Session X31: Focus Session: Topological Insulators: Synthesis and Characterization - Scanning Tunneling Spectroscopy

Scanning tunneling spectroscopic studies of Dirac fermions and impurity resonances in the surface-state of a strong topological insulator Bi$_{2}$Se$_{3}$

Scanning tunneling spectroscopic studies of MBE-grown Bi$_{2}$Se$_{3}$ epitaxial films on Si (111) revealed surface-state (SS) characteristics of Dirac fermions and signatures of strong impurity resonances. The impurity resonances in this three-dimensional strong topological insulator (3D-STI) occurred near the Dirac energy (E$_{D})$ and diverged as the Fermi level (E$_{F})$ approached E$_{D}$. They were also highly localized within a region of radius $\sim $ 0.2 nm, beyond which the SS spectra of the 3D-STI recovered quickly, suggesting robust topological protection against non-magnetic impurities. Similar spectral characteristics and separations between E$_{F}$ and E$_{D}$ were also observed in the MBE-grown Bi$_{2}$Se$_{3}$ films on CdS. For sufficiently thin samples, opening of an energy gap due to wave-function overlap between the surface and interface layers was observed. The Rashba-like spin-orbit splitting further gave rise to spin-preserving quasiparticle interferences. Finally, the effect of different impurities (e.g. Cr and Mn) on the SS spectra of Bi$_{2}$Se$_{3}$ as a function of magnetic field will be reported. This work was supported by FENA and DARPA.   

Scanning tunneling spectroscopy studies of Bi2Te2Se

Topological insulators are a class of semiconductors characterized by the presence of current-carrying helical surface states lying within the bulk gap. The surface states of these materials possess massless Dirac-like dispersion. Helical spin texture of the surface states leads to suppression of backscattering in these materials. Results of scanning tunneling spectroscopy study of Bi2Te2Se (BTS) topological insulator will be presented.~~Similar to previously studied Bi2Te3 and Bi2Se3 the new material shows a relatively large band gap and a simple surface band structure. High bulk resistivity and high surface electron mobility make it a compound of interest for potential applications. Differential conductance mapping with scanning tunneling microscope is used to visualize surface states of this novel highest-bulk-resistivity topological insulator. These experiments enable us to assess the variation of local density of states in this compound under different growth conditions and to correlate the findings with transport properties   

Scanning tunneling spectroscopy of stripe induced spatial modulations in the electronic structure of Bi2Te3+d

To take full advantage of the unique properties of topological insulators, a huge amount of effort has been spent in exploring the tunability of their charge carrier density or magnetism by chemical doping, creating an ever increasing need for probing and understanding the real-space electronic response. In this study, we investigate the effects of a 1D periodic modulation (stripes) on the electronic structure of Bi2Te3+d. Using a combination of spectroscopic mapping, Fourier transform spectroscopy and Landau level measurements we map out the energy-momentum dispersion of the surface state over a large energy range and show that the stripes have a non-trivial effect on the bulk and surface state band structure.   

Scanning tunneling microscopy study of ultrathin topological insulator Bi$_2$Te$_3$ nanoribbons

Currently there is an increasing interest in the study of topological insulators (TI) nanostructures as a result of their large surface-to-volume ratio, which allows the manifestation of surface conduction states without masking by bulk carriers. We performed low-temperature scanning tunneling microscopy (STM) measurements of the surface of TI Bi$_2$Te$_3$ nanostructures grown on HOPG. Although surface states of TIs are inherently robust against almost any surface modifications, these materials are prone to various surface chemical reactions which are taken into account when preparing samples for devices and STM study. Our STM measurements reveal the presence of ultrathin nanostructures (nanoribbons and nanoplates) which show an important growth anisotropy, with lateral dimension growing much faster than the vertical thickness dimension. Nanoribbons group mainly in bunches, with an aspect ratio of 1:300 and thickness down to 6 nm (6 quintuple layers). Nanoplates with hexagonal morphology of lower than 20 quintuple layers thick were also found, suggesting both one- and two-dimensional preferential growth.   

The field dependence of quantized Landau levels on Bi$_{2}$Te$_{3+d}$ via scanning tunneling spectroscopy

Measurements of Landau level (LL) spectra by scanning tunneling spectroscopy can provide important information on quasi-particle lifetime, effective g-factor as well as the dispersion of surface state bands. We have studied the effect of magnetic fields on spatially inhomogeneous samples of the topological insulator Bi$_{2}$Te$_{3+d}$. Using spatial maps of Landau levels as a function of magnetic field, we show that the surface state electrons near the Dirac point are surprisingly sensitive to perturbations. The magnetic field dependence also allows us to obtain an upper limit to the effective g-factor for the surface state electrons.   

