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Register a new userSTM imaging of impurity resonances on Bi$_2$Se$_3$
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Added by
Zhanybek Alpichshev
Publication date
2011
fields
Physics
and research's language is
English
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In this paper we present detailed study of the density of states near defects in Bi$_2$Se$_3$. 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 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.
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Detailed study of the LDOS associated with the surface-state-band near a step-edge of the strong topological-insulator Bi2Te3, reveal a one-dimensional bound state that runs parallel to the stepedge and is bound to it at some characteristic distance. This bound state is clearly observed in the bulk gap region, while it becomes entangled with the oscillations of the warped surface band at high energy, and with the valence band states near the Dirac point. Using the full effective Hamiltonian proposed by Zhang et al., we obtain a closed formula for this bound state that fits the data and provide further insight into the general topological properties of the electronic structure of the surface band near strong structural defects.
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 concave, oscillations appear which disperse with a particular wave-vector that may result from an unconventional hexagonal warping term.
The influence of individual impurities of Fe on the electronic properties of topological insulator Bi$_2$Se$_3$ is studied by Scanning Tunneling Microscopy. The microscope tip is used in order to remotely charge/discharge Fe impurities. The charging process is shown to depend on the impurity location in the crystallographic unit cell, on the presence of other Fe impurities in the close vicinity, as well as on the overall doping level of the crystal. We present a qualitative explanation of the observed phenomena in terms of tip-induced local band bending. Our observations evidence that the specific impurity neighborhood and the position of the Fermi energy with respect to the Dirac point and bulk bands have both to be taken into account when considering the electron scattering on the disorder in topological insulators.
We report scanning tunnelling microscopy and spectroscopy (STM/STS) studies on one and two unit cell (UC) high FeSe thin films grown on Bi$_2$Se$_3$(0001). In our thin films, we find the tetragonal phase of FeSe and dumb-bell shaped defects oriented along Se-Se bond directions. In addition, we observe striped moire patterns with a periodicity of ($7.3pm 0.1$) nm generated by the mismatch between the FeSe lattice and the Bi$_2$Se$_3$ lattice. We could not find any signature of a superconducting gap in the tunneling spectra measured on the surface of one and two UC thick islands of FeSe down to 6.5 K. The spectra rather show an asymmetric behavior across and a finite density of states at the Fermi level ($E_F$) resembling those taken in the normal state of bulk FeSe.
The ferromagnetic topological insulator V:(Bi,Sb)$_2$Te$_3$ has been recently reported as a quantum anomalous Hall (QAH) system. Yet the microscopic origins of the QAH effect and the ferromagnetism remain unclear. One key aspect is the contribution of the V atoms to the electronic structure. Here the valence band of V:(Bi,Sb)$_2$Te$_3$ thin films was probed in an element-specific way by resonant photoemission spectroscopy. The signature of the V $3d$ impurity band was extracted, and exhibits a high density of states near Fermi level. First-principles calculations support the experimental results and indicate the coexistence of ferromagnetic superexchange and double exchange interactions. The observed impurity band is thus expected to contribute to the ferromagnetism via the interplay of different mechanisms.
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