1,721,242 research outputs found
Influence of strain on the Stark effect in InP/GaInP quantum discs
No full textIn InP/GaInP quantum discs it is shown that strain induces a type I to type II transition with increasing thickness of the disc. When an external electric field is applied along the cylindrical axis of the disc, the exciton energy exhibits a Stark effect, which for the light hole exciton becomes linear even for a small field value, while for the heavy hole it is more quadratic. (C) 2004 Elsevier B.V. All rights reserved
Diffusion of interacting particles in discrete geometries: Equilibrium and dynamical properties
No full textAbstract: We expand on a recent study of a lattice model of interacting particles [Phys. Rev. Lett. 111, 110601 (2013)]. The adsorption isotherm and equilibrium fluctuations in particle number are discussed as a function of the interaction. Their behavior is similar to that of interacting particles in porous materials. Different expressions for the particle jump rates are derived from transition-state theory. Which expression should be used depends on the strength of the interparticle interactions. Analytical expressions for the self-and transport diffusion are derived when correlations, caused by memory effects in the environment, are neglected. The diffusive behavior is studied numerically with kinetic Monte Carlo (kMC) simulations, which reproduces the diffusion including correlations. The effect of correlations is studied by comparing the analytical expressions with the kMC simulations. It is found that the Maxwell-Stefan diffusion can exceed the self-diffusion. To our knowledge, this is the first time this is observed. The diffusive behavior in one-dimensional and higher-dimensional systems is qualitatively the same, with the effect of correlations decreasing for increasing dimension. The length dependence of both the self-and transport diffusion is studied for one-dimensional systems. For long lengths the self-diffusion shows a 1/L dependence. Finally, we discuss when agreement with experiments and simulations can be expected. The assumption that particles in different cavities do not interact is expected to hold quantitatively at low and medium particle concentrations if the particles are not strongly interacting
Design and synthesis of novel p-type TCOs: From computational screening towards film deposition
No full textOver the last decades, great progress has been made in the field of n-type TCO materials for various opto-electronic applications. This already resulted in the device implementation and commercialization of many oxides. However, p-type TCO materials research is still in a fundamental stage, and materials science has yet to find a solution to the generally low carrier mobilities in these materials. Especially for the design of transparent electronics, the availability of transparent p-n junctions is critical for many electronic components.
Using modern day’s widespread availability of material databases in combination with powerful computational techniques allows for high-throughput screenings of thousands of oxides in search of various desired properties. For potential TCO candidates, parameters such as the carrier effective mass, the optical band gap and the (extrinsic) dopability are having high priorities.
The candidates resulting from these screenings are often complex multi-metal oxides, in which the matrix material typically requires the addition of external acceptor dopants. All in all, these requirements contribute to the fact that the synthesis of these “designer oxides” is often not straightforward. Additionally, the synthesis should be implemented in a process that ultimately leads to the fabrication of functional oxide films. By depositing these thin films using chemical solution deposition (CSD) instead of more traditional vacuum techniques, we can overcome some challenges that come along with the deposition of multi-metal oxides, such as homogeneity issues and control of the stoichiometry, including dopant addition.
In this work, two recent p-type TCO candidate materials were selected for further study: Li-doped Cr2MnO4 and Na-doped Ln2SeO2 (Ln: lanthanide). Both of these materials are identified as primary candidates from their respective computational screening procedures. Aqueous precursor solutions for both materials are developed, in which the metal ions are stabilized by α-hydroxy carboxylic acids which fulfill the role of ligands. Thin films of these materials are deposited via spin-coating. Ultimately, a thermal treatment (combined with a selenization step for material 2) leads to crystalline films of the desired phase. The resulting films are characterized further via XRD (phase formation) and SEM (morphology). In addition, issues regarding phase formation are investigated and circumvented by optimizing the synthesis and processing steps
Design and synthesis of novel p-type TCOs: From computational screening towards film deposition
No full textOver the last decades, great progress has been made in the field of n-type TCO materials for various opto-electronic applications. This already resulted in the device implementation and commercialization of many oxides. However, p-type TCO materials research is still in a fundamental stage, and materials science has yet to find a solution to the generally low carrier mobilities in these materials. Especially for the design of transparent electronics, the availability of transparent p-n junctions is critical for many electronic components.
