A Large Effective Phonon Magnetic Moment in a Dirac Semimetal

We investigated the magnetoterahertz response of the Dirac semimetal Cd$_3$As$_2$ and observed a particularly low frequency optical phonon, as well as a very prominent and field sensitive cyclotron resonance. As the cyclotron frequency is tuned with field to pass through the phonon, the phonon become circularly polarized as shown by a notable splitting in their response to right- and left-hand polarized light. This splitting can be expressed as an effective phonon magnetic moment that is approximately 2.7 times the Bohr magneton, which is almost four orders of magnitude larger than ab initio calculations predict for phonon magnetic moments in nonmagnetic insulators. This exceedingly large value is due to the coupling of the phonons to the cyclotron motion and is controlled directly by the electron-phonon coupling constant. This field tunable circular-polarization selective coupling provides new functionality for nonlinear optics to create light-induced topological phases in Dirac semimetals.Comment: 15 pages for main text and SI; To appear in Nano Letters (2020

THz electrodynamics of mixed-valent YbAl

We present our results from time-domain THz spectroscopy measurements of thin films of mixed-valent YbAl$$_3$$ and its structural analogue LuAl$$_3$$. Combined with Fourier transform infrared (FTIR) spectroscopy, the extended Drude formalism is utilized to study the quasiparticle scattering rate and effective masses in YbAl$$_3$$. We find that LuAl$$_3$$ demonstrates conventional Drude transport whereas at low temperatures YbAl$$_3$$ demonstrates a renormalized Drude peak and a mid-infrared (MIR) peak in the conductivity, indicative of the formation of a mass-enhanced Fermi liquid (FL). In YbAl$$_3$$ the extended Drude analysis demonstrates consistency with FL behavior below the FL coherence temperature $$T^* < 40$$ K with the scattering rate following $$T^2$$ proportionality and a moderate mass enhancement. Despite not observing a clear $$\omega ^2$$ Fermi liquid-like frequency dependence the evidence is consistent with a moderate mass Fermi liquid, albeit one with a smaller mass than observed in single crystals. The extended Drude analysis also demonstrates a slow crossover between the FL state and the normal state above the $$T^*$$ in YbAl$$_3$$, indicative of incoherent hybridization effects persisting to high temperatures