Optoelectronic Response of Nodal Line Semimetal PbTaSe2

The discovery and synthesis of topological semimetals is an active field of research as new materials are realized and their applications are unlocked. Weyl semimetals, first discovered in 2015 [1], are topological materials characterized by linearly dispersing cones of Weyl fermions in their electron band structure with a single valence-conduction band touching point. Another class of topological semimetals are the nodal line semimetals (NLSM), which are characterized by topologically protected nodal lines within electron band structure, where the valence-conduction band touching points are sustained along a one-dimensional curve in the three-dimensional Brillouin zone [2,3] The nodal line states are manifested as protected surface states [4]. Early experiments have measured the anomalous and unique optical responses of nodal semimetals [5]. Possible applications for these materials include use in spintronics, nonreciprocal energy transport [6], as well as infrared and THz applications [7,8]. Given various exciting applications of nodal semimetals, it is imperative to fully characterize their opto-electronic response as well as assess the tunability of this response via external stimuli.<br/>In our work, we analyze the optical response of PbTaSe2, a NLSM, under static and alternating electric fields. Using density functional theory (DFT), we predict the electronic structure and elucidate its optoelectronic properties. Ellipsometry is employed to gather information on polarizability, supported by the dielectric function derived from first-principles calculations.<br/>To evaluate the electrical and optical conductivity, we use conductive atomic force microscopy (C-AFM) and Scanning Photocurrent Microscopy (SPCM), respectively. C-AFM measures the electrical conductivity under a DC electric field, while SPCM assesses photocurrent generation under illumination. These techniques provide a comprehensive understanding of the performance of PbTaSe₂ under different conditions.<br/>Further, we explore the material's optoelectronic response under strain to assess its tunability. This study allows us to correlate theoretical predictions of polarizability and electronic structure with experimental measurements of optical and DC conductivity. Our findings demonstrate how strain impacts the optoelectronic properties of PbTaSe2, showcasing the range of tunability achievable through nanoengineering techniques.<br/><br/>References:<br/><br/>[1] Xu, SY., Alidoust, N., Belopolski, I. <i>et al.</i> Discovery of a Weyl fermion state with Fermi arcs in niobium arsenide. <i>Nature Phys</i> <b>11</b>, 748–754 (2015). https://doi.org/10.1038/nphys3437<br/>[2] AA Burkov, MD Hook, L Balents, Topological nodal semimetals. <i>Phys Rev B</i> <b>84</b>, 235126 (2011).<br/>[3] C Fang, Y Chen, HY Kee, L Fu, Topological nodal line semimetals with and without spin-orbital coupling. <i>Phys Rev B</i> <b>92</b>, 081201 (2015).<br/>[4] Bian, G., Chang, TR., Sankar, R. <i>et al.</i> Topological nodal-line fermions in spin-orbit metal PbTaSe₂. <i>Nat Commun</i> <b>7</b>, 10556 (2016). https://doi.org/10.1038/ncomms10556<br/>[5] Yang, M. X., Luo, W., & Chen, W. (2022). Quantum transport in topological nodal-line semimetals. <i>Advances in Physics: X</i>, <i>7</i>(1). https://doi.org/10.1080/23746149.2022.2065216<br/>[6] Singh, S. <i>et al.</i> Anisotropic Nodal-Line-Derived Large Anomalous Hall Conductivity in ZrMnP and HfMnP. <i>Advanced Materials</i> <b>33</b>, 2104126 (2021).<br/>[7] Liu, J., Xia, F., Xiao, D., García de Abajo, F. J. & Sun, D. Semimetals for high-performance photodetection. <i>Nature Materials 2020 19:8</i> <b>19</b>, 830–837 (2020).<br/>[8] Tomarchio, L. <i>et al.</i> THz Generation from the Topological Nodal Line Semimetal Co2MnGa. <i>ACS Appl. Electron. Mater.</i> <b>5</b>, 1437–1443 (2023).<br/><br/>This work was supported in part by ARO MURI (Grant No. W911NF-19-1-0279) via U. Michigan.