Polarized mid-infrared spectroscopy as a potential tool for investigating Weyl semimetals

Date

Oct 30, 2019

Time

11:00 AM - 12:00 PM

Speaker

Prof. Leigh M. Smith

Affiliation

University of Cincinnati, OH, USA

Language

en

Main Topic

Chemie

Other Topics

Chemie

Host

Dr. Y. Sun

Description

Time-integrated and femtosecond transient spectroscopies have been used for years to probe electronic states in semiconductors, understanding the band structure, intra- and inter-band transitions, and interactions with phonons, nanostructures and defects. We use polarized Raman spectroscopy and polarized transient reflectivity (TR) to investigate the band-to-band transitions and relaxation dynamics in tellurium which has a 0.35 eV gap. Tellurium is a chiral material with complex nested conduction and valence bands, and which has recently been shown by ARPES to exhibit spin textures resulting from a Weyl node near the H6 conduction band minimum. TR spectra show transitions between the H4, H5 and H6 VBs and the H6 CB, while the dynamics shows evidence for scattering of carriers to indirect valleys. Weyl semimetals occur when the gap collapses (usually because of strong spin-orbit coupling) causing band crossings and inversions over limited parts of the Brillouin zone, while well defined gaps are seen elsewhere. Because the gap collapses only partially, mid-infrared pulses in principle can interrogate transitions from bands near the Fermi energy to higher lying states in the same region of k space. We use resonant Raman scattering, polarized transient reflectivity, and polarized photothermal and photogalvanic spectroscopies in a device to investigate the electronic structure of the Type-II Weyl semimetal NbIrTe4. This material has broken inversion symmetry which results in 16 Weyl nodes which are less than 100 meV from the Fermi energy. In principle, the chiral Weyl nodes should couple strongly with circularly polarized light. We use linear- and circular-polarized mid-infrared to visible spectroscopies ranging from 0.3 to 3 eV to show there is some evidence for band-to-band electronic transitions, and also that the nanoflakes couple strongly to circularly polarized light, particularly at low energies approaching the Weyl nodes. Some possible insights are gained by comparison of these very preliminary results with DFT calculations of the phonon modes and band structure. The potential for extensions of these measurements to low temperatures, magnetic and electric fields, and other new materials will be discussed. We acknowledge the support of the NSF through grants DMR 1507844, DMR 1531373 and ECCS 1509706. Peide Ye (Purdue Univ) supplied the Tellurium nanoflakes, Stephen Wilson (UCSB) supplied the NbIrTe4 single crystals, Fuchun Zhang (Kavli Institute, Beijing) and Congcong Le (now at MPI Dresden) have provided theoretical support. _________________________________________________________________________________ Leigh M. Smith is Professor and Head of the Department of Physics. He has been at the University of Cincinnati since 1990. He received his BA in Mathematics and Physics at the University of Virginia, PhD in Condensed Matter Physics from the University of Illinois at Urbana-Champaign, and was Postdoctoral Fellow at the IBM T.J. Watson Research Center. He has been Visiting Faculty at the Danish Technical University and Australian National University.

Links

• MPI CPfS

Last modified: Oct 31, 2019, 12:07:36 AM