Constraining Forces Stabilizing Superconductivity in Bismuth

As shown in former papers, the nonadiabatic Heisenberg model presents a novel mechanism of Cooper pair formation generated by the strongly correlated atomic-like motion of the electrons in narrow, roughly half-filled "superconducting bands". These are energy bands represented by optimally localized spin-dependent Wannier functions adapted to the symmetry of the material under consideration. The formation of Cooper pairs is not the result of an attractive electron-electron interaction but can be described in terms of quantum mechanical constraining forces constraining the electrons to form Cooper pairs. There is theoretical and experimental evidence that only this nonadiabatic mechanism operating in superconducting bands may produce eigenstates in which the electrons form Cooper pairs. These constraining forces stabilize the Cooper pairs in any superconductor, whether conventional or unconventional. Here we report evidence that also the experimentally found superconducting state in bismuth at ambient as well as at high pressure is connected with a narrow, roughly half-filled superconducting band in the respective band structure. This observation corroborates once more the significance of constraining forces in the theory of superconductivity

K-space magnetism as the origin of superconductivity

The nonadiabatic Heisenberg model presents a nonadiabatic mechanism generating Cooper pairs in narrow, roughly half-filled "superconducting bands" of special symmetry. Here we show that this mechanism may be understood as the outcome of a special spin structure in the reciprocal space, hereinafter referred to as k-space magnetism. The presented picture permits a vivid depiction of this new mechanism highlighting the height similarity as well as the essential difference between the new nonadiabatic and the familiar Bardeen-Cooper-Schrieffer mechanism

Nonadiabatic Atomic-like State Stabilizing Antiferromagnetism and Mott Insulation in MnO

In this paper I report evidence that the antiferromagnetic and insulating ground state of MnO is caused by a nonadiabatic atomic-like motion as it is evidently the case in NiO. In addition, I show that the experimental findings of Goodwin et al. [Phys. Rev. Lett. (2006), 96,~047209] corroborate my suggestion that the rhombohedral-like distortion in antiferromagnetic MnO as well as in antiferromagnetic NiO is an inner distortion of the monoclinic base-centered Bravais lattice of the antiferromagnetic phases.Comment: arXiv admin note: text overlap with arXiv:1911.0819

Multi-scale defect engineering and interface modifications for enhancement of thermoelectric properties in nanostructured bulk materials

Among various static energy conversion technologies, the thermoelectric (TE) energy conversion has gained the considerable interest due to its reliability and ability to directly convert waste heat into electricity. In TE conversion technology, physical properties such as thermopower (α), electrical (σ) and thermal conductivity (κ) are exploited simultaneously to convert waste heat into electricity. The efficiency of such conversion depends upon various factors such as temperature, figure of merit (ZT= α2σ/ κ) etc. An inherent coupling among α, σ and κ limits ZT and thereby constrains one in achieving high efficiency. Many efforts have been carried out in decoupling TE properties. The inherent coupling of TE properties resulted in a saturation of the field for several years. Recently, it is realized that the inclusion of multi-length scale defects plays an important role in tuning the TE properties of various materials. Defects are often perceived as imperfections in materials that could adversely affect their performance. On the contrary, because of the limited size scale of nanomaterials, the power of defects could be effectively utilized to selectively scatter phonons and filter low-energy carriers. Hence, it is important to control the length scale and nature of these defects to improve the desired TE properties. This dissertation is focused on answering this important question: `Can one achieve control over the nature and length scale of these defects to decouple and and tune the temperature dependence of ZT in nanostructured bulk materials?\u27 To answer this question, three different materials systems were studied in this work demonstrating the role of various length scales and nature of defects. Firstly, the effects of extrinsic point defects, such as rattlers (Ce, In, Ba, Yb), dopants (Co, Ni) and secondary phases on FeSb3 and CoSb3 based p-type skutterudites on the transport and magnetic properties is studied. `Phonon glass and electron crystal\u27 like behavior was observed in Ni-doped skutterudites. Interestingly, we found that the addition of In facilitated the formation of secondary phases with various morphologies upon surpassing the filling fraction limits. Such in-situ secondary phases were in fact found to be beneficial to the system altering their electrical transport properties, and thereby increasing the ZT of the system as compared to that of the parent compound. The highest ZT value of 0.9 at 650 K was reported for p-type skutterudite sample with nominal composition In0.1Ce0.9Fe3.5Ni0.5Sb12. In the low temperature regime (T \u3c 150K), the electrical transport and magnetic susceptibility exhibited single-ion Kondo-like behavior. The crystal field effects due to the splitting of ground state of Ce (4f level) in presence of cubic crystalline field were observed to dictate the magnetic properties below 100 K. Further, our magnetic susceptibility data is consistent with a crystal field splitting gap of ~39 meV (~450 K). The intrinsic surface or interfacial defects in elemental Bismuth were introduced by controlling the surface-to-volume ratio using a combination of high energy ball-milling and spark plasma sintering (SPS) processes. The obtained ball-milled powders were SPS processed with different ON-OFF time ratios of the DC current pulses to further modify the nature and extent of these surfaces. The `double decoupling\u27 (simultaneous optimization of the thermopower, electrical conductivity and thermal conductivity) in single element polycrystalline Bi was observed via a combination of an increase in the surface-to-volume ratio achieved by ball milling process and an interface (or grain boundary) modification by the SPS process. As a result, a greater than six-fold improvement in the PF, and hence ZT, was achieved in polycrystalline bulk Bi samples. Our detailed studies of the effect of SPS conditions on the transport properties of polycrystalline Bi strongly suggests that surface states play a prominent role in enhancing the TE performance of Bi. Lastly, planar or two-dimensional defects were introduced by chemical exfoliation of layered chalcogenide n-type Bi2Te3. Particularly, chemical exfoliation allows for the introduction of micro-structured scattering centers at multiple length scales while preserving the basal plane properties needed for high ZT values. Mechanical process such as, grinding, sintering and exfoliation are known to generate donor- like defects. In this method, the possible introduction of positively charged defects (TeBi antisites/Te vacancies) on the grain boundaries resulted in: i) the injection of electrons into the bulk increasing carrier concentration, and ii) a potential barrier that selectively filtered low-energy minority carriers (holes in case of n-type Bi2Te3 samples) and thereby, shifting the bipolar (two carrier contribution) effects to higher temperatures. This effect is clearly reflected in the thermopower and thermal conductivity data. Thus, the shift in the bipolar effects results in the shift of ZT maxima to higher temperature, where peak ZT is broadened over a wide temperature range of ~ 150 K. In addition to this, the compatibility factor of our samples exhibits smaller changes over the broad operating temperature regime, making it a good candidate for potential device design

