TOWARD A UNIFIED THEORY OF URBACHS RULE AND EXPONENTIAL ABSORPTION EDGES
TOWARD A UNIFIED THEORY OF URBACHS RULE AND EXPONENTIAL ABSORPTION EDGES
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DOI：
10.1103/physrevb.5.594
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发表时间：
1972-01-01
期刊：
影响因子：
--
通讯作者：
REDFIELD, D
中科院分区：
文献类型：
--
作者：
DOW, JD;REDFIELD, D
在离子（乌尔巴赫规则：g= σ k B T*）和共价材料中均观察到指数吸收边α= A e g （h ω−h ω 0）。给出的论据表明，指数吸收边的统一理论必须(i)依赖于电场作为原因，（ii）包括激子效应和电子与空穴之间的最终态相互作用，以及（iii）将乌尔巴赫规则归因于激子的相对内部运动。用均匀微场的统计分布代替非均匀微场进行了近似计算；该计算是对Redfield和Dexter先前强弱场理论中物理相关的中强度场的推广。与其他微场模型通过对微场分布进行平均来获得指数谱形状不同，本理论获得了一个定量的指数边缘作为其固有特征。在各种材料中，边缘的温度依赖性在定性上遵循微场源的性质。从该模型中获得了离子晶体中乌尔巴赫规则的特定温度依赖性，并补充了解释场非均匀性的参数。
Exponential absorption edges α= A e g (ℏ ω− ℏ ω 0) have been observed in both ionic (Urbach's rule: g= σ k B T* and covalent materials. Arguments are given to show that a unified theory of exponetial absorption edges must (i) rely on electric microfields as the cause,(ii) include exciton effects and the final-state interaction between the electron and the hole, and (iii) ascribe Urbach's rule to the relative, internal motion of the exciton. An approximate calculation has been made in which the nonuniform microfields are replaced by a statistical distribution of uniform microfields; this calculation is a generalization to physically relevant intermediate-strength fields of previous strong-and weak-field theories of Redfield and Dexter. In contrast with the other microfield models, which obtain the exponential spectral shape by averaging over microfield distributions, the present theory obtains a quantitatively exponential edge as an inherent feature. The temperature dependences of the edges in various materials follow qualitatively from the nature of the microfield sources. The specific temperature dependence of Urbach's rule in ionic crystals is obtained from this model, with supplementary arguments to account for nonuniformity of the fields.