Using modern day’s widespread availability of material databases in combination with powerful computational techniques allows for high-throughput screenings of thousands of oxides in search of various desired properties. For potential TCO candidates, parameters such as the carrier effective mass, the optical band gap and the (extrinsic) dopability are having high priorities.
The candidates resulting from these screenings are often complex multi-metal oxides, in which the matrix material typically requires the addition of external acceptor dopants. All in all, these requirements contribute to the fact that the synthesis of these “designer oxides” is often not straightforward. Additionally, the synthesis should be implemented in a process that ultimately leads to the fabrication of functional oxide films. By depositing these thin films using chemical solution deposition (CSD) instead of more traditional vacuum techniques, we can overcome some challenges that come along with the deposition of multi-metal oxides, such as homogeneity issues and control of the stoichiometry, including dopant addition.
In this work, two recent p-type TCO candidate materials were selected for further study: Li-doped Cr2MnO4 and Na-doped Ln2SeO2 (Ln: lanthanide). Both of these materials are identified as primary candidates from their respective computational screening procedures. Aqueous precursor solutions for both materials are developed, in which the metal ions are stabilized by α-hydroxy carboxylic acids which fulfill the role of ligands. Thin films of these materials are deposited via spin-coating. Ultimately, a thermal treatment (combined with a selenization step for material 2) leads to crystalline films of the desired phase. The resulting films are characterized further via XRD (phase formation) and SEM (morphology). In addition, issues regarding phase formation are investigated and circumvented by optimizing the synthesis and processing steps
Charge transport in magnetic topological insulators
No full textAbstract: Novel quantum phases of matter and developing practical control over their characteristics is one of the primary aims of current condensed matter physics. It offers the potential for a new generation of energy, electronic and photonic technologies. Among all the newly found phases of matter, topological insulators are novel phases of quantum matter with fascinating bulk band topology and surface states protected by specific symmetries. For example, at the boundary of a strong topological insulator and a trivial insulator, metallic surface states appear that are protected by time-reversal symmetry. As a result, the bulk continues to be insulating, while the surface can support exotic high-mobility spin-polarized electronic states. Since there is no such thing as a clean system, impurities and other disorders are always present in materials. Even while impurities appear to be unfavorable to a system at first look, doping the host system with impurities allows us to engineer different electronic properties of systems, such as the Fermi level or electron density. Because of the symmetry protected metallic states in topological insulators, charge transport responds distinctively to magnetic and non-magnetic impurities. This doctoral dissertation explores how the longitudinal charge transport in magnetic topological thin films and the anomalous Hall effect on the surface of 3D magnetic topological insulators is influenced by point-like and randomly distributed dilute magnetic impurities. We are interested in how charge transport in these systems responds to the orientation of the magnetization orientation and how this response evolves based on the system's main characteristics, such as the magnitude of the Fermi level or gate voltage. Because topological insulators have a strong spin-orbit coupling, the interaction between conducting electrons and local magnetic impurities is very anisotropic. We will show that this anisotropy even enhances when magnetic topological thin films are exposed to a substrate or gate voltage. Therefore, to properly capture this anisotropy in charge transport calculations, we rely on a generalized Boltzmann formalism together with a modified relaxation time scheme. We show that magnetic impurities affect the charge transport in topological insulators by inducing a transition selection rule that governs scatterings of electrons between various electronic states. We see that this selection rule is highly sensitive to the spin direction of the magnetic impurities as well as the position of the Fermi level. According to this selection rule and depending on the position of the Fermi level, two different transport regimes are realized in magnetic topological thin films. In one of these regimes, our findings show that a dissipation less charge current can be generated. In other words, even if there are many magnetic impurities in the system, electrons do not notice them and, remarkably, conduct charge without dissipation. Outside this regime, the charge transport is always dissipative and its sensitivity to the spatial orientation of the magnetic impurities can be effectively modulated by a substrate or gate voltage. In this doctoral thesis, we also explore the anomalous Hall effect (AHE) on the surface of 3D magnetic topological