Session A51: Topological Materials: Chiral and Semimetals

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Solids with topologically robust electronic states exhibit unusual electronic and optical transport properties that do not exist in other materials. A particularly interesting example is chiral charge pumping, the so-called chiral anomaly, in recently discovered topological Weyl semimetals, where simultaneous application of parallel DC electric and magnetic fields creates an imbalance in the number of carriers of opposite topological charge (chirality). Here, using time-resolved terahertz measurements on the Weyl semimetal TaAs in a magnetic field, we optically interrogate the chiral anomaly by dynamically pumping the chiral charges and monitoring their subsequent relaxation. Theory based on Boltzmann transport shows that the observed effects originate from an optical nonlinearity in the chiral charge pumping process. Our measurements reveal that the chiral population relaxation time is much greater than 1 ns. The observation of terahertz-controlled chiral carriers with long coherence times and topological protection suggests the application of Weyl semimetals for quantum optoelectronic technology.   

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Quantum interference effects play an important role in the optical properties of topological materials. One example is Fano resonance, featured by an asymmetric Breit-Wigner-Fano (BWF) line shape that describes interference between discrete and continuous scattering states. In this work, we present the anisotropic Fano resonance in a type-II Weyl semimetal candidate LaAlSi by polarized Raman spectroscopy using five excitation wavelengths from UV to near-infrared. The B₁ phonon mode for 532-nm laser excitation shows anisotropic BWF line shape whose frequency, linewidth, and asymmetry factor all depend on the laser polarization angle. Such anisotropic Fano resonance is unique and rarely observed in solid-state materials. Meanwhile, a scattering background occurs along with the asymmetry constituting the continuous states, which can be explained by double resonant Raman scattering due to flat phonon dispersions in the density functional theory (DFT) calculations. The experimental observations combined with DFT calculations provide valuable insights into the microscopic scattering pathways of LaAlSi, which is essential in understanding the optical scattering process and electron-phonon interactions in topological quantum materials.   

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Topological semimetal candidate CaSn₃ shows superconductivity at low temperatures, providing a promising platform for understanding the interplay between topological physics and superconductivity [1,2]. To clarify the intercorrelation between topology and superconductivity, not only the investigation of superconducting gap structure, but also a detailed study of the Fermiology in the normal state is essential. Here, we present de Haas van Alphen quantum oscillations study for topological semimetal candidate CaSn₃ single crystal via torque magnetometry in high magnetic fields up to 35T. Comparing the density functional theory calculations with our experimental observations in the quantum oscillations, we will discuss the detailed Fermi surfaces, spin-splitting due to the Zeeman effect, and the non-trivial topological aspects of CaSn₃.

[1] S. Gupta, R. Juneja, R. Shinde, and A. K. Singh, Journal of Applied Physics 121, 214901 (2017).
[2] X. Luo, D. F. Shao, Q. L. Pei, J. Y. Song, L. Hu, Y. Y. Han, X. B. Zhu, W. H. Song, W. J. Lu, and Y. P. Sun, J. Mater. Chem. C 3, 11432 (2015).   

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Topological phases in materials are intimately linked to the symmetry of the crystal lattice. Understanding the effect of symmetry breaking on the electronic properties allows for tuning of the topological properties and enables the study of novel quantum phenomena. ZrSiS is a topological semimetal showing coexistence of mirror symmetry and non-symmorphic symmetry-protected Dirac states. The four-fold rotational symmetry of the lattice can be broken by applying a uniaxial strain and consequently affect the topological bands. Here, we present transport measurements on single crystal ZrSiS as a function of uniaxial strain. The effect of strain on the Shubnikov de Haas (SdH) oscillations provides evidence that the topological phase can be tuned by strain.   

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Here, we present a study on the Fermi-surface of the Dirac type-II semi-metallic candidate NiTe₂ via the temperature and angular dependence of the de Haas-van Alphen (dHvA) effect measured in single-crystals grown through Te flux. In contrast to its isostructural compounds like PtSe₂, band structure calculations predict NiTe₂ to display a tilted Dirac node very close to its Fermi level that is located along the Γ to A high symmetry direction within its first Brillouin zone (FBZ). The angular dependence of the dHvA frequencies is found to be in agreement with the first-principle calculations when the electronic bands are slightly shifted with respect to the Fermi level (ε_F), and therefore provide support for the existence of a Dirac type-II node in NiTe₂. Despite the coexistence of Dirac-like fermions with topologically trivial carriers, samples of the highest quality display an anomalous and large, either linear or sub-linear magnetoresistivity. This suggests that Lorentz invariance breaking Dirac-like quasiparticles dominate the carrier transport in this compound.   

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EuCd₂As₂ is predicted to host a pair of Weyl points, and Weyl cone like features which may arise from ferromagnetic fluctuations have been observed. [1,2] Here, we present the synthesis details and characterization of single crystals of EuCd₂As₂ that, with varying initial stoichiometries, can be tuned from an antiferromagnetic ground state (T_N ~ 9.5 K) to a ground state with a clear ferromagnetic component (T_C ~ 30 K). [3] We have further attempted to understand this variation in magnetic behavior by doping Na on the Eu site. Magnetization, magneto-optical Kerr rotation, specific heat and resistivity data will be presented and discussed.
1. Wang, L.-L. et. al., Phys. Rev. B 99, 245147 (2019).
2. Ma, J.-Z. et. al., Science Advances, Vol. 5, no. 7, (2019)
3. Jo, N. H. et. al., Phys. Rev. B 101, 140402(R) (2020).   

