【作者】 韩超群；

【导师】 钱冬；

【作者基本信息】 上海交通大学 ， 凝聚态物理专业， 2016， 博士

【摘要】 近年来,以石墨烯、过渡族金属硫族化合物以及拓扑绝缘体等为代表的一系列具有特殊二维电子能带结构的低维材料受到人们广泛关注。这些材料体系由于其新奇的电学、光学、磁学等性质在电子学、自旋电子学、光电器件以及新能源等领域表现出极其重要的潜在应用价值。本论文利用高分辨率角分辨光电子能谱（ARPES）结合分子束外延（MBE）、扫描隧道显微镜（STM）以及原位微区四探针技术,针对一些有趣的低维晶体材料,包括黑磷、锶掺杂超导拓扑绝缘体（Sr_xBi₂Se₃）以及超薄铋（Bi）/拓扑绝缘体异质薄膜等体系的电子能带结构和基本电输运性质等问题进行了系统地研究,主要结果如下:一、利用基于同步辐射的高分辨率角分辨光电子能谱,通过细致调节入射光子能量以及光偏振,精确地测定了层状材料黑磷单晶样品的能带结构,并同已有的第一性原理结果进行了比较。实验上确定了黑磷能隙在布里渊区中的位置,澄清了理论计算上存在的一些不确定性。发现现有的理论计算结果和实验确定的能带结构存在一定差异。相比计算结果,实验上发现费米能级附近能带发生额外劈裂,导致在垂直二维面方向上电子的能带色散被压制,表现出比理论预计更强的二维特性。同时,我们还首次在黑磷表面发现存在表面共振态能带。二、用熔融法制备了高质量的超导Sr_xBi₂Se₃单晶样品,并对其超导特性、电子结构和原子结构进行了详细研究。ARPES对不同Sr掺杂组分的Sr_xBi₂Se₃单晶样品的能带结构进行了研究,发现Sr原子的进入为Bi₂Se₃提供了非常弱的电子掺杂,掺杂样品的费米能级位置相比未掺杂母体只改变了几十个毫电子伏特（meV）。ARPES测量结果还明确地表明超导Sr_xBi₂Se₃样品仍然存在拓扑表面态,该表面态和体态在动量空间完全彼此分离。同时,STM测量结果显示样品表面存在超导能隙。基于实验观察到的费米能级位置、表面态能带特性以及超导态的存在,我们认为Sr_xBi₂Se₃样品完全满足理论上实现拓扑超导的要求,该体系很可能是一个新的拓扑超导体。另外,STM和高分辨透射电镜（TEM）的实验结果表明绝大多数掺杂的Sr原子不是在范德瓦尔斯层间,而是在Bi和Se构成的五层结构中,这能够很好地解释为什么该掺杂体系的载流子浓度较低。三、利用分子束外延方法在Bi₂Se₃和Bi₂Te₃单晶表面外延生长了高质量的单原胞和双原胞层Bi（111）薄膜,利用基于四探针扫描隧道显微镜系统的原位微区四探针对单晶固体以及异质薄膜体系进行了真空原位电输运性质的测量。实验结果表明电导信号来自于表面区域二维导电通道,电导率在1×10^-3S量级。我们发现Bi₂Se₃和Bi₂Te₃固体上呈现出不同的电子输运行为,Bi₂Te₃具有较强的电声子散射效应,而Bi₂Se₃体系中电声子作用很弱。更多还原

【Abstract】 Recently,the two dimensional materials such as graphene,topological insulators,transition metal chalcogenide and so on have attracted tremendous attentions due to their exotic electronic,magnetic,optical properties and many potential applications.In this thesis,we have detailed and systematically study of the transport proper-ties,topological properties,and electronic band structure of Black Phosphorus single crystal,Sr-doped superconduting topological insulators（Sr_xBi₂Se₃）,and heterostruc-ture of ultrathin Bi（111）grown on topological insulators（Bi₂Se₃,Bi₂Te₃）,using high-resolution angular resolved photoemission electron spectroscopy（ARPES）,molecular beam epitaxy（MBE）,and scanning tunneling spectroscopy（STM）.The main results are as follows:1.Electronic structures of single crystalline black phosphorus were studied by state-of-art angle-resolved photoemission spectroscopy.Through high resolution pho-ton energy dependence measurements,the band dispersions along out-of-plane and in-plane directions are experimentally determined.The electrons were found to be more localized in the ab-plane than that is predicted in calculations.Beside the k_z-dispersive bulk bands,resonant surface state is also observed in the momentum space.2.Using high-resolution angle-resolved photoemission spectroscopy and scan-ning tunneling microscopy/spectroscopy,the atomic and low energy electronic struc-ture of the Sr-doped superconducting topological insulators（Sr_xBi₂Se₃）were studied.Scanning tunneling microscopy shows that most of the Sr atoms are not in the van der Waals gap.After the Sr doping,the Fermi level was found to move a little bit upwards compared with the parent compound Bi₂Se₃,which is consistent with the low carrier density in this system.The topological surface state was clearly observed and the posi-tion of the Dirac point was determined in all doped samples.The surface state is well separated from the bulk conduction bands in the momentum space.The persistence of separated topological surface state combining with small Fermi energy makes this new superconducting material a very promising candidate of the time reversal invariant topological superconductor.3.Electronic transport properties of topological insulators（TIs）Bi₂Se₃,Bi₂Te₃and ultra thin Bi（111）/TI were studied using in situ scanning-tunneling-microscope-based micro-four-probe technique.The measured conductivity were found to be dom-inated by two dimensional（2D）conducting channels.The 2D conductivity is in the order of 1×10^-3S,Temperature dependence measurements indicate that the electron-phonon scattering is much stronger in Bi₂Te₃than that in Bi₂Se₃.Our results suggest that the in situ micro-four-probe measurements can be used to detect the intrinsic elec-tronic transport properties of exotic 2D electron systems.更多还原