insulators. The AHE is generated by three mechanisms: the intrinsic effect (owing to a nonzero Berry curvature), the side jump effect, and the skew scattering effect. They compete to dominate the AHE in distinct regimes. Analytically, we calculate the contributions of all three mechanisms to the scattering of massive Dirac fermions by magnetic impurities. Our results reveal three transport regimes based on the relative importance of the engaged mechanisms. The identification of these three distinctive transport regimes can assist experimentalists in achieving a regime in which each contribution is dominant over the others, allowing them to measure them separately. Typically, this is not feasible empirically since the total value of the experimentally observed AHE conceals the specific information of each of the three contributions. Based on our analytical calculations, we prove that the AHE can change sign by varying the orientation of the surface magnetization, the concentration of impurities, and the location of the Fermi level, which is consistent with previous experimental findings. In addition, we show that by suitably adjusting the given parameters, any contribution to the AHE, or even the entire AHE, can be turned off. For example, in a system with in-plane magnetization, one can turn off the AHE by pushing the system into the completely metallic regime. Furthermore, we demonstrate that any contribution to the AHE, or even the whole AHE, can be turned off by appropriately changing the provided parameters. For example, in a system with in-plane magnetization, the AHE can be turned off by pushing the system into the fully metallic regime
Active sound localisation through Bayesian inference
No full textAbstract: While the human auditory system is proficient on its own at discerning the direction of incoming sounds, it operates in concert with other sensory modalities to reach accurate spatial awareness. Many studies have investigated the integration of auditory and visual information, but much less attention has been given to the importance of proprioceptive and vestibular information in the localisation process. The vestibular and proprioceptive systems aid in discerning self-motion from source motion and, through this, can stabilise perception and provide additional cues for sound localisation. The aim of this PhD thesis was to better understand how head movement and position estimation affects sound localisation. To this end, an ideal-observer model, based on Bayesian principles, was developed as a tool to predict dynamic sound localisation in humans and to test how performance is affected by the available information. Behavioural experiments were conducted in conjunction with model simulations to determine the acoustic cues and head motions that are relevant to dynamic sound localisation. The results from the psychoacoustic experiments were found to be in general agreement with the model output, though some quantitative differences indicated that dynamic sound localisation may involve processes that can be considered non-ideal. These studies offer valuable insights for the field of psychoacoustics and for auditory engineering applications in modern technologies such as hearing aids and virtual or augmented reality
Geautomatiseerde in silico ontwikkeling van materialen voor energie- en plasmatoepassingen
No full textAbstract: Materials and their properties play a vital role in most applications we use on a daily basis. Many of the revolutions in industry are instigated by the discovery, by accident or design, of a new material that makes an application commercially viable. Historically, the study of materials has largely relied on an intuition-driven trial and error approach. However, considering the enormous design space of possible materials, as well as the fact that many of the straightforward materials have already been discovered, this process has become too expensive and time-consuming. Since the middle of the 20th century, a new paradigm in materials science has been developing, where computer simulations are used to calculate the properties of materials from rst principles. This new approach has steadily become more and more successful, pushed forward by the ever-increasing performance of modern computers and rapid progress in theoretical methods. During the past few decades, computational materials science has started evolving more and more into a predictive tool instead of simply oering theoretical insight into the physical processes of materials of interest. In combination with increasingly available tools for automating the required calculations, this has led to the concept of in silico materials design, where large numbers of compounds are investigated using computer simulations in order to gauge their potential for a specic application. Among the most successful theoretical frameworks for computational materials science is density functional theory, which can determine the electronic structure of many compounds with ever increasing accuracy using a reasonable amount of computational resources. However, the connection between the electronic structure of a material and the property of interest for a specic application is rarely trivial. The main goal of this thesis is to provide or improve this connection, by