Condensed Matter/Biological Physics Seminar: Prof. Lei Fang, Northwestern University  Add To Calendar

  • Date(s): Thursday, 1/21 2:00 PM to Thursday, 1/21 3:00 PM
  • Speaker: Prof. Lei Fang
  • Host: Hyowon Park
  • Campus Address: 2214 SES (Science and Engineering South)

Title: Design, Synthesis and Characterization of Topological Insulators and 2D Materials beyond Graphene

Abstract: In modern condensed matter physics, the notion of spontaneous symmetry breaking accompanied with the formation of an order parameter is generally applied to phase transitions in various macroscopic systems. An exception to this universal rule is the quantum matter protected by topological invariance. Benchmark systems with nontrivial topological invariance are the recently discovered topological insulators (TIs). These materials are bulk insulators, but possess conducting edges or surfaces (1). TIs demonstrate a variety of appealing properties, such as the back-scattering immunity, Dirac cone-like electronic structure, and the spin-momentum locking (2). These remarkable properties have significant impact on energy applications, spintronics and quantum computing techniques.
In this seminar, I will firstly overview the physics of topological invariance, the electronic structure of TIs and their potential applications. The second part of my talk focuses on TI nanomaterials. I will introduce our catalyst-free physical vapor deposition method that enables the growth of millimeter-long topological insulator Bi2Se3 nanoribbons. Angle dependent quantum oscillation measurements on single nanoribbon show clear evidences for topological surface states and π of the Berry’s phase (3). The ultralong nanoribbons open a new vista for fabricating multiple devices on single TI nanoribbon. In addition to TI nanomaterials, I will briefly talk about how to rationally design new TIs using the so-called structure motifs engineering method (4). We have successfully discovered a new TI system [PbSe]5[Bi2Se3]3m, m=1,2 and observed superconductivity by tuning doping and structure (5). In the last part of my talk, I will introduce our latest progress on 2D heterostructure [Pb2BiS3][AuTe2]. This material exhibits a wealth of interesting properties such as Dirac-like linear band dispersions (6), extremely large electrical anisotropy, unusually strong spin-orbit coupling, weak antilocalization, and atomically thin nanosheets. Moreover, the first-principles calculations show a helical-like spin texture on the Fermi surface. The helical-like spin texture obtains an unusual winding number of the spin vector that may give rise to a nontrivial Berry’s phase (7)


(1) X.-L. Qi and S.-C. Zhang, Phys. Today 63(1), 33 (2010).

(2) M. Z. Hasan and C. L. Kane, Rev. Mod. Phys. 82, 3045 (2010).

(3) L. Fang, et al., Nano Lett. 12, 6164(2012).

(4) L. Fang, et al., J. Am. Chem. Soc. 136, 11079 (2014).

(5) L. Fang, et al., Phys. Rev. B (R) 90, 020504 (2014).

(6) L. Fang, et al., J. Am. Chem. Soc. doi:10.1021/ja5111688 (2015).

(7) L. Fang, et al., Submitted to Nature Phys. 

Cm-Bio 1-21 Fang