Designer Topological Insulator with Enhanced Gap and Suppressed Bulk Conduction in Bi2Se3/Sb2Te3 Ultrashort-Period Superlattices.

A novel approach to suppress bulk conductance in three-dimensional (3D) topological insulators (TIs) using short-period superlattices (SLs) of two TIs is presented. Evidence for superlattice gap enhancement (SGE) was obtained from the reduction of bulk background doping from 1.2 × 1020 cm-3 to 8.5 × 1018 cm-3 as the period of Bi2Se3/Sb2Te3 SLs is decreased from 12 nm to 5 nm. Tight binding calculations show that, in the ultrashort-period regime, a significant SGE can be achieved for the resulting SL. Ultrathin short-period SLs behave as new designer TIs with bulk bandgaps up to 60% larger than the bandgap of the constituent layer of largest bandgap, while retaining topological surface features. Evidence for gap formation was obtained from ellipsometric measurements. Analysis of the weak antilocalization cusp in low-temperature magneto-conductance confirms that the top and bottom surfaces of the SL structure behave as Dirac surfaces. This approach represents a promising platform for building truly insulating TIs.

Bi2Se3 van der Waals Virtual Substrates for II-VI Heterostructures.

We report on the growth and characterization of optical quality multiple quantum well structures of Zn x Cd1-x Se/Zn x Cd y Mg1-x-y Se on an ultra-thin Bi2Se3/CdTe virtual substrate on c-plane Al2O3 (sapphire). Excellent quality highly oriented films grown along the (111) direction were achieved as evidenced by reflection high energy electron diffraction and X-ray diffraction studies. We also observed room temperature and 77 K photoluminescence emission with peak energies at 77 K of 2.407 eV and linewidths of 56 meV comparable to those achieved on structures grown on InP. Exfoliation of the structures is also possible due to the van der Waals bonding of Bi2Se3. Exfoliated (substrate free) films exhibit photoluminescence emission nearly identical to that of the supported film. Additionally, contactless electroreflectance measurements show good agreement with simulations of the multiple quantum well structure as well as evidence of excited state levels. These results open new avenues of research for substrate independent epitaxy and the possibility of ultra-thin electronics.

High Curie temperature ferromagnetic structures of (Sb2Te3)1-x(MnSb2Te4)x with x = 0.7-0.8.

Magnetic topological materials are promising for realizing novel quantum physical phenomena. Among these, bulk Mn-rich MnSb2Te4 is ferromagnetic due to MnSb antisites and has relatively high Curie temperatures (TC), which is attractive for technological applications. We have previously reported the growth of materials with the formula (Sb2Te3)1-x(MnSb2Te4)x, where x varies between 0 and 1. Here we report on their magnetic and transport properties. We show that the samples are divided into three groups based on the value of x (or the percent septuple layers within the crystals) and their corresponding TC values. Samples that contain x < 0.7 or x > 0.9 have a single TC value of 15-20 K and 20-30 K, respectively, while samples with 0.7 < x < 0.8 exhibit two TC values, one (TC1) at ~ 25 K and the second (TC2) reaching values above 80 K, almost twice as high as any reported value to date for these types of materials. Structural analysis shows that samples with 0.7 < x < 0.8 have large regions of only SLs, while other regions have isolated QLs embedded within the SL lattice. We propose that the SL regions give rise to a TC1 of ~ 20 to 30 K, and regions with isolated QLs are responsible for the higher TC2 values. Our results have important implications for the design of magnetic topological materials having enhanced properties.

Self-assembled Bismuth Selenide (Bi2Se3) quantum dots grown by molecular beam epitaxy.

We report the growth of self-assembled Bi2Se3 quantum dots (QDs) by molecular beam epitaxy on GaAs substrates using the droplet epitaxy technique. The QD formation occurs after anneal of Bismuth droplets under Selenium flux. Characterization by atomic force microscopy, scanning electron microscopy, X-ray diffraction, high-resolution transmission electron microscopy and X-ray reflectance spectroscopy is presented. Raman spectra confirm the QD quality. The quantum dots are crystalline, with hexagonal shape, and have average dimensions of 12-nm height (12 quintuple layers) and 46-nm width, and a density of 8.5 × 109 cm-2. This droplet growth technique provides a means to produce topological insulator QDs in a reproducible and controllable way, providing convenient access to a promising quantum material with singular spin properties.