Session A29: Three Dimensional Topological Insulators: Chalcogenides

Superconductivity in a topological insulator Sb$_2$Te$_3$

We report an observation of superconductivity in a topological material Sb$_2$Te$_3$ synthesized under modest pressure ($\sim$ 5.5~MPA ) that has the zero-field superconducting transition temperature $T_c = 8.3$~K -- the highest among any topological systems reported thus far. High resolution TEM and XRD Rietveld refinement analysis of the superconducting crystals show that while there is a 0.2\% elongation of the lattice parameter in the \textit{c}-direction, the rhombohedral van der Walls unit cell structure is preserved. The upper critical field $H_{c2}$ anisotropy is surprisingly small, only $\sim 1.5$, much smaller than the crystalline anisotropy of $\sim 8$. This anisotropy appears consistent with the paramagnetically limited critical field, given the reported large value ($\sim 10$) of the \textit{g}-factor. The diamagnetic state of this new superconductor is also unusual, since even in the normal state the system supports large orbital currents. We will discuss our observations in the context of topological superconductivity and Dirac energy-momentum dispersion of the surface states.   

Bi$_{1-x}$Sb$_{x}$(110): A non-closed packed surface of a topological insulator

Topological insulators are characterised by an insulating bulk band structure, but topological considerations require their surfaces to support gap-less, metallic states. Meanwhile, many examples of such materials have been predicted and found experimentally, but experimental effort has concentrated on the closed-packed (111) surface of these materials. Thus, the theoretical picture of an insulating bulk embedded in a metallic surface from all sides of a crystal still needs to be confirmed. Here we present angle-resolved photoemission spectroscopy results from the (110) surface of the topological insulator Bi$_{1-x}$Sb$_{x}$ ($x \approx 0.15$). The observed band structure and Fermi contour are in excellent agreement with theoretical predictions and slightly different from the electronic structure of the parent surface Bi(110), in particular around the $X_1$ time-reversal invariant momentum. We argue that the preparation of surfaces different from (111) opens the possibility to tailor the detailed electronic structure and properties of the topological surface states.   

Mass acquisition of Dirac fermions in the presence of magnetic doping in the topological insulator Sb$_{2}$Te$_{3}$

The nontrivial bulk band topology and time reversal symmetry yield gapless surface states in three dimensional topological insulators. The gapless nature of surface states in strong topological insulator is predicted to be violated by time-reversal-symmetry breaking perturbations, which opens back-scattering channels between Kramers pairs and induces a massive gap near the Dirac point of surface states. Such a massive Dirac fermion system gives rise to an unconventional magnetoelectric response relating to many exotic phenomena such as half-quantized anomalous Hall effect, topological quantized magnetoelectric effect and even the magnetic monopole. Here we introduce time-reversal-symmetry breaking by doping Cr atoms into the topmost quintuple layer or into the bulk of Sb$_{2}$Te$_{3}$ thin films. We demonstrate for the first time by Landau level spectroscopy the deviation of zero modes, which indicates the acquirement of a mass term in the presence of surface or bulk magnetic doping. We also show that the magnitude of the mass term in the surface states depends on both the Cr doping level and the magnetic field, offering a new way of measuring the doping- and field-dependence of local magnetization of dopants. Our observation suggests Cr-doped Sb$_{2}$Te$_{3}$ is a promising candidate for realization of proposed novel magnetoelectric effects.   

Possible topological insulating state in bismuth doped with arsenic: magneto-optical study

Bismuth and its alloys with antimony have attracted attention in recent years due to possible realization of topological insulating state. In this study we have used infrared and magneto-optical spectroscopies to probe the electrodynamic response of bismuth doped with 1.0 $\%$ of arsenic. The spectra will be presented for temperatures down to 5 K, and in magnetic fields as high as 18 Tesla. The results reveal strong magneto-optical activity, especially around the plasma minimum in reflectance. These findings will be compared and contrasted with magneto-optical results on topological insulator Bi$_{1-x}$Sb$_x$.   

