Session B60: Topological Materials: Weyl, Dirac, Chiral, and Other Semimetals

Theoretical and Experimental Discovery of Topological Chiral Crystals

We have shown that Kramers-Weyl fermions are a universal topological electronic property of all non-magnetic chiral crystals with spin-orbit coupling and are guaranteed by structural chirality, lattice translation and time-reversal symmetry. We determined that all point-like nodal degeneracies in non-magnetic chiral crystals with relevant spin-orbit coupling carry non-trivial Chern numbers. Kramers–Weyl materials can exhibit a monopole-like electron spin texture and topologically non-trivial bulk Fermi surfaces over an unusually large energy window [G. Chang et al. Nature Materials 17, 978-985 (2018)]. Among all the materials, we predicted the RhSi family to exhibit the ideal topological band structures, displaying the largest possible momentum separation of compensative chiral fermions, the largest proposed topologically nontrivial energy window, and the longest possible Fermi arcs on its surface [G. Chang et al. PRL 119, 206401 (2017)]. We present the theory of exotic nonlinear optical responses of topological chiral crystals, including quantized photogalvanic effect in RhSi and robust photocurrents from Fermi arc surface states [G. Chang et al. PRL 119, 206401 (2017) and arXiv:1906.03207 (2019)]. We also discuss the experimental discovery of RhSi as topological chiral crystals [D. S. Sanchez et al. Nature 567, 500-505 (2019)] and additional experiments that our discovery has enabled. (This work is in collaboration with D. S. Sanchez, I. Belopolski, T. A. Cochran, B. J. Wieder, F. Schindler, J. Yin, S. S. Zhang, S. Huang, B. Singh, T. Chang, A. Bansil, T. Neupert, S.-Y. Xu, H. Lin, and M. Zahid Hasan)   

Unconventional Topological Fermions in Orthorhombic RhSi

Topological semimetals with different types of band crossings, distinct from conventional Dirac and Weyl nodes, can give rise to novel fermionic excitations that may not have analogues in high-energy physics. Exotic multifold topological excitations have been predicted theoretically and observed in a family of transition metal silicides (e.g. β-RhSi). These compounds adopt the cubic FeSi structure type (space group No. 198), for which the calculations predict Chern numbers > 1 at specific high symmetry points within their Brillouin zone.
Through Density Functional Theory (DFT) calculations, we report that such multi degenerate fermions also exist in α-RhSi, which adopts an orthorhombic structure type (space group No. 62). Group symmetry analysis combined with DFT calculations indicate the coexistence of multiple types of multifold Dirac fermions at or near high symmetry points. Measurements of the angular dependence of the de Haas–van Alphen effect, indicate a Fermi surface whose topography is in reasonable agreement with the DFT calculations. We observe a pronounced anomaly in the magnetic torque of α-RhSi suggesting the possibility of topological phase-transitions.   

Ultrasound Studies of the Magnetic Weyl Semimetal Co3Sn2S

Co₃S₂Sn₂ is a magnetic Weyl semimetal that crystallizes in a shandite structure with the Co atoms forming a kagome lattice. It orders ferromagnetically at about 174 K with easy axis perpendicular to the kagome plane. Co₃S₂Sn₂ displays a giant anomalous Hall effect as well as other transport anomalies characteristic of Weyl semimetals. Here we present Resonant Ultrasound (RUS) measurements on single crystals of Co₃S₂Sn₂ as a function of temperature and magnetic field. RUS measures the mechanical resonances of the crystals and is highly sensitive to phase transitions and other thermodynamic irregularities in the sample.

  

Epitaxial growth and characterization of topological semimetals from the 2D transition metal dichalcogenides family

Prototypical topological Weyl and Dirac semimetals are 3D crystal structures and are typically grown in the form of bulk. Discovering and engineering topological semimetals from the family of 2D TMDs could open the way for exploitation of their topological properties by fabricating thin epitaxial films and devices on suitable substrates. In this work, we show that using molecular beam epitaxy, single and few layers of HfTe₂^[1] and ZrTe₂^[2] can be grown epitaxially on AlN and InAs substrates. By combining ARPES and DFT calculations we provide evidence that they are type-I and type-II Dirac semimetals, respectively, with the Dirac point located at the Fermi energy. Dirac cones are maintained down to the single-layer, suggesting that they could be considered as the electronic analogues of graphene. Moreover, we report the first direct observation at room temperature of the orthorhombic Weyl semimetal phase of MoTe₂^[3]. DFT calculations predict eight type-II Weyl nodes which are located just below the Fermi level, making them accessible to electronic transport, thus creating prospect for practical applications.
[1] S. A. Giamini et al., 2D Mater. 4, 015001 (2017).
[2] P. Tsipas et al., ACS Nano 12, 1696 (2018).
[3] P. Tsipas et al., Adv. Funct. Mater. 28, 1802084 (2018).   

