Publications

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Ultrafast optical microscopy is an important tool for examining fundamental phenomena in semiconductor nanowires with high temporal and spatial resolution. Here, we used this technique to study carrier dynamics in single GaN/InGaN core-shell nonpolar multiple quantum well nanowires. We find that intraband carrier-carrier scattering is the main channel governing carrier capture, while subsequent carrier relaxation is dominated by three-carrier Auger recombination at higher densities and bimolecular recombination at lower densities. The Auger constants in these nanowires are approximately 2 orders of magnitude lower than in planar InGaN multiple quantum wells, highlighting their potential for future light-emitting devices.

ZrTe₅, a topological semimetal, has recently attracted great attention due to its extraordinary electronic properties. Extensive studies have been carried out in ZrTe₅ on their charge transport properties. However, there are few studies on their spin properties. One well-developed technique to study spin degeneracy of a Landau level (LL) in a two-dimensional system is by tilting magnetic field. It is known that the Landau level energy is proportional to the magnetic field normal component while the Zeeman energy scales with the total magnetic field. Therefore, these two energy scales can be tuned relatively to each other in a tilted magnetic field.

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A three-dimensional (3D) Dirac semimetal (DS) is an analogue of graphene, but with linear energy dispersion in all (three) momentum directions. 3D DSs have been a fertile playground in discovering novel quantum particles, for example Weyl fermions, in solid state systems. Many 3D DSs were theoretically predicted and experimentally confirmed. We report here the results in exfoliated ZrTe 5 thin flakes from the studies of aberration-corrected scanning transmission electron microscopy and low temperature magneto-transport measurements. Several unique results were observed. First, a π Berry phase was obtained from the Landau fan diagram of the Shubnikov-de Haas oscillations in the longitudinal conductivity σxx. Second, the longitudinal resistivity ρxx shows a linear magnetic field dependence in the quantum limit regime. Most surprisingly, quantum oscillations were also observed at fractional Landau level indices N = 5/3 and 7/5, demonstrating strong electron-electron interaction effects in ZrTe5.

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Resistive switches are non-volatile memory cells based on nano-ionic redox processes that offer energy efficient device architectures and open pathways to neuromorphics and cognitive computing. However, channel formation typically requires an irreversible, not well controlled electroforming process, giving difficulty to independently control ionic and electronic properties. The device performance is also limited by the incomplete understanding of the underlying mechanisms. Here, we report a novel memristive model material system based on self-assembled Sm-doped CeO 2 and SrTiO 3 films that allow the separate tailoring of nanoscale ionic and electronic channels at high density (â 1/410 12 inch â '2). We systematically show that these devices allow precise engineering of the resistance states, thus enabling large on-off ratios and high reproducibility. The tunable structure presents an ideal platform to explore ionic and electronic mechanisms and we expect a wide potential impact also on other nascent technologies, ranging from ionic gating to micro-solid oxide fuel cells and neuromorphics.

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Metamaterials made of nanoscale inclusions or artificial unit cells exhibit exotic optical properties that do not exist in natural materials. Promising applications, such as super-resolution imaging, cloaking, hyperbolic propagation, and ultrafast phase velocities have been demonstrated based on mostly micrometer-scale metamaterials and few nanoscale metamaterials. To date, most metamaterials are created using costly and tedious fabrication techniques with limited paths toward reliable large-scale fabrication. In this work, we demonstrate the one-step direct growth of self-assembled epitaxial metal-oxide nanocomposites as a drastically different approach to fabricating large-area nanostructured metamaterials. Using pulsed laser deposition, we fabricated nanocomposite films with vertically aligned gold (Au) nanopillars (∼20 nm in diameter) embedded in various oxide matrices with high epitaxial quality. Strong, broad absorption features in the measured absorbance spectrum are clear signatures of plasmon resonances of Au nanopillars. By tuning their densities on selected substrates, anisotropic optical properties are demonstrated via angular dependent and polarization resolved reflectivity measurements and reproduced by full-wave simulations and effective medium theory. Our model predicts exotic properties, such as zero permittivity responses and topological transitions. Our studies suggest that these self-assembled metal-oxide nanostructures provide an exciting new material platform to control and enhance optical response at nanometer scales.

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Recently, it has been predicted that topological crystalline insulators (TCIs) may exist in SnTe and Pb_1-xSn_xTe thin films [1]. To date, most studies on TCIs were carried out either in bulk crystals or thin films, and no research activity has been explored in heterostructures. We present here the results on electronic transport properties of the 2D electron gas (2DEG) realized at the interfaces of PbTe/ CdTe (111) heterostructures. Evidence of topological state in this interfacial 2DEG was observed.