Ultrafast photoinduced band splitting and carrier dynamics in chiral tellurium nanosheets

Giriraj Jnawali,Yongsheng Leng,Yuan Xiang,Ruoxing Wang,Gang Qiu,Iraj Abbasian Shojaei,Bryan M Wong,Leigh M Smith,Peide D Ye,Howard E Jackson,Samuel M Linser,Chao Lian,Wenzhuo Wu

doi:10.1038/s41467-020-17766-5

Abstract

Trigonal tellurium (Te) is a chiral semiconductor that lacks both mirror and inversion symmetries, resulting in complex band structures with Weyl crossings and unique spin textures. Detailed time-resolved polarized reflectance spectroscopy is used to investigate its band structure and carrier dynamics. The polarized transient spectra reveal optical transitions between the uppermost spin-split H4 and H5 and the degenerate H6 valence bands (VB) and the lowest degenerate H6 conduction band (CB) as well as a higher energy transition at the L-point. Surprisingly, the degeneracy of the H6 CB (a proposed Weyl node) is lifted and the spin-split VB gap is reduced upon photoexcitation before relaxing to equilibrium as the carriers decay. Using ab initio density functional theory (DFT) calculations, we conclude that the dynamic band structure is caused by a photoinduced shear strain in the Te film that breaks the screw symmetry of the crystal. The band-edge anisotropy is also reflected in the hot carrier decay rate, which is a factor of two slower along the c-axis than perpendicular to it. The majority of photoexcited carriers near the band-edge are seen to recombine within 30 ps while higher lying transitions observed near 1.2 eV appear to have substantially longer lifetimes, potentially due to contributions of intervalley processes in the recombination rate. These new findings shed light on the strong correlation between photoinduced carriers and electronic structure in anisotropic crystals, which opens a potential pathway for designing novel Te-based devices that take advantage of the topological structures as well as strong spin-related properties.

Highlights

Trigonal tellurium (Te) is a chiral semiconductor that lacks both mirror and inversion symmetries, resulting in complex band structures with Weyl crossings and unique spin textures
H- or H′-points in the Brillouin zone is lifted in momentum space due to the spin-orbit interaction (SOI) and the breaking of inversion symmetry while the lowest conduction band (CB) is doubly spin-degenerate and protected by the 3-fold screw symmetry of the helices[7,8,9,10]
The topological phase transition from a trivial semiconductor to a Weyl semimetal (WSM) is predicted under applied external pressure when the spinpolarized uppermost valence bands (VB) and CB are inverted across the band gap[8,24]

Summary

Introduction

Trigonal tellurium (Te) is a chiral semiconductor that lacks both mirror and inversion symmetries, resulting in complex band structures with Weyl crossings and unique spin textures. Beyond graphene and the transition metal dichalcogenides (TMDs), a variety of exotic materials such as topological insulators (TIs)[5] and Weyl semimetals (WSMs)[6] have been recently discovered, which display unique electronic structure and chiral spin texture on the Fermi surface. Most of these materials involve heavy elements such as Bi, Sb, Te, Se, etc., suggesting that strong spin-orbit interaction (SOI) is the key to the complex electronic structure in these materials. The unique chiral electronic structure offers the ability to control the electronic charge and spin degrees of freedom in the absence of a magnetic field for applications in spintronics

Methods

Results

Conclusion