Session P04: Dirac/Weyl Semimetals -- Optical and Spectroscopy

Optical Control of Chiral Charge Pumping in a Topologcial Weyl Semimetal

Solids with topologically robust electronic states exhibit unusual electronic and optical transport properties not seen in other materials. A peculiar example is chiral charge pumping (also called chiral anomaly) in recently discovered topological Weyl semimetals, where simultaneous application of parallel static electric and magnetic fields causes an imbalance in the number of topological chiral carriers. Here, using time-resolved terahertz measurements on the Weyl semimetal tantalum arsenide (TaAs) in a magnetic field, we uncover the optical control of chiral anomaly by dynamically pumping the chiral charges and monitor their relaxation. Rigorous theory based on Boltzmann transport shows that the observed optically-induced control of chiral anomaly is governed by the optical nonlinearity in the chiral charge pumping process. Our measurements reveal that the chiral pumping relaxation is much longer than 1 ns. The optically-controlled long-lived chiral carriers observed here in a Weyl semimetal exhibit similar behavior to the valley-polarized carriers in 2D semiconductors, but with the extra feature of topological protection, suggesting possible valleytronic applications of Weyl semimetals.   

Circular photogalvanic effect of layered type-II Weyl semimetal MoTe2 in mid-infrared band

T_d-Phase Molybdenum ditelluride (MoTe2) has attracted enormous research interests as a Type-II Weyl semimetallic state and the fascinating properties it exhibits. By applying circularly polarized 4.0-μm and 10.6-μm excitation at normal incidence, circular photogalvanic effect (CPGE) has been observed in topological T_d-phase MoTe₂. The CPGE signal is not allowed by the C_2V symmetry by second order nonlinear effect under our experimental geometry. This work is one of the first experimental realizations of this effect in type-II Weyl semimetals with the MIR spectral regime, which is suitable to unveil the physics related to Weyl cones. The occurrence of CPGE in MoTe₂ is attributed to a built-in electric field, which can remove full C_2v symmetry, resulting in nonzero injection current via an third order nonlinear optical effect. The field is produced by the work function difference between samples and electrodes or photothermoelectric effect after photo-excitation. Additionally, the existence of CPGE in our experiments is an unique signature of the T_d phase，which is in consistence with the recently-reported Barkhausen effect during phase transition of MoTe₂.   

Anisotropic Broadband Photo Response of Layered Type-II Weyl semimetals

T_d-MoTe₂ and TaIrTe₄, two-dimensional layered Type-II Weyl semimetal, are expected to share several unique advantages with Dirac semimetals for photodetection. The extremely high mobility and the chiral Fermions near the Weyl node lead to several advantages over traditional semiconductor-based photodetectors. Here, we report the realization of broadband self-powered photodetectors based on T_d-MoTe₂ and TaIrTe₄. Broadband responses from visible to mid-infrared are experimentally tested. The PC generation mechanisms are investigated and it is found that PTE effect plays critical role in PC generation. Furthermore, we found that the anisotropy is wavelength dependent, and the degree of anisotropy increases as the excitation wavelength gets closer to the Weyl nodes which can be used for polarization-sensitive photo detection. Thus T_d-MoTe₂ and TaIrTe₄ provide a new class of material platform for high performance photodetection applications   

Second Harmonic Generation Spectroscopy of Inversion Symmetry Breaking Weyl Semimetals

Weyl semimetals with acentric point groups permit bulk second order nonlinear optical processes by the very nature of their broken inversion symmetry. A common example of such an effect is second harmonic generation, in which two photons of frequency ω combine to produce a photon of frequency 2ω in a manner dependent not only on the symmetry of the crystalline lattice, but also on the electronic states involved in the optical transitions at these frequencies. Here, we present a spectroscopic second harmonic generation study of a Weyl semimetal, and discuss data taken as a function of incident and emitted polarization angle over a range of incident frequencies from the visible to the infrared.   

Optical Response Characteristics of the Weyl Semimetals WTe2 and MoTe2

Weyl semimetals are 3D systems exhibiting linear dispersion band crossings which act as Berry curvature monopoles in the reciprocal space. A key signature for Weyl semimetals is the appearance of surface Fermi arcs as a result of the projection of the bulk Weyl points on the surface of the material. Here we report first-principles calculations for the dynamical conductivity tensor, a basic quantity for the light-matter interactions, of the Weyl semimetals MoTe₂ and WTe₂. The calculations rely on the Kubo formula, computed based on a Wannier interpolation scheme, which takes into account the realistic electronic structure of these materials. The different components of the optical conductivity tensor are determined and studied as a function of frequency. Contributions from the bulk and surface states of the studied materials are also distinguished. The relation of the calculated results with the experimental ones, as well as the temperature effects, are also discussed.
  

