Spin-State Engineering of Single Titanium Adsorbates on Ultrathin Magnesium Oxide

TL;DR

This study demonstrates the ability to control and understand the spin states of individual titanium atoms on ultrathin magnesium oxide surfaces, revealing two distinct spin configurations and their dependence on local environment, advancing surface-based quantum computing prospects.

Contribution

It provides experimental and theoretical insights into the spin states of single Ti adsorbates on MgO/Ag(100), showing tunable spin and charge states for quantum applications.

Findings

Two distinct spin states, S=1/2 and S=1, depend on adsorption site and MgO thickness.

Density functional theory indicates Ti$^+$ configuration with ~3 electrons in valence shells.

Site-dependent spin states are explained by charge redistribution among orbitals.

Abstract

Single atomic adsorbates on ultrathin insulating films provide a promising route toward bottom-up quantum architectures based on atomically identical yet individually addressable spin qubits on solid surfaces. A key challenge in engineering quantum-coherent spin nanostructures lies in understanding and controlling the spin state of individual adsorbates. In this work, we investigate single titanium (Ti) atoms adsorbed on MgO/Ag(100) surfaces using a combined scanning tunneling microscopy and electron spin resonance. Our measurements reveal two distinct spin states, $S = 1/2$ and $S = 1$, depending on the local adsorption site and the thickness of the MgO film. Density functional theory calculations suggest a Ti$^+$ configuration for the Ti adsorbates with approximately 3 electrons in the 4$s$ and 3$d$ valence shells. Using a multi-orbital atomic multiplet calculations the site dependence of the spin can be rationalized as a charge redistribution between spin-polarizing and depolarizing orbitals. These findings underscore the potential of surface-supported single atoms as spin qubits with tunable spin and charge states, enabling atom-by-atom control in the realization of a versatile quantum platform on surfaces.