Quantum Transport in Topological Magnets

In the past several years, a new field of symmetry-protected topological materials has emerged in condensed matter physics, based on the wide range of consequences that result from the realization that certain properties of physical systems can be expressed as topological invariants, which are insensitive to local perturbations. This new class of materials hosts unique surface/edge states, such as the first known topological system – quantum Hall insulator with dissipationless chiral edge states, and massless spin-helical Dirac surface states in 3D topological insulators that are unlike any other known 1D or 2D electronic systems. In this thesis, to understand the role and significance of topology in real materials we focus on time-reversal symmetry (TRS) protected 3D topological insulators (TIs) and on the chiral edge states that emerge in the quantum anomalous Hall (QAH) state after TRS is broken. While the interior/bulk of topological insulators is no different from ordinary semiconductors with narrow bandgap, the spins of surface electrons are locked at a 90◦ to their momentum and the conduction channels for up-spin and down-spin particles are separated, akin to a two-way highway with a barrier in between to prevent the collisions. This phenomenon is a consequence of the TRS protection at the surface. If we break this symmetry and remove one of the conduction channels, for example with an out-of-plane magnetization, a QAH state with the dissipationless chiral edge state can emerge, while both the interior of the surface and the bulk are now free of itinerant charge carriers. These dissipationless chiral conduction channels are technologically important as they can transform modern electronics into quantum electronics for supreme energy efficiency. Even though QAH was shown possible theoretically on a 2D honeycomb lattice under staggered magnetic flux by Haldane in 1988, it has been only recently realized in a topological magnet after the discovery of topological insulators. The first quantum anomalous Hall demonstration was under most restrictive materials engineering constraints, severely limiting the exploration of fundamental physics and technological applications. The access to the chiral channels in magnetically doped TIs (such as Cr, V heavily doped (Bi, Sb)2Te3 thin films) is challenging – it requires tuning the Fermi level precisely into the small Dirac mass gap (~10 meV) uniformly across a mesoscopic sample. The doping disorders limit the observed QAH temperature to ∼300 mK range and only to samples with thicknesses of 5 - 10 nm. A recently discovered new class of van der Waals (vdW) materials in the MnBi2Te4 and MnSb2Te4 class opened vast new opportunities in materials design. These intrinsic topological magnets do not require doping with magnetic ions, for their crystal structure comprises seven atomic layers (septuple layers, SL) blocks with a single Mn layer in the middle, each aligned ferromagnetically (FM) out-of-plane. Such FM exchange interaction has been predicted to open a very large Dirac mass gap (∼70 meV), making it in principle easier to achieve QAH state. However, the interaction between SLs was determined to be antiferromagnetic (AFM), and QAH has been possible only in the small odd number of SLs where the net surface magnetic order was still FM-like. This new class of topological magnets can potentially host chiral edge states at higher temperatures without thickness limits, but currently it has two outstanding challenges, namely tuning the Fermi level into the Dirac exchange gap in the presence of naturally occurring charged defects, and the AFM coupling in the bulk. In this thesis, we aim to resolve these two challenges by (1) modifying the magnetism of the bulk to ferromagnetic by inserting a topological Bi2Te3 non-magnetic spacer in between each SL layer, i.e., MnBi2Te4/Bi2Te3 superlattice, and (2) developing a new chemical potential tuning method | hydrogenation using an aqueous solution of hydrogen chloride to tune the Fermi level of the bulk without impacting the surface electron mobility. This dissertation consists of four chapters. In Chapter 1 we give an introduction to topological effects, with a focus on the theoretical and experimental progress in our understanding of QAH. We then summarize the current status of knowledge regarding the recently discovered intrinsic topological magnets and lay out the challenges in this new quantum materials class. In Chapter 2 we describe the experimental methods used in our study. In Chapter 3 we present our discovery of a previously unknown Berry-curvature-driven anomalous Hall regime (‘Q-window’) at above-Kelvin temperatures in the magnetic topological bulk crystals where through growth Mn ions self-organize into a period-ordered MnBi2Te4/Bi2Te3 superlattice. In Chapter 4 we discuss a new chemical tuning method involving hydrogen ions. We demonstrate that hydrogenation resolves an outstanding challenge in chalcogenide classes of three-dimensional (3D) topological insulators and magnets | the control of intrinsic bulk conduction that denies access to quantum surface transport. We demonstrate those carrier densities are easily tuned by over 1020 cm-3, allowing moving the Fermi level into the bulk bandgap to enter surface/edge current channels. We show that the hydrogen-tuned topological materials are stable at room temperature and tunable disregarding bulk size, opening a breadth of platforms for harnessing emergent topological states

