such complex materials to viability for QIS technologies, including proposal of an easily measured figure of merit for rapid materials iteration and optimization.