Session R51: Dirac and Weyl Semimetals: Materials and Modeling--Optical and Spectroscopy

Live

In materials that break inversion symmetry, light can generate a DC current via the photogalvanic effect. In this talk I will describe a surprising topological feature of the photogalvanic response to circularly polarized light: In chiral crystalline materials featuring point-like band crossings known as Weyl nodes, the circular photocurrent is exactly quantized in terms of fundamental constants only. The reason behind this is that the photocurrent directly measures the monopole charge of a Weyl node, a rare example of quantization in a gapless system at finite frequency. After presenting the basic theory behind the effect, I will discuss whether it can occur for multiband symmetry-protected crossings, such as those recently found in CoSi and related materials. I will also review the conditions to extract the quantized coefficient in different approaches, including DC photocurrents, THz spectroscopy and difference frequency generation, and discuss the recent experimental progress on this front.   

Live

The study of nonlinear optical and optoelectronic properties in Weyl semimetals has attracted much interest since the first observation of giant anisotropic second harmonic generation (SHG) in the type-I Weyl semimetal TaAs. The recently discovered class of chiral Weyl semimetals adds a unique perspective to this research because the lack of mirror symmetries allows for the energy separation of paired Weyl points. This separation allows light of micron-scale wavelengths to be used to directly probe one of the two paired Weyl cones, which is not possible in the more familiar type-I and type-II Weyl semimetals. In this talk, I will present our experimental study of two second order nonlinear effects, SHG and photocurrent generation, in an archetypal chiral Weyl semimetal, CoSi. I will quantify these effects and compare their strengths to observations of previously studied nonlinear crystals. Finally, I will apply field-symmetry-based arguments to distinguish between the AC effect of SHG and the DC effects of the shift current and circular photogalvanic effect.   

Live

Symmetry plays a key role in both conventional and topological phases of matter, making the ability to drive symmetry changes optically a critical step in developing future technologies that rely on such control. Here, we use femtosecond optical pulses to transiently lower symmetry in the prototypical type-I Weyl semimetal TaAs. Using second harmonic generation spectroscopy, we observe an ultrafast reduction of magnetic point group symmetry without any structural change, indicating the electronic origin of the transition. We argue that this effect is brought on by photocurrent generation, which breaks both spatial and time-reversal symmetry through introducing an asymmetry in the non-equilibrium distribution of charge carriers following photoexcitation. Our results demonstrate that optically driven photocurrents explicitly break electronic symmetry in a generic fashion, opening up the possibility of driving phase transitions between symmetry-protected states on ultrafast time scales.   

Live

We theoretically study the bulk and surface polariton modes in Type-I Weyl semimetals with broken time-reversal symmetry. We apply the fully quantum-mechanical approach to derive the bulk and surface conductivity tensors by not only considering both intraband and interband optical transitions, but also taking into account all possible combinations of bulk-to-bulk, bulk-to-surface, and surface-to-surface transitions. We explore the potential of popular tip-enhanced optical spectroscopy techniques for studies of bulk and surface topological electron states in these materials. Strong anisotropy, anomalous dispersion, and the optical Hall effect for surface polaritons launched by a nanotip provide information about the Weyl node position and separation in the Brillouin zone, the value of the Fermi momentum, and the matrix elements of the optical transitions involving both bulk and surface electron states.   

Live

Second harmonic generation (SHG) is a fundamental nonlinear optical phenomenon widely used both for experimental probes of materials and for application to optical devices. Even-order nonlinear optical responses including SHG generally require breaking of inversion symmetry, and thus have been utilized to study noncentrosymmetric materials. In this talk, we will talk about our theoretical study of the SHG in inversion-symmetric Dirac and Weyl semimetals under a DC current which breaks the inversion symmetry by creating a nonequilibrium steady state [1]. We find that Dirac and Weyl semimetals exhibit strong SHG upon application of finite current. Our experimental estimation for a Dirac semimetal Cd₃As₂ and a magnetic Weyl semimetal Co₃Sn₂S₂ suggests that the induced susceptibility χ⁽²⁾ for practical applied current densities can reach 10⁵ pm V⁻¹ with mid-IR or far-IR light. This value is 100-10000 times larger than those of typical nonlinear optical materials.

[1] KT, T. Morimoto, J. Orenstein, and J. E. Moore, arXiv:2007.08887.   

Live

Graphene-like 2D Dirac materials are gapless only within certain approximations, e.g., if spin-orbit coupling (SOC) is neglected. It has recently been reported that materials with non-symmorphic crystal lattice possess symmetry-enforced Dirac-like band dispersion around certain high-symmetry momenta even in presence of SOC [1]. Here we calculate the optical absorption coefficient of non-symmorphic semimetals, such as α-bismuthene, which host two anisotropic Dirac cones with different Fermi velocities along x and y directions. We find that the optical absorption coefficient depends strongly on the anisotropy factor and the photon polarization. When a magnetic field is applied perpendicular to the plane the material, the absorption coefficient also depends on an internal parameter we termed the “mixing angle” of the band structure. Due to the anisotropy present in our model, the two Dirac cones contribute differently to the optical absorbance. Consequently, it is possible to selectively address one or the other cone and to control the magnitude of the optical absorbance by varying photon polarization .

[1] P.J. Kowalczyk et al., ACS Nano 14, 1888−1894 (2020).   

