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Register a new userExperimental verification of the surface termination in the topological insulator TlBiSe$_{2}$ using core-level photoelectron spectroscopy and scanning tunneling microscopy
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The surface termination of the promising topological insulator TlBiSe$_{2}$ has been studied by surface and bulk sensitive probes. Our scanning tunneling microscopy has unmasked for the first time the unusual surface morphology of TlBiSe$_{2}$ obtained by cleaving, where islands are formed by residual atoms on the cleaved plane. The chemical condition of these islands was identified using core-level spectroscopy. We observed thallium core-level spectra that are strongly deformed by a surface component in sharp contrast to the other elements. We propose a simple explanation for this behavior by assuming that the sample cleaving breaks the bonding between thallium and selenium atoms, leaving the thallium layer partially covering the selenium layer. These findings will assist the interpretation of future experimental and theoretical studies on this surface.
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Electrical field control of the carrier density of topological insulators (TI) has greatly expanded the possible practical use of these materials. However, the combination of low temperature local probe studies and a gate tunable TI device remains challenging. We have overcome this limitation by scanning tunneling microscopy and spectroscopy measurements on in-situ molecular beam epitaxy growth of Bi2Se3 films on SrTiO3 substrates with pre-patterned electrodes. Using this gating method, we are able to shift the Fermi level of the top surface states by 250 meV on a 3 nm thick Bi2Se3 device. We report field effect studies of the surface state dispersion, band gap, and electronic structure at the Fermi level.
Through a combination of experimental techniques we show that the topmost layer of the topo- logical insulator TlBiSe$_2$ as prepared by cleavage is formed by irregularly shaped Tl islands at cryogenic temperatures and by mobile Tl atoms at room temperature. No trivial surface states are observed in photoemission at low temperatures, which suggests that these islands can not be re- garded as a clear surface termination. The topological surface state is, however, clearly resolved in photoemission experiments. This is interpreted as a direct evidence of its topological self-protection and shows the robust nature of the Dirac cone like surface state. Our results can also help explain the apparent mass acquisition in S-doped TlBiSe$_2$.
Using low-temperature scanning tunneling microscopy and spectroscopy, we have studied the proximity effect at the interfaces between superconducting Pb island structures and metallic Pb-induced striped-incommensurate phase formed on a Si(111) substrate. Our real-space observation revealed that the step structures on the two-dimensional metallic layer exhibit significant roles on the propagation of the superconducting pair correlation; the proximity effect is terminated by the steps, and in the confined area by the interface and the steps the effect is enhanced. The observed results are explained quantitatively with an elastic reflection of electrons at the step edges based on calculations with the quasi-classical Greens function formulation using Usadel equation.
The discovery of graphene has put the spotlight on other layered materials including transition metal dichalcogenites (TMD) as building blocks for novel heterostructures assembled from stacked atomic layers. Molybdenum disulfide, MoS2, a semiconductor in the TMD family, with its remarkable thermal and chemical stability and high mobility, has emerged as a promising candidate for post-silicon applications such as switching, photonics, and flexible electronics. Since these rely on controlling the position of the Fermi energy (EF), it is crucial to understand its dependence on doping and gating. Here we employed scanning tunneling microscopy (STM) and spectroscopy (STS) with gating capabilities to measure the bandgap and the position of EF in MoS2, and to track its evolution with gate voltage. For bulk samples, the measured bandgap (~1.3eV) is comparable to the value obtained by photoluminescence, and the position of EF (~0.35eV) below the conduction band, is consistent with n-doping reported in this material. Using topography together with spectroscopy we traced the source of the n-doping in bulk MoS2 samples to point defects, which we attribute to S vacancies. In contrast, for thin films deposited on SiO2, we found significantly higher levels of n-doping that cannot be attributed to S vacancies. By combining gated STS with transport measurements in a field effect transistor (FET) configuration, we demonstrate that the higher levels of n-doping in thin film samples is due to charge traps at the sample-substrate interface.
Following the intense studies on topological insulators, significant efforts have recently been devoted to the search for gapless topological systems. These materials not only broaden the topological classification of matter but also provide a condensed matter realization of various relativistic particles and phenomena previously discussed mainly in high energy physics. Weyl semimetals host massless, chiral, low-energy excitations in the bulk electronic band structure, whereas a symmetry protected pair of Weyl fermions gives rise to massless Dirac fermions. We employed scanning tunneling microscopy/spectroscopy to explore the behavior of electronic states both on the surface and in the bulk of topological semimetal phases. By mapping the quasiparticle interference and emerging Landau levels at high magnetic field in Dirac semimetals Cd$_3$As$_2$ and Na$_3$Bi, we observed extended Dirac-like bulk electronic bands. Quasiparticle interference imaged on Weyl semimetal TaAs demonstrated the predicted momentum dependent delocalization of Fermi arc surface states in the vicinity of the surface-projected Weyl nodes.
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