Transition between tetramer and monomer phases driven by vacancy configuration entropy on Bi/Ag(001)
Transition between tetramer and monomer phases driven by vacancy configuration entropy on Bi/Ag(001)
复制标题
DOI：
10.1103/physrevb.75.155409
复制
发表时间：
2007-04-01
影响因子：
3.7
通讯作者：
Aruga, Tetsuya
中科院分区：
文献类型：
--
作者：
Nakagawa, Takeshi;Ohgami, Osamu;Aruga, Tetsuya
利用低能电子衍射、扫描隧道显微镜（STM）和角分辨紫外光电子能谱（ARUPS）研究了Bi/Ag（001）上低温c（4x 4）和高温c（2x2）相之间的相变。STM实验表明，c（4x 4）表面由正方形点阵排列的Bi四聚体组成，而四聚体在c（2x2）相中分解为单体。ARUPS显示在从c（4x 4）到c（2x2）的转变后，Bi诱导的表面共振带在（Tau）周围的bar上向上移动，这表明Bi四聚体通过构成四聚体的Bi原子之间的局部共价键合而稳定。ARUPS还表明Bi诱导的表面共振带的大Rashba自旋轨道分裂。相变与中温区有关，在该中温区中，两相共存于表面上，其面积随温度变化而逐渐变化。空位密度越高，相变温度越低。提出了空位组态熵驱动相变的模型。由于四聚体的分解，c（2x2）相中较大的空位组态熵解释了通过两相共存区的相变行为。
A phase transition between low-temperature c(4x4) and high-temperature c(2x2) phases on Bi/Ag(001) has been investigated using low energy electron diffraction, scanning tunneling microscopy (STM), and angle resolved ultraviolet photoelectron spectroscopy (ARUPS). STM experiments show that the c(4x4) surface is composed of Bi tetramers arrayed on a square lattice, while the tetramers are decomposed to monomers in the c(2x2) phase. ARUPS shows an upward shift of a Bi-induced surface resonance band around (Tau) over bar upon the transition from c(4x4) to c(2x2), which suggests that the Bi tetramer is stabilized by local, covalent bonding between Bi atoms constituting a tetramer. ARUPS also indicates large Rashba spin-orbit splitting of the Bi-induced surface resonance band. The phase transition is associated with an intermediate temperature region in which the two phases coexist on the surface with their area changing gradually with changing temperature. The transition temperature is lowered for surface with higher vacancy density. A model is presented to show that vacancy configuration entropy drives the phase transition. Larger vacancy configuration entropy in the c(2x2) phase, due to the decomposition of tetramers, accounts for the behavior of the phase transition via the two-phase coexisting region.