Exciton Condensation and Charge Fractionalization in a Topological Insulator Film

Abstract

An odd number of gapless Dirac fermions is guaranteed to exist at a surface of a strong topological insulator. We show that in a thin-film geometry and under external bias, electron-hole pairs that reside in these surface states can condense to form a novel exotic quantum state which we propose to call "topological exciton condensate" (TEC). This TEC is similar in general terms to the exciton condensate recently argued to exist in a biased graphene bilayer, but with different topological properties. It exhibits a host of unusual properties including a stable zero mode and a fractional charge +/-e/2 carried by a singly quantized vortex in the TEC order parameter.

Observation of a large-gap topological-insulator class with a single Dirac cone on the surface

“…Our theoretical calculation supported by our experimental data suggests that in insulating simplest topological surface spectrum would make it possible to observe and study many exotic quantum phenomena predicted in topological field theories, such as the Majorana fermions [14], magnetic monopole image [9,10] or topological exciton condensates [15], by transport probes.…”

“…Two such phases are the quantum spin Hall insulator [4] and the strong topological insulator [5,6,7] both realized in the vicinity of a Dirac point but yet quite distinct from graphene [16]. The strong-topological-insulator phase contains surface states (SSs) with novel electromagnetic properties [7,8,9,10,11,12,13,14,15]. It is currently believed that the Bi 1−x Sb x insulating alloys realize the only known topological-insulator phase in the vicinity of a threedimensional Dirac point [5], which can in principle be used to study topological electromagnetic and interface superconducting properties [8,9,10,14].…”

Interaction-enhanced coherence between two-dimensional Dirac layers

“…Spontaneous coherence in topological insulators is favored by their reduced number of degenerate Dirac cones 20 . On the other hand the 3D bulk of the material is expected to substantially screen the interactions between the surface Dirac electrons, strongly reducing their effective fine structure constant from its nominal value in vacuum α 0 = e 2 /v D .…”

Ultrathin Topological Insulator Bi2Se3 Nanoribbons Exfoliated by Atomic Force Microscopy

“…12-17 Many exotic physical phenomena are predicted to emerge in low dimensional nanostructures of Bi 2 Se 3 . 18,19 For example, ultra-thin Bi 2 Se 3 down to a few (≤5) nanometers is expected to exhibit topologically non-trivial edge states, which serves as a new platform for the 2D QSHE 18 .In addition, tuning of the chemical potential becomes easier than thick Bi 2 Se 3 due to the suppression of bulk contribution. Fortunately, such ultra-thin Bi 2 Se 3 can be naturally obtained due to its layered rhombohedral crystal structure; two Bi and three Se atomic sheets are covalently bonded to form one quintuple layer (QL, ~1 nm thick) (Figure 1b), where adjacent QLs are coupled by relatively weak van der Waals interaction.…”