Live

One of the special properties of chiral Weyl semimetals is that their Weyl points are separated in energy due to the lack of mirror plane symmetries of the crystalline lattice. Another special property of chiral crystal structures is that they can also host topological phonon Weyl and Dirac points with similar characteristics to the electronic quasiparticles that have attracted the bulk of the scientific community's interest. These phonon modes can be observed by time-resolved ultrafast spectroscopies in which a femtosecond timescale laser pulse impulsively excites collective excitations that are measured in the time-domain by a separate mechanically delayed pulse. Depending upon the symmetry of the mode, this probe pulse either experiences a time-dependent change in reflectance that is measured as pump-probe spectroscopy, or a change in polarization that is measured as a time-dependent Kerr rotation. In this talk, we describe our time-resolved pump-probe and Kerr effect measurements of these phonon modes in space group 198 compounds and discuss our observations in terms of the electronic Weyl band structure.   

Live

(TaSe₄)₂I is a Weyl semimetal candidate that undergoes a charge density wave (CDW) transition on its quasi-one-dimensional lattice. This makes it an ideal system to study the interplay between topological effects and strong correlations. In particular, the onset of CDW order links two Weyl nodes near the Fermi surface to open up a gap leading to axion electrodynamics. In this work, we explore the band structure of (TaSe₄)₂I in the proximity of Weyl nodes with high momentum resolution laser-based angle-resolved photoemission (ARPES). Our results show strong circular dichroism in the measured ARPES spectra both above and below the CDW transition temperature (~260 K). We will discuss the implications of our results based on the predicted spin/orbital texture and effects of CDW on the electronic band structure of (TaSe₄)₂I near the Weyl nodes.   

Live

Topological physics and strong electron-electron correlations in quantum materials are typically studied independently. However, there have been rapid recent developments in quantum materials in which topological phase transitions emerge when the single-particle band structure is modified by strong interactions. In [Shi*, Wieder*, Meyerheim*, et al., Nat. Phys. (2021)], we demonstrate that the room-temperature phase of (TaSe4)₂I is a Weyl semimetal with 24 pairs of Weyl nodes. Owing to its quasi-1D structure, (TaSe4)₂I hosts an established CDW instability just below room temperature. Using X-ray diffraction, angle-resolved photoemission spectroscopy, and first-principles calculations, we find that the CDW in (TaSe4)₂I couples the bulk Weyl points and opens a band gap. The correlation-driven topological phase transition in (TaSe4)₂I provides a route towards observing condensed-matter realizations of axion electrodynamics in the gapped regime, topological chiral response effects in the semimetallic phase, and represents an avenue for exploring the interplay of correlations and topology in a solid-state material.   

Live

In this work, we describe the results of ¹²⁵Te NMR measurements vs crystal orientation on an aligned-crystal sample of the topological chalcogenide ZrTe₅. Results measured with B along the a crystal axis indicate a Lifshitz transition with the chemical potential crossing a gapped Dirac node. With a magnetic field of B = 9T parallel to the b axis we probe the quantum-limit behavior of the Dirac-type carriers very close to E_F. In this orientation, with T decreasing toward the Lifshitz transition, we observe an abrupt reduction in spin-lattice relaxation rate, indicating appearance of a gapped or very low-carrier phase in the n-type region. At lower temperatures, we find a further change, with a large decrease in shift for the zigzag Te site commencing at 60 K. This change is consistent with the appearance of a spin-polarized state at low temperatures. These transformations are also consistent with a recently reported B-T phase diagram [1], and we discuss the nature of these phases based on the NMR results.

[1] Tang et al., Nature 569, 537 (2019).   

Live

In close analogy to electronic semimetals, a variety of Weyl, Dirac points and nodal lines are predicted for phonons recently by ab initio calculations. However, very few experimental confirmations have been made, and most of them focused on bulk dispersions only. In this talk, we will present the results of our Inelastic Neutron Scattering (INS) experiments on MnSi and CoSi crystals. The measured phononic band structures and coherent cross sections are in remarkable agreement with our DFPT calculations, which enabled us to explore multiple types of unconventional Weyl and Dirac points in the Brillioun zone. Furthermore, being aware of neutron scattering's capability of wavefunction detection and polarization determination, we derived an explicit relation between the topological invariant of a Weyl node and the INS intensities nearby. Apart from protected surface states and novel transport phenomena, our work suggests that topological nodes also leave some observable traces in spectroscopic patterns.   

Live

Weyl semimetals (WSMs) have unusual optical responses originating from unique topological properties of their bulk and surface electron states. Their third-order optical nonlinearity is expected to be very strong, especially at long wavelengths, due to linear dispersion and high Fermi velocity of three-dimensional Weyl fermions. Here we derive the third-order nonlinear optical conductivity of WSMs in the long-wavelength limit and calculate the intensity of the nonlinear four-wave mixing signal as it is transmitted through the WSM film or propagates away from the surface of the material in the reflection geometry. All results are analytic and show the scaling of the signal intensity with variation of all relevant parameters. The nonlinear generation efficiency turns out to be surprisingly high for a lossy material, of the order of several mW per W³ of the incident pump power. Optimal conditions for maximizing the nonlinear signal are realized in the vicinity of bulk plasma resonance. This indicates that ultrathin WSM films of the order of skin depth in thickness could find applications in compact optoelectronic devices.   

Live

Extreme magnetoresistance (XMR), magnetic and structural phase transition, and possible non-trivial topological phases in the rare-earth monopnictide family have recently invigorated intense research interest. Recent publications have experimentally revealed the presence of a Dirac-like semimetallic phases in some of the lighter rare earth monopnictide materials. This study investigates the electronic structure of other members with higher f-electrons in this family. In particular, here we present a combined study using angle-resolved photoemission spectroscopy (ARPES) and first-principles calculations on DySb. A bulk band gap that lacks band inversion is observed at the (X) point of the BZ, which is further supported by our first-principles calculations. Our study establishes DySb as a trivial semimetal and provides a direction to develop a deeper understanding of XMR through rare-earth correlated materials.