Session R59: Weyl semimetals, nonlinear optics

Fermi surface instability in the antiferromagnetic Dirac material Ca1-xNaxMnBi2

The quasi-two-dimensional bismuth layer-like AMnBi₂ (A= alkaline as well as rare earth atom) lately advanced as an arena for the investigation of low-energy quasiparticle excitations in topological materials. The A= Sr or Ca compositions have attracted special attention because of the coexistence of the anisotropic Dirac cones with antiferromagnetic order. In a broader context, the title compound also provides an opportunity to study low-dimensional magnetism and its putative relationship to the electronic properties. This work describes novel results of reflectivity measurements from the far-infrared up to the ultraviolet that probe the optical response as a function of temperature. This gives access to the optical conductivity which captures the relevant energy scales shaping the electronic structure. We discover [1] a reshuffling of spectral weight, defined as the integral of the real part of the optical conductivity, for energy scales up to 0.2 eV. This occurs at the onset of the spin reorientation transition which also manifests as an anomaly in the dc transport data of the title compound. This may reveal the inclination towards a Fermi surface instability in topological materials.
[1] M. Corasaniti et al., Phys. Rev. B100, 041107(R) (2019).   

Evidence for Strong Electron-Phonon Coupling in Weyl Semimetals

Since the experimental discovery of topological semimetals (TSM) there has been a flurry of activity focused on identifying signatures of topological transport. However, many of the transport properties display behavior that is dominated by non-topological interactions with the phonon system. A deeper understanding of these TSMs therefore requires investigation of the coupling between the topological electronic system and the non-topological vibrational system. In this presentation we discuss our recent temperature dependent Raman measurements on multiple TSMs which reveal unusual temperature dependence of the linewidths. These linewidths indicate strong electron-phonon coupling in these materials, and furthermore, they show a preponderance of optical phonon decay into electron-hole pairs which seems to be a relatively general feature in TSMs.   

Second Order Nonlinear Responses in Chiral Weyl Semimetal CoSi

The origin of the nonlinear responses in Weyl semimetals has become an increasingly contentious issue in recent years, with some of these effects being attributed to topology and others to symmetry. Part of this contention comes from the fact that the Weyl nodes are required by mirror symmetries in non-chiral Weyl semimetals to appear at exactly the same energy in the band structure, making it impossible to probe a single cone. Recently, chiral Weyl semimetals – with no mirror symmetries – have been observed with paired Weyl cones at significantly different energies (>100meV), allowing for cone discrimination based on choice of laser wavelength. Here, we present our study of the second order nonlinear responses in one such chiral Weyl semimetal, CoSi. In particular, we look at variation of the second harmonic generation (SHG) at multiple crystalline facets of CoSi, and the spatial inhomogeneity and temporal dynamics of its induced SHG. Further, we compare the SHG response strength to a variety of second order nonlinear crystals. Finally, we examine the spontaneous photocurrent response in CoSi.   

Magnetic-Field Dependence of Chirality-Resolved Optical Phonon in Weyl semimetal

Weyl semimetal (WSM), the solid-state realization of chiral Weyl fermions, exhibits chiral zeroth Landau levels in the magnetic field contributing to novel transport and optical properties. While electronic transport has been a focus of studies, the optical response of WSM is relatively under-explored, yet it can also reflect the topological nature of WSM. For example, the chirality of the Weyl fermions can be determined by measuring the photocurrent using circularly polarized mid-infrared light. In this work, we study the chirality-resolved optical phonons in type-I WSM TaP under magnetic field, using Raman spectroscopy with excitations of both linear and circularly polarized lasers. Under magnetic field, the B₁¹ mode prohibited at zero field for 633 nm excitation can be evidently observed for linear polarization. With circularly polarized excitation, the measured phonon modes break the classical Raman tensor theory, and the symmetry-prohibited modes can be observed. Moreover, the phonon mode intensity exhibits non-reciprocity with polarizations of incident and scattered light (σ+/σ+ and σ+/σ-). Our observation is closely related to the chiral Landau levels and shows the capability of Raman spectroscopy in studying the magnetic field response of WSMs.   

Unique Optical Responses of Weyl Semimetals

The isolation of electrons of different chirality in Weyl semimetals led to predictions of novel transport phenomena. Amongst these are the nonlinear responses and chiral anomaly. Here I will discuss our recent results on a range of Weyl semimetals. First I will focus on their nonlinear responses. Next, I will cover how Raman reveals surprisingly large electron-phonon coupling, that is relevant to their high mobility and potential for chiral transport.   

Weyl Semimetals in Nanophotonics and Quantum Optoelectronics

Weyl semimetals obtain their topological classification from the breaking of inversion or time-reversal symmetry. Consequently, they hold the potential for large optical nonlinearities and nonreciprocal behavior, making Weyl semimetals a very interesting materials class for optoelectronic and photonic applications. In this talk, we discuss our first principles calculations of optoelectronic properties of Weyl semimetals for integration with nanophotonic and plasmonic architectures. We discuss the coupling of atom-like emitters to cavities made from Weyl semimetals, designed with the idea of minimizing the loss penalty of conventional metals, as well as the use of Weyl semimetals for nonreciprocal thermal radiation control, which could avoid the applied magnetic field required in previous nanophotonic device designs. Finally, as an outlook, we will present a general computational approach to develop low-loss, highly nonlinear Dirac and Weyl semimetal materials for photonic information science.   

