Spintronics and valleytronics are emerging quantum electronic technologies that rely on using electron spin and multiple extrema of the band structure (valleys), respectively, as additional degrees of freedom. There are also collective properties of electrons in semiconductor nanostructures that potentially could be exploited in multifunctional quantum devices. Specifically, plasmonic semiconductor nanocrystals offer an opportunity for interface-free coupling between a plasmon and an exciton. However, plasmon-exciton coupling in single-phase semiconductor nanocrystals remains challenging because confined plasmon oscillations are generally not resonant with excitonic transitions. Here, we demonstrate a robust electron polarization in degenerately doped InO nanocrystals, enabled by non-resonant coupling of cyclotron magnetoplasmonic modes with the exciton at the Fermi level. Using magnetic circular dichroism spectroscopy, we show that intrinsic plasmon-exciton coupling allows for the indirect excitation of the magnetoplasmonic modes, and subsequent Zeeman splitting of the excitonic states. Splitting of the band states and selective carrier polarization can be manipulated further by spin-orbit coupling. Our results effectively open up the field of plasmontronics, which involves the phenomena that arise from intrinsic plasmon-exciton and plasmon-spin interactions. Furthermore, the dynamic control of carrier polarization is readily achieved at room temperature, which allows us to harness the magnetoplasmonic mode as a new degree of freedom in practical photonic, optoelectronic and quantum-information processing devices.

Controlling plasmonic properties of aliovalently doped semiconductor nanocrystals in mid-infrared (MIR) spectral region is of a particular current interest, because of their potential application in heat-responsive devices and near-field enhanced spectroscopies. However, a lack of detailed understanding of the correlations among the electronic structure of the host lattice, dopant ions, and surface properties hampers the development of MIR-tunable plasmonic nanocrystals (NCs). In this article, we report the colloidal synthesis and spectroscopic properties of two new plasmonic NC systems based on In2O3, antimony- and titanium-doped In2O3 NCs, and comparative investigation of their electronic structure using the combination of the Drude–Lorenz model and density functional theory. The localized surface plasmon resonances (LSPRs) lie at lower energies and have smaller bandwidths for Ti-doped than for Sb-doped In2O3 NCs with similar doping levels, indicating lower free electron density. Surprisingly, the Fermi level is found to be higher in Ti-doped In2O3 than in Sb-doped In2O3, suggesting the formation of electron trap states on nanocrystal surfaces, which reduce carrier density without significantly impacting their mobility. Controlling the competition between doping concentration and electron trapping allowed us to generate LSPR in Ti-doped In2O3 nanocrystals deep in the MIR region, and tune the absorption spectra from 650 cm–1 to 8000 cm–1. We also demonstrated the possibility to enhance the intensity of LSPR in these new plasmonic NCs by adjusting the synthesis and post-synthesis treatment conditions. The results of this work allow for an expansion of the tuning range of LSPR of colloidal metal oxide NCs by controlling the electronic structure of aliovalent dopant and charge carrier trapping.

We extend the classical Gauss-Bonnet formula to the case of compact Riemann surfaces with cone or cusp metrics under the hypothesis that the Gaussian curvature is an L 1 function with respect to the metric.

Germanium telluride (GeTe), a phase-change material, is known to exhibit four different structural states: three at room-temperature (one amorphous and two crystalline, α and γ) and one at high temperature (crystalline, β). Because transitions between the amorphous and crystalline states lead to significant changes in material properties (e.g., refractive index and resistivity), GeTe has been investigated as a phase-change material for photonics, thermoelectrics, ferroelectrics, and spintronics. Consequently, the temperature-dependent phase transitions in GeTe have been studied for bulk and thin-film GeTe, both fabricated by sputtering. Colloidal synthesis of nanoparticles offers a more flexible fabrication approach for amorphous and crystalline GeTe. These nanoparticles are known to exhibit size-dependent properties, such as an increased crystallization temperature for the amorphous-to-α transition in sub-10 nm GeTe particles. The α-to-β phase transition is also expected to vary with size, but this effect has not yet been investigated for GeTe. Here, we report time-resolved x-ray diffraction of GeTe nanoparticles with different diameters and from different synthetic protocols. We observe a non-volatile amorphous-to-α transition between 210 C and 240 C and a volatile α-to-β transition between 370 C and 420 C. The latter transition was reversible and repeatable. While the transition temperatures are shifted relative to the values known for bulk GeTe, the nanoparticle-based samples still exhibit the same structural phases reported for sputtered GeTe. Thus, colloidal GeTe maintains the same general phase behavior as bulk GeTe while allowing for more flexible and accessible fabrication. Therefore, nanoparticle-based GeTe films show great potential for applications such as in active photonics.