Three dimensional (3D) topological Dirac/Weyl semimetals are currently a new frontier and focus topic in condensed matter physics and materials science. Their low energy electronic excitations are located around a few Dirac/Weyl nodes in momentum space and resemble the relativistic Dirac/Weyl fermions in high energy physics.

Recently Prof. Nanlin Wang’s group in ICQM performed magneto infrared spectroscopy on ZrTe5 single crystals, which were identified to be a 3D Dirac semimetal earlier by transport and ARPES measurements. Their initial optical spectroscopy measurement without magnetic field revealed a linear rising of the optical conductivity with frequency over a relatively broad frequency range, which is a hallmark of 3D massless Dirac fermions. This result was published in Physical Review B 92, 075107 (2015). When magnetic field was applied, transitions between Landau levels were clearly observed in optical reflectance. The transition energies follow the sequence of 1:1+√2:√2+√3:… and are proportional to the square root of the magnetic field in the low field regime, which is a strong evidence of massless Dirac dispersion. It is also found that an exceptionally low magnetic field (less than 1 Tesla) was capable of driving the compound into its quantum limit, which demonstrates that ZrTe5 is an extremely clean system and ideal platform for studying 3D massless Dirac fermions. Most significantly, the splitting of Landau levels at relatively small magnetic field was explicitly identified in the reflectance data. Such Zeeman splitting has never been observed in 2D massless Dirac fermions, such as in graphene. Prof. Fa Wang’s theoretical group at the ICQM built an effective model for this material and analyzed the experimental data. They found that the ZrTe5 compound was transformed into a linenode semimetal under the current experimental configuration with Zeeman field along the crystalline b axis. According to this analysis the Dirac node of this material will be split to two Weyl nodes if the Zeeman field is applied along the c axis, leaving open the possibility of controlled realization of Weyl semimetals in this compound.

This is the first magneto-infrared measurement on 3D Dirac materials. This work has been published in Physical Review Letter 115, 176404 (2015) and selected as Editor’s Suggestion. Rongyan Chen (post doctor in Prof. Nanlin Wang’s group) and Zhiguo Chen (former student of Prof. Nanlin Wang) are leading authors and contribute equally to this work. The samples are provided by Prof. Genda Gu’s group from Brookhaven national laboratory in the US. Prof. Nanlin Wang and Prof. Fa wang are both corresponding authors of this paper.

The work was supported by the National Science Foundation of China, the 973 project of the Ministry of Science and Technology of China, and the Collaborative Innovation Center of Quantum Matter in Beijing, China.