Landau quantization and the thickness limit of topological insulator thin films of Sb$_{2}$Te$_{3}$

We report the experimental observation of Landau quantization of molecular beam epitaxy grown Sb$_{2}$Te$_{3}$ thin films by a low-temperature scanning tunneling microscope. Different from all the reported systems, the Landau quantization in Sb$_{2}$Te$_{3}$ topological insulator is not sensitive to the intrinsic substitutional defects in the films. As a result, a nearly perfect linear energy dispersion of surface states as 2D massless Dirac fermion system is observed. We demonstrate that 4 quintuple layers are the thickness limit for Sb$_{2}$Te$_{3}$ thin film being a 3D topological insulator.   

Multiband quasiparticle interference in the topological insulator Cu$_x$Bi$_2$Te$_3$

One of the main interests in topological materials is their purported robustness against disorder. In the Bi$_{2}$X$_{3}$ family (X=Se,Te) cubic spin-orbit coupling terms play an important role through their effect on the dispersion of the surface states. The cubic terms are also responsible for the restoration of Friedel like oscillations around impurity sites, which may be important in surface transport processes. We have therefore investigated how impurity scattering affects the surface states using a combination of Fourier-transform scanning tunneling spectroscopy (FT-STS) and calculations of the charge density oscillations expected from the cubic spin-orbit coupling terms [1]. FT-STS allows us to map out the energy-momentum relation of the important scattering wave-vectors, which can be compared to scattering vectors predicted from a self-consistent single impurity scattering calculation. To fully explain the features observed in our experiments we need to take the condu ction band into account. Namely, for energies where the surface states and the conduction band overlap, the dominant scattering process turns out to be interband impurity scattering. \\[4pt] [1] E. van Heumen et al., arXiv: 1110.4406   

LT-STM study of the topological insulator Bi$_{2}$Se$_{3}$ with superconducting and/or magnetic over layers

Superconducting and/or magnetic over layers structures on topological insulator (TI) surfaces were suggested as a potential candidate to create Majorana bound states and 1D Majorana fermions (spin $\raise.5ex\hbox{$\scriptstyle 1$}\kern-.1em/ \kern-.15em\lower.25ex\hbox{$\scriptstyle 2$} $, particle = antiparticle). The latter can serve as a platform for topological quantum computation since two Majorana bound states equal one fermion bound state and two degenerate states (full/empty) forming one qubit. In this work we study the TI Bi$_{2}$se$_{3}$ using a low temperature STM. The data shows the signature energy gap $\Delta \quad \sim $ 0.3 eV, metallic surface states and typical defects. Following the experimental challenges to create and detect Majorana bound or chiral edge states, we present our study of Nb and/or Fe films on the Bi$_{2}$se$_{3}$ surface.   

Visualizing the inhomogeneous response of Dirac surface states to bulk disorder in topological insulators

The Dirac dispersion and helical spin texture of surface states in topological insulators render them resilient to backscattering. This remarkable property, assured by time reversal symmetry, should give rise to enhanced surface conductivity as the Dirac states anti-localize in presence of disorder. We have used scanning tunneling microscopy (STM) and spectroscopy to study the response of the surface states to both magnetic and non-magnetic dopants [HB et al. Nat. Phys. doi:10.1038/nphys2108]. We find that helicity provides protection from scattering, irrespective of the magnetic nature of the individual scaterrers and even of ferromagnetic correlations among them. However, the charged defects in the bulk induce pronounced fluctuations in energy, momentum and helicity of the surface states. Agreement with a theoretical model, derived for the response of Dirac states to charged disorder in graphene, further implies that such fluctuations limit the attainable surface mobility. Although we show that the potential energy landscape induced by the bulk defects does not localize the Dirac surface states, our results suggest that reducing charged defects content is essential for tuning the chemical potential to the Dirac energy and enhancing mobility of the novel surface states.   