Exotic magnetic properties of diluted magnetic binary chalcogenides

Using first-principles Green function approach we studied electronic and magnetic properties of diluted magnetic binary chalcogenides A$_2$B$_3$, doped with transition metals substituing the A element. The electronic structure of the impurities in the chalcogenides is mainly featured by the crystal field splitting. We found that two main mechanisms are responsible for long-range magnetic order in these materials: hole mediated magnetism within the layer of A atoms and indirect interaction between magnetic moments via a B atom. We also estimated Curie temperature of these systems, which was found in good agreement with the available experimental data. Our results shed light on the understanding of magnetic interaction and control in toplogical insulators.   

Magneto-transport properties of the ternary topological insulator (Bi$_{0.5}$Sb$_{0.5})_{2}$Te$_{3}$ in the presence of electrostatic gating and magnetic impurity

A three-dimensional topological insulator, (Bi$_{0.5}$Sb$_{0.5})_{2}$Te$_{3}$, is used to characterize the electronic properties of the spin helical conducting surface state. Epitaxial films are grown via MBE on (111) SrTiO$_{3}$ substrate, which serves as the gate dielectric. Magnetoresistance (MR) and Hall effect measurements have been performed at various back gate voltages. Ambipolar field effect has been observed, enabling effective tuning of the Fermi level across the band gap. Weak antilocalization effect is identified and used to differentiate the surface state. The Hikami-Larkin-Nagaoka (HLN) equation is used to analyze the MR data and the results show the top and bottom surfaces become decoupled when the Fermi level is in the bulk band gap. We also examine the effects of paramagnetic impurity (MI), which introduces time reversal symmetry breaking scattering, on the TI surface states. Taking advantage of the unique capability of \textit{in situ} deposition in a customized dilution refrigerator, paramagnetic Cr atoms were incrementally quench-condensed onto the sample surface and transport measurements were performed at each MI density. The procedure eliminates any sample-to-sample variation and complications from air exposure. Pronounced changes in the weak antilocalization effect and the sample carrier density with increasing MI concentration were observed. Possible origins of these observations will be discussed.   

Visualizing Landau levels of Dirac electrons in Bi$_{2}$Te$_{3}$ in a one dimensional potential

When a magnetic field is applied to a solid, the electrons fall into discrete, highly degenerate Landau levels. In each Landau level the wavefunction has a certain characteristic spread, which increases with the energy (index) of the level. This has important physical consequences especially in the presence of spatial inhomogeneity. Using scanning tunneling spectroscopy, we have examined the Dirac electrons on Bi$_{2}$Te$_{3}$ under a magnetic field and subject to a smooth one-dimensional periodic potential. We find that the lowest Landau levels track the potential variation, but the higher levels are more homogeneous. Through a calculation of the Landau level wavefunctions, we form a coherent picture of how their spread interacts with the potential landscape, explaining the experimental data. Our findings have important implications for transport and magneto-resistance measurements in Dirac materials with engineered potential landscapes.   

Topological States Ruled by Stacking Faults in Bi$_{2}$Se$_{3}$ and Bi$_{2}$Te$_{3}$

Extended defects like stacking faults (SF) can originate topologically protected metallic states in bulk topological insulators (TI). These induced topological states are a response to the weakening of the inter-layer van der Waals interactions due to the SF defect. In TI thin films the degeneracy of Dirac bands of opposite surfaces can be lifted upon the formation of SF defects. Such slab asymmetry can promote a net spin current, absent of backscattering processes, in thin film made of TIs. These results have been obtained by fully relativistic first principles calculations.   

