Claudia Felser2 Johannes Gooth2 Satya Guin2 Chenguang Fu2 Sarah Watzman1

2, Solid State Chemistry, Max Planck Institute Chemical Physics of Solids, Dresden, , Germany
1, Department of Mechanical and Materials Engineering, University of Cincinnati, Cincinnati, Ohio, United States

Topology, a mathematical concept, became recently a hot topic in condensed matter physics and materials science. The topology of the electronic structure of a material determines the electronic, thermal and magnetic properties of solids. All known materials can be reclassified through the lens of topology [1]. Beside of Weyl and Dirac fermions, new fermions can be identified via linear and quadratic 3-, 6- and 8- band crossings stabilized by space group symmetries [2]. Weyl semimetals, a new class of topological phases were found in NbP, NbAs. TaP, MoP and WP2. [3-13]. In NbP nano wires we have observed the chiral and a mixed gravitational anomaly [9, 10]. Additionally, NbP [10] and WP2 [11] show evidence for a hydrodynamic flow of electrons, which violated strongly the Wiemann Franz law. MoP and WP2 show exceptional transport properties such as high conductivity (better than copper), high mobilties and a large magneto-resistance [6, 7]. In WP2 a transition from a hydrodynamic electron fluid below 15 K into a conventional metallic state at higher temperatures is ovebserved in thermal and magnetoelectric transport experiments [11]. The hydrodynamic regime is characterized by a viscosity-induced dependence of the electrical resistivity on the square of the channel width that coincides with as strong violation of the Wiedemann-Franz law. Single crystalline NbP shows an exceptional high Nernst-effect with a strong magnetic field depedence [12, 13]. Even polycrystalline, spark plasma sintered samples of NbP show still a large Nernst thermopower value of ~90 µV/K and power factor of ~35×10-4 Wm-1K-2 at 9 Tesla. Also in magnetic samples, such as Co2MnAl and Co3Sn2S2, we can design giant Nernst effects due to an strongly enhanced Berry curvature [14,15]. In general, the concept of topology might enable us to design more energy efficient materials for thermoelectric applications and beyond.

[1] B Bradlyn et al., Nature 547 298, (2017) and M. G. Vergniory, et al. preprint arXiv:1808.01163
[2] B. Bradlyn, et al., Science 353, aaf5037A (2016).
[3] C. Shekhar, et al., Nature Physics 11, 645 (2015)
[4] Z. K. Liu, et al., Nature Materials 15, 27 (2016)
[5] L. Yang, et al., Nature Physics 11, 728 (2015)
[6] C. Shekhar, et al. preprint arXiv:1703.03736
[7] N. Kumar, et al. Nature Communications 8, 1642 (2017)
[8] A. C. Niemann et al., Sci. Rep. 7, 43394 (2017)
[9] J. Gooth et al. Nature Communications 9, 4093 (2018)
[10] J Gooth et al., Nature 547 324, (2017)
[11] S. J. Watzman,et al.,Phys. Rev. B 97, 161404 (2017)
[12] U. Stockert, et al., Journal Physics: Condensed. Matter 29, 325701 (2017)
[13] Chenguang Fu, et al., Energy & Environmental Science 11, 2813 (2018)
[14] Enke Liu, et al., Nature Physics online, preprint arXiv:1712.06722
[15] Satya N. Guin, et al., preprint arXiv:1806.06753