# High Energy Physics Seminar

*,*UCLA

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The existence of the axion was postulated in 1977 to resolve the strong charge-parity problem in quantum chromodynamics (QCD), and has gained popularity due to its candidacy as dark matter. Despite significant efforts, no QCD axion has been detected to date. The axion response in condensed matter falls into two categories: (i) the background axion field which can become quantized in the presence of pseudoscalar-symmetry, giving 3D topological insulators their characteristic θ=π, and (ii) the dynamical axion where spontaneous symmetry breaking promotes the axion angle to a dynamical field, mimicking the QCD axion. In condensed matter systems where axions naturally emerge as collective excitations, the electromagnetic field couples nonlinearly to a pseudoscalar polarization term, the axion field. In time-reversal invariant topological insulators, the topological magnetoelectric response is quantized, hence the axion is static and acts as the topological invariant. Emergent dynamical axions also arise in certain topological systems that spontaneously break pseudoscalar symmetry, and are an exact analog to the QCD axion. Among these materials which promote the static axion to a fluctuating field are topological insulators with antiferromagnetic order and Weyl semimetals in the presence of charge density waves. The presence of a dynamical axion field in CM systems makes them a highly motivated candidate for eventual real-world detection. The difficulty of detecting axionic signatures not only plagues high-energy experiments, but also presents significant challenges in CM systems, necessitating new approaches. I will discuss recent theoretical proposals that postulate a two-step optical protocol resulting in excitation and unambiguous detection of the dynamical axion field in topological quantum matter. The first step is to induce collective oscillations of the axion mode by two-photon absorption with the magnetoelectric nature of the excitation manifesting in the polarization dependence of the excitation beams. Next, these axion oscillations would manifest in time-resolved Kerr-rotation, which again carries signature polarization dependence due to the magnetoelectric nature of the coupling. Time permitting, I will discuss our recent work in axion-field enabled nonreciprocity in magnetic Weyl semimetals. The giant optical nonreciprocity in Weyl semimetals can be compactly explained by the presence of a static axion field, whose steady-state prediction is closely related to the time-resolved signature of the dynamical axion. Thus, the giant optical nonreciprocity is a direct manifestation of the underlying axion field, and therefore a probe of it.

* This work in the NarangLab is driven by Olivia Liebman, Drs. Emily Been, Ioannis Petrides, and Jon Curtis, and supported by the Quantum Science Center (QSC), a National Quantum Information Science Research Center of the U.S. Department of Energy (DOE).

The talk is in 469 Lauritsen.

Contact theoryinfo@caltech.edu for Zoom link prior to 3:30pm.