Proximity effect in MBE grown bismuth chalcogenide thin films

Topological insulators (TIs) comprise a new state of matter which provides access to novel physics. Of the set of materials that have exhibited spectroscopic evidence of topologically protected surface states, bismuth chalcogenide systems have garnered particular interest due to their relatively large nominal bulk band gap and single Dirac cone near the Fermi surface. We are studying the superconducting proximity effect in MBE grown thin films of Bi$_{2}$Se$_{3}$, Bi$_{2}$Te$_{3}$, and ternary compounds. After \textit{in situ} deposition of a low temperature superconductor, the films are patterned into devices containing a matrix of superconducting islands of tunable size and density on top of the TI layer. We discuss growth optimization, device processing, the role of the superconductor-TI interface, and proximity effect transport results.   

Topological phase transition induced by atomic displacements in PbS and PbTe

Discovery of 3D topological insulator initiates exploration of finding new materials having topological insulating phase or mechanisms for topological phase transitions. Introducing interactions or strains into non-interacting electron systems, for example, can produce non-trivial topological phases in them otherwise having trivial band insulating phase at equilibrium conditions. Using first-principles methods, we study emerging topological phases in band insulating PbS and PbTe, which are induced by selective atomic displacements. Phonon modes corresponding to the displacements are identified and conditions of inducing the topological phase transition are suggested. We show that surface states develop flickering Dirac cones at band-inversion k-points upon dynamic atomic displacements with sufficient amplitude. Our results demonstrate that elementary excitation modes like phonon can induce topological phases in trivial band insulators.   

Optical selection rules for electron-hole pair excitation in 3D topological insulators

Experiments using ARPES, which is based on the photoelectric effect, have shown that the surface states in 3D topological insulators (TI) are helical. Here we consider Weyl interface fermions due to band inversion in narrow-bandgap semiconductors, such as $Pb_{1-x} Sn_{x} Te$ and $Bi_{1-x} Sb_{x}$. We determine the optical selection rules of electron-hole pair (EHP) excitation by means of the solutions of the 3D Dirac equation. While EHPs in graphene are generated through intraband transitions, we show that in 3D TI they are generated through both intraband and interband transitions. For their analysis, we calculate explicitly the electric dipole matrix elements by means of bandstructure calculations for $Pb_{1-x} Sn_{x} Te.$ We will introduce a spin helicity operator in 3D TI. Our results are crucial for future opto-spintronic devices based on 3D TI.   

Disorder tuned anomalous Hall effect in thin films of Cr doped topological insulators

The anomalous Hall effect (AHE) -- an appearance of a voltage transverse to the electric current in the absence of an external magnetic field -- is a process that arises from the spin-orbit coupling between current and magnetic moments that has been fundamentally linked to the topological nature of the Hall current. Recent first-principle calculations predict that when topological insulators (TIs) are doped with transition metal ions, such as Cr or Fe, a novel \emph{magnetically ordered} insulating state will form -- a state that in thin samples may support a \textit{quantized} anomalous Hall conductance. Here we report an observation of AHE in \textit{rf} sputtered thin Cr doped films of Bi$_2$Te$_3$. The anomalous Hall resistivity $\rho_{xy}$ scales with the longitudinal resistivity squared, $\rho_{xx}^2$, and a distinct ferromagnetic hysteretic response (loops) at temperatures below 10 K with coercive fields of the order of 0.5 T is observed. In as-deposited films the resistivity is below the resistivity quantum $h/e^2$. Using 2.5 MeV electron beam irradiation with varying fluence we can tune the resistivity upward by orders of magnitude. A large effect of controlled quenched point disorder on the quantization of AHE in Bi$_2$Te$_3$ will be discussed.   

Imaging single-atom impurities in topological materials

We use low temperature spectroscopic scanning tunneling microscopy to study topological materials in which the surface states are protected by time reversal symmetry. We image the local density of states around a variety of single-atom impurities in the presence of a magnetic field. On a subset of these impurities, we observe broad peaks in the local density of states at energies around the Dirac point. Furthermore, we use Landau level spectroscopy and quasiparticle scattering to discuss the interplay between impurities and the surface states.