THEORETICAL INVESTIGATION OF QUANTUM TRANSPORT PHENOMENA IN AN ATOM BRIDGES CONSTRUCTED BETWEEN AN STM TIP AND A METAL SURFACE

当扫描隧道显微镜（STM）的尖端与金属表面接触并向上抬起时，尖端与金属表面之间形成原子大小的接触（点接触）。随着尖端进一步上升，距离表面不到几纳米，在STM尖端和金属表面之间形成一个原子大小的线（几个原子的尺寸）。由于电子被限制在与原子桥轴垂直的方向上，原子桥表现出量子化的电导。我们使用单波段哈伯德模型，研究了这些由磁性材料制成的原子大小的桥中可观察到的量子输运现象。我们发现，在有限温度下，由于来自强电子库仑相互作用的子带间散射，相应的电导(G)与长度图显示出与特征阶梯结构的偏差，特别是在电导平台和电导台阶之间的过渡区域。我们还提出了一种方法，如何使用STM尖端，我们可以操纵这些原子桥的磁性和量子输运性质。通过第一性原理计算，我们还确定了铁原子桥可能的稳定结构，一个和两个原子在维度上跨越。此外，我们已经确定了如何通过合金化改变原子线的组成来控制/操纵这些原子桥的磁性和量子输运特性，并获得具有流过它们的电流的特定自旋极化比的原子线。以铁α ni1 -α原子桥为例。我们发现这种合金原子桥的磁矩比块状材料的Pauling-Slater曲线预测的磁矩大得多。此外，我们已经确定了少数自旋对相应原子桥电导的贡献强烈依赖于Ni浓度，尽管大多数自旋的贡献保持不变。这些性质是由相应的s波段和d波段的能量变化决定的。因此，基于本研究获得的信息的技术将为我们提供一种设计和实现功能弹道自旋导体的新方法。少

When a scanning tunneling microscope (STM) tip is brought into contact with a metal surface and then lifted up, an atom-sized contact (point contact) is formed between the tip and the surface. As the tip is further lifted up, by about less than a few nanometers from the surface, an atom-sized wire (a few-atoms across in dimensions) is formed between the STM tip and the metal surface. Because the electrons are confined in the direction normal to the atom bridge axis, the atom bridge exhibits quantized conductance. We have investigated the quantum transport phenomena observable in these atom-sized bridges, made from magnetic materials, using the single-band Hubbard model. We found that, at finite temperatures, because of inter-subband scattering, which originates from the strong electron Coulomb interaction, the corresponding Conductance (G) vs. Length plot shows deviations from the characteristic step-like structure, particularly in the transition region between a conductance plateau an … More d a conductance step. We have also proposed a method on how, using the STM tip, we could manipulate the magnetic and quantum transport properties of these atom bridges. By performing first-principles calculations, we have also determined the possible stable structures for Fe atom bridges, one and two atoms across in dimensions. Furthermore, we have determined how we could control/manipulate the magnetic and quantum transport properties of these atom bridges by changing the composition of the atom wire through alloying and obtain, e.g., atom wires with specific spin polarization ratios of the electric current flowing through them. Consider for example an FeαNi1-α, atom bridge. We found that this alloyed atom bridge exhibits a magnetic moment considerably larger than that predicted by the Pauling-Slater curve for bulk materials. Furthermore, we have determined that the minority spin contribution to the corresponding atom bridge conductance strongly depends on the Ni concentration, although the majority spin contribution remains the same. These properties are determined by the energetic variations in the corresponding s- and d-bands. Thus, a technology based on information gained from this study would provide us with a novel means of designing and realizing functional ballistic spin conductors. Less