AI 中文总结
本研究通过光学方法研究低温下LiYF4中Ho3+离子的电子-核能级交叉，发现核自旋扩散独立于声子促进平衡，产生长寿命非平衡态，为多组分量子系统热化提供新见解。
AI 中文摘要
物理学以及工程学中的一个基本问题是平衡化。例如，热涨落和量子涨落的调控使得复杂系统的热退火和量子退火成为可能。一个关键问题是不同子系统，如电子、核自旋和声子，如何各自平衡以及彼此之间如何平衡。能级交叉在耦合自由度的动力学中扮演着特殊角色，因为正是在这里纠缠可以被最大化以加速弛豫。这里我们使用光学方法建立电子-核能级图，包括避免交叉和非避免交叉，并研究盐（LiYF$_4$）中稀土离子（Ho$^{3+}$）的平衡化，这是一个具有量子涨落的模型系统，可以通过施加垂直于四方宿主晶体学长轴的外磁场来调谐。我们通过监测作为扫描纵向场函数的能级布居数来跟踪系统状态，并发现低温下，取决于实验方案，会出现截然不同的非平衡态。我们找到证据表明核自旋激发在无声子辅助的情况下扩散并平衡，且倾向于获得比电子自旋和更冷的晶格更高的有效温度。热辅助隧穿理论排除了标准的声子辅助隧穿情景，反而表明在能级反交叉处的快速动力学完全由核自旋扩散促进。这为致密、多组分相互作用量子系统中的热化及相关慢弛豫现象提供了新的理解。
英文摘要
A fundamental problem in physics as well as engineering is equilibration. For example, the regulation of thermal and quantum fluctuations enables thermal and quantum annealing of complex systems. A key question is how different subsystems, such as electrons, nuclear spins and phonons equilibrate on their own as well as with each other. Level crossings play a special role in the dynamics of coupled degrees of freedom, for it is here that entanglement can be maximized to speed up relaxation. Here we use optical methods to establish the electronuclear level scheme, including avoided and unavoided crossings, and to examine equilibration of a rare earth ion (Ho$^{3+}$) in a salt (LiYF$_4$), a model system with quantum fluctuations which can be tuned via an external magnetic field transverse to the crystallographic long axis of the tetragonal host. We track the state of the system by monitoring the populations of the levels as a function of swept longitudinal fields, and discover that at low temperatures, depending on the experimental protocol, vastly different non-equilibrium states arise. We find evidence that nuclear spin excitations diffuse and equilibrate without the assistance of phonons, and tend to acquire higher effective temperatures than the electronic spins and the yet cooler lattice. A theory of thermally assisted tunneling rules out the standard scenario of phonon -assisted tunneling, and instead suggests that fast dynamics at level anti-crossings are facilitated entirely by nuclear spin diffusion. This provides a new understanding of thermalization and related slow relaxation phenomena in dense, multi-component interacting quantum systems.
Comments63 pages, 13 figures
