Session D42: Topological Insulators: Experiment

Topological insulator Sb$_{2}$Te$_{3}$ thin films grown by MBE

Atomically flat Sb$_{2}$Te$_{3}$ thin films were grown by molecular beam epitaxy (MBE) on Si(111) substrate. Layer-by-layer growth was characterized by real time reflection high-energy electron diffraction (RHEED) intensity oscillations. \textit{In situ }angle-resolved photoemission spectroscopy (ARPES) and scanning tunneling microscopy (STM) measurements reveal that the as-grown films are stoichiometric Sb$_{2}$Te$_{3}$. When the thickness is larger than two quintuple layers, the films show the predicted topological property with a single Dirac-cone on the surface. The measured band structure for the films with a thickness from one to five quintuple layers agrees well with our first principle calculations.   

Surface States on Topological Insulators: Angle-Resolved Photoemission Spectroscopy Studies

We have studied the electronic structure of several topological insulators using high resolution Angle-Resolved Photoemission Spectroscopy. We have observed two types of metallic surface states: a) pairs of spin-orbit split states with opposite spin chirality and b) unpaired spin chiral state. Through the adsorption of various atomic/molecular species, we were able to vary the filling, tune the spin-orbit coupling and alter the spin texture of these states.   

Experimental demonstration of the topological surface states protected by the time-reversal symmetry

In this work, we use a low temperature scanning tunneling microscope (STM) to observe the standing waves formed by the nontrivial surface states of topological insulator Bi$_{2}$Te$_{3}$. Molecular beam epitaxy was used to grow Bi$_{2}$Te$_{3}$(111) film on Si(111) substrate. The interference fringes are caused by the scattering of the topological states off Ag impurities and step edges on the Bi$_{2}$Te$_{3}$(111) surface. By studying the voltage-dependent standing wave patterns, we determine the energy dispersion E(k), which confirms the Dirac cone structure of the topological states. We show that the backscattering of the topological states by nonmagnetic impurities is completely suppressed, which is very different from the conventional surface states. The absence of backscattering is a spectacular manifestation of the time-reversal symmetry, which offers a direct proof of the topological nature of the surface states.   

STM imaging of electronic waves on the surface of Bi2Te3: topologically protected surface states and hexagonal warping effects

Scanning tunneling spectroscopy studies on high-quality Bi$_2$Te$_3$ crystals exhibit perfect correspondence to ARPES data, hence enabling identification of different regimes measured in the local density of states (LDOS). Oscillations of LDOS near a step are analyzed. Within the main part of the surface band oscillations are strongly damped, supporting the hypothesis of topological protection. At higher energies, as the surface band becomes convex, oscillations appear which disperse with a particular wave-vector that may result from an unconventional hexagonal warping term.   

Quantization Rules for Topological Surface States

Recently, a new class of chiral electronic states on the surface of insulators with strong spin-orbit interaction has been discovered [1,2]. One of the unusual properties of this material is the elimination of the possibility of backscattering between states of orthogonal spins [3]. An unexplored aspect of these materials is the question of how the absence of backscattering affects the energy quantization in a confined geometry. We have studied the variations in the local density of states in the regions confined between adjacent atomic step edges on Sb(111) using a low temperature scanning tunneling microscopy. We have found a remarkably Dirac-like quantization with E$_{n}$= nE$_{0}$ (n=integer) for the confined surface states of Sb over a wide range of energies. Our experiments also demonstrate the absence of confinement and quantization in the regime where backscattering cannot occur. [1] L. Fu, C. L. Kane, and E. J. Mele, Phys. Rev. Lett. 98, 106803 (2007) [2] D. Hsieh \textit{et al.}, Nature 452, 970 (2008) [3] P. Roushan et al., Nature 460, 1106 (2009)   

The warped Fermi contours of Bi$_{2}$Te$_{3}$ topological surface states

It has been predicted and experimentally verified [1,2] that novel two-dimensional metallic states exist on the surface of certain insulators known as topological insulators. Among the known topological insulators, Bi$_{2}$Te$_{3}$ emerges as the most practical for device applications due to its large bulk band gap and surface states consisting of only a single Dirac cone. To study the Dirac Fermions on the surface of Bi$_{2}$Te$_{3}$, we have used a cryogenic scanning tunneling microscope. Close to atomic step edges, energy resolved conductance mapping of surface states shows interference pattern resulting from scatterings. The interference pattern observed enabled us to investigate the dispersion, the spin chirality, and the role of Fermi surface warping in scattering for these surface states. We will present these results in connection with angle-resolved photoemission measurements of the contours of constant energy. In addition, we touch upon the physical consequence of broken symmetry at atomic step edges for the surface states, which so far has been overlooked.   

