We demonstrate that within the model of massless Dirac fermions, graphene has a strong nonlinear optical response in the terahertz regime. It is found that the nonlinear contribution significantly alters both the single frequency and frequency tripled optical response at experimentally relevant field strengths. The optical activity of single layer graphene is significantly enhanced by nonlinear effects, and the frequency tripled response opens the gateway to photonic and optoelectronic device applications.

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The 12442-type Fe-based superconductor is the only system that possesses two FeAs layers between neighboring insulating layers, which is worth the in-depth investigations. In this work, millimeter-sized single crystals of KCa2Fe4As4F2 were grown using a self-flux method. The chemical compositions and crystal structure were characterized carefully. Superconductivity with the critical transition T c = 33.5 K was confirmed by both the resistivity and magnetic susceptibility measurements. Moreover, the upper critical field H c2 was studied by the resistivity measurements under different magnetic fields, where an anisotropy of 8 was revealed near the superconducting transition. Importantly, a rather steep increase for the in-plane H c2 ab with cooling, dμ0 H c2 ab /dT| T c = −50.9 T/K, was observed. This value is several times higher than that of other systems of Fe-based superconductor and indicates an extremely high upper critical field. Possible origins for this behavior were discussed. The finding in our work is a great promotion both for understanding the physical properties and for the high-field applications of 12442-type Fe-based superconductors.

The topological electronic structure plays a central role in the nontrivial physical properties in topological quantum materials. A minimal, "hydrogen-atom-like" topological electronic structure is desired for research. In this work, we demonstrate an effort toward the realization of such a system in the intrinsic magnetic topological insulator MnBi 2 Te 4 , by manipulating the topological surface state (TSS) via surface modification. Using high resolution laser-and synchrotron-based angle-resolved photoemission spectroscopy (ARPES), we found the TSS in MnBi 2 Te 4 is heavily hybridized with a trivial Rashba-type surface state (RSS), which could be efficiently removed by the in situ surface potassium (K) dosing. By employing multiple experimental methods to characterize K dosed surface, we attribute such a modification to the electrochemical reactions of K clusters on the surface. Our work not only gives a clear band assignment in MnBi 2 Te 4 but also provides possible new routes in accentuating the topological behavior in the magnetic topological quantum materials.

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