Session K68: Weyl semimetals

Anomalies in topological semimetals

Topological semimetals are a new class of metallic materials, which exist at band fillings that ordinarily correspond to insulators or compensated accidental semimetals with zero Luttinger volume. Their metallicity is a result of nontrivial topology in momentum space and crystal symmetry, wherein topological charges may be assigned to point or line band-touching nodes, preventing gap opening, unless protecting crystal symmetries are violated. These topological charges, however, are defined from noninteracting band eigenstates, which raises the possibility that the physics of topological semimetals may be modified qualitatively by electron-electron interactions. Here we ask the following question: what makes the topological semimetals nontrivial beyond band theory? Alternatively, can strong electron-electron interactions open a gap in topological semimetals without breaking the protecting symmetries or introducing topological order? We demonstrate that the answer is generally no, and what prevents it is their topological response, or quantum anomalies. While this is familiar in the case of magnetic Weyl semimetals, where the topological response takes the form of an anomalous Hall effect, analogous responses in other types of topological semimetals are more subtle and involve crystal symmetry as well as electromagnetic gauge fields. Physically these responses are detectable as fractional symmetry charges induced on certain gauge defects, or as spontaneous fractional electric polarization.    

Methods for enhancing the anomalous Nernst effect in Weyl materials

The anomalous Nernst effect (ANE) is a thermoelectric transport phenomenon that is closely related to the anomalous Hall effect. Both effects are significantly enhanced in the presence of large Berry curvatures. As Weyl nodes are associated with Berry curvature singularities, Weyl semimetals are promising candidates for realizing large ANE currents and are thus exciting targets for various functional applications. Here we introduce a method for enhancing ANE signals by inducing constructive superposition of opposite-chirality Weyl nodes via a magnetic field. We illustrate this effect with multi-band models, discuss some design principles for magnetic topological materials which may exhibit this effect, and present density functional theory results for selected candidate materials.   

Weyl hydrodynamics in a strong magnetic field

We study the hydrodynamic transport of electrons in a Weyl semimetal in a strong magnetic field. Impurity scattering in a Weyl semimetal with two Weyl nodes is strongly anisotropic as a function of the direction of the field and is significantly suppressed if the field is perpendicular to the separation between the nodes in momentum space. This allows for convenient access to the hydrodynamic regime of transport, in which electron scattering is dominated by interactions rather than by impurities. In a strong magnetic field, electrons move predominantly parallel to the direction of the field, and the flow of the electron liquid in a Weyl-semimetal junction resembles the Poiseuille flow of a liquid in a pipe. We compute the viscosity of the Weyl liquid microscopically and find that it weakly depends on the magnetic field and has the temperature dependence $\eta(T)\propto T^2$. The hydrodynamic flow of the Weyl liquid can be generated by a temperature gradient.  The hydrodynamic regime in a Weyl-semimetal junction can be probed via the thermal conductance $G_q(B,T)\propto B^2 T$ of the junction.   

Effects of doping on thermoelectric and thermomagnetic transport in polycrystalline NbP and WTe2

Weyl semimetals (WSM) combine both topological and semimetallic effects, making them candidates for interesting thermoelectric transport properties. Single-crystalline NbP, a Type I WSM, was previously shown to have a large Nernst thermopower, a small Seebeck thermopower, and no magneto-Seebeck effect [1]. However, Skinner and Fu predict this class of materials contains a large, linear-in-field magneto-Seebeck effect [2]. Here, we present results on polycrystalline NbP indicating a large Nernst effect, albeit reduced from that seen in a single crystal, existing simultaneously with an enhanced Seebeck thermopower and a large magneto-Seebeck effect. We propose that slight doping, found present in the polycrystalline sample of NbP, alters the location of the Fermi level enough to allow this to be observed. The presence of both a large Nernst and magneto-Seebeck thermopower is uncommon and could have unique device advantages if used additively. We extend our work to WTe₂, a Type II WSM, through characterization of both longitudinal and transverse thermoelectric and thermomagnetic transport measurements.