Surface States of Doped Topological Insulators with Superconducting and Magnetic Correlations

The proximity effects induced by superconducting and magnetic correlations are expected to alter the nature of the topological Dirac surface states. Certain schemes that involve the deposition of both a superconductor and a ferromagnet are predicted to allow the creation, detection, and manipulation of the elusive Majorana Fermionic excitations. In light of those suggestions, we studied the implications doping-induced bulk superconductivity and ferromagnetism has on the Dirac surface states of topological insulators. We utilized high resolution scanning tunneling microscopy and spectroscopy to measure the behavior on the surface of Bi$_{2}$Se$_{3}$ whose bulk turns superconducting when intercalated with Cu. We also investigated Mn doped Bi$_{2}$Te$_{3}$ that was recently shown to become ferromagnetic. Such magnetic impurities also relieve the spin-selective scattering dictated by the surface-state chirality, thus enabling backscattering of the Dirac fermions which is absent in stoichiometric Bi$_{2}$Te$_{3}$.   

Strong scattering between surface and bulk electrons in a parent topological insulator Bi$_2$Se$_3$

We report on angle resolved photoemission spectroscopic studies on a parent topological insulator, Bi$_2$Se$_3$. The line width of the spectral function (inverse of the quasi-particle lifetime) shows an anomalous behavior. This behavior can be reasonably accounted for by assuming decay of the quasi-particles predominantly into bulk electronic states through electron-electron interaction and defect scattering. Studies on aged surfaces reveal that topological metallic states are very much unaffected by the potentials created by absorbed atoms or molecules on the surface, indicating that topological states are indeed protected against weak perturbations.   

Coexistence of 2D-surface and 3D-bulk Fermi surfaces in Bi$_{2}$Se$_{3}$

The resurgence of interest in Bi$_{2}$Se$_{3}$, and related compounds, has been driven by the prediction that these materials can fulfill the requirements for the observation of a topological insulating state, i.e. the electronic spectrum being fully gapped in the bulk with a gapless, dissipationless surface state. The key to interpreting transport data from these materials is resolving the relative contributions of surface and bulk conductivity. To this end, we use high magnetic field, rf- and microwave-spectroscopy to selectively couple to the surface conductivity at high frequency. In the frequency range of a few GHz we observe a crossover from quantum oscillations indicative of a small 3D Fermi surface, to cyclotron resonance indicative of a 2D surface state. The implications of this observation with respect to the existence of a topologically protected Dirac-cone will be discussed.   

Mapping the band structure of three-dimensional topological insulator Bi2Se3 in two-dimensional limit

In this work, with in situ angle-resolved photoemission spectroscopy, we have investigated the thickness dependent band structure of molecular beam epitaxy grown Bi2Se3, a typical three-dimensional insulator, from 1 quintuple layer (QL) up to 200QL. An energy gap is observed in the topologically protected metallic surface states of bulk Bi2Se3 below the thickness of 6QL, due to the coupling between the surface states from two opposite surfaces of the Bi2Se3 film. The gapped surface states exhibit sizable Rashba-type spin-orbit splitting, resulting from breaking of structural inversion symmetry induced by 6H-SiC substrate. The spin-splitting can be controlled by tuning the potential difference between the two surfaces, which can be utilized into electrical spin manipulation.   

Berry Phase Measurements of Topological Surface States

In special two-dimensional systems, such as graphene and topological insulators, electrons can mimic the behavior of relativistic Dirac particles and exhibit a quantized Berry's phase of $\pi $. In this experiment we harness the power of complimentary interferometry methods to quantify the quantum phase present in pure antimony, a topological insulator parent matrix. We extract the Berry's phase from the quantum magneto-oscillations measured in low-temperature transport (Shubnikov-de Haas effect) and in local tunneling conductance (TMCO). We also combine scanning tunneling spectroscopy (STS) with band structure determined by angle-resolved photoemission spectroscopy (ARPES) to develop a novel technique for quantifying the Berry's phase of surface electrons. These techniques enable robust determination of the Berry's phase that can remarkably be extended to high magnetic fields, providing a new window on the dynamics of these unique spin-polarized carriers.   

