Bias induced spin state transition mediated by electron excitations

TL;DR

This paper proposes a mechanism where bias-induced electron excitations, specifically MLCT and MC, drive spin state transitions in molecular junctions, aligning with experimental voltage thresholds and bias polarity effects.

Contribution

It introduces a theoretical model linking bias-induced electron excitations to spin state transitions, supported by first-principles calculations and experimental data.

Findings

MLCT leads to LS to HS transition at certain voltages

MC causes HS to LS transition at specific voltages

Bias polarity influences the activation of excitations and spin transitions

Abstract

Recent experiments reported that spin-state transitions were realized by applying bias voltages. But these bias-induced spin state transitions (BISSTs) are not fully understood, especially the mechanism. It is well known that the metal-to-ligand charge transfer excitation (MLCT) and the metal-centered excitation (MC) activated by light radiation can induce the transition from low spin (LS) to high spin (HS) and that from HS to LS. Moreover, electronic excitations are accessible by inelastic cotunneling in molecular junctions with bias voltages applied. Based on these two experimental facts, we propose the MLCT basically leads to the BISST from LS to HS, and the MC results in the BISST from HS to LS. The rationality of the mechanism is demonstrated by comparing first-principles results and experimental observations. The calculated voltage threshold for activating the MLCT (MC) is close to the experimental voltage for observing the BISST from LS to HS (from HS to LS). The activation of MLCT (MC) depends on the bias polarity, which can explain the bias-polarity dependence of BISST in the experiment. Our study is important for further design of molecular spintronic devices working on spin state transition.