Session Y05: Novel optical responses in topological semimetals and other materials

Giant nonlinear optical response in transition metal monopnictide Weyl semimetals

The second-order nonlinear conductivity σ⁽²⁾ describes the current generated in proportion to the square of the applied electric field and is equal to zero unless the medium breaks inversion symmetry. Recently, we reported a giant, anisotropic σ⁽²⁾ at photon energy 1.5 eV in TaAs, a polar Weyl semimetal, that is larger than previously measured in any crystal. Subsequently, we measured the spectrum of σ⁽²⁾(ω) from 0.4 to 1.6 eV and found that the response at 1.5 eV is, in fact, the high-energy tail of a sharp resonance at 0.7 eV. Our discovery of a giant anisotropic σ⁽²⁾(ω) in TaAs raises the following questions: what is special about TaAs and/or polar metals that accounts for large resonant optical nonlinearity, and, is there a fundamental upper bound on σ⁽²⁾(ω) in such inversion breaking crystals? After describing the experimental findings, I will describe a simple model based on the band-geometric theory of nonlinear optical response that addresses these questions. The model is relevant to applications that attempt to use intrinsic inversion breaking to convert optical power to electrical current.   

Optical gyrotropy as a probe of the Berry curvature and intrinsic orbital moment on the Fermi surface

In crystals without a center of inversion, the Bloch states acquire a Berry curvature and an intrinsic orbital moment in momentum space. Out of the 21 noncentrosymmetric crystal classes, 18 are "gyrotropic" (optically active). In gyrotropic metals, the Berry curvature and intrinsic orbital moment on the Fermi surface can be probed by low-frequency optical experiments. The former gives rise to an intraband photocurrent induced by circularly-polarized light, and the latter to intraband natural optical rotation. I will begin this presentation by introducing the phenomenology and microscopic theory of these effects. As an application, I will discuss first-principles results for p-doped trigonal tellurium. We find that the experimentally observed sign reversal with temperature of the circular photocurrent can be explained by the presence of Weyl points near the bottom of the conduction band acting as sources and sinks of Berry curvature.   

Theory of nonlocal transport in metals with nontrivial band geometry

I will discuss the topological and geometric aspects of optical and transport phenomena in metals with nontrivial band geometry. Motivated by the chiral anomaly and the chiral magnetic effect in Weyl metals, I will outline the full theory of linear-in-q contribution to the non-local conductivity in a disordered metal. Physical applications of the theory include the natural optical activity of metals and the dynamic chiral magnetic effect, as well as the kinetic magnetoelectric effect/the current-induced magnetization in metallic systems. The theory is similar in spirit to the one of the anomalous Hall effect in metals, and is directly applicable to the analysis of the typical optical and transport measurements (e.g. Faraday rotation, current-induced magnetization) in the THz frequency range.   

Topological aspects of nonlinear optical effects

The responses of materials to high intensity light, i.e., nonlinear optical responses, constitute a vast field of physics and engineering. While topology has been playing a central role in recent studies of condensed matters, topological aspects of nonlinear optical effects have not been fully explored so far. In this talk, I will show a few examples of nonlinear optical effects that have topological origins. First, I discuss that the second-order nonlinear optical effects including the shift-current and second harmonic generation (SHG) are described by topological quantities, the Berry connection of Bloch wave functions. Next, I show that the topological formula well explains giant SHG responses in Weyl semimetal that were observed in TaAs. Finally, I discuss that another second-order nonlinear effect, circular photogalvanic effect (CPGE), is governed by Berry curvature and measuring CPGE in Weyl semimetals allows an access to monopole physics of Weyl fermions.   

Unexpected emergent states of matter created out of equilibrium in tantalum disulphide.

The idea that many body systems in complex materials may self-organise into long range order under highly non-equilibrium conditions suggests that entirely new emergent states with new and unexpected functionalities might be created in this way. The route for creation of such states is not limited to symmetry-breaking phase transitions, and entirely novel mechanisms may come into play. I will discuss recent experimental findings of mesoscopically textured metastable states with long range order created through density-driven non-equilibrium transitions. Combining ultrafast optical pulse excitation with scanning tunneling microscopy under moderate non-equilibrium conditions we find unambiguous evidence for long range electronic order appearing through a topological transition. Further out of equilibrium, under warm dense matter conditions we observe an entirely new metastable state of hyperuniform electronic matter. These discoveries open the way to new mechanisms for the creation of new states of matter created under highly non-equilibrium conditions.