One of the most frequently used methods for characterizing thin films is UV–Vis absorption. The near‐edge region can be fitted to a simple expression where the intercept gives the band gap and the fitting exponent identifies the electronic transition as direct or indirect. [See Tauc et al., Physica Status Solidi 15, 627 (1966); naturally, these are usually called “Tauc” plots.] In earlier work, we found that direct band gaps fitted using Tauc's method can be quite accurate, to ∼1% [see Viezbicke et al., Phys. Status Solidi B 252, 1700 (2015)]. Still, slopes of these Tauc plots are less frequently quantified, even though the slopes are directly rooted in key band‐structure parameters. In this study, we examine the reproducibility of Tauc plot slopes for ZnO as a model direct‐gap material and compare these experimental values with the theoretically derived slope. In contrast to the band gap accuracy, the experimental slope values varied by several orders of magnitude. The histogram of slope values was significantly more compact for Tauc plots exhibiting less Urbach tail contribution. In these cases, the Tauc slopes can provide an order‐of‐magnitude quantification of other key band characteristics such as carrier effective mass.

The origins of hydrodynamic transport in strongly interacting Dirac and Weyl semimetals have remained elusive in theoretical descriptions and experimental measurements. We investigate the structure and microscopic properties of transport in WP 2 , a type-II Weyl semimetal, to probe the emergence of hydrodynamic phenomena. We characterize the quantum behavior underlying the hydrodynamic transport regime as a function of temperature through ab initio calculations of the relevant microscopic scattering processes, including electron-phonon, electron-electron, and phonon-mediated electron-electron lifetimes. We present a fundamentally new approach to calculate phonon-drag, a mechanism that is invoked in numerous recent experiments, and remains the subject of active debate in the field. Further, we show unique and unexpected features of the lifetime-resolved Fermi surfaces of WP 2 in the hydrodynamic regime and quantify the degree of anisotropy in electron and hole pockets. This description of the microscopic dynamics in hydrodynamic systems like WP 2 indicates the importance of electron-phonon interactions in understanding connections between transport in hydrodynamic materials and strongly correlated quantum systems including unconventional metals and high Tc superconductors.

Weyl semimetals are 3D phases of matter with topologically protected states that have remarkable macroscopic transport behaviors. As phonon dynamics and electron-phonon scattering play a critical role in the electrical and thermal transport, we pursue a fundamental understanding of the origin of these effects in type-I Weyl semimetals NbAs and TaAs. In the temperature-dependent Raman spectra of NbAs, we reveal a previously unreported Fano lineshape, a signature stemming from the electron-phonon interaction. Additionally, the temperature dependence of the A 1 phonon linewidths in both NbAs and TaAs strongly deviate from the standard model of anharmonic decay. To capture the mechanisms responsible for the observed Fano asymmetry and the atypical phonon linewidth, we present first principles calculations of the phonon self-energy correction due to the electronphonon interaction. Finally, we investigate the relationship between Fano lineshape, electron-phonon coupling, and locations of the Weyl points in these materials. Through this study of the phonon dynamics and electronphonon interaction in these Weyl semimetals, we consider specific microscopic pathways which contribute to the nature of their macroscopic transport. arXiv:1903.07550v1 [cond-mat.mes-hall]

Weyl semimetals are materials with topologically nontrivial band structure both in the bulk and on the surface, hosting chiral nodes which are sinks and sources of Berry curvature. Weyl semimetals have been predicted, and recently measured, to exhibit large nonlinear optical responses. This discovery, along with their high mobilities, makes Weyl semimetals relevant to a broad spectrum of applications in optoelectronic, nanophotonic and quantum optical devices. While there is growing interest in understanding and characterizing the linear and nonlinear behavior of Weyl semimetals, an ab initio calculation of the linear optical and optoelectronic responses at finite temperature remains largely unexplored. Here, we specifically address the temperature dependence of the linear optical response in type-I Weyl semimetals TaAs and NbAs. We evaluate from first principles the scattering lifetimes due to electron-phonon and electron-electron interaction and incorporate these lifetimes in evaluating an experimentally relevant frequency-, polarization-and temperature-dependent complex dielectric function for each semimetal. From these calculations we present linear optical conductivity predictions which agree well where experiment exists (for TaAs) and guide the way for future measurements of type-I Weyl semimetals. Importantly, we also examine the optical conductivity's dependence on the chemical potential, a crucial physical parameter which can be controlled experimentally and can elucidate the role of the Weyl nodes in optoelectronic response. Through this work, we present design principles for Weyl optoelectronic devices that use photogenerated carriers in type-I Weyl semimetals.Weyl semimetals, one class of materials with topologically nontrivial electronic behavior, have generated considerable recent attention 1-3 for their novel responses to applied electric and magnetic fields. These materials exhibit linearly dispersive electronic band touchings in the bulk states of the crystal, which can be described by the Weyl equation 4 and appear in pairs of opposite chirality. 5,6 Weyl nodes are connected by characteristic Fermi arcs when projected onto the surface Brillouin zone; therefore Weyl semimetals are distinguished from other topological systems in having both unique bulk and surface states which are protected only by translational symmetry. Perhaps most promising, due to their separation of chirality, the Weyl nodes have diverging Berry connection, leading to predictions 7-12 and observations 13-17 of strongly nonlinear optical responses in Weyl semimetals. In the context of the study of optoelectronic materials, nonlinearity has proven to be a powerful method for elucidating material properties, including the symmetries of electronic structure and their associated Berry curvature. Technologically, nonlinearity and harmonic generation is a key mechanism to generate high frequencies for electronics and optoelectronics, enable light-sources and lasers across broad wavelength ranges, and allow single and pair photon ge...

Single-layer transition metal dichalcogenides (TMDCs) can adopt two distinct structures corresponding to different coordination of the metal atoms. TMDCs adopting the T-type structure exhibit a rich and diverse set of phenomena, including charge density waves (CDW) in a √ 13 × √ 13 supercell pattern in TaS2 and TaSe2, and a possible excitonic insulating phase in TiSe2. These properties make the T-TMDCs desirable components of layered heterostructure devices. In order to predict the emergent properties of combinations of different layered materials, one needs simple and accurate models for the constituent layers which can take into account potential effects of lattice mismatch, relaxation, strain, and structural distortion. Previous studies have developed ab initio tight-binding Hamiltonians for H-type TMDCs [1]. Here we extend this work to include T-type TMDCs. We demonstrate the capabilities of our model using three example systems: a 1-dimensional sinusoidal ripple, the 2×2 CDW in TiSe2, and the √ 13 × √ 13 CDW in TaS2. Using the technique of band unfolding we compare the electronic structure of the distorted crystals to the pristine band structure and find excellent agreement with direct DFT calculations, provided the magnitude of the distortions remains in the linear regime.