Abstract-Electric field modulated topological magnetoelectric effect in Bi 2 Se 3

Mintu Mondal, Dipanjan Chaudhuri, Maryam Salehi, Cheng Wan, N. J. Laurita, Bing Cheng, Andreas V. Stier, Michael A. Quintero, Jisoo Moon, Deepti Jain, Pavel P. Shibayev, James R. Neilson, Seongshik Oh, and N. P. Armitage

https://journals.aps.org/prb/abstract/10.1103/PhysRevB.98.121106

Topological insulators have been predicted to exhibit a variety of interesting phenomena including a quantized magnetoelectric response and novel spintronics effects due to spin textures on their surfaces. However, experimental observation of these phenomena has proved difficult due to the finite bulk carrier density which may overwhelm the intrinsic topological responses that are expressed at the surface. Here, we demonstrate an ionic gel gating technique to tune the chemical potential of ${\mathrm{Bi}}_2{\mathrm{Se}}_3$thin films while simultaneously performing THz spectroscopy. We can tune the carrier concentration by an order of magnitude and shift the Fermi energy $E_F$ to as low as $\simeq 10$ meV above the Dirac point. At high-bias voltages and magnetic fields, we observe a quantized Faraday angle consistent with the topological magnetoelectric effect that can be tuned by ionic gel gating through a number of plateau states.

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Abstract-Anomalous gap-edge dissipation in disordered superconductors on the brink of localization

Bing Cheng, Liang Wu, N. J. Laurita, Harkirat Singh, Madhavi Chand, Pratap Raychaudhuri, and N. P. Armitage

https://journals.aps.org/prb/accepted/6307cY98Z8e1bc50801465e805c29452f5d55bb8f

Superconductivity in disordered systems close to an incipient localization transition has been an area of investigation for many years, but many fundamentally important aspects are still not understood. It has been noted that in such highly disordered superconductors, anomalous spectral weight develops in their conductivity near and below the superconducting gap energy. In this work we investigate the low frequency conductivity in disordered superconducting NbN thin films close to the localization transition with time-domain terahertz spectroscopy. In the normal state, strong deviations from the Drude form due to incipient localization are found. In the superconducting state we find substantial spectral weight at frequencies well below the superconducting gap scale derived from tunneling. We analyze this spectral weight in the context of a model of disorder induced broadening of the quasiparticle density of states. We find that aspects of the optical and tunneling data can be consistently modeled in terms of this effect of mesoscopic disorder, showing that in this disorder and frequency range, quasiparticle effects and not collective modes are the source of low energy absorption. Interestingly, we also find that as a function of frequency the optical conductivity recovers to the normal state value much faster than any model predicts. This points to the non-trivial interplay of superconductivity and disorder close to localization.