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Register a new userDetermination of the quantized topological magneto-electric effect in topological insulators from Rayleigh scattering
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Topological insulators (TIs) exhibit many exotic properties. In particular, a topological magneto-electric (TME) effect, quantized in units of the fine structure constant, exists in TIs. In this Letter, we study theoretically the scattering properties of electromagnetic waves by TI circular cylinders particularly in the Rayleigh scattering limit. Compared with ordinary dielectric cylinders, the scattering by TI cylinders shows many unusual features due to the TME effect. Two proposals are suggested to determine the TME effect of TIs simply based on measuring the electric-field components of scattered waves in the far field at one or two scattering angles. Our results could also offer a way to measure the fine structure constant.
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Axion field induced topological magneto-electric response has attracted lots of attentions since it was first proposed by Qi et al. in 2008. Here we find a new type of anti-commutative magneto-electric response $beta^{xi}(omega)$, which can induce a dynamical magneto-electric current driven by a time-varying magnetic field. Unlike the Chern-Simons Axion term, this magneto-electric response term is gauge-independent, and manifests in the systems breaking the symmetries of the time-reversal, inversion and mirror. In particular, we predict Mn$_{2}$Bi$_{2}$Te$_{5}$ as a material candidate to observe dynamical magneto-electric current.
A key challenge in condensed matter research is the optimization of topological insulator (TI) compounds for the study and future application of their unique surface states. Truly insulating bulk states would allow the exploitation of predicted surface state properties, such as protection from backscattering, dissipationless spin-polarized currents, and the emergence of novel particles. Towards this end, major progress was recently made with the introduction of highly resistive Bi$_2$Te$_2$Se, in which surface state conductance and quantum oscillations are observed at low temperatures. Nevertheless, an unresolved and pivotal question remains: while room temperature ARPES studies reveal clear evidence of TI surface states, their observation in transport experiments is limited to low temperatures. A better understanding of this surface state suppression at elevated temperatures is of fundamental interest, and crucial for pushing the boundary of device applications towards room-temperature operation. In this work, we simultaneously measure TI bulk and surface states via temperature dependent optical spectroscopy, in conjunction with transport and ARPES measurements. We find evidence of coherent surface state transport at low temperatures, and propose that phonon mediated coupling between bulk and surface states suppresses surface conductance as temperature rises.
Granular conductors form an artificially engineered class of solid state materials wherein the microstructure can be tuned to mimic a wide range of otherwise inaccessible physical systems. At the same time, topological insulators (TIs) have become a cornerstone of modern condensed matter physics as materials hosting metallic states on the surface and insulating in the bulk. However it remains to be understood how granularity affects this new and exotic phase of matter. We perform electrical transport experiments on highly granular topological insulator thin films of Bi$_2$Se$_3$ and reveal remarkable properties. We observe clear signatures of topological surface states despite granularity with distinctly different properties from conventional bulk TI systems including sharp surface state coupling-decoupling transitions, large surface state penetration depths and exotic Berry phase effects. We present a model which explains these results. Our findings illustrate that granularity can be used to engineer designer TIs, at the same time allowing easy access to the Dirac-fermion physics that is inaccessible in single crystal systems.
Quantum anomalous Hall effect (QAHE) has been experimentally observed in magnetically doped topological insulators. However, ultra-low temperature (usually below 300 mK), which is mainly attributed to inhomogeneous magnetic doping, becomes a daunting challenge for potential applications. Here, a textit{nonmagnetic}-doping strategy is proposed to produce ferromagnetism and realize QAHE in topological insulators. We numerically demonstrated that magnetic moments can be induced by nitrogen or carbon substitution in Bi$_2$Se$_3$, Bi$_2$Te$_3$, and Sb$_2$Te$_3$, but only nitrogen-doped Sb$_2$Te$_3$ exhibits long-range ferromagnetism and preserve large bulk band gap. We further show that its corresponding thin-film can harbor QAHE at temperatures of 17-29 Kelvin, which is two orders of magnitude higher than the typical temperatures in similar systems. Our proposed textit{nonmagnetic} doping scheme may shed new light in experimental realization of high-temperature QAHE in topological insulators.
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