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Tuning the properties of magnetic topological materials is of interest to realize exotic physical phenomena, new quantum phases and quasiparticles, and topological spintronic devices. However, current topological materials exhibit Curie temperature (TC) values far below those needed for practical applications. In recent years, significant progress has been made to control and optimize TC, particularly through defect-engineering of these structures. Most recently, we reported TC values up to 80 K for (MnSb2Te4)x(Sb2Te3)1−x when 0.7 ≤ x ≤ 0.85 by controlling the composition x and the Mn content in these structures during molecular beam epitaxy growth. In this study, we show further enhancement of the TC, as high as 100 K, by maintaining high Mn content and reducing the growth rate from 0.9 nm/min to 0.5 nm/min. Derivative curves of the Hall resistance and the magnetization reveal the presence of two TC components contributing to the overall value and suggest TC1 and TC2 have distinct origins: excess Mn in MnSb2Te4 septuple layers (SLs) and high Mn content in Sb2−yMnyTe3 quintuple layer (QL) alloys, respectively. To elucidate the mechanisms promoting higher TC values in this system, we show evidence of enhanced structural disorder due to the excess Mn that occupies not only Sb sites but also Te sites, leading to the formation of a new crystal structure for these materials. Learning to control defects that enhance desired magnetic properties and understanding the mechanisms that promote high TC in magnetic topological materials such as (Mn1+ySb2−yTe4)x(Sb2−yMnyTe3)1−x is of great importance to achieve practical quantum devices.

 

Developing alternative material platforms for use in superconductor–semiconductor hybrid structures is desirable due to limitations caused by intrinsic microwave losses present in commonly used III/V material systems. With the recent reports on tantalum superconducting qubits that show improvements over the Nb and Al counterparts, exploring Ta the superconductor in hybrid material systems is promising. Here, we study the growth of Ta on semiconducting Ge (001) substrates grown via molecular beam epitaxy. We show that at a growth temperature of 400 °C, the Ta diffuses into the Ge matrix in a self-limiting nature resulting in smooth and abrupt surfaces and interfaces with roughness on the order of 3–7 Å as measured by atomic force microscopy and x-ray reflectivity. The films are found to be a mixture of Ta5Ge3 and TaGe2 binary alloys and form a native oxide that seems to form a sharp interface with the underlying film. These films are superconducting with a TC∼1.8−2 K and HC⊥∼1.88 T, HC∥∼5.1 T. These results show this tantalum germanide film to be promising for future superconducting quantum information platforms.

 

Abstract Magnetic topological materials are promising for realizing novel quantum physical phenomena. Among these, bulk Mn-rich MnSb 2 Te 4 is ferromagnetic due to Mn Sb antisites and has relatively high Curie temperatures (T C ), which is attractive for technological applications. We have previously reported the growth of materials with the formula (Sb 2 Te 3 ) 1−x (MnSb 2 Te 4 ) 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 T C values. Samples that contain x < 0.7 or x > 0.9 have a single T C value of 15–20 K and 20–30 K, respectively, while samples with 0.7 < x < 0.8 exhibit two T C values, one (T C1 ) at ~ 25 K and the second (T C2 ) 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 T C1 of ~ 20 to 30 K, and regions with isolated QLs are responsible for the higher T C2 values. Our results have important implications for the design of magnetic topological materials having enhanced properties. 

We report the growth of self-assembled Bi₂Se₃quantum 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.

 

We report on the growth and characterization of optical quality multiple quantum well structures of Zn_xCd_1−xSe/Zn_xCd_yMg_1−x−ySe 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.