Session T10: Topological Insulators: Synthesis

Manipulation of topological states in a topological-insulator heterostructure

The Dirac fermions in the Topological insulators (TIs) are immune to backward scattering by nonmagnetic impurities or disorder. While experimental realizations of novel topological phenomena depend crucially on the inherent robustness of the topological surface states against perturbations, it turned out to be difficult to maintain stable surface properties under ambient atmosphere. This situation has been a hindrance for realizing novel topological phenomena and device applications of TIs. We present a novel approach to solve this problem- the heterostructure engineering where one can alter the stacking sequence of layers or insert different building blocks into the crystal. We have performed angle-resolved photoemission spectroscopy on (PbSe)$_5$(Bi$_2$Se$_3$)$_{3m}$, which forms a natural multilayer heterostructure consisting of a TI and an ordinary insulator. For $m = 2$, we observed a gapped Dirac-cone state within the bulk-band gap, suggesting that the topological interface states are effectively encapsulated by block layers. These results demonstrate that utilization of TI heterostructures is a new promising strategy for manipulating the topological states and realizing exotic quantum phenomena.   

Topologically tunable ultrafast Fano interference dynamics

Asymmetric Fano resonance arises from interference between continuum and discrete state. The asymmetric profile has attracted strong interests in understanding light-induced optoelectronic responses and corresponding applications. In conventional solids, however, the tunability of Fano resonance is generally limited by material's intrinsic property. Topological insulator is unique class of matters embodying both conducting Dirac surface and insulating bulk. If it is possible to manipulate the two coexisting states, then it should form an ideal laboratory for realizing a topologically tunable Fano system. In this work, with recently discovered topological phase transition in (Bi$_{1-x}$In$_{x})_{2}$Se$_{3}$, we report novel Fano interference phenomena. By engineering the spatial overlap between surface Dirac electrons and bulk phonon, we continuously tune, abruptly switch, and dynamically modulate the Fano profile. Ultrafast optical-pump terahertz-probe spectroscopy reveals that the controlled spatial overlap is responsible for the picosecond tunability of the asymmetric Fano profile, suggesting potentials toward optically controllable topological Fano systems.   

Adsorption and dissociation of H2O molecules on the topological insulator surface, Bi$_{2}$Se$_{3}$ (111)

Three-dimensional strong topological insulators (TIs) such as Bi$_{2}$Se$_{3}$, Bi$_{2}$Te$_{3}$ and Bi$_{2}$Sb$_{3}$ are intriguing for their surface metallicity in contrast to the insulating bulk. The metallic surface states of TI are known to be topologically protected and robust for impurities and disorders. In this work, we report first-principles calculations on the adsorption and dissociation of H$_{2}$O molecules on Bi$_{2}$Se$_{3}$ (111) to understand the chemical reactivity and the effect of oxidation on the surface metallicity. On the pristine (111) surface, H$_{2}$O molecule is found to chemisorb on a subsurface Bi atom and form two additional hydrogen bonds with neighboring surface Se atoms. The adsorbed H$_{2}$O molecule can be dissociated into a hydroxyl (OH) and H. The dissociated OH takes a surface Se site, and pushes up the Se atom that is bonded with the dissociated H. We examined a subsequent dissociation reaction of OH. The final reaction products are a substitutional O (O$_{\mathrm{Se}})$ and an H$_{2}$Se molecule floating in the vacuum. By examining the electronic structure, we found that the chemisorbed OH induces n-type doping. On the other hand, the O$_{\mathrm{Se}}$ and the adsorbed H$_{2}$O result in p-type doping. Throughout the whole chemical processes studied, the metallic surface state remains intact.   

