Nonlinear nanoelectrodynamics of a Weyl metal

Chiral Weyl fermions with linear energy-momentum dispersion in the bulk accompanied by Fermi-arc states on the surfaces prompt a host of enticing optical effects. While new Weyl semimetal materials keep emerging, the available optical probes are limited. In particular, isolating bulk and surface electrodynamics in Weyl conductors remains a challenge. We devised an approach to the problem based on near-field photocurrent imaging at the nanoscale and applied this technique to a prototypical Weyl semimetal TaIrTe4. As a first step, we visualized nano-photocurrent patterns in real space and demonstrated their connection to bulk nonlinear conductivity tensors through extensive modeling augmented with density functional theory calculations. Notably, our nanoscale probe gives access to not only the in-plane but also the out-of-plane electric fields so that it is feasible to interrogate all allowed nonlinear tensors including those that remained dormant in conventional far-field optics. Surface- and bulk-related nonlinear contributions are distinguished through their “symmetry fingerprints” in the photocurrent maps. Robust photocurrents also appear at mirror-symmetry breaking edges of TaIrTe4 single crystals that we assign to nonlinear conductivity tensors forbidden in the bulk. Nano-photocurrent spectroscopy at the boundary reveals a strong resonance structure absent in the interior of the sample, providing evidence for elusive surface states.

Quantum theory of the nonlinear Hall effect

The nonlinear Hall effect is an unconventional response, in which a voltage can be driven by two perpendicular currents in the Hall-bar measurement. Unprecedented in the family of the Hall effects, it can survive time-reversal symmetry but is sensitive to the breaking of discrete and crystal symmetries. It is a quantum transport phenomenon that has deep connection with the Berry curvature. However, a full quantum description is still absent. Here we construct a quantum theory of the nonlinear Hall effect by using the diagrammatic technique. Quite different from nonlinear optics, nearly all the diagrams account for the disorder effects, which play decisive role in the electronic transport. After including the disorder contributions in terms of the Feynman diagrams, the total nonlinear Hall conductivity is enhanced but its sign remains unchanged for the 2D tilted Dirac model, compared to the one with only the Berry curvature contribution. We discuss the symmetry of the nonlinear conductivity tensor and predict a pure disorder-induced nonlinear Hall effect for point groups C3, C3h, C3v, D3h, D3 in 2D, and T, Td, C3h, D3h in 3D. This work will be helpful for explorations of the topological physics beyond the linear regime.

Electrical, Photoluminescence and Optical Investigation of ZnO Nanoparticles Sintered at Different Temperatures

We report here structural, electrical, photoluminescence (PL), and optical investigations of ZnO nanoparticles. The ZnO samples are initially sintered at various temperatures (T s ) (600-1200 o C) temperatures and their size is reduced twice to nanoscale by using ball friction at 200 rpm rotational speed and 30 minutes duration. It is found that the T s do not influence the well-known peaks associated with the ZnO hexagonal structure, whereas the constants of the lattice and the average crystallite diameters are affected. Although the nonlinear area is observed for all samples in the I-V curves, the breakdown field E B and nonlinear coefficient β are moved to lower values as T s increases, while the residual voltage K r and nonlinear conductivity (σ 2 ) are increased. The empirical relations for K r , E B , and β as a function of T s are; K r = 0.004 T s – 0.487, E B = -1.786T s +2559.5 and β = -0.052 T s +75.19. On the other hand, a maximum UV absorption shift (A max ) is obtained at 412 nm, 400 nm, 384 nm, and 326 nm as the T s increases up to 1200 o C. For each sample, two different energy band gap values are obtained; the first is called the basic bandgap (E gh ) and its value above 3 eV, while the second is called the optical band gap (E gL ), and its value below 2.1 eV. Moreover, the empirical relations of them are E gh = 0.002 T s - 0.24, E gl = -0.0033 T s +5.242 and ∆E = - 0.0015 T s +5.002. Furthermore, the values of (N/m*) and lattice dielectric constant ε L are increased by increasing T s up to 1200 o C, while the vice is versa for the interatomic distance R. The dielectric loss tan δ is almost linear above 4 eV for all samples, and it decreases sharply as the T s increases. The optical and electrical conductivities σ opt and σ ele are decreased as the T s increases up to 1200 o C. Finally, the characteristic of UV band edges against the optimum value of PL intensity for the samples shows 8-continuous peaks. Furthermore, the PL intensity of the peaks is decreased by increasing T s and also by shifting the UV wave number towards the IR region.