二氧化钒中的电子关联与涨落晶格畸变

中文摘要 AI 辅助

关联材料中的金属-绝缘体相变通常伴随着晶体结构的变化。这类相变通常采用相干晶格近似来描述，该近似将电子的关联处理与由最小化能量所固定的一个或几个经典畸变坐标所表示的晶格相结合。在这种近似下，结构自由度本身不携带熵，尽管相变熵在电子与晶格之间的分配往往决定了相变温度。在此，我们发展了一种动力学平均场理论的随机半经典扩展，其中关联电子与涨落晶格畸变共同演化，将先前的公式从线性电子-声子相互作用推广到非线性情形，并从单一晶格模式推广到多个耦合晶格模式。由此得到的非保守朗之万方程具有由相互作用电子子系统自洽产生的确定性力、阻尼和关联噪声。将该方法应用于包含二氧化钒单斜相两种对称性不同畸变的最小双轨道模型，我们发现这两种畸变在温度明显分离的情况下熔化，从而产生绝缘相、中间金属相和高对称相。这两个相变具有不同的起源：二聚化的熔化由与关联电子的耦合驱动，并已在相干晶格近似内被捕获，而无畸变结构的恢复则需要涨落晶格的熵。由于电子子系统是在非平衡动力学平均场理论框架内处理的，该框架可在未来工作中扩展到具有非热电子分布的光激发系统。

英文摘要

Metal-insulator transitions in correlated materials are often accompanied by a change of crystal structure. They are commonly described within a coherent lattice approximation, which combines a correlated treatment of the electrons with a lattice represented by one or a few classical distortion coordinates fixed by minimizing an energy. The structural degrees of freedom then carry no entropy of their own, even though the partition of the transition entropy between electrons and lattice is often what decides the transition temperature. Here we develop a stochastic semiclassical extension of dynamical mean-field theory in which correlated electrons and fluctuating lattice distortions are evolved together, extending previous formulations from linear to nonlinear electron-phonon interactions and from a single to several coupled lattice modes. The resulting non-conservative Langevin equations have deterministic forces, damping, and correlated noise generated self-consistently by the interacting electronic subsystem. Applying the approach to a minimal two-orbital model containing the two symmetry-distinct distortions of the monoclinic phase of vanadium dioxide, we find that the two distortions melt at well separated temperatures, giving an insulating, an intermediate metallic, and a high-symmetry phase. The two transitions have different origins: the melting of the dimerization is driven by the coupling to the correlated electrons and is captured already within the coherent lattice approximation, whereas the restoration of the undistorted structure requires the entropy of the fluctuating lattice. Since the electronic subsystem is treated within nonequilibrium dynamical mean-field theory, the framework can be extended in future work to photoexcited systems with nonthermal electronic distributions.

发表机构

• Collège de France, PSL Research University（法兰西学院，PSL研究大学）
• Institute of Theoretical Physics, University of Hamburg（汉堡大学理论物理研究所）
• The Hamburg Centre for Ultrafast Imaging（汉堡超快成像中心）
• RWTH Aachen University（亚琛工业大学）
• Institut für Theoretische Physik und Astrophysik and Würzburg-Dresden Cluster of Excellence ct.qmat, Universität Würzburg（维尔茨堡大学理论物理与天体物理研究所及维尔茨堡-德累斯顿卓越集群ct.qmat）

机构由 AI 辅助整理，请以论文原文为准。

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23 pages, 6 figures