利用非弹性中子散射实验量化分子量子比特中的自旋-声子耦合
Experimental Quantification of Spin-Phonon Coupling in Molecular Qubits using Inelastic Neutron Scattering
AI总结：
通过非弹性中子散射实验量化分子量子比特中的自旋-声子耦合，揭示不同温度下晶格振动对自旋弛豫的影响及分子结构对耦合强度的调控。
摘要：
电子自旋叠加态通过其对局部环境的敏感性实现了纳米级传感，但它们对振动运动的敏感性也限制了相干时间。在分子自旋系统中，化学可调性和原子级分辨率伴随着一个密集且热力学可访问的声子谱，这引入了高效的自旋弛豫路径。尽管有大量理论研究，但关于哪些振动能量主导自旋弛豫或分子结构如何控制自旋-声子耦合（SPC）尚缺乏实验共识。我们提出了一种完全实验的方法，通过结合温度依赖的振动光谱（来自非弹性中子散射）与电子顺磁共振测得的自旋弛豫率，来量化SPC系数。我们将此框架应用于两个模型S = 1/2系统，铜（II）酞菁（CuPc）和铜（II）八乙基卟啉（CuOEP）。两种不同的弛豫模式出现：在40 K以下，弱耦合的晶格模式低于50 cm⁻¹主导，而在40 K以上，光学声子高于约185 cm⁻¹被热力学填充并驱动弛豫，SPC系数几乎大三数量级。CuOEP中的结构畸变破坏了平面对称性，使晶格变软并增强非谐振散射，但同时也提高了分子核心中自旋所在处的伸缩模式能量。这将振动能量重新分布到分子外围和非平面区域，最终使SPC相对于CuPc减少，并在CuOEP中实现室温自旋相干。尽管我们的方法不提供特定模式的SPC系数，但它量化了不同频谱区域的贡献，并建立了广泛适用的、完全实验的晶格结构、晶格动力学和自旋弛豫之间的联系。
英文摘要：
Electronic spin superposition states enable nanoscale sensing through their sensitivity to the local environment, yet their sensitivity to vibrational motion also limits their coherence times. In molecular spin systems, chemical tunability and atomic-scale resolution are accompanied by a dense, thermally accessible phonon spectrum that introduces efficient spin relaxation pathways. Despite extensive theoretical work, there is little experimental consensus on which vibrational energies dominate spin relaxation or how molecular structure controls spin-phonon coupling (SPC). We present a fully experimental method to quantify SPC coefficients by combining temperature-dependent vibrational spectra from inelastic neutron scattering with spin relaxation rates measured by electron paramagnetic resonance. We apply this framework to two model S = 1/2 systems, copper(II) phthalocyanine (CuPc) and copper(II) octaethylporphyrin (CuOEP). Two distinct relaxation regimes emerge: below 40 K, weakly coupled lattice modes below $50~\mathrm{cm}^{-1}$ dominate, whereas above 40 K, optical phonons above ~$185~\mathrm{cm}^{-1}$ become thermally populated and drive relaxation with SPC coefficients nearly three orders of magnitude larger. Structural distortions in CuOEP that break planar symmetry soften the crystal lattice and enhance anharmonic scattering, but also raise the energy of stretching modes at the molecular core where the spins reside. This redistributes vibrational energy toward the molecular periphery and out of plane, ultimately reducing SPC relative to CuPc and enabling room-temperature spin coherence in CuOEP. Although our method does not provide mode-specific SPC coefficients, it quantifies contributions from distinct spectral regions and establishes a broadly applicable, fully experimental link between crystal structure, lattice dynamics, and spin relaxation.
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