[1] S. J. Watzman et al. Phys. Rev. B 97(16), 161404(R) (2018).

[2] B. Skinner and L. Fu. Sci. Adv. 4(5) (2018).   

Nonlinear Hall effect in epitaxial WTe2 thin film

In exfoliated few-layer T_d-WTe₂, it has previously been shown that an applied longitudinal electrical current can generate a nonlinear anomalous Hall effect in time-reversal symmetric conditions. This nonlinear Hall effect is a manifestation of the Berry dipole that originates from the opposite Berry curvatures of the two Weyl nodes in T_d-WTe₂, which is known as a Weyl semimetal. In this talk, we present our studies of the nonlinear Hall effect in epitaxial few-layer Td-WTe2 samples grown by molecular beam epitaxy. By measuring the 2nd harmonic response of the transverse voltage in the presence of an applied longitudinal electrical current, we have observed a voltage signal that scales quadratically with respect to the current. The signal exists at zero magnetic field and is independent of the frequency used in the measurement – mimicking the nonlinear Hall effect. We discuss the relationship of this signal with respect to the lattice symmetry in WTe2, which is calibrated using in-situ reflection high-energy electron diffraction during the growth. We will further discuss the temperature and angle dependence of the nonlinear Hall signal in epitaxial WTe2 samples and correlate it to magneto-transport studies.   

Non local spin voltage in DSM, WSM and Role of Impurities

In this talk, we look at nonlocal spin transport in 3D topological insulators, Dirac, and Weyl semimetals. We further investigate the role of scattering and impurities in the nonlocal spin transport signature of spin momentum locked materials. We combine different materials with different properties, such as spin momentum locking and diffusion length, to investigate the interfacial effects of heterostructures on nonlocal spin transports. Finally, we look at recent experimental results and explain their results based on our quantum transport modeling. Furthermore, The role of the emergent magnetic field generated from the Weyl nodes is investigated in nonlocally precessing the spins.   

High Fermi velocities and small cyclotron masses in LaAlGe

We report quantum oscillation measurements of LaAlGe, a Lorentz-violating type-II Weyl semimetal with tilted Weyl cones. Very small quasiparticle masses and very high Fermi velocities were detected at the Fermi surface. Whereas three main frequencies have been observed, the angular dependence of two Fermi surface sheets indicates possible two-dimensional (2D) character despite the absence of the 2D structural features such as van der Waals bonds. Such conducting states may offer a good platform for low-dimensional polarized spin current in magnetic RAlGe (R = Ce, Pr) materials.   

Weyl-Kondo semimetal: Extreme topological tunability and nonlinear optical exploration

The recent surge of interest in exploring strongly correlated variants of topological semimetals has raised compelling questions about how to engineer, control, and detect their gapless topological nature. One amenable platform to map out this landscape is heavy fermion materials, which can harbor the theoretically established¹ and experimentally indicated² Weyl-Kondo semimetal (WKSM) phase, or a nodal line Kondo semimetal phase predicted based on the cooperation of strong correlations and space-group symmetry³. However unlike weakly correlated topology, where the chiral anomaly or electronic dispersion can be observed, wholly different experimental signatures of nontrivial topology are needed for strongly correlated systems. In the first part of this talk, I will show theoretical work on fine control of the Weyl-Kondo nodes under a relatively small Zeeman field through multiple topological phase transitions⁴, and survey the experiments on the WKSM candidate Ce₃Bi₄Pd₃ which indicate strongly correlated topology and its control by magnetic field⁵. Finally, owing to the high sensitivity of the WKSM, photoinduced phase transitions can be detected through emission spectra exhibiting high-harmonic generation, in addition to information about electronic structure and band topology. In the second part of the talk, I will present this recent work on nonlinear optical effects in the WKSM, towards an ultimate control of electronic topology on-demand.

