PHYSICAL REVIEW MATERIALS 6, 054204 (2022) Unconventional Landau level transitions in Weyl semimetal NbP Minhao Zhao, 1, 2 , * Zhongbo Yan, 3 , * Xiaoyi Xie, 1, 2 Yunkun Yang, 1, 2 Pengliang Leng, 1, 2 Mykhaylo Ozerov , 4 Dayu Yan, 5, 6 Youguo Shi, 5, 6 Jinshan Yang, 7 , † Faxian Xiu, 1, 2, 8, 9, 10 , ‡ and Shaoming Dong 7 1 State Key Laboratory of Surface Physics and Department of Physics, Fudan University, Shanghai 200433, China 2 Shanghai Qi Zhi Institute, 41st Floor, AI Tower, No. 701 Yunjin Road, Xuhui District, Shanghai 200232, China 3 School of Physics, Sun Yat-Sen University, Guangzhou 510275, China 4 National High Magnetic Field Laboratory, Tallahassee, Florida 32310, USA 5 Institute of Physics and Beijing National Laboratory for Condensed Matter Physics, Chinese Academy of Sciences, Beijing 100190, China 6 School of Physical Sciences, University of Chinese Academy of Sciences, Beijing 100190, China 7 State Key Laboratory of High Performance Ceramics & Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China 8 Institute for Nanoelectronic Devices and Quantum Computing, Fudan University, Shanghai 200433, China 9 Zhangjiang Fudan International Innovation Center, Fudan University, Shanghai 201210, China 10 Shanghai Research Center for Quantum Sciences, Shanghai 201315, China (Received 30 June 2021; accepted 7 April 2022; published 31 May 2022) Weyl semimetals were experimentally discovered as a new quantum phase of matter that exhibits topologically protected states characterized by separated Weyl points. Similar to other topological materials, the research of Landau level transitions can provide abundant information on Fermi surfaces. Extensive experimental efforts on low-temperature transport and optical properties have been dedicated to investigating the nontrivial topology structure. However, there are some theoretically predicted unconventional Landau level transitions in Weyl semimetals that have not been observed experimentally. And, their contribution to the electromagnetic response of Weyl semimetal remains elusive. Here, we report the magneto-optical study of Landau quantization and selection rules in Weyl semimetal NbP under Voigt geometry. By changing the direction of the electric vector of the incoming light under Voigt and Faraday geometry, abundant Landau level transition modes which contain  = 0, ±1, ±2 selection rules are obtained. The richness of the optical spectra, particularly the ones from the Voigt geometry, allows us to determine the location of Fermi energy and its evolution with magnetic fields. We further extract the transitions between Landau levels and Fermi energy and deduce the Landau index. Our results reveal the track of Fermi energy under varying magnetic fields based on the Landau level transitions under Voigt geometry and suggest that a combination of the magneto-optical spectra under Voigt and Faraday geometry can effectively determine the key properties of topological materials that are otherwise inaccessible. DOI: 10.1103/PhysRevMaterials.6.054204 I. INTRODUCTION Since the discovery of the TaAs family [1–3], numerous materials were identified as Weyl semimetals both theoret- ically and experimentally [2,4–9]. The electrons in Weyl semimetals not only host a linear E-k dispersion in the mo- mentum space but also carry chirality, giving rise to unique physical properties and electromagnetic responses such as Fermi arcs [5,6,10,11] and dynamic chiral anomaly [12–14]. However, under a varying magnetic field, the track of the Fermi energy in transport becomes inaccessible because the band structures of Weyl semimetals are commonly very com- plicated and a number of Weyl points are present near the Fermi energy. In the presence of a magnetic field, the track of Fermi energy in Weyl semimetals is even more complex * These authors contributed equally to this work. † jyang@mail.sic.ac.cn ‡ Faxian@fudan.edu.cn than in Dirac semimetals in which the carriers near the Fermi surface tend to gather at zeroth Landau level with an in- creasing magnetic field. Traditional transport measurements can provide basic information on Fermi energy including Fermi energy and Fermi velocity. In contrast, magneto- optical spectroscopy is rather effective to investigate Landau level quantizations and selection rules in topological ma- terials [15–21]. Some theoretical and experimental efforts were recently devoted to unconventional selection rules un- der Faraday geometry [22,23]. To this end, the N =±1 selection rule and √ B-type energy quantization were identi- fied in Dirac semimetals [24–27] and topological insulators [18,19,28,29]. For Weyl semimetals, however, their unique electromagnetic response originates from θ E · B in electro- magnetic Lagrangian [30], which requires a Voigt geometry to probe the topological structure. For instance, the dynamic chiral anomaly in a Weyl semimetal NbAs has been discov- ered by magneto-optical measurement under Voigt geometry recently [31]. Besides that, so far there are no salient physi- cal properties unveiled based on the unconventional Landau 2475-9953/2022/6(5)/054204(8) 054204-1 ©2022 American Physical Society