We reanalyze some of the critical transport experiments and provide a coherent understanding of the current generation of topological insulators (TIs). Currently TI transport studies abound with ...widely varying claims of the surface and bulk states, often times contradicting each other, and a proper understanding of TI transport properties is lacking. According to the simple criteria given by Mott and Ioffe–Regel, even the best TIs are not true insulators in the Mott sense, and at best, are weakly-insulating bad metals. However, band-bending effects contribute significantly to the TI transport properties including Shubnikov de-Haas oscillations, and we show that utilization of this band-bending effect can lead to a Mott insulating bulk state in the thin regime. In addition, by reconsidering previous results on the weak anti-localization (WAL) effect with additional new data, we correct a misunderstanding in the literature and generate a coherent picture of the WAL effect in TIs.
Bismuth Selenide (Bi sub(2) Se sub(3)) is a topological insulator exhibiting helical spin polarization and strong spin-orbit coupling. The spin-orbit coupling links the charge current to spin current ...via the spin Hall effect (SHE). We demonstrate a Bi sub(2) Se sub(3) spin detector by injecting the pure spin current from a magnetic permalloy layer to a Bi sub(2) Se sub(3) thin film and detect the inverse SHE in Bi sub(2) Se sub(3). The spin Hall angle of Bi sub(2) Se sub(3) is found to be 0.0093 + or - 0.0013 and the spin diffusion length in Bi sub(2) Se sub(3) to be 6.2 + or - 0.15 nm at room temperature. Our results suggest that topological insulators with strong spin-orbit coupling can be used in functional spintronic devices.
In a topological insulator (TI), if its spin–orbit coupling (SOC) strength is gradually reduced, the TI eventually transforms into a trivial insulator beyond a critical point of SOC, at which point ...the bulk gap closes: this is the standard description of the topological phase transition (TPT). However, this description of TPT, driven solely by the SOC (or something equivalent) and followed by closing and reopening of the bulk band gap, is valid only for infinite-size samples, and little is known how TPT occurs for finite-size samples. Here, using both systematic transport measurements on interface-engineered (Bi1–x In x )2Se3 thin films and theoretical simulations (with animations in the Supporting Information), we show that description of TPT in finite-size samples needs to be substantially modified from the conventional picture of TPT due to surface-state hybridization and bulk confinement effects. We also show that the finite-size TPT is composed of two separate transitions, topological-normal transition (TNT) and metal–insulator transition (MIT), by providing a detailed phase diagram in the two-dimensional phase space of sample size and SOC strength.
The availability of native substrates is a cornerstone in the development of microelectronic technologies relying on epitaxial films. If native substrates are not available, virtual substrates - ...crystalline buffer layers epitaxially grown on a structurally dissimilar substrate - offer a solution. Realizing commercially viable virtual substrates requires the growth of high-quality films at high growth rates for large-scale production. We report the stoichiometric growth of SrTiO
exceeding 600 nm hr
. This tenfold increase in growth rate compared to SrTiO
grown on silicon by conventional methods is enabled by a self-regulated growth window accessible in hybrid molecular beam epitaxy. Overcoming the materials integration challenge for complex oxides on silicon using virtual substrates opens a path to develop new electronic devices in the More than Moore era and silicon integrated quantum computation hardware.
Material defects remain as the main bottleneck to the progress of topological insulators (TIs). In particular, efforts to achieve thin TI samples with dominant surface transport have always led to ...increased defects and degraded mobilities, thus making it difficult to probe the quantum regime of the topological surface states. Here, by utilizing a novel buffer layer scheme composed of an In2Se3/(Bi0.5In0.5)2Se3 heterostructure, we introduce a quantum generation of Bi2Se3 films with an order of magnitude enhanced mobilities than before. This scheme has led to the first observation of the quantum Hall effect in Bi2Se3.
The unusual electronic states found in topological materials can enable a new generation of devices and technologies, yet a long‐standing challenge has been finding materials without deleterious ...parallel bulk conduction. This can arise either from defects or thermally activated carriers. Here, the criteria that materials need to meet to realize transport properties dominated by the topological states, a necessity for a topological device, are clarified. This is demonstrated for 3D topological insulators, 3D Dirac materials, and 1D quantum anomalous Hall insulators, though this can be applied to similar systems. The key parameters are electronic bandgap, dielectric constant, and carrier effective mass, which dictate under what circumstances (defect density, temperature, etc.) the unwanted bulk state will conduct in parallel to the topological states. As these are fundamentally determined by the basic atomic properties, simple chemical arguments can be used to navigate the phase space to ultimately find improved materials. This will enable rapid identification of new systems with improved properties, which is crucial to designing new material systems and push a new generation of topological technologies.
What does it take to make a room‐temperature electronic device based on the unusual states found in topological materials? The answer is a materials‐redesign, which delicately balances bandgap, dielectric constant, and effective mass. Identifying this new generation of materials can pave the way to realistic quantum technologies.
Driven by an ever‐expanding interest in new material systems with new functionality, the growth of atomic‐scale electronic materials by molecular beam epitaxy (MBE) has evolved continuously since the ...1950s. Here, a new MBE technique called hybrid‐MBE (hMBE) is reviewed that has been proven a powerful approach for tackling the challenge of growing high‐quality, multicomponent complex oxides, specifically the ABO3 perovskites. The goal of this work is to (1) discuss the development of hMBE in a historical context, (2) review the advantageous surface kinetics and chemistry that enable the self‐regulated growth of ABO3 perovskites, (3) layout the key components and technical challenges associated with hMBE, (4) review the status of the field and the materials that have been successfully grown by hMBE which demonstrate its general applicability, and (5) discuss the future of hMBE in regards to technical innovations and expansion into new material classes, which are aimed at expanding into industrial realm and at tackling new scientific endeavors.
The development of hybrid molecular beam epitaxy is described from a historic, scientific, and technical perspective. The rapid progress in this expanding field is comprehensively summarized and emphasis is placed on the growth aspects of ternary and quaternary perovskite oxides with different chemistries, the control over cation stoichiometry and its direct ramification in thin film quality.
High entropy oxides are emerging as an exciting new avenue to design highly tailored functional behaviors that have no traditional counterparts. Study and application of these materials are bringing ...together scientists and engineers from physics, chemistry, and materials science. The diversity of each of these disciplines comes with perspectives and jargon that may be confusing to those outside of the individual fields, which can result in miscommunication of important aspects of research. In this Perspective, we provide examples of research and characterization taken from these different fields to provide a framework for classifying the differences between compositionally complex oxides, high entropy oxides, and entropy stabilized oxides, which is intended to bring a common language to this emerging area. We highlight the critical importance of understanding a material’s crystallinity, composition, and mixing length scales in determining its true definition.
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