Low-Dimensional Electronic Excitations in a Metallic Monolayer of Finite Area Adsorbed on a Semiconductor Surface

在Si(111)-7×7表面沉积一层银原子可以形成Si(111)-√<3>×√<3> -Ag表面，该表面的表面态带提供了理想的二维导电电子系统（2DES）。该电子系统被限制在每个√<3> ×√<3>-Ag畴内，被原子台阶或相外边界包围。这种2DES中的电子激发涉及一些有趣的特性，如低维数、量子尺寸效应以及边缘和区域激发的存在。首先，我们研究了无限面积金属单层中的二维等离子体激元。该研究表明，由于低维数，交换相关效应开始随着PL波数q的增加而显著出现，尽管2DES具有较高的有效密度。我们的计算几乎定量地描述了高分辨率电子能量损失谱（HREELS）的实验结果。其次，利用局部密度近似，我们研究了被限制在恒宽D无限长条形区域的2DES中的低维PL。我们发现了一系列的PL色散分支，这些分支的节点数随着能量的上升而依次增加。在这里，节点数是由诱导电子密度δn在条带上的分布指定的。0节点和1节点模式是δn分布中具有不同宇称的边PL。随着D的减小，0节点模式演化为一维PL模式。每个多节点高阶模态的δn分布在较小的q区域都有驻波分布，随着q的增加，驻波分布演变为PL区域。在多节点模态中，驻波分布一直持续到较大的q范围。随着宽度D的增大，相邻PL色散分支之间的能量分离变小，并且在较小的q值下发生向PL区域的演化。我们对HREELS的分析表明，如果探针电子沿着条带区域附近的轨迹运动，则由一系列PL色散分支引起的损失峰应该在能量损失谱中清晰地出现，并且损失峰应该具有可观察的强度。少

The Si(111)-√<3>×√<3> -Ag surface can be formed by depositing one monolayer of Ag atoms on a Si(111)-7×7 surface, and the surface-state band at this surface provides an ideal two-dimensional conduction-electron system (2DES). This electron system is confined in each √<3> ×√<3>-Ag domain surrounded by atomic steps or out-of-phase boundaries. Electronic excitations in this 2DES involves some interesting characteristics, such as low dimensionality, quantum-size effects, and presence of both edge and area excitations.First, we have investigated two-dimensional plasmons (PL's) in the metallic monolayer of infinite area. This investigation has shown that, owing to low dimensionality, the exchange-correlation effects begin to appear remarkably with an increase in PL wave number q, though the 2DES has a high effective density. Our calculation has given a virtually quantitative description of the experimental results of high-resolution electron energy-loss spectroscopy (HREELS).Next, by means o … More f the local-density approximation, we have examined low-dimensional PL's in a 2DES restricted to a strip region with constant width D and infinite length. We have found a series of PL dispersion branches where the node number increases one by one with ascending energy. Here, the node number is specified from the distribution of the induced electron density δn across the strip. The 0-node and 1-node modes are edge PL's with different parity in the δn distribution. With a decrease in D, the 0-node modes evolve into one-dimensional PL's. Each higher-order mode with more than one node has a standing-wave pattern in its δn distribution in a smaller q region, which evolves into area PL's with an increase in q. In a mode with more nodes, the standing-wave pattern persists up to a larger q range. As the width D becomes larger, the energy separation between neighboring PL dispersion branches becomes smaller, and the evolution into the area PL's occurs at smaller q values. Our analysis in relation to HREELS has shown that loss peaks due to a series of PL dispersion branches should appear clearly in the energy-loss spectrum, and that the loss peaks should have observable intensity, if a probe electron follows a trajectory in the vicinity of the strip region. Less