¹H.-H. Lai, S. E. Grefe, S. Paschen, Q. Si, PNAS 115 1 2018 & PRB 101 075138 2020

²S. Dzsaber et al., PRL 118 246601 2017 & PNAS 118 8 2021

³L. Chen et al., 2021 arXiv:2107.10837; C. Cao, G.-X. Zhi, J.-X. Zhu, PRL 124 166403 2020

⁴S. E. Grefe, H.-H. Lai, S. Paschen, Q. Si, 2020 arXiv:2012.15841

⁵S. Dzsaber et al., 2019 arXiv:1906.01182   

Topological semimetal driven by strong correlations and crystalline symmetry

Weyl-Kondo semimetal has emerged as a rare example of gapless topological states driven by strong correlations [2,3]. The Kondo effect produces Weyl nodes near the Fermi energy with highly renormalized Fermi velocity. In this work [1], we develop a general framework for Kondo-driven topological semimetal phases and for the design of such materials. We propose that the space group symmetry constrain the topology of correlation-driven low-energy electronic excitations such as the heavy composite fermions. This framework leads to different types of Kondo-driven topological semimetals depending on the space-group symmetry constraints. We illustrated this general approach in square-net systems, with and without inversion symmetry. In these cases, strong correlations cooperate with the nonsymmorphic mirror symmetry to produce Weyl-Kondo nodal-line semimetals, with nodes pinned to the Fermi energy. Finally, we propose several Ce-based heavy fermion materials to realize such phases and suggest means to experimentally probe them.

 

[1] L. Chen et al., arXiv:2107.10837.

 

[2] H.-H. Lai, et al., PNAS 115, 93 (2018); S. E. Grefe et al., PRB 101, 075138 (2020).

 

[3] S. Dzsaber et al., PNAS 118, e2013386118 (2021); Phys. Rev. Lett. 118, 246601 (2017).   

Possible Weyl-Kondo Semimetallic State in Disordered Mn-doped VAl3 Compounds

Dirac- or Weyl-Kondo semimetal is particularly interested in condensed matter physics because the Kondo effect induces band hybridization, resulting in a Kondo gap, contrary to the topologically protected Weyl semimetallic state. The Weyl-Kondo semimetal was theoretically proposed but not experimentally realized yet, as we know. Here we suggest a possible coexistence of the Weyl semimetal and Kondo effect in disordered Mn-doped VAl3. The VAl3 is known as a type-II Dirac semimetal. Dilute Mn-doping in the VAl3 Dirac semimetal induces a dilute Kondo effect at low temperature, confirmed by the temperature-dependent electrical resistivity, magnetic susceptibility, and specific heat measurements. The magnetic doping in Dirac semimetal breaks time-reversal symmetry, lifting the band degeneracy, manifests the Weyl semimetallic phase transition in Mn-doped VAl3. From the angle-dependent magneto-resistance measurement, we found the phase transition from Dirac-semimetal VAl3 to Weyl semimetal in Mn-doped VAl3. The Weyl-Kondo semimetal driven by dilute magnetic doping element in Dirac semimetal can be a good platform for investigating the non-trivial properties of competing effects between topological and correlated electronic states.   

Withdrawn

Topological semimetals exhibit novel quantum phases and properties of matter, such as surface/edge states, topological Fermi arc, highly anisotropic negative longitudinal magnetoresistance, and exotic Hall transports. Recently, a new type of Hall effect is discovered in time-reversal symmetric topological semimetals in which the Hall conductivity is proportional to the square of the applied voltage. This unusual Hall signal is resulting from Berry curvature dipole (BCD) allowed from broken crystalline symmetry. In this talk, we will present electronic and transport properties of a topological semimetal exhibiting BCD-induced nonlinear Hall effect using angle-resolved photoemission spectroscopy, first-principles calculations, and transport measurements. We find that substantial nonlinear Hall conductivity, mainly contributed from the BCD around Weyl points located near the Fermi energy.