Electronic Transport in Topological Insulators – IsoTop

A new class of quantum states of matter with remarkable properties has been recently discovered: the topological insulators. These topological phases result from very special band structures driven by a strong intrinsic spin-orbit coupling. It was first realized by Kane et al. in 2005 that intrinsic spin orbit in a graphene sheet could lead to a new insulating phase characterized by a bulk gap but having remarkable metallic edge states, analogous to the quantum Hall effect. However, this so-called quantum spin Hall (QSH) phase occurs in the absence of any time reversal symmetry breaking and possesses counter-propagating edge current of spin up and spin down electrons. Moreover, these edges states are robust to smooth perturbations respecting time-reversal symmetry and their presence is a topological property of the phase. Hence this QSH state constitutes a new topological phase in 2D, occurring in the absence of any magnetic field. Later, following a theoretical proposal by Bernevig at al., the predicted edge states were soon after reported by the experimental group of Würzburg throurgh conductance measurements in HgTe/CdTe quantum wells.

It was later realized that the QSH state allows for an amazing 3D generalization: the so-called topological insulators. The existence of surface states of these insulators is driven by the strong intrinsic spin orbit interaction. The dispersion of these surface states is described by a single (or an odd number of) Dirac cone(s) as opposed to Graphene (or other 2D crystals). The search for materials allowing for the appearance of this new state of matter started very actively in the USA. The BiSb alloy was the first candidate identified, and more recently a whole class of materials including Bi2Te3 , Bi2Se3 and Sb2Te3 has been proposed. Several spin resolved photoemission experiments have confirmed the existence the Dirac surface states in BiSb and Bi2Se3 .

It is now widely accepted that topological insulators exist but very few experimental studies on their edge/surface transport properties are available. It is the purpose of the present project to explore thoroughly the transport properties of these topological edge/surface states comparing them systematically with those of conventional metals. Three principal directions will be considered. First, given the coherent character of the surface states, it is natural to analyze how the physics of weak Anderson localization and universal conductance fluctuations is modified in these systems. Second, the physics of magnetic impurities in topological insulators will be explored including the Kondo effect and the Ruddermann-Kittel-Kasuya-Yosida interaction mediated by the surface states. Third, one also expects nontrivial proximity effect between a topological insulators and a superconductor related to the formation of Majorana states. Each task will provide some insight in the novel properties of topological insulators and the results will provide a deeper understanding of the novel properties of these materials.
The proposed project will benefit from ongoing international collaborations between the principal investigators and leading groups in the field of topological insulators (Berkeley, Stanford, Würzbürg).