analyzing existing metrics for aws or anomalies, and developing new descriptors of material properties as well as the tools for calculating them using automated work ows. These methods are then applied to a set of topics, including solar cells, Li-ion batteries and ion-induced secondary electron emission. The structure of the thesis is as follows: Chapter 1 brie y introduces the concept of in silico materials design, and provides a guide to the reader of this thesis for navigating and consulting the available resources. Chapter 2 explains the density functional theory framework, as well as some practical computational techniques for calculating the electronic structure using this framework. Chapter 3 outlines the work ows used for the automation of the required density functional theory calculations of each descriptor or metric. Chapter 4 discusses the Shockley-Queisser limit and spectroscopic limited maximum eciency, two metrics used to determine the potential of a material as the absorber layer of a single-junction solar cell. Next, it makes a comparison of the CuAu-like and chalcopyrite phase in the context of thin-lm photovoltaics. Chapter 5 presents an investigation of the stability of the oxygen framework of Li-rich Li2MnO3 and Li2IrO3 battery cathodes, as well as a limited substitution of Mn as a potential recipe for improving the structural stability of these materials. Moreover, it discusses the energy landscapes of LiCB11H12 and NaCB11H12 polyborane salts in the context of solid electrolytes. Chapter 6 discloses a new model for calculating the secondary electron emission yield from ions neutralized at a semiconductor and metal surface, and applies this descriptor to a set of elemental surfaces spanning the periodic table in a high-throughput approach
Atomistic modeling of the structural and electronic properties of Cr-based oxides and their potential application as TCO materials
No full textFirst-principles study of polarons in WO\u2083
No full textAbstract: Polarons are quasiparticles emerging in materials from the interaction of extra charge carriers with the surrounding atomic lattice. They appear in a wide va- riety of compounds and can have a profound impact on their properties, making the concept of a polaron a central and ubiquitous topic in material science. Al- though the concept is known for about 75 years, the origin of polarons is not yet fully elucidated. This thesis focuses on WO 3 as a well-known prototypical system for studying polarons, which inherent polaronic nature is linked to its remark- able electrical and chromic properties. The primary objective of this research is to provide a comprehensive atomistic description and understanding of polaron formation in WO 3 using first-principles density functional theory (DFT) calcula- tions. Additionally, the investigation explores the interactions between polarons and the possibility of bipolaron formation. Following a systematic strategy, we first extensively analyze the dielectric and lattice dynamical properties of WO 3 in both the room-temperature P 2 1 /n and ground-state P 2 1 /c phases. Our specific focus is on characterizing the zone-center phonons, which serve as the founda- tion for identifying the phonon modes involved in the polaron formation and charge localization process. Subsequently, we examine the impact of structural distortions on the electronic structure of WO 3 to elucidate the interplay between structural distortions and electronic properties, thereby laying the groundwork for understanding electron-phonon couplings. By incorporating these critical fac- tors, we address our primary research goals. The most common explanation for the polaron formation is associated with the electrostatic screening of the extra charge by the polarizable lattice. Here, we show that, even in ionic crystals, this is not necessarily the case. We demonstrate that polarons in this compound arise primarily from non-polar atomic distortions. We then unveil that this unexpected behavior originates from the undoing of distortive atomic motions, which lowers the bandgap. As such, we coin the name of anti-distortive polaron and validate its appearance through a simple quantum-dot model, in which charge localization is the result of balancing structural, electronic, and confinement energy costs. Then, we also study the polaron-polaron interaction and present the formation of the antiferromagnetic W 4+ bipolaronic state with relatively large formation energy. Our analysis of the W 4+ bipolaronic distortions on the global structure reveals the same behavior as in experiments where the highly distorted monoclinic phase transforms into a tetragonal phase as a function of doping. Additionally, leveraging our previous findings on asymmetric polaronic distortion and examin- ing different merging orientations, we stabilize the antiferromagnetic W 5+ -W 5+ bipolaronic state with an energy lower than the W 4+ state. This thesis clari- fies the formation of unusual medium-size 2D polarons and bipolarons in WO3,which might be relevant to the whole family of ABO 3 perovskites, to which WO 3 is closely related. The simplicity of the concept provides also obvious guidelines for tracking similar behavior in other families of compounds
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