Morphology and Optical Properties of Thin Cd3As2 Films of a Dirac Semimetal Compound

Metasurface Enabled Photothermoelectric Photoresponse of Semimetal Cd3As2 for Broadband Photodetection

The artificial engineering of photoresponse is crucial for optoelectronic applications, especially for photodetectors. Here, we designed and fabricated a metasurface on a semimetallic Cd₃As₂ nanoplate to improve its thermoelectric photoresponse. The metasurface can enhance light absorption, resulting in a temperature gradient. This temperature gradient can contribute to thermoelectric photoresponse through the photothermoelectric effect. Furthermore, power-dependent measurements showed a linearly dependent photoresponse of the Cd₃As₂ metasurface device, indicating a second-order photocurrent response. Wavelength-dependent measurements showed that the metasurface can efficiently separate photoexcited carriers in the broadband range of 488 nm to 4 μm. The photoresponse near the metasurface boundaries exhibits a responsivity of ∼1 mA/W, which is higher than that near the electrode junctions. Moreover, the designed metasurface device provided an anisotropic polarization-dependent photoresponse rather than the isotropic photoresponse of the original Cd₃As₂ device. This study demonstrates that metasurfaces have excellent potential for artificial controllable photothermoelectric photoresponse of various semimetallic materials.

Bi Layer Properties in the Bi–FeNi GMR-Type Structures Probed by Spectroscopic Ellipsometry

Bismuth (Bi) having a large atomic number is characterized by a strong spin–orbit coupling (SOC) and is a parent compound of many 3D topological insulators (TIs). The ultrathin Bi films are supposed to be 2D TIs possessing a nontrivial topology, which opens the possibility of developing new efficient technologies in the field of spintronics. Here we aimed at studying the dielectric function properties of ultrathin Bi/FeNi periodic structures using spectroscopic ellipsometry. The [Bi(d)–FeNi(1.8 nm)]N GMR-type structures were grown by rf sputtering deposition on Sitall-glass (TiO2) substrates. The ellipsometric angles Ψ(ω) and Δ(ω) were measured for the grown series (d = 0.6, 1.4, 2.0, and 2.5 nm, N = 16) of the multilayered film samples at room temperature for four angles of incidence of 60∘, 65∘, 70∘, and 75∘ in a wide photon energy range of 0.5–6.5 eV. The measured ellipsometric angles, Ψ(ω) and Δ(ω), were simulated in the framework of the corresponding multilayer model. The complex (pseudo)dielectric function spectra of the Bi layer were extracted. The GMR effects relevant for the studied Bi–FeNi MLF systems were estimated from the optical conductivity zero-limit (optical GMR effect). The obtained results demonstrated that the Bi layer possessed the surface metallic conductivity induced by the SOC effects, which was strongly enhanced on vanishing the semimetallic-like phase contribution on decreasing the layer thickness, indicating its nontrivial 2D topology properties.