First-Principles Investigation of Spin–Phonon Coupling in Vanadium-Based Molecular Spin Quantum Bits

Paramagnetic molecules
can show long spin-coherence times, which make them good candidates
as quantum bits (qubits). Reducing the efficiency of the spin–phonon
interaction is the primary challenge toward achieving long coherence
times over a wide temperature range in soft molecular lattices. The
lack of a microscopic understanding about the role of vibrations in
spin relaxation strongly undermines the possibility of chemically
designing better-performing molecular qubits. Here we report a first-principles
characterization of the main mechanism contributing to the spin–phonon
coupling for a class of vanadium­(IV) molecular qubits. Post-Hartree–Fock
and density functional theory methods are used to determine the effect
of both intermolecular and intramolecular vibrations on modulation
of the Zeeman energy for four molecules showing different coordination
geometries and ligands. This comparative study provides the first
insight into the role played by coordination geometry and ligand-field
strength in determining the spin–lattice relaxation time of
molecular qubits, opening an avenue to the rational design of new
compounds.

顺磁分子可展现出较长的自旋相干时间（spin-coherence times），这使其成为量子比特（qubits）的优质候选材料。在软分子晶格中，实现宽温度区间内的长相干时间，首要挑战在于降低自旋-声子相互作用（spin–phonon interaction）的效率。目前，对于振动在自旋弛豫（spin relaxation）中所起作用的微观认知匮乏，这极大限制了通过化学手段设计性能更优的分子量子比特的可能性。本文针对一类钒(IV)分子量子比特，报道了其自旋-声子耦合主要机制的第一性原理表征（first-principles characterization）。研究采用后哈特里-福克（Post-Hartree–Fock）与密度泛函理论（density functional theory）方法，针对四种具有不同配位几何构型与配体的分子，探究了分子间与分子内振动对塞曼能量（Zeeman energy）调制的影响。这项对比研究首次揭示了配位几何构型与配体场强度（ligand-field strength）在决定分子量子比特自旋-晶格弛豫时间（spin–lattice relaxation time）中发挥的作用，为新型化合物的理性设计开辟了路径。