Flexible Thermoelectric Fabrics Based on Layered Topological Insulator Bi$_{2}$Se$_{3}$ Nanoplates/Polyvinylidene Fluoride Composite

We report a highly-flexible and ultrathin thermoelectric fabrics based on topological insulator (TI) Bi$_{2}$Se$_{3}$ Nanoplates/PVDF Composite, which show a room temperature Seebeck coefficient, electrical conductivity, and figure of merit ZT -8 $\mu$V/K, 5000 S/m, 0.02, respectively. This results demonstrate that Bi$_{2}$Se$_{3}$ Nanoplates/PVDF composite exhibit favorable thermoelectric characteristics, which opens a new avenue to fabricate highly-flexible and lightweight sustainable energy sources that could be compatible with portable/wearable electronic devices. The low thermal conductivity of the composites ($\sim$ 0.42 W/(mK)) suggests the nonconducting host polymer matrix PVDF serves to bind the conducting topological insulator (TI) Bi$_{2}$Se$_{3}$ while still maintaining an adequate power factor and figure of merit. The flexible thermoelectric fabrics based on layered topological insulator Bi$_{2}$Se$_{3}$ Nanoplates/PVDF composite that with comparable thermoelectrical efficiency is only a typical example that showing the promising of the present method for further applications of 2D topological insulator like Bi$_{2}$Se$_{3}$, Bi$_{2}$Te$_{3}$ and Sb$_{2}$Te$_{3}$ At their current performance, if enough thermal energy is available, the composites could be used to provide sufficient thermoelectric power for low powered personal and portable electronics.   

Fermi level tuning and weak localization/weak antilocalization competition of bulk single crystalline Bi$_{\mathrm{2-x}}$Sb$_{\mathrm{x}}$Se$_{2}$Te compounds

In the investigation of the electrical transport properties of single crystalline Bi$_{\mathrm{2-x}}$Sb$_{\mathrm{x}}$Se$_{2}$Te (x $=$ 0.0, 0.6, 0.8, 1.0, 1.2, and 1.4) compounds, we observed a systematic change of the Fermi level from n-type metallic (x $=$ 0.0, 0.6) or small-gap semiconducting ( x $=$ 0.8) to p-type semiconducting (x $=$ 1.0) and metallic (x $=$ 1.2, 1.4), respectively, with increasing Sb-substitution concentration from the temperature-dependent electrical resistivity $\rho \left( T \right)$ and Hall resistivity $\rho_{xy} \left( T \right)$ measurements, respectively. The parent compound Bi$_{2}$Se$_{2}$Te exhibits linear negative magnetoresistance measurements at low temperatures. From the Hikamii-Larkin-Nagaoka analysis of the compounds (x $=$ 0.8 and 1.0), we found that there is a competing behavior between WL and WAL in terms of Sb-doping and magnetic field strength.   

Emergence of Decoupled Surface Transport Channels in Bulk Insulating Bi2Se3 Thin Films

In ideal topological insulator (TI) films the bulk state, which is supposed to be insulating, should not provide any electric coupling between the two metallic surfaces. However, transport studies on existing TI films show that the topological states on opposite surfaces are electrically tied to each other at thicknesses far greater than the direct coupling limit where the surface wave functions overlap. Here, we show that as the conducting bulk channels are suppressed, the parasitic coupling effect diminishes, and the decoupled surface channels emerge as expected for ideal TIs. In Bi2Se3 thin films with fully suppressed bulk states, the two surfaces, which are directly coupled below $\sim$10 QL, become gradually isolated with increasing thickness and are completely decoupled beyond $\sim$20 QL. On such a platform, it is now feasible to implement transport devices whose functionality relies on accessing the individual surface layers without any deleterious coupling effects.   