Epitaxial growth of Kagome semimetal FeSn and Mn3Ge

The Kagome lattice is hexagonal and composed of corner sharing triangles. Materials with layered Kagome lattice allow the interplay of topological properties and interactions in flat bands, and can give rise to spin liquids, skyrmions and other novel phases. There is broad interest in realizing high-quality topological semimetal materials such as Mn₃X (Sn, Ge and others) [1], FeSn [2] and Co₃Sn₂S₂ [3], in layered Kagome structure. Both angle-resolved photoemission spectroscopy (ARPES) and transport measurement indicate they are topologically nontrivial. Until now, most measurements are performed on bulk samples which can be a limitation for both physics and application. By using molecular beam epitaxy and sputtering, We synthesized Kagome structured FeSn and Mn₃Ge films. Both in-situ and ex-situ characterizations indicate these films are highly crystalline and c-axis oriented. Highly ordered atomic layers are captured by scanning transmission electron microscopy. Anomalous Nernst effect has been measured which can be related to non-vanishing Berry curvature.   

Magnetotransport properties of Dirac semimetal TaSe3

We have investigated the magnetotransport properties of the transition metal trichalcogenide TaSe₃, which is a Dirac semimetal candidate. While the electrical resistivity exhibits metallic behavior at zero field, a magnetic field (H)-induced upturn is observed at low temperatures for H > 4 T, applied perpendicular to the current direction. The extremely large, non-saturating transverse magnetoresistance (MR) reaches ~ 7000 % for H = 14 T at 1.9 K. For H > 6 T, Shubnikov-de Haas (SdH) oscillations are observed with frequencies F_α ≈ 98 T and F_β ≈ 186 T. By constructing the Landau fan diagram for each oscillation, we calculated the Berry phase to be 1.1 π and 0.2 π for F_α and F_β , respectively . The underlying physics will be discussed.
  

Beyond topology in TaAs

Topological Weyl and Dirac semimetals have excited much research interest in the last few years. Much of the study of these materials has focused on the existence of exotic signatures of topology, such as Fermi arc surface states, nonlocal transport, and observations of boundary effects such as the quantum Hall effect. One feature shared by many of these materials is a low carrier density. Landau quantization in low carrier density topological semimetals in moderate magnetic fields produces large changes in the band structure of these materials, allowing the formation of tunable, strongly interacting correlated states. We present thermal and thermodynamic signatures of these strong correlations in the prototypical Weyl semimetal TaAs.   

A non-linear Hall effect at zero field in a chiral nonmagnetic compound

The Hall effect arising from the Berry curvature is one of the most important topological phenomena. In the linear response regime, such a Hall effect appears only in the time-reversal symmetry breaking condition; this is well-known as an anomalous Hall effect. By contrast, the recent studies have proposed that in the nonlinear response regime, a Hall voltage can be observed due to the Berry curvature even in time-reversal invariant systems. These works demonstrate that the new-type of Hall effect is a powerful probe for investigating Berry physics in the nonmagnetic topological materials. However, experimental studies on this nonlinear Hall effect are still lacking.
Here, we report the observation of nonlinear Hall effect in a chiral nonmagnetic compound. We find that an electric current induces a nonlinear Hall voltage under time-reversal invariant conditions. The sign of the voltage at 4.2 K depends strongly on the direction of the chirality of crystal structure, which is consistent with the theoretical calculations. Furthermore, this nonlinear Hall effect becomes smaller at higher temperatures but is still observable even at room temperature.   