Large longitudinal circular photogalvanic effect in a chiral Weyl semimetal

Breaking all of the mirror symmetries in a chiral Weyl semimetal lifts the energy
degeneracy of two Weyl points with opposite Chern numbers. One of the conse-
quences is the large or even quantized circular photogalvanic effect (CPGE)
, which originates from one of the Weyl points by keeping the other node
Pauli-blocked. Here, we report a large photocurrent generated by circularly
polarized light in a chiral Weyl semimetal. Symmetry analysis shows that
the photocurrent is consistent with the longitude CPGE in bulk. The second-order
conductivity spectrum of CPGE in near-infrared and mid-infrared regions is then obtained and
the condition of quantization is examined.   

Polarized Transient Reflectance Spectroscopy of Type II Weyl Semimetal NbIrTe4

Weyl semimetals have attracted intense interest for exhibiting chiral Fermion states. Strong nonlinear effects such as 2^nd harmonic generation and DC photocurrents via circular photogalvanic effect are seen. Less understood is the dynamic response of said chiral carriers. We present polarized transient reflectance of single nanoflakes of NbIrTe₄, employing ultrafast (140 fs) NIR pump and MIR probe pulses. We use a photo-elastic modulator to produce a ~50 kHz oscillation of the circular probe polarization from left to right handedness. DFT calculations predict the presence of 8 pairs of Weyl nodes with opposite chirality in the BZ of NbIrTe₄. We observe a typical transient response composed of a rapid (~500 fs) decay followed by a slow (~1 ns) relaxation. A sign change in the transient response is observed at ~0.5 eV, suggestive of a band-to-band transition between a band below the Fermi energy and near the Weyl nodes to some higher-lying state.   

Angle and Polarization-Resolved Resonant Raman Spectroscopy of the Type-II Weyl Semimetal NbIrTe4

We perform polarized Raman spectroscopy measurements of single nanoflakes exfoliated from a single crystal of the layered ternary compound NbIrTe₄. By varying the angle between the polarization direction of incident light and the crystalline “a” axis, spectra indicate strongly anisotropic Raman peaks which are consistent with the broken inversion symmetry of the crystal, which is an essential enabling condition for a type II Weyl semimetal. We have used both 633 nm and 514 nm laser excitation with incoming and backscattered rays parallel to the c-axis normal to the flake with the “a” and “b” axes in the plane of the nanoflake. The comparison of these spectra shows an enhancement of Raman peaks and also changes in rotational symmetry for 633 nm excitation. This suggest a possible unresolved resonance with a state at lower energies. Density Functional Theory (DFT) calculations of this crystal (space group Pmn2₁) show close correspondence with both the frequency and symmetries of the modes detected in our measurement.   

Investigating the Energy Dependent Photogalvanic Effect in the Type-II Weyl Semimetal NbIrTe4

We explore the photogalvanic effect in NbIrTe₄ , a type II Weyl semimetal. NbIrTe4 exhibits 8 pairs of Weyl nodes which are monopoles of Berry curvature with opposite ±1 chiralities. We measured the photoresponse for a full rotation of a quarter wave plate for energies ranging from 0.3 to 1 eV. The Photothermoelectric (PT), and Circular (CPGE) and Linear (LPGE) Photogalvanic responses were extracted by fitting the angular dependence of the signal. The PT response shows an onset at an energy at 0.3 eV, consistent with DFT calculations. The LPGE response is relatively constant over this energy range, while the CPGE response shows a strong increase at low energies as the excitation approaches the Weyl nodes at the Fermi energy. Surprisingly, a strong peak in the CPGE response is also seen in the range 0.5 to 0.7 eV. This suggests that the CPGE response may be enhanced by a transition between bands near the Weyl point and a higher lying state.   

Investigation of optical properties of 3D Dirac materials

3D Dirac semimetals are newly discovered topological materials that are characterized by Weyl nodes with double degeneracy in the bulk. The electronic bands are protected from gapping by crystal symmetries and touch at special points in the BZ zone. A Dirac semimetal band structure has been observed in with ARPES but the study of their electric-optical properties has yet to be further developed Here, we present spectral measurements on the 3D Dirac semimetal . Using FITR technics in reflectivity configuration, we have observed the electromagnetic response of the 3D Dirac material from the FIR up to the NIR frequency range in different electrical field polarization configuration. Our measurements reveal the presence of unusual collective excitations near the screened plasma frequency. We attribute these excitations as arising from the extra -term in the action of the electromagnetic field that originates from the chiral shift between the Weyl nodes. We will also present the optical response in an applied magnetic field.