Epitaxial Growth of Artificial Graphene on Conventional Semiconductor Surface towards Room Temperature Topological Quantum States

Graphene is a 2D hexagonal lattice made of \textit{sp}$^{2}$ hybridized carbon. Fundamental understanding of graphene has recently spurred a surge of searching for 2D topological quantum phases in solid-state materials. Here we demonstrate the epitaxial growth of artificial graphene, in which the carbon atoms are replaced by other elements, on conventional semiconductor surface to realize large-gap topological quantum phases. We show that Si(111) surface functionalized with 1/3 monolayer of halogen atoms [Si(111)-$\sqrt 3 \times \sqrt 3 $X (X$=$Cl, Br, I)] exhibiting a trigonal superstructure, provides an ideal template for epitaxial growth of heavy metals, such as Bi, which self-assemble into a hexagonal lattice with high kinetic and thermodynamic stability. Remarkably, the Bi overlayer show the feature of a ($p_{\mathrm{x}}$, $p_{\mathrm{y}})$ analogue of graphene that exhibits quantum spin Hall state with an energy gap as large as $\sim$ 0.8 eV. Growth of transition metals lead to the discovery of a new 2D material, \textit{sd}$^{2}$ graphene, characterized with bond-center electronic hopping, which surprisingly transforms the atomic hexagonal lattice into a hidden kagome lattice and exhibits a wide range of topological quantum phases. For example, quantum anomalous Hall states can be realized in W@Si(111)-$\sqrt 3 \times \sqrt 3 $-Cl, with an energy gap of $\sim$ 0.1 eV. These findings may pave the way for future exploration of Si-based topological quantum phases, by exploiting epitaxial growth and current available semiconductor technology. This research was supported by DOE (Grant No: DEFG02-04ER46148).   

Population dynamics of Floquet-Bloch states on the surface of a topological insulator

Floquet-Bloch bands emerge in solid-state systems due to a coherent interaction between light and matter. Using time and angle resolved photoemission spectroscopy; we demonstrate that intense ultrashort midinfrared pulses hybridize with the surface Dirac fermions of a topological insulator to form states periodic in both momentum and energy. These states exhibit band gaps at avoided crossings which are dependent on the incident light polarization and the electron momentum. Circularly polarized light induces an additional gap at the Dirac point by naturally breaking time reversal symmetry. We further characterize these bands as a function of the incident light intensity and pump-probe delay time to understand the excitation and decay mechanisms of electrons in the Floquet-Bloch states.   

Surface and interface states of Bi$_{2}$Se$_{3}$ thin films investigated by second harmonic generation

Topological insulators (TIs) behave as a charge-gapped insulator in its interior, but hosting a spin-momentum-locked Dirac state at the surface. When the Fermi level crosses over conduction/valence band, undesirable bulk charge transport disturbs to exploit the surface nature, so that thin film TIs have been highlighted as a method to reduce bulk carrier effect due to large surface to volume ratio. In this presentation, we discuss surface and/or interface states for Bi$_{2}$Se$_{3}$ in form of film by exploiting second harmonic generation technique. Based on nonlinear susceptibility deduced from the model fitting, we investigate the details of band bending such as its direction and strength which were further addressed by examining terahertz field profile emitted from the sample. Finally, we discuss the evolution of these properties as a function of film thickness.   

Investigation of the Dirac states in a topological insulator, Bi(2)Se(3)

Topological insulators are bulk insulators with metallic surface states protected by time reversal symmetry. These surface states are spin-polarized, backscattering free and exhibit Dirac cone in their energy band structure, and thus are potential candidates for technological advances and realizing exotic phenomena. However, experiments show appearance of such topological order on the surface of metallic bulk and instability of the Dirac states in most of the materials studied. Thus, doping foreign elements to engineer the electronic states and get access to the surface states has become an outstanding problem. We studied the detailed electronic structure of Bi2Se3 using ARPES and DFT calculations and observe different behavior for different surface terminations. Se terminated surface exhibits an electron doping scenario with aging in contrast to the hole doped scenario in Bi terminated surface. The Dirac cone on Bi terminated surface is found to be most stable even in presence of impurities and is most suitable to engineer topological insulators. In addition, we observe that the Dirac cone has strong contribution from interface states between top two quintuple layers. All these results provide an insight of the emergence of topological order on real materials.   

