Session J51: Dirac and Weyl Semimetals: Materials and Modeling -- Transport

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This work explores electron transport in Weyl semimetals in the presence of magnetic field and charged impurities in various regimes of impurity concentrations and the magnitude of the field. We demonstrate that for sufficiently strong magnetic fields, the scattering of quasiparticles exhibits a strong anisotropy as a function of the direction of the magnetic and is strongly suppressed if the field is perpendicular to the separation of the Weyl nodes in momentum space, which allows one to easily access the regime of hydrodynamic transport in experiment. We study the viscosity of the electron liquid in a Weyl semimetal for weak and strong magnetic fields and discuss possible manifestations of the hydrodynamic transport in experiment.   

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Bulk Dirac semimetals such as Na3Bi and Cd2As3 have received attention due to their non-trivial band structure topology and the novel physics associated with it, including anomalous transport regimes and protected surface Fermi arcs. At present, first-principles studies of the scattering mechanisms and charge transport properties of Dirac semimetals remain limited. In this talk, we will present first-principles calculations of the electron mobility (and its governing scattering mechanisms), magneto-transport and spin relaxation times in selected Dirac semimetals. These studies employ the PERTURBO code developed in our group to efficiently compute electron scattering mechanisms and solve the Boltzmann transport equation. We overcome key challenges associated with Dirac semimetals, including calculating electron-phonon scattering processes near the Dirac cones using effective interpolation techniques, correctly treating spin-orbit coupling, and handling materials with large unit cells. Our results attempt to clarify the microscopic origin of the large mobilities and magnetoresistance of topological semimetals, and characterize their spin dynamics, for which few experimental results or theoretical predictions exist.   

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Spin-valley locking in the band structure of in monolayers of group-VI transition metal dichalcogenides such as MoS₂ has attracted enormous interests. In this talk, we report a non-centrosymmetric Dirac semimetal BaMnSb₂ is characterized by spin-valley locking^[1]. This is revealed by comprehensive studies using first-principle calculations, tight-binding and effective model analyses, and angle-resolved photoemission spectroscopy. Moreover, this material also exhibits a stacked quantum Hall effect. The spin-valley degeneracy extracted from the plateau height of quantized Hall resistivity is close to 2, further confirming the spin-valley locking picture. In the extreme quantum limit, we have also observed a two-dimensional chiral metal at the side surface. These findings establish BaMnSb₂ as a rare platform for exploring coupled spin and valley physics in bulk single crystals and accessing 3D interacting topological states.
[1] Liu et al., arXiv:1907.06318   

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In non-centrosymmetric materials, spin-orbit coupling induces the momentum-dependent spin splitting of the bands. In 2D systems with in-plane inversion asymmetry, such as monolayer transition metal dichalcogenides (TMDCs), the Zeeman-type spin splitting leads to the valley-contrasting spin polarization[1]. However, the variety of such spin-valley coupling is limited for TMDCs, since the valley configuration is fixed to that for graphene. In this talk, we report the spin-valley coupling is tunable in the layered semimetal BaMnX₂ (X = Bi, Sb), where the distorted X square net layer with in-plane polarization realizes the spin-valley coupled Dirac fermion. Our structural analysis reveals that the lattice distortion for X = Bi is approximately one-tenth of that for X = Sb. Consequently, the first-principles calculation predicts a different configuration of spin-polarized Dirac valleys; X = Sb has two valleys[2], while X = Bi has six. This was experimentally observed as a clear difference in the Shubnikov-de Haas oscillation at high fields between the two materials. Thus, various spin-valley coupling can be controlled by the chemical substitution in BaMnX₂.
References:
[1] Di Xiao et al., Phys. Rev. Lett. 108, 196802 (2012).
[2] H. Sakai et al., Phys. Rev. B 101, 081104(R) (2020).   

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Previous work has shown that Dirac/Weyl semimetals with a single pocket of carriers can exhibit a large enhancement of thermopower when the system reaches the extreme quantum limit, where only a single Landau level is occupied. Here we study the magnetothermoelectric properties of compensated semimetals, for which pockets of electron- and hole-type carriers coexist at the Fermi level. We show that, when the compensation is nearly complete, such systems exhibit a huge enhancement of thermopower beginning at a much smaller magnetic field, such that ω_c τ>1. We discuss our results in light of recent measurements on the compensated Weyl semimetal tantalum phosphide, in which an enormous magnetothermoelectric effect was observed.   

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Weyl semimetals are excellent candidates for transverse thermoelectric transport via the Nernst effect. The Nernst effect is a thermoelectric phenomenon in which a temperature gradient is applied orthogonal to an applied magnetic field, resulting in a voltage in the mutually perpendicular direction. Single-crystal NbP has shown a large Nernst thermopower, exceeding 800 mV K-1 at 109 K and 9 T [1]. Published work in bulk polycrystalline NbP, with an average grain size of ~100 microns, maintains a large Nernst thermopower, albeit decreased by a factor of ~8 [2]. In this work, we present a grain size study on bulk polycrystalline samples of NbP, where different annealing times result in varying grain sizes. Nernst thermopower continues decrease with decreasing grain size. However, thermal conductivity is also greatly reduced, which is an advantage for thermoelectric transport applications. Via microstructural characterization and thermomagnetic transport measurements, we can determine the optimal grain size for transverse thermoelectric transport applications.
[1] S. J. Watzman et al. Phys. Rev. B 97(16), 161404(R) (2018).
[2] C. Fu et al. Energy Environ. Sci. 11(10), 2813-2830 (2018).   