Impacts of economic development on ecosystem risk in the Yellow River Delta

AbstractWith the economic development and population growth, humans have changed ecosystems more rapidly and extensively to meet the rapidly growing demand for food, fresh water, timber, fiber and fuel. This has led to a substantial and largely irreversible loss of the biodiversity on earth. The ecosystem risk is created as a new concept to understand the environmental problems. Therefore, it is important to develop quantitative methods for regional ecosystem risk analysis. Yellow River Delta is the widest, most intact and youngest delta both in China and in the world; its ecosystem environment is much more vulnerable due to its special location and industrial structure. Therefore, it is very important to manage them wisely and strategically. Therefore, Yellow River Delta is selected as the case area to reveal the impacts of economic development on ecosystem risk in this study. This study selected the ecological quality index to show the potential ecosystem risk and estimated the impacts of economic development on ecosystem risk using the panel data model on the pixel level based on the GIS, RS technique. It's found that the economic development will have impacts on the ecological environment to a certain degree, however, these impacts can exchange to a greater degree with the development. Then more funds and advanced technologies can be used to promote the intensive development of land use, which may decrease the impacts of economic development on the environment. Therefore, we need to ensure the coordinated development of the economy and ecological environment. The research results provide meaningful decision-making information for the urbanization process and environmental protection in the Yellow River Delta

Singular robust room-temperature spin response from topological Dirac fermions

Topological insulators are a class of solids in which the nontrivial inverted bulk band structure gives rise to metallic surface states that are robust against impurity scattering. In three-dimensional (3D) topological insulators, however, the surface Dirac fermions intermix with the conducting bulk, thereby complicating access to the low energy (Dirac point) charge transport or magnetic response. Here we use differential magnetometry to probe spin rotation in the 3D topological material family (Bi$_2$Se$_3$, Bi$_2$Te$_3$, and Sb$_2$Te$_3$). We report a paramagnetic singularity in the magnetic susceptibility at low magnetic fields which persists up to room temperature, and which we demonstrate to arise from the surfaces of the samples. The singularity is universal to the entire family, largely independent of the bulk carrier density, and consistent with the existence of electronic states near the spin-degenerate Dirac point of the 2D helical metal. The exceptional thermal stability of the signal points to an intrinsic surface cooling process, likely of thermoelectric origin, and establishes a sustainable platform for the singular field-tunable Dirac spin response.Comment: 20 pages, 14 figure

3D Stretchable Arch Ribbon Array Fabricated via Grayscale Lithography.

Microstructures with flexible and stretchable properties display tremendous potential applications including integrated systems, wearable devices and bio-sensor electronics. Hence, it is essential to develop an effective method for fabricating curvilinear and flexural microstructures. Despite significant advances in 2D stretchable inorganic structures, large scale fabrication of unique 3D microstructures at a low cost remains challenging. Here, we demonstrate that the 3D microstructures can be achieved by grayscale lithography to produce a curved photoresist (PR) template, where the PR acts as sacrificial layer to form wavelike arched structures. Using plasma-enhanced chemical vapor deposition (PECVD) process at low temperature, the curved PR topography can be transferred to the silicon dioxide layer. Subsequently, plasma etching can be used to fabricate the arched stripe arrays. The wavelike silicon dioxide arch microstructure exhibits Young modulus and fracture strength of 52 GPa and 300 MPa, respectively. The model of stress distribution inside the microstructure was also established, which compares well with the experimental results. This approach of fabricating a wavelike arch structure may become a promising route to produce a variety of stretchable sensors, actuators and circuits, thus providing unique opportunities for emerging classes of robust 3D integrated systems

Stable topological insulators achieved using high energy electron beams

Topological insulators are transformative quantum solids with immune-to-disorder metallic surface states having Dirac band structure. Ubiquitous charged bulk defects, however, pull the Fermi energy into the bulk bands, denying access to surface charge transport. Here we demonstrate that irradiation with swift ($\sim 2.5$ MeV energy) electron beams allows to compensate these defects, bring the Fermi level back into the bulk gap, and reach the charge neutrality point (CNP). Controlling the beam fluence we tune bulk conductivity from \textit{p}- (hole-like) to \textit{n}-type (electron-like), crossing the Dirac point and back, while preserving the Dirac energy dispersion. The CNP conductance has a two-dimensional (2D) character on the order of ten conductance quanta $G_0 =e^2/h$, and reveals, both in Bi$_2$Te$_3$ and Bi$_2$Se$_3$, the presence of only two quantum channels corresponding to two topological surfaces. The intrinsic quantum transport of the topological states is accessible disregarding the bulk size.Comment: Main manuscript - 12 pages, 4 figures; Supplementary file - 15 pages, 11 figures, 1 Table, 4 Note