Electrostatic Coupling between Two Surfaces of a Topological Insulator Nanodevice

We report on electronic transport measurements of dual-gated nanodevices of the low-carrier density topological insulator (TI) Bi$_{\mathrm{1.5}}$Sb$_{\mathrm{0.5}}$Te$_{\mathrm{1.7}}$Se$_{\mathrm{1.3}}$. In all devices, the upper and lower surface states are independently tunable to the Dirac point by the top and bottom gate electrodes. In thin devices, electric fields are found to penetrate through the bulk, indicating finite capacitive coupling between the surface states. A charging model allows us to use the penetrating electric field as a measurement of the intersurface capacitance C$_{\mathrm{TI}}$ and the surface state energy-density relationship $\mu $(n), which is found to be consistent with independent angle-resolved photoemission spectroscopy measurements. At high magnetic fields, increased field penetration through the surface states is observed, strongly suggestive of the opening of a surface state band gap due to broken time-reversal symmetry.   

A top-down approach to prepare bismuth bi-layer terminated Bi$_2$Se$_3$(0001)

A bi-layer of bismuth (Bi) in the (111) plane possesses strong spin-orbit coupling and represents a prototype 2D topological insulator (TI). In this study, we propose a novel top-down approach to prepare Bi bi-layer on 3D TI Bi$_2$Se$_3$(0001). Combining scanning tunneling microscopy, X-ray crystal truncation rod analysis and Auger electron spectroscopy we demonstrate that under controlled exposure of Bi$_2$Se$_3$ to atomic hydrogen flux the selenium is removed from the topmost quintuple layer and a flat Bi bi-layer terminated Bi$_2$Se$_3$, with Bi$_2$Se$_3$-terrace-size lateral extension is achieved. Our results suggest new prospectives to manipulate the electronic properties of both Bi and Bi$_2$Se$_3$ TIs.   

Efficient Dual-Gate Tuning of Fermi Level in Thin-Film Topological Insulator

Experimental studies of novel quantum phenomena predicted for three-dimensional (3D) topological insulators (TIs) often require tuning of the Fermi level across the Dirac point. Both back gating and top gating techniques have been successfully applied to 3D TI thin films, however, with a single gate the chemical potential of only one surface can be controlled effectively, which is not sufficient for many applications. Recently, we have succeeded in developing a comprehensive method for fabricating dual-gated devices on TI thin films grown by molecular beam epitaxy. The method combines 1) the transfer of a high-quality bulk-insulating (Bi$_{1-x}$Sb$_{x}$)$_{2}$Te$_{3}$ thin film grown on sapphire onto a Si/SiO$_{2}$ wafer, which serves as a back gate, and 2) the fabrication of a top gate by using a low-temperature deposition of SiN$_{x}$. We demonstrate that the dual gating allows effective tuning of the chemical potentials of the top and bottom surfaces across the Dirac point, which is manifested in a large peak of the sheet resistance accompanied by a sharp sign change of R$_{yx}$ upon sweeping the top and bottom gate voltages. This dual-gating method opens exciting opportunities for realization of various novel phenomena expected for 3D TIs.   

Air-Stable Electron Depletion of Bi$_2$Se$_3$ into the Topological Regime using Molybdenum Trioxide

Bismuth selenide (Bi$_2$Se$_3$) is a three-dimensional strong topological insulator of particular interest due to its relatively large bulk band gap (300 meV) and single set of topologically non-trivial surface states. However, persistent doping makes routine electronic access to the topological regime difficult. Here we explore surface transfer doping via molecular deposition as a route to bring the Fermi level into the topological regime and protect against ambient degradation. Bi$_2$Se$_3$ single crystals are cleaved in ultra-high vacuum and X-ray photoemission spectroscopy is used to measure the shifts in work function, Bi core levels, and charge state of Mo during deposition of MoO$_3$ molecules; the data indicate that MoO$_3$ can lower the Fermi level to within $\sim$ 100 meV of the Dirac point. Thin film transport demonstrates that $\sim$ 10$^{13}$ electrons can be depleted from the Bi$_2$Se$_3$ and that an MoO$_3$ capping layer is stable for days after exposure to ambient.