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Register a new userSpin texture in weak topological insulators: the role of bulk states and band bending
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We derive the spin texture of a weak topological insulator via a supersymmetric approach that includes the roles of the bulk gap edge states and surface band bending. We find the spin texture can take one of four forms: (i) helical, (ii) hyperbolic, (iii) hedgehog, with spins normal to the Dirac-Weyl cone of the surface state, and (iv) hyperbolic hedgehog. Band bending determines the winding number in the case of a helical texture, and for all textures can be used to tune the spin texture polarization to zero. For the weak topological insulator SnTe, we show that inclusion of band bending is crucial to obtain the correct texture winding number for the (111) surface facet $Gamma$-point Dirac-Weyl cone. We argue that hedgehogs will be found only in low symmetry situations.
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We reanalyze some of the critical transport experiments and provide a coherent understanding of the current generation of topological insulators (TIs). Currently TI transport studies abound with widely varying claims of the surface and bulk states, often times contradicting each other, and a proper understanding of TI transport properties is lacking. According to the simple criteria given by Mott and Ioffe-Regel, even the best TIs are not true insulators in the Mott sense, and at best, are weakly-insulating bad metals. However, band-bending effects contribute significantly to the TI transport properties including Shubnikov de-Haas oscillations, and we show that utilization of this band-bending effect can lead to a Mott insulating bulk state in the thin regime. In addition, by reconsidering previous results on the weak anti-localization (WAL) effect with additional new data, we correct a misunderstanding in the literature and generate a coherent picture of the WAL effect in TIs.
Non-invasive local probes are needed to characterize bulk defects in binary and ternary chalcogenides. These defects contribute to the non-ideal behavior of topological insulators. We have studied bulk electronic properties via $^{125}$Te NMR in Bi$_2$Te$_3$, Sb$_2$Te$_3$, Bi$_{0.5}$Sb$_{1.5}$Te$_3$, Bi$_2$Te$_2$Se and Bi$_2$Te$_2$S. A distribution of defects gives rise to asymmetry in the powder lineshapes. We show how the Knight shift, line shape and spin-lattice relaxation report on carrier density, spin-orbit coupling and phase separation in the bulk. The present study confirms that the ordered ternary compound Bi$_2$Te$_2$Se is the best TI candidate material at the present time. Our results, which are in good agreement with transport and ARPES studies, help establish the NMR probe as a valuable method to characterize the bulk properties of these materials.
The main scientific activity in the field of topological insulators (TIs) consists of determining their electronic structure by means of magneto-transport and electron spectroscopy with a view to devices based on topological transport. There is however a caveat in this approach. There are systematic experimental discrepancies on the electronic structure of the most pristine surfaces of TI single crystals as determined by Shubnikov de Haas (SdH) oscillations and by Angle Resolved PhotoElectron Spectroscopy (ARPES). We identify intense ultraviolet illumination -that is inherent to an ARPES experiment- as the source for these experimental differences. We explicitly show that illumination is the key parameter, or in other words the trigger, for energetic shifts of electronic bands near the surface of a TI crystal. This finding revisits the common belief that surface decoration is the principal cause of surface band bending and explains why band bending is not a prime issue in the illumination-free magneto-transport studies. Our study further clarifies the role of illumination on the electronic band structure of TIs by revealing its dual effect: downward band bending on very small timescales followed by band flattening at large timescales. Our results therefore allow us to present and predict the complete evolution of the band structure of TIs in a typical ARPES experiment. By virtue of our findings, we pinpoint two alternatives of how to approach flat band conditions by means of photon-based techniques and we suggest a microscopic mechanism that can explain the underlying phenomena.
Detection and manipulation of electrons spins are key prerequisites for spin-based electronics or spintronics. This is usually achieved by contacting ferromagnets with metals or semiconductors, in which the relaxation of spins due to spin-orbit coupling limits both the efficiency and the length scale. In topological insulator materials, on the contrary, the spin-orbit coupling is so strong that the spin direction uniquely determines the current direction, which allows us to conceive a whole new scheme for spin detection and manipulation. Nevertheless, even the most basic process, the spin injection into a topological insulator from a ferromagnet, has not yet been demonstrated. Here we report successful spin injection into the surface states of topological insulators by using a spin pumping technique. By measuring the voltage that shows up across the samples as a result of spin pumping, we demonstrate that a spin-electricity conversion effect takes place in the surface states of bulk-insulating topological insulators Bi1.5Sb0.5Te1.7Se1.3 and Sn-doped Bi2Te2Se. In this process, due to the two-dimensional nature of the surface state, there is no spin current along the perpendicular direction. Hence, the mechanism of this phenomenon is different from the inverse spin Hall effect and even predicts perfect conversion between spin and electricity at room temperature. The present results reveal a great advantage of topological insulators as inborn spintronics devices.
We have investigated spin-electricity conversion on surface states of bulk-insulating topological insulator (TI) materials using a spin pumping technique. The sample structure is Ni-Fe|Cu|TI trilayers, in which magnetic proximity effects on the TI surfaces are negligibly small owing to the inserted Cu layer. Voltage signals produced by the spin-electricity conversion are clearly observed, and enhanced with decreasing temperature in line with the dominated surface transport at lower temperatures. The efficiency of the spin-electricity conversion is greater for TI samples with higher resistivity of bulk states and longer mean free path of surface states, consistent with the surface spin-electricity conversion.
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