Scanning Tunneling Microscopy and Spectroscopy of Topological Insulators

We performed cryogenic scanning tunneling microscopy and spectroscopy measurements of the surface and defect states in topological insulators Bi2Se3 and Bi2Te3. Our measurements demonstrate gapless surface states in both materials. While Bi2Te3 is naturally p-doped, we observed both p- and n-doped regions on the cleaved surface. These observations can be correlated with local cleaving within quintuple layers seen from topography measurements, suggesting a new route for surface doping of topological insulators. We also describe unique clover-shaped defects states appearing inside the bulk valence band. Our band structure calculations show that these states originate from a combination of bonding anisotropy and surface band bending, resulting in formation of subsurface resonances.   

Topological Insulator Nanoribbons and Nanosheets Studied by Scanning Tunneling Microscopy

Topological insulators (TIs) have recently emerged as a new phase of matter characterized by topologically-protected gapless surface states. These conductive surface states open the door for novel dissipationless electronic and spintronic devices based on a growing library of TI materials. Nanoscale TIs are particularly attractive because their high surface-to-volume ratio enhances the relative contribution of the surface states. We report a simple method for producing samples of Bi$_{2}$Se$_{3}$ nanostructures on a gold substrate. We present the first scanning tunneling microscopy (STM) measurements on TI nanostructures, and characterize them from their largest dimension ($>$10 micron) down to atomic scale. We observe single crystal structures that show preferential growth in both one and two dimensions (nanoribbons and nanosheets), with both morphologies exhibiting atomically precise edges. Our observations confirm the high-quality nature of the TI nanostructures, and their suitability for use in a new generation of dissipationless nanoscale electronics.   

Growth and electronic structure of topological insulator Bi$_{2}$Se$_{3}$ thin films on graphene/SiC(0001) substrate

We report on the growth of atomically smooth and stoichiometric Bi$_{2}$Se$_{3}$ thin films on bilayer graphene prepared on SiC(0001) substrate using molecular beam epitaxy. It is found that the position of the Dirac point changes with film thickness, and that the Dirac transport regime appears in Bi$_{2}$Se$_{3}$ film as thin as 10 quintuple layers. By mapping the step edge-induced interference patterns with scanning tunneling microscopy, a linear Dirac surface state band is obtained. By manipulating the charge states of subsurface Fe acceptors, we demonstrate the presence of a bulk bandgap in the Bi$_{2}$Se$_{3}$ films. The high quality films pave a way for fundamental research of topological insulators, and are important for potential spintronic and quantum-computing applications at room temperature.   

Systematic control of surface Dirac fermion density on topological insulator Bi$_2$Te$_3$

Three dimensional (3D) topological insulators are quantum materials with a spin-orbit induced bulk insulating gap that exhibit quantum-Hall-like phenomena in the absence of applied magnetic fields. They feature surface states that are topologically protected against scattering by time reversal symmetry. The proposed applications of topological insulators in device geometries rely on the ability to tune the chemical potential on their surfaces in the vicinity of the Dirac node. Here, we demonstrate a suite of surface control methods based on a combination of photo-doping and molecular-doping to tune the Dirac fermion density on the topological (111) surface of Bi$_2$Te$_3$. Their efficacy is demonstrated via direct electronic structure measurements using high resolution angle-resolved photoemission spectroscopy. These results open up new opportunities for probing topological behavior of Dirac electrons in Bi$_2$Te$_3$. At least one of the methods demonstrated here can be successfully applied to other topological insulators (Bi$_{1-x}$Sb$_x$, Sb$_2$Te$_3$ and Bi$_2$Se$_3$). More importantly, our methods of topological surface state manipulation demonstrated here are highly suitable for future spectroscopic studies of topological phenomena which will complement the transport results gained from the traditional electrical gating techniques.