Optical evidence of chiral Landau level in Weyl semimetal

Weyl semimetal has been extensively studied with exotic phenomena such as chiral anomaly. Despite the numerous experimental efforts to explore the chiral anomaly in different materials, electrical transport is the main approach where negative magneto-resistivity works as a criterion. More evidence of chiral anomaly, such as the existence of chiral Landau level, is desired for further study. Here we report a magneto-optical study in Weyl semimetal. Allowed optical transition under magnetic field can reveal the Landau level structure. The Chiral Landau levels are experimentally found in typical Weyl semimetals NbAs while the system is driven close to the quantum limit. The related optical transition exhibits distinct difference from other normal Landau levels. The z momentum can be finite with unique magnetic field evolution. Meanwhile, multiple carriers with different topological origins are found, indicate complex band structure of the crystal. Our results demonstrate an effective tool for studying complex topological systems. Further evidence of chiral magnetic effect will also be discussed.   

Second harmonic generation studies on the Weyl semimetal WTe2

Tungsten ditelluride (WTe2) is a layered material that is both a ferroelectric and a Weyl semimetal. The coexistence of these two properties stems from inversion symmetry breaking, a condition that is sensitive to the atomic-scale lattice distortions in this material. Here, we performed second harmonic generation (SHG) studies on tungsten ditelluride. We will report on unusual and large-amplitude temperature dependent responses of the SHG as a probe of the inversion-breaking phase in WTe2.   

Terahertz Generation Measurements in Inversion-Symmetry Breaking Weyl Semimetals with Spectrally Resolved Excitation Fields

Topological materials, and in particular Weyl semimetals, offer a playground for diverse optical properties. When inversion symmetry is broken, such materials additionally allow second-order bulk nonlinear effects. We present terahertz generation data with excitation fields ranging across the near infrared spectrum. Measurements were taken with both linear- and circular-polarized light in a variety of geometries.   

Chiral Landau levels in Weyl semimetal NbAs with multiple topological carriers

Recently, Weyl semimetals have been experimentally discovered in both inversion-symmetry-breaking and time-reversal-symmetry-breaking crystals. The non-trivial topology in Weyl semimetals can manifest itself with exotic phenomena which have been extensively investigated by photoemission and transport measurements. Despite the numerous experimental efforts on Fermi arcs and chiral anomaly, the existence of unconventional zeroth Landau levels, as a unique hallmark of Weyl fermions which is highly related to chiral anomaly, remains elusive owing to the stringent experimental requirements. Here, we report the magneto-optical study of Landau quantization in Weyl semimetal NbAs. High magnetic fields drive the system towards the quantum limit which leads to the observation of zeroth chiral Landau levels in two inequivalent Weyl nodes. As compared to other Landau levels, the zeroth chiral Landau level exhibits a distinct linear dispersion in z momentum direction and allows the optical transitions without the limitation of zero z momentum or sqrt_B magnetic field evolution. The magnetic field dependence of the zeroth Landau levels further verifies the predicted particle-hole asymmetry of the Weyl cones. Meanwhile, the optical transitions from the normal Landau levels exhibit the coexistence of multiple carriers including an unexpected massive Dirac fermion, pointing to a more complex topological nature in inversion-symmetry-breaking Weyl semimetals. Our results provide insights into the Landau quantization of Weyl fermions and demonstrate an effective tool for studying complex topological systems.   

Optical Signatures of the Chiral Anomaly in Mirror-Symmetric Weyl Semimetals

The chiral anomaly is a phenomenon characteristic of Weyl fermions, which has condensed matter realizations in Weyl semimetals. Efforts to observe a smoking gun signatures of the chiral anomaly in Weyl semimetals have so far focused on a negative magnetoresistance in electronic transport. Unfortunately, disentangling the chiral anomaly contribution in transport measurements has proven non-trivial. Recent works have proposed an alternative approach of probing phonon dynamics for signatures of the chiral anomaly in non-mirror-symmetric crystals. Here, we show that such phononic signatures can be extended to mirror symmetric crystals, broadening the pool of candidate materials. We show that the background magnetic field can break mirror symmetry strongly enough to yield observable signatures of chiral anomaly even in mirror-symmetric materials. Specifically for mirror-symmetric Weyl semimetals such as TaAs and NbAs, including the Zeeman interaction at B~10T, we predict an IR reflectance peak will develop with an E_IR●B dependence.   

Raman Insights into the Phonon Dynamics of Weyl Semimetals

Weyl semimetals (WSM) are a recently realized class of topological materials with low energy electronic excitations characterized by a three-dimensional, linear dispersion. While much focus has been directed at characterizing and understanding the electronic properties of various WSMs, little attention has been paid to the vibrational properties. In the WSMs NbAs and TaAs such considerations are crucial for understanding the anomalously high mobility and thermal response. We present temperature and polarization dependent Raman spectra which reveal anomalous lineshapes, as well as deviations from standard anharmonic behavior.   

Optical Evidence of Chiral Magnetic Anomaly in Weyl Semimetal TaAs

Chiral pumping from optical electric fields oscillating at THz frequencies is observed in the Weyl material TaAs with electric and magnetic fields aligned along both the a- and c-axes. Free carrier spectral weight enhancement is measured directly for the first time, confirming theoretical expectations of chiral pumping. A departure from linear field-dependence of the Drude weight is observed at the highest fields in the quantum limit, providing direct evidence of field-dependent Fermi velocity of the chiral Landau level. Implications for the chiral magnetic effect in Weyl semimetals from the optical f-sum rule are discussed