Using angle resolved photoemission spectroscopy we studied the evolution of the surface electronic structure of the topological insulator $\text{Bi}_2\text{Se}_3$ as a function of water vapor exposure. We find that a surface reaction with water induces a band bending shifting the Dirac point deep into the occupied states and creating quantum well states with a strong Rashba-type splitting. The surface is thus not chemically inert, but the topological state remains protected. The band bending is traced back to Se-abstraction leaving positively charged vacancies at the surface. Due to the presence of water vapor, a similar effect takes place when $\text{Bi}_2\text{Se}_3$ crystals are left in vacuum or cleaved in air, which likely explains the aging effect observed in the $\text{Bi}_2\text{Se}_3$ band structure.
Gas Doping on the Topological Insulator Bi2Se3 Surface
利用第一性原理计算研究了拓扑绝缘体Bi 2 Se 3表面存在Se空位时的气体分子掺杂.与实验一致，NO2和O2被发现占据Se空位位置，去除空位掺杂电子并恢复完美表面的能带结构。相比之下，NO和H2不有利于钝化这种空位。有趣的是，我们已经揭示了NO2的解离过程，可以很好地解释最近的实验报道的推测引入的“光子掺杂”效应。实验策略来验证这种机制。讨论了不同钝化剂的选择和钝化效果。这一步骤为在利用鲁棒拓扑表面状态的器件应用中使用此类材料铺平了道路。
Based on a self-consistent $t$-matrix approximation, we explore the influence of magnetic and nonmagnetic doping on the surface electronic states and conductivity of topological insulators. We show that warping parameter has a crucial impact on the density of states and dc conductivity of the doped surfaces. As the warping strength is increased, the surface density of states at high energies is suppressed and the resonant states induced by impurities in the vicinity of the Dirac point gradually disappear. It is found that nonmagnetic impurities break electron-hole symmetry at low warping stren…
Quantum anomalous Hall effect (QAHE) has been experimentally observed in magnetically doped topological insulators. However, ultra-low temperature (usually below 300 mK), which is mainly attributed to inhomogeneous magnetic doping, becomes a daunting challenge for potential applications. Here, a \textit{nonmagnetic}-doping strategy is proposed to produce ferromagnetism and realize QAHE in topological insulators. We numerically demonstrated that magnetic moments can be induced by nitrogen or carbon substitution in Bi$_2$Se$_3$, Bi$_2$Te$_3$, and Sb$_2$Te$_3$, but only nitrogen-doped Sb$_2$Te$_3…
Proximity Driven Enhanced Magnetic Order at Ferromagnetic Insulator / Magnetic Topological Insulator Interface
磁绝缘体/拓扑绝缘体（MI/TI）界面的磁交换驱动邻近效应为新现象提供了一个丰富的舞台，同时也为实现低能量耗散量子器件提供了一种途径。在这里，我们报告了一个戏剧性的增强邻近交换耦合的MI /磁TI EuS / Sb $_{2-x}$ V $_x$ Te $_3$混合异质结构，其中V掺杂是用来驱动TI（Sb $_{2}$ Te $_3$）磁性。我们观察到一个人工的反铁磁结构附近的MI/TI接口，这可能是增强的邻近耦合。邻近效应和掺杂之间的相互作用提供了深入了解可控工程的磁秩序使用混合异质结构。
Innovations in imaging technology involves finding strategies and materials suitable for detection applications over the entire infrared range. Herein, we propose a new design concept based on the unique feature of an excitonic insulator, namely, negative exciton transition energy ($E_t$). We demonstrate this concept using first-principles $GW$-BSE calculations on one-dimensional organometallic wire (CrBz)$_\infty$. The pristine (CrBz)$_\infty$ exhibits an excitonic instability due to a negative $E_t$ for the lowest exciton. Substitutional doping can continuously tune the $E_t$ from $\sim$0 to…
The observation of helical surface states in Bi-based three-dimensional topological insulators has been a challenge since their theoretical prediction. The main issue raises when the Fermi level shifts deep into the bulk conduction band due to the unintentional doping. This results in a metallic conduction of the bulk which dominates the transport measurements and hinders the probing of the surface states in these experiments. In this study, we investigate various strategies to reduce the residual doping in Bi-based topological insulators. Flakes of Bi$_2$Se$_3$ and Bi$_{1.5}$Sb$_{0.5}$Te$_{1.…
Inducing Lifshitz transition by extrinsic doping of surface bands in topological crystalline insulator Pb$_{1-x}$Sn$_{x}$Se
研究了窄隙半导体Pb $_{1-x}$ Sn $_{x}$ Se在其岩盐结构相中的拓扑保护表面态，x=0.45、0.23、0.15和0。角分辨光电子能谱学的本质上p掺杂的样品显示出两个狄拉克锥，偏心的时间反转不变点X的表面布里渊区的所有，但x=0的样品的明确指示。碱金属的吸附逐渐使表面带充满电子，驱动x>0拓扑晶体绝缘体系统通过Lifshitz转变，并从空穴到电子的费米表面。x>0样品中的电子掺杂带显示出狄拉克锥的完整构型，也证实了表面带的电子-空穴对称性。
The magnetic, magneto-transport and ARPES studies of Fe and S co-doped Bi2Se3 were investigated. With doping concentration magneto-resistance (MR) gradually decreases and for a certain doping concentration giant negative MR is observed which persists up to room temperature. Magnetic measurement indicates that the negative MR is observed when ferromagnetic ordering is induced with Fe doping. The magnetic ordering can be attributed with the RKKY interaction. Positive MR reappears with larger doping concentration which may be attributed to the decrease of FM ordering due to the turning off of the…
Correlated electron materials (CEMs) host a rich variety of condensed matter phases. Vanadium dioxide (VO2) is a prototypical CEM with a temperature-dependent metal-to-insulator (MIT) transition with a concomitant crystal symmetry change. External control of MIT in VO2 - especially without inducing structural changes - has been a long-standing challenge. In this work, we design and synthesize modulation-doped VO2-based thin film heterostructures that closely emulate a textbook example of filling control in a correlated electron insulator. Using a combination of charge transport, hard x-ray pho…
The quantum anomalous Hall (QAH) insulator is a topological quantum state with quantized Hall resistance and zero longitudinal resistance in the absence of an external magnetic field. The QAH insulator carries spin-polarized dissipation-free chiral edge current and thus provides a unique opportunity to develop energy-efficient transformative information technology. Despite promising advances on the QAH effect over the past decade, the QAH insulator has thus far eluded any practical applications. In addition to its low working temperature, the QAH state in magnetically doped topological insulat…
In this paper we explore possible phases arising from doping neutral excitons into a Mott insulator in the context of moiré bilayers. We consider two moiré layers coupled together only through inter-layer repulsion and there is a U(2)$\times$ U(2) symmetry. The densities of the two layers can be tuned to be $n_t=x,n_b=1-x$ with $n_t+n_b=1$. $x=0$ limit is a layer polarized Mott insulator and small $x$ regime can be reached by doping inter-layer excitons at density $x$. Charge gap can remain finite at small $x$, as is demonstrated experimentally in the WSe$_2$-hBN-WSe$_2$/WS$_2$ system. To capt…
