Electric-field-tuned topological phase transition in ultrathin Na3Bi

Abstract

The electric field induced quantum phase transition from topological to conventional insulator has been proposed as the basis of a topological field effect transistor [1-4]. In this scheme an electric field can switch 'on' the ballistic flow of charge and spin along dissipationless edges of the two-dimensional (2D) quantum spin Hall insulator [5-9], and when 'off' is a conventional insulator with no conductive channels. Such a topological transistor is promising for low-energy logic circuits [4], which would necessitate electric field-switched materials with conventional and topological bandgaps much greater than room temperature, significantly greater than proposed to date [6-8]. Topological Dirac semimetals (TDS) are promising systems in which to look for topological field-effect switching, as they lie at the boundary between conventional and topological phases [3,10-16]. Here we use scanning probe microscopy/spectroscopy (STM/STS) and angle-resolved photoelectron spectroscopy (ARPES) to show that mono-and bilayer films of TDS Na3Bi [3,17] are 2D topological insulators with bulk bandgaps >400 meV in the absence of electric field. Upon application of electric field by doping with potassium or by close approach of the STM tip, the bandgap can be completely closed then re-opened with conventional gap greater than 100 meV. The large bandgaps in both the conventional and quantum spin Hall phases, much

Quantum Spin Hall Materials

“…Even though graphene in itself does not have the physical conditions necessary to support realistic QSH states, and even though semiconductor quantum well systems have taken the trophy of being the first experimentally realized QSH systems, the 2D materials and thin film material family is catching up fairly quickly. 2D QSH materials with strong experimental evidence includes monolayer WTe 2 , Bismuthene, and Na 3 Bi ultrathin film . There are also attractive theoretical proposals of QSH materials such as other monolayer transition metal dichalcogenides MX 2 (M = (W, Mo) and X = (Te, Se, S)), graphene heterostructures with strong spin–orbit coupling 2D materials, etc.…”

“…Apart from layered or 2D materials, several 3D materials were predicted to be QSH insulators at the ultrathin form, such as Bi, transition metal oxide Na 2 IrO 3 , Si 2 Te 2 , as well as 3D topological Dirac semimetals Cd 3 As 2 and Na 3 Bi . Recently, QSH effect was observed in mono‐ and bilayer films of Na 3 Bi . Few‐layer Na 3 Bi(001) was grown on Si(111) substrate, which has been demonstrated to have “ON” and “OFF” features by applying vertical electric field (see Section 2.2 for discussions on driving the transition between topologically nontrivial and topologically trivial states in QSH materials).…”

“…The bandgap is extracted from the STS spectra and a critical electric field is shown above which the material will turn into a trivial insulator. The electric field is tuned by changing the distance between the STM tip and the Na 3 Bi ultrathin film …”

“…The electric field close the quantum spin Hall bandgap and re‐open it as a trivial insulator. Reproduced with permission . Copyright 2018, Springer Nature.…”

Electric field manipulation of surface states in topological semimetals

“…For example, monolayer and bilayer films of Na 3 Bi have bulk bandgaps greater than 300 meV, suggesting that topological properties in these thin films will survive at room temperature. 23,24 The topological response of WSMs and DSMs comprises the manifestation of the chiral anomaly in a large negative magnetoresistance, in the presence of both electric and magnetic fields, and in an anomalous Hall effect (in WSMs), due to the transport of the surface states. 1,5 Therefore, the renormalization of the Hamiltonian parameters due to time-dependent external fields has been the subject of intense research.…”