Probing surface state conductance of topological insulator Bi$_{2}$Se$_{3}$ with scanning tunneling potentiometry

Topological insulators such as Bi$_{2}$Se$_{3}$ have unique surface states. How do electrons actually flow on the surface of a real Bi$_{2}$Se$_{3}$ sample? We study this question using scanning tunneling potentiometry. In this measurement, a lateral current flows in the sample while the local potential is mapped on the surface using a scanning tunneling microscope. This technique can be used to identify with atomic resolution the potential drops in the current-carrying pathways at the surface, and is ideally suited to measure the properties of quasi-2D materials such as graphene. Our topological insulator samples are MBE grown films of Bi$_{2}$Se$_{3}$ on a sapphire substrate. We will describe both the surface morphology and its effect on the current carrying pathways in the material.   

Probing high energy, unoccupied states on the surface of pristine and Fe doped Bi$_{2}$Te$_{3}$ by scanning tunneling spectroscopy

We probe the surface state of pristine and Fe-doped topological Insulator Bi$_{2}$Te$_{3}$ by using Fourier Transform Scanning Tunneling Spectroscopy (FT-STS). FT-STS allows us to probe the surface state dispersion far above the Fermi energy, a regime inaccessible to angle resolved photoemission spectroscopy. We report the observation of novel multi scattering channels that emerge at high energies along the $\Gamma $M and $\Gamma $K directions. The possible origins of these channels including spin-orbit scattering will be discussed.   

STS studies of the surface of Bi2Se3

We apply scanning tunneling spectroscopy to characterize the surface of the topological insulator Bi2Se3. Spectroscopy reveals that the minimum in the local density of states (LDOS) does not actually vanish in the region where Dirac cone states exist. We demonstrate with density functional theory calculations that this can be understood in terms of an asymmetric addition to the LDOS associated with a contribution from the bulk valence band that overlaps in energy with the Dirac point. We will discuss the origin of the fluctuations in the LDOS seen in the experiment near 0.2 eV above the Dirac point, which are associated with tunneling into the lowest conduction band states.   

STM imaging of impurity resonances on Bi2Se3

In this paper we present detailed study of the density of states near defects in Bi2Se3. In particular, we present data on the commonly found triangular defects in this system. While we do not find any measurable quasiparticle scattering interference effects, we do find localized resonances, which can be well fitted by theory [1] once the potential is taken to be extended to properly account for the observed defects. The data together with the fits confirm that while the local density of states around the Dirac point of the electronic spectrum at the surface is significantly disrupted near the impurity by the creation of low-energy resonance state, the Dirac point is not locally destroyed. We discuss our results in terms of the expected protected surface state of topological insulators.   

Probing Dirac Fermions in Bi-based Semimetals by Cryomagnetic Scanning Tunneling and Point Contact Spectroscopy

The topological semimetal Bi$_2$Se$_3$ is distinguished by the presence of two-dimensional Dirac fermions with strong spin-orbit coupling. The linear dispersion of Dirac fermions in Bi$_2$Se$_3$ was recently observed by scanning tunneling spectroscopy measurements of the Landau level spacing versus magnetic field. In this work we extend the field-dependent spectroscopy study of Dirac fermions to other Bi-based semimetals, for both topological and non-topological cases, using cryomagnetic scanning tunneling and point contact probes on single crystals down to 300 mK and up to 9 T. The spectral dependences on field strength and field direction are examined, in an effort to elucidate the role of spin-orbit coupling in each case.   

Thickness and Wave-Vector Dependence in the Inter-Surface Coupling of Topological Surface States of Sb(111) Films

Sb is a semimetal but possesses topological surface states (SSs) [1]. Taking advantage of quantum confinement effect for bulk states and topological protection for SSs, an Sb thin film could be a topological insulator. We explore this possibility using Fourier-transform scanning tunneling spectroscopy (FT-STS) and \textit{ab initio} calculations for Sb(111) films of thickness $\le $ 30 bilayers (BL). Quasiparticle interference (QPI) patterns of SSs and calculated band structures exhibit dramatic dependence on film thicknesses, reflecting variation of inter-surface coupling of SSs with film thickness and wave vector \textbf{k}. One Kramers-pair of SSs forming a Dirac point at \textbf{k} = 0 exist on each surface for 6-BL or thicker films. The inter-surface coupling of SSs not far away from \textbf{k} = 0 is significant in the QPI patterns at $\sim $ 10 BL. Such coupling is due to a relative large penetration depth of these SSs results in unpolarized states with large wave function amplitude in the interior of the film. [1] D. Hsieh et al., Science 323, 919 (2009); K. K. Gomes et al., e-print arXiv:0909.0921 (2009); J. Seo et al., Nature 466, 343 (2010).