Topological Insulators

Recently, several compounds have vaulted into the forefront of the condensed matter field after being named prime candidates for the physical realization of a new class of materials, known as topological insulators. A topological insulator has an energy gap in the bulk, however, unlike traditional insulators or semiconductors, topological insulators are topologically inequivalent to the vacuum. A fundamental consequence and principle experimental indicator of the non-trivial topology is one or more robust metallic surface states that are protected both from localization and backscattering by time-reversal symmetry. Beyond the inherent importance of exploring a complex phase of quantum matter, these systems are of great interest for device applications such as quantum computing and photonics.

Faraday Rotation Due to Surface States in the Topological Insulator (Bi1–xSbx)2Te3s
Shao et al. Nano Letters 17, 980 (2017) Ref. [237]

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Using magneto-infrared spectroscopy, we have explored the charge dynamics of (Bi,Sb)2Te3 thin films on InP substrates. From the magneto-transmission data we extracted three distinct cyclotron resonance (CR) energies that are all apparent in the broad band Faraday rotation (FR) spectra. This comprehensive FR-CR data set has allowed us to isolate the response of the bulk states from the intrinsic surface states associated with both the top and bottom surfaces of the film. The FR data uncovered that electron- and hole-type Dirac Fermions reside on opposite surfaces of our films, which paves the way for observing many exotic quantum phenomena in topological insulators.

 

Topological insulators are tunable waveguides for hyperbolic polaritons
Wu et al. Physical Review B 92 205430 (2015). Article [217]

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picture51Iron-based superconductors have been found to exhibit an intimate interplay of orbital, spin, and lattice degrees of freedom, dramatically affecting their low-energy electronic properties, including superconductivity. Albeit the precise pairing mechanism remains unidentified, several candidate interactions have been suggested to mediate the superconducting pairing, both in the orbital and in the spin channel. Here, we employ optical spectroscopy (OS), angle-resolved photoemission spectroscopy (ARPES), ab initio band-structure, and Eliashberg calculations to show that nearly optimally doped NaFe0.978Co0.022As exhibits some of the strongest orbitally selective electronic correlations in the family of iron pnictides.

Sum-Rule Constraints on the Surface State Conductance of Topological Insulators
Post et al. Physical Review Letters 115, 116804 (2015). Article [212]

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picture52We report the Drude oscillator strength D and the magnitude of the bulk band gap Eg of the epitaxially grown, topological insulator BiSb2Te3. The magnitude of Eg, in conjunction with the model independent f-sum rule, allows us to establish an upper bound for the magnitude of D expected in a typical Dirac-like system composed of linear bands. The experimentally observed D is found to be at or below this theoretical upper bound, demonstrating the effectiveness of alloying in eliminating bulk charge carriers. Moreover, direct comparison of the measured D to magnetoresistance measurements of the same sample supports assignment of the observed low-energy conduction to topological surface states.