Topology Classification using Chiral Symmetry: Chiral Zak Phase and Spin Correlations

Topology classification theory has been broadly applied to explain and gain deeper insight of many physical phenomena. On the other hand, the power of this theory for one-dimensional (1D) systems has not been as widely used. Quasi 1D graphene nanoribbons (GNRs) can now be synthesized with atomic precision via bottom-up molecular precursor techniques and are discovered to possess nontrivial topological properties, as predicted by Cao, Zhao and Louie under a Z₂ classification using time reversal and spatial symmetries. However, the use of these symmetries constrains the classification to nonmagnetic materials with symmetric unit cell. Here we develop another approach to use only chiral symmetry (based on chiral Zak phase) to classify 1D materials, which gives an Z classification. Using bulk-edge correspondence, we connect the chiral Zak phase with an edge-index that predicts the characters of topology-induced states at junctions of two 1D materials belonging to different classes. Moreover, using the GNRs as an example, we investigate the spin-spin correlations between topologically protected junction states via first-principles calculations.   

Anisotropic thermoelectric properties of topological semimetal YbMnSb2 single crystal

Topological materials can be a suitable platform for the study of thermoelectric transport properties due to the high mobility and exotic transport behaviors. Some topological materials are also good thermoelectric materials, for example, Bi₂Te₃ alloys. In this work, we report the thermoelectric properties of topological semimetal YbMnSb₂ single crystal. Although being a semimetal, YbMnSb₂ still show promising Seebeck coefficient due to the much larger contribution of valence band than the conduction band. Large anisotropy is found in resistivity and thermal conductivity, while Seebeck is almost isotropic. Furthermore, we investigate the thermoelectric transport properties under magnetic field, wherein a maximum enhancement of the thermoelectric figure of merit (zT) reached 200 % at 100 K under 9 T. With further reduction of the thermal conductivity by alloying, microstructure engineering et al., we expect better thermoelectric performance. The present work can be instructive for the search of other new high-performing thermoelectric materials.   

STM/STS of Few-Layer Topological Semimetal NiTe2 at 78 K

NiTe₂ is a semimetal which has garnered interest due to recent observation of topological states and Dirac nodes near the Fermi level (E_F). Here, Au-assisted exfoliation of bulk NiTe₂ is performed to generate a few-layer NiTe₂ surface to better understand the evolution of the band structure as a function of film thickness. Scanning tunneling microscopy and spectroscopy measurements are performed at 78 K to probe the local density of electronic states (LDOS) at the surface and around defects. Multiple peaks in the LDOS are observed near E_F and are compared to those obtained computationally from density functional theory (DFT) to reveal the nature of these features in the electronic structure.
  

Topological light meets topological semimetal

A Weyl semimetal carries topological charges at the Weyl nodes; a light beam can also carry a topological charge, when it has an orbital angular momentum (OAM) from spatial phase and/or polarization gradients. Recently there has been a lot of interest in understanding how the spin angular momentum (SAM) of light interacts with materials to induce photocurrents (circular photogalvanic effect, CPGE), but not many studies have focused on photocurrents generated by the OAM of light. Here we report a unique orbital photogalvanic effect (OPGE) in a type-II Weyl semimetal WTe₂, featured by a photocurrent winding around the axis of OAM-carrying beams, whose intensity is directly proportional to the topological winding number of the light field, and can be attributed to a discretized dynamical Hall effect. In addition to obtaining new microscopic insights into light-matter interactions with topological light, these measurements show promise for fabricating on-chip photodetectors on Weyl semimetals capable of detecting OAM modes, which can be useful for building the next-generation, high-capacity optoelectronic circuits.   

Quenching of Relaxation Pathway In Weyl Semimetal TaAs

Following the topological classification of materials, Weyl semimetal has been predicted theoretically and has since been experimentally confirmed. Experimental investigation on Weyl semimetal shows promise in the field of optoelectronics due to its wide spectral response from visible to near infrared regime. However, the transient dynamics of Weyl semimetal has not been clearly established. In this work, we studied the ultrafast response of tantalum arsenide (TaAs) in the optical regime through transient reflection spectroscopy with varying pump fluence. We noted two relaxation pathways for the hot injected electrons in TaAs by the pump pulse. We identified the origin of these two relaxation pathways through pump fluence dependent study. We noticed that depending on the energy of the probing wavelength, one of the relaxation pathways is quenched. The understanding of this quenching mechanism provides a reliable way to study the transient dynamics in Weyl semimetal.