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Materials with multifold band degeneracy provide a unique opportunity to study massless fermions without elementary particle counterparts. Recently CoSi has been shown to host both six- and four-fold band degenerate points[1,2].
Here we present a fermiology study of CoSi based on angle-resolved Shubnikov–de Haas oscillation measurements. The focused ion beam technique is used to fabricate microstructures with large length to cross section ratios to increase signal[3]. Two clear oscillation frequencies are observed at all angles. Despite the isotropic Fermi surface evident by the angle-independent oscillation frequency, the oscillation amplitude shows a sharp minimum with field applied along [100] direction, unexpected for the isotropic Fermi surfaces of a cubic material. Ab initio calculations reveal an angle-dependent electron-phonon lifetime, demonstrates the possible origin of the angle-dependent oscillation amplitude. These results indicate the existence of Fermi surface “hot” spots and its possible relation to band topology needs to be further investigated.

[1] Z. Rao et al., Nature 567, 496 (2019).
[2] D. S. Sanchez et al., Nature 567, 500–505 (2019).
[3] P. J. W. Moll, Annu. Rev. Condens. Matter Phys. 9, 147 (2018).   

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One of the most exciting phenomena from a single Weyl/multifold node in response to the electromagnetic wave is the quantized circular photogalvanic effect (CPGE), where a universal photo-current generation rate is expected regardless of the light frequency. By doing the terahertz emission measurement with widely tunable excitation energy from 0.2 to 1.1 eV, we exam two candidates for the quantized CPGE, RhSi and CoSi. We find both materials show giant but non-quantized CPGE. Combining the experimental results with density functional theory calculations, we explain the origins for the absence of the quantization and further propose to reach quantized CPGE in these two compounds by altering the Fermi level and increasing the hot-carrier lifetime. References: Ni, et al. arXiv. arXiv:2006.09612; Ni, et al. arXiv:2005.13473; Xu, et al. PNAS 10.1073/pnas.2010752117 (2020).   

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Chiral topological semimetals (which possess neither mirror nor inversion symmetries) are expected host numerous novel phenomena, such as multifold fermions with large topological charge, long Fermi-arc surface states, unusual magnetotransport and lattice dynamics, unconventional superconductivity and other correlated phenomena, and exotic optical effects, such as a quantized response to circularly polarized light. However, until recently, all experimentally confirmed topological semimetals crystallized in space groups that contain mirror operations, which means that the aforementioned phenomena must vanish.

Here, I will present evidence from angle-resolved photoelectron spectroscopy that a family of intermetallic catalysts, including PtAl and PdGa [1,2], are chiral topological semimetals. We directly visualize the multifold fermions in these compounds and show that they carry the largest possible Chern number that can be realized in any material. We also show experimentally that there is a direct relationship between the handedness of the crystal structure and the electronic chirality (i.e. the Chern number sign) of the multifold fermions. This finding demonstrates that structural chirality can be used as a control parameter to manipulate phenomena that are sensitive to electronic chirality, such as the direction of topological photocurrents.

I will then present our latest experimental results about new directions in the field of chiral topological semimetals.

[1] N. B. M. Schröter et al., Nat. Phys. 15, 759–765 (2019).
[2] N. B. M. Schröter et al., Science 369, 179 (2020).   

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The way electrons interact with phonons determines many physical processes that occur within electronic devices and material systems, but it has been difficult to investigate due to the weak strength of the electron-phonon interaction (EPI). An unusual kind of EPI is the Kohn anomaly, first discovered in the 1950s, whereby phonon softening is observed due to the divergence of the electron screening. This has been previously observed in several material systems, ranging from elemental metals to carbon-based allotropes, but not yet in topological materials whose topology imparts an inherent robustness against perturbations. In this talk, I will describe the observation of a chiral Kohn anomaly in the Weyl semimetal tantalum phosphide using inelastic x-ray and neutron scattering, which were guided upon by theoretical calculations [1]. In particular, the strong agreement between theory and calculations related to the unique features of Kohn anomalies in this topological material can help shed light on the strength of the EPI and on fundamental processes that would underlie some of these exotic materials.

[1] Nguyen, T. et al. Phys. Rev. Lett. 124, 236401 (2020).   

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The first observation of chiral plasmon modes in 3D Dirac and Weyl materials Na₃Bi and CoSi is reported through optical spectroscopy. Reflection measurements on single-crystals, over the spectral range from 20 mev to 2.5 eV and temperature ranging from 5K to 300K, show absorption features locked to the strongly temperature-dependent plasma edges. The frequency shifts of chiral plasmon features from the plasma edge are consistent with the Berry curvature associated with a Dirac or Weyl pair. The Lifshitz gap associated with inter-Fermi arcs transitions in CoSi is also reported.