Galperin Michael, Nitzan Abraham, Ratner Mark A, Stewart Duncan R
Department of Chemistry and Materials Research Center, Northwestern University, Evanston, Illinois 60208, USA.
J Phys Chem B. 2005 May 5;109(17):8519-22. doi: 10.1021/jp0457500.
Inelastic electron tunneling spectroscopy (IETS) measurements are usually carried out in the low-voltage ("Ohmic", i.e., linear) regime where the elastic conduction/voltage characteristic is symmetric to voltage inversion. Inelastic features, normally observed in the second derivative d(2)I/dV(2) are also symmetric (in fact antisymmetric) in many cases, but asymmetry is sometimes observed. We show that such asymmetry can occur because of different energy dependences of the two contact self-energies. This may be attributed to differences in contact density of states (different contact material) or different energy dependence of the coupling (STM-like geometry or asymmetric positioning of molecular vibrational modes in the junction). The asymmetry scales with the difference between the energy dependence of these self-energies and disappears when this dependence is the same for the two contacts. Our nonequilibrium Green function approach goes beyond proposed WKB scattering theory in properly accounting for Pauli exclusion, as well as providing a path to generalizations, including consideration of phonon dynamics and higher-order perturbation theory.
非弹性电子隧穿谱（IETS）测量通常在低电压（“欧姆”，即线性） regime 中进行，其中弹性传导/电压特性对于电压反转是对称的。非弹性特征通常在二阶导数 d(2)I/dV(2) 中观察到，在许多情况下也是对称的（实际上是反对称的），但有时会观察到不对称性。我们表明，这种不对称性可能由于两个接触自能的不同能量依赖性而出现。这可能归因于态密度的接触差异（不同的接触材料）或耦合的不同能量依赖性（类似STM的几何结构或分子振动模式在结中的不对称定位）。不对称性随着这些自能的能量依赖性之间的差异而缩放，并且当两个接触的这种依赖性相同时消失。我们的非平衡格林函数方法超越了所提出的WKB散射理论，能够正确考虑泡利不相容原理，并且还提供了一种推广途径，包括考虑声子动力学和高阶微扰理论。
