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تسجيل مستخدم جديدEnhanced thermoelectricity by controlled local structure in bismuth-chalcogenides
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Spectroscopic techniques, including photoelectron spectroscopy, diffuse reflectance, and x-ray absorption, are used to investigate the electronic structure and the local structure of LaOBiS$_{2-x}$Se$_x$ thermoelectric material. It is found that Se substitution effectively suppresses local distortion, that can be responsible for the increased carrier mobility together with a change in the electronic structure. The results suggest a possible way to control thermoelectric properties by tuning of the local crystal structure of these materials.
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Bismuth chalcogenides Bi$_2$Se$_3$ and Bi$_2$Te$_3$ are semiconductors, which can be both thermoelectric materials (TE) and topological insulators (TI). Lattice defects arising from vacancies, impurities, or dopants in these materials are important i
n that they provide the charge carriers in TE applications and compromise the performance of these materials as TIs. We present the first solid-state nuclear magnetic resonance (NMR) study of the $^{77}$Se and $^{125}$Te NMR resonances in polycrystalline powders of Bi$_2$Se$_3$ and Bi$_2$Te$_3$, respectively. The spin-lattice ($T_1$) relaxation is modeled by at most two exponentials. Within the framework of this model, the NMR measurement is sensitive to the distribution of native defects within these materials. One component corresponds to a stoichiometric fraction, an insulator with a very long $T_1$, whereas the other component is attributed to a sample fraction with high defect content with a short $T_1$ resulting from interaction with the conduction carriers. The absence of a very long $T_1$ in the bismuth telluride suggests defects throughout the sample. For the bismuth selenide, defect regions segregate into domains. We also find a substantial difference in the short $T_1$ component for $^{125}$Te nuclei (76 ms) and $^{77}$Se (0.63 s) in spite of the fact that these materials have nearly identical lattice structures, chemical and physical properties. Investigations of the NMR shift and Korringa law indicate that the coupling to the conduction band electrons at the chalcogenide sites is much stronger in the telluride. The results are consistent with a stronger spin-orbit coupling (SOC) to the $p$-band electrons in the telluride. If most parameters of a given material are kept equal, this type of experiment could provide a useful probe of SOC in engineered TI materials.
This study reports on the synthesis of ball-like bismuth ferrite BiFeO3 nanoflowers by means of microwave assisted hydrothermal process and also on their composition and mechanism of growth. It turns out that the petals of the nanoflowers are compose
d of the nanocrystals with the size about 35-39 nm whereas their thickness and size depends on the concentration of surfactants. The petals contain BiFeO3 phase and traces of Bi2O3 oxide and metallic Bi and Fe deposited mainly at their surface. Amounts of impurity phases are more pronounced in nanoflowers synthesized during short time, and become almost negligible for longer microwave processing. The nanoflowers contain also mixed Fe valence, with the Fe2+/Fe3+ ratio depending on the time of synthesis. The growth and shape of the nanoflowers result from the process of diffusion in the initial stages of hydrothermal reaction.
Bismuth chalcogenides are the most studied 3D topological insulators. As a rule, at low temperatures thin films of these materials demonstrate positive magnetoresistance due to weak antilocalization. Weak antilocalization should lead to resistivity d
ecrease at low temperatures; in experiments, however, resistivity grows as temperature decreases. From transport measurements for several thin films (with various carrier density, thickness, and carrier mobility), and by using purely phenomenological approach, with no microscopic theory, we show that the low temperature growth of the resistivity is accompanied by growth of the Hall coefficient, in agreement with diffusive electron-electron interaction correction mechanism. Our data reasonably explain the low-temperature resistivity upturn.
Recently published discoveries of acoustic and optical mode inversion in the phonon spectrum of certain metals became the first realistic example of non-interacting topological bosonic excitations in existing materials. However, the observable physic
al and technological use of such topological phonon phases remained unclear. In this work we provide a strong theoretical and numerical evidence that for a class of metallic compounds (known as triple point topological metals), the points in the phonon spectrum, at which three (two optical and one acoustic) phonon modes (bands) cross, represent a well-defined topological material phase, in which the hosting metals have very strong thermoelectric response. The triple point bosonic collective excitations appearing due to these topological phonon band-crossing points significantly suppress the lattice thermal conductivity, making such metals phonon-glass like. At the same time, the topological triple-point and Weyl fermionic quasiparticle excitations present in these metals yield good electrical transport (electron-crystal) and cause a local enhancement in the electronic density of states near the Fermi level, which considerably improves the thermopower. This combination of phonon-glass and electron-crystal is the key for high thermoelectric performance in metals. We call these materials topological thermoelectric metals and propose several newly predicted compounds for this phase (TaSb and TaBi). We hope that this work will lead researchers in physics and materials science to the detailed study of topological phonon phases in electronic materials, and the possibility of these phases to introduce novel and more efficient use of thermoelectric materials in many everyday technological applications.
Direct visualizations of spin accumulation due to the enhanced spin Hall effect (SHE) in bismuth (Bi) - doped silicon (Si) at room temperature are realized by using helicity-dependent photovoltage (HDP) measurements. Under application of a dc current
to the Bi-doped Si, clear helicity-dependent photovoltages are detected at the edges of the Si channel, indicating a perpendicular spin accumulation due to the SHE. In contrast, the HDP signals are negligibly small for phosphorus-doped Si. Compared to a platinum channel, which has a large spin Hall angle, more than two-orders of magnitude larger HDP signals are obtained in the Bi-doped Si.
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