Session G68: Topological Semimetals II

Optical fingerprints of unconventional carriers in kagome metals

Kagome metals are the newly emerging class of quantum materials, where the peculiar kagome structure along with the itinerant character of the electrons give rise to a non-trivial combination of entangled magnetic states, electronic correlations, and topological orders. Two dissimilar features of massive, localized carriers at dispersionless flat bands and the massless Dirac fermions at the linearly dispersing bands coexist in their electronic structure, giving rise to various exotic phenomena. While the magnetic kagome metals are at the center of interest due to the proximity of the flatband magnetism, newly discovered non-magnetic counterparts possess a peculiar density wave order along with the unconventional superconductivity.

Optical spectroscopy is a useful method that shows excellent sensitivity to both linear and flat bands. Furthermore, (partial)gaps at the density of state at Fermi energy can be identified with clear signatures in optical spectra and the energy scale can be discussed. The opportunity to combine with other external stimuli such as magnetic field and pressure offers a viable route to disentangle different contributions. In this talk, I will highlight the optical fingerprints of unconventional carriers in these unique materials.   

Spin waves in graphene with Zeeman-type spin splitting

We present a computational study of spin waves in doped paramagnetic single-layer graphene with adjustable Zeeman-type band splitting. The spin waves are described using time-dependent spin-density-functional response theory, treating dynamical exchange-correlation effects within the Slater and Singwi-Tosi-Land-Sjolander approximations. We obtain spin-wave dispersions and spin stiffnesses as a function of doping and spin polarization, and discuss prospects for their experimental observation.   

Superconductor vortex spectrum of the Fermi arc metal in Weyl semimetals

The surface of a Weyl semimetal hosts open Fermi arcs that form closed contours with Fermi arcs on the opposite surface. The resulting 2D Fermi surface, when subjected to a magnetic field, exhibits characteristic quantum oscillations stemming from semiclassical cyclotron orbits that involve both bulk and surface travel and obey a subtle form of Bohr-Sommerfeld quantization conditions. In this work, we use similar semiclassical ideas to derive the superconductor vortex spectrum of this 2D metal. We first recall that each Weyl node produces a chiral Majorana fermion in the bulk inside the vortex. We then show that the vortex spectrum is governed by the total Berry phase acquired by a wavepacket traversing a closed orbit consisting of Fermi arcs on opposite surfaces and the bulk chiral modes. In particular, the vortex gap closes and reopens when the total Berry phase sweeps through $(2n+1)\pi$  . The Berry phase contributed by the chiral modes depends on the vortex orientation relative to the separation between the Weyl nodes. We, therefore, predict characteristic features in the vortex tilt-dependence of the local density of states that can be probed by standard scanning tunneling measurements.   

Evolution of Charge Order in Ba1-xSrxAl4

The BaAl₄ type structure also hosts a variety of interesting and exotic properties. Recent results have shown an emergence of CDW order in SrAl₄ at 243K, together with a hysteretic structural transition at 87K that is thought to be a structural transition to a monoclinic system. The tetragonal structure of BaAl₄ has also recently been shown to have topologically non-trivial bands and has lattice parameters very close to SrAl₄. In order to study the interplay between these features we have done a chemical substitution study to track the reported transitions. We have performed electrical resistivity measurements, along with x-ray and neutron scattering on the Ba_1-xSr_xAl₄ family. We show a suppression of the CDW order with increased Ba substitution. Neutron scattering and x-ray measurements show the nature of the CDW.   

Electric quantum oscillations in Weyl semimetals

Electronic transport in Weyl semimetals is quite extraordinary due to the topological property of the chiral anomaly generating the charge pumping between two distant Weyl nodes with opposite chiralities under parallel electric and magnetic fields. Here, we develop a full nonequilibrium quantum transport theory of the chiral anomaly, based on the fact that the chiral charge pumping is essentially nothing but the Bloch oscillation. Specifically, by using the Keldysh nonequilibrium Green function method, it is shown that there is a rich structure in the chiral anomaly transport, including the negative magnetoresistance, the non-Ohmic behavior, the Esaki-Tsu peak, and finally the resonant oscillation of the dc electric current as a function of electric field, called the electric quantum oscillation. We argue that, going beyond the usual behavior of linear response, the non-Ohmic behavior observed in BiSb alloys can be regarded as a precursor to the occurrence of electric quantum oscillation, which is both topologically and energetically protected in Weyl semimetals.   

Discovering Dirac materials with quad-helicoid surface states

    Surface states of Dirac materials consist of two opposite helicoid surfaces, which cross each other along a line in the Brillouin zone and will hybridize with each other even against symmetry-allowed perturbations [1]. However, recent studies show that Dirac points with nonzero Z2 monopole charge can host gapless surface states under certain conditions [1-3], and such Dirac points can even have topologically protected quad-helicoid surface states which have never been explored in electronic and phononic systems. 

    In this talk, we will show all the possible momenta and space groups that may obtain the Dirac points with quad-helicoid surface states by three conditions summarized in our work. Those conditions are universal for systems with/without spin-orbit coupling, e.g., electronic systems, phononic systems, and photonic systems. We also offer two material candidates to show the topological property of Dirac points carrying quad-helicoid surface states in the spinless electronic band structure and phonon band structure, respectively. Topological phase transition from Z2 Dirac points to Weyl dipoles by breaking certain symmetries will also be discussed.    

Electronic structures of twisted few-layer black phosphorus

Few-layer black phosphorus (BP), a gapped two-dimensional (2D) semi-Dirac material, has pseudospin features even in its semiconducting phase [1,2]. When its band gap is closed, few-layer BP has a semi-Dirac point, and when its band gap is inverted, it has two anisotropic Dirac cones [1-3]. Twisted bilayers of gapped 2D semi-Dirac materials may have interesting emergent phenomena due to directional mismatch of in-plane anisotropy of each layer. In our present work, we perform density functional theory (DFT) calculations of 90-degree twisted few-layer BP. We control the band gap of twisted few-layer BP by changing structural parameters and analyze electronic band structures near the chemical potential. We construct effective Hamiltonian based on DFT results and obtain relevant pseudospin representations. We also discuss effects of externally applied electric fields.   

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WTe₂ and MoTe₂ monolayers provide a rich atomically thin platform to explore topological physics, electrically switchable circular photogalvanic effect, rectification, and quantum nonlinear Hall effects. For realization of the nonlinear quantum phenomena, a nonzero Berry-curvature dipole (BCD) is required, which has been demonstrated in pristine WTe₂ and MoTe₂ monolayers [1-4]. In this work, we theoretically design a new monolayer W_0.5Mo_0.5Te₂ using the pristine T_d-WTe₂ and T_d-MoTe₂ monolayers [5]. We confirm the dynamical, elastic, and mechanical stability of W_0.5Mo_0.5Te₂ monolayer by phonon and elastic constants calculations. Our electronic band structure calculations reveal the presence of a relatively much smaller bandgap near the Fermi energy compared to that of in pristine T_d-WTe₂ and T_d-MoTe₂ monolayers. Such a narrow bandgap leads to divergence of Berry curvature near the Fermi energy yielding a large enhancement of BCD in W_0.5Mo_0.5Te₂ monolayer, as predicted by our first-principles calculations.

[1] You et al., Phys. Rev. B 98, 121109 (2018).

[2] Zhang et al., 2D Materials 5, 044001 (2018).

[3] S.-Y. Xu et al., Nat. Phys. 14, 900 (2018).

[4] Ma et al., Nature 565, 337–342 (2019).

[5] Karki et al., arXiv:2109.08725 (2021).