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Topological electronic materials host exotic boundary modes, that cannot be realized as standalone states, but only at the boundaries of a topologically classified bulk. Topological Weyl semimetals, whose bulk electrons exhibit chiral Weyl-like dispersion, host Fermi-arc states on their surfaces. The Fermi-arc surface bands disperse along open momentum contours terminating at the surface projections of bulk Weyl nodes with opposite chirality. Such reduction of the surface degrees of freedom by their segregation to opposite surfaces of the sample, that reoccurs in all topological states of matter and even exhibited by topological defects [1], provides topological protection from their surface elimination. We have confirmed the Weyl topological classification of both the inversion symmetry broken compound TaAs [2] and the time reversal symmetry broken Co3Sn2S2 [3] by spectroscopic visualization of their Fermi-arc surface states through the interference patterns those electrons embed in the local density of states. This has allowed us to examine their unique nature and level of protection against perturbations. In TaAs the Fermi arc bands are found to be much less affected by the surface potential compared to trivial bands that also exist on its surfaces. In contrast, in Co3Sn2S2 the dispersion of the topological Fermi-arc bands, and even their inter-Weyl node connectivity, are found to vary with the surface termination. A possible resolution of this discrepancy will be discussed.

[1] Abhay Kumar Nayak et al, “Resolving the Topological Classification of Bismuth with Topological Defects” Science Advances 5, eaax6996 (2019)
[2] Rajib Batabyal et al, “Visualizing weakly bound surface Fermi arcs and their correspondence to bulk Weyl fermions” Science Advances 2, e1600709 (2016)
[3] Noam Morali et al, “Fermi-arc diversity on surface terminations of the magnetic Weyl semimetal Co3Sn2S2”Science 365, 1286 (2019)   

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Extremely large non-saturating magnetoresistance has recently been reported for a large number of both topologically trivial and non-trivial materials. Different mechanisms have been proposed to explain the observed magnetotransport properties, yet without arriving to definitive conclusions or portraying a global picture. We investigate the transverse magnetoresistance of materials by combining the Fermi surfaces calculated from first principles with the Boltzmann transport theory approach relying on the semiclassical model and the relaxation time approximation. We show that in all cases we investigated charge carrier compensation and open-orbit mechanisms are responsible for non-saturating magnetoresistance. We address in detail magnetotransport phenomena in some representative materials: copper, bismuth as well as in topological semiletals tungsten diphosphide WP₂[1] and zirconium silicon sulfide ZrSiS[2], finding excellent agreement with experimental results.

[1] S. N. Zhang et al., Phys. Rev. B 99, 035142 (2019)
[2] M. Novak et al., Phys. Rev. B 100, 085137 (2019)   

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The electron correlation in topological semimetals is an important subject of topological material physics. The quantum limit(QL), wherein electrons are quasi-one-dimensionally confined under the magnetic field, offers a fertile playground to study correlation induced non-trivial phases such as the topological CDW or excitonic insulator, but these phases remain to be fully explored experimentally.
In this study, we report the magnetoresistance up to 55 T for CaIrO₃, which is a strongly-correlated Dirac semimetal on the verge of Mott transition[1,2]. We observed the giant positive longitudinal magnetoresistance in the QL, which steeply increases at 9 T, shows a peak around 18 T, and decreases at higher field. The activation energy derived by the Arrhenius plot also shows non-monotonic magnetic field dependence, which agrees well with the theoretical model of CDW instability[3]. Combining with the results of non-linear current-voltage property, we propose that disordered CDW is formed in the intermediate field region (9-25 T) of QL in CaIrO₃.

[1] M. Zeb and H. Kee, PRB 86, 085149 (2012)
[2] J. Fujioka et al., Nat. Commun. 10, 362 (2019)
[3] H. Fukuyama, Solid State Commun. 26, 783 (1978)   

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In time-reversal invariant materials, Berry curvature dipole (BCD) gives rise to an anomalous nonlinear Hall effect (NLHE) when the inversion symmetry is broken. Although NLHE is observed in bilayer^[1] and few-layer^[2] WTe₂, BCD-induced NLHE in bulk single-crystals remains elusive. In this talk, we will report a bulk NLHE in a spin-valley locked Dirac semimetal BaMnSb₂^[3]. In this material, the distorted Sb zig-zag chains generate two gapped Dirac cones with spin-valley locking near the X point in the Brillouin zone, which creates a BCD. Our experiments show when an alternating current flows along the zig-zag chain direction across the crystal, a second-order Hall signal is present and exhibits the quadratic dependence on current. Through structure and domain tuning, we further demonstrate the observed NLHE is intrinsic. This finding shows NLHE measurements can serve as a probe for Berry curvature for non-centrosymmetric bulk crystals.

[1] Q. Ma, et al., Nature 565, 337 (2019).
[2] K. Kang, et al., Nat. Mater. 18, 324 (2019).
[3] J. Y. Liu, et al., ArXiv:1907.06318.   

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The proximity of the Fermi energy to its Dirac node make ZrTe₅ a suitable platform in which to study the influence of topology in transport properties. In particular, it has been shown that the Berry curvature in ZrTe₅ gives rise to an anomalous contribution to its Hall signal. We report measurements of the Anomalous Hall Effect in ZrTe₅ at different stages of a temperature induced Lifshitz transition, in which the Fermi level decreases with increasing temperature. An enhancement of the anomalous Hall signal is observed for temperatures at which the Fermi level approaches the vicinity of the Dirac node, providing an insight of the interplay between topology and electronic states in the system. Our results show that ZrTe₅ is a highly tunable platform at which to study the influence of topology in electronic transport.