Synthesis, Structure, and Optoelectronic Properties of Hybrid Metal Iodides

The focus of this dissertation was on the preparation and analysis of new materials related to hybrid halide perovskites, AMX3, where A = a small organic cation, M = a divalent heavy metal, and X = a halogen. It sought to understand the fundamental reasons for why the hybrid perovskites work as excellent optoelectronic materials, and to use this information to design new, stable, and less toxic materials.This dissertation first sought to understand whether disparate slabs of metaliodide octahedra could be electronically coupled using electronically functional organic molecules, as this could create more stable and potentially more functional materials. A well-known organic compound made famous during the advent of organic metals, tetrathiafulvalene, was used to prepare three new hybrid materials [(TTF)Pb2I5, (TTF)BiI4, and (TTF)4BiI6] with some found to behave like semiconductors with 3D electronic connectivity, even though they were structurally 1D or 2D materials. This discovery supported our original hypothesis that a more stable, layered, hybrid perovskite material could potentially exist using functional organic molecules as both structural and electronic components.This work then extended its focus to study platinum based hybrid perovskites, A2PtI6 (A = small organic cation), and examined the effect of increasing cation size in Pt based vacancy ordered hybrid perovskite materials (VOHPs). This study was pertinent to our fundamental understanding of VOHPs, as by establishing structural trends caused by the small organic cations, and how different metals (Pt) behaved in these systems, future avenues for material design could be opened. It was found that the hydrogen bonding of the small organic cation to the iodides of the [PtI6]2- octahedra were quite significant as cation size increased, as these interactions dictated final structure and subsequent optical properties of these materials.In the process of understanding iodide containing hybrid perovskites, serendipitous discoveries of polyiodides were made along the way. These works include understanding the effect of hydrogen bonding in the formation of hybrid platinum oligo- and polyiodides, as well as resolving the 200 years old mystery as to what happens when starch combines with iodine.In summary, the work presented herein is reflective of the many diverse preparation and characterization techniques needed to complete these projects: solution and solid state synthesis, and single crystal and bulk material characterization techniques ranging from crystallography, calorimetry, and optical spectroscopy. It is also a testament to the social aspect of science, as to complete this work, many collaborations needed to be formed

Stress Management as an Enabling Technology for High-Field Superconducting Dipole Magnets

This dissertation examines stress management and other construction techniques as means to meet future accelerator requirement demands by planning, fabricating, and analyzing a high-field, Nb_(3)Sn dipole. In order to enable future fundamental research and discovery in high energy accelerator physics, bending magnets must access the highest fields possible. Stress management is a novel, propitious path to attain higher fields and preserve the maximum current capacity of advanced superconductors by managing the Lorentz stress so that strain induced current degradation is mitigated. Stress management is accomplished through several innovative design features. A block-coil geometry enables an Inconel pier and beam matrix to be incorporated in the windings for Lorentz Stress support and reduced AC loss. A laminar spring between windings and mica paper surrounding each winding inhibit any stress transferral through the support structure and has been simulated with ALGOR®. Wood’s metal filled, stainless steel bladders apply isostatic, surface-conforming preload to the pier and beam support structure. Sufficient preload along with mica paper sheer release reduces magnet training by inhibiting stick-slip motion. The effectiveness of stress management is tested with high-precision capacitive stress transducers and strain gauges. In addition to stress management, there are several technologies developed to assist in the successful construction of a high-field dipole. Quench protection has been designed and simulated along with full 3D magnetic simulation with OPERA®. Rutherford cable was constructed, and cable thermal expansion data was analysed after heat treatment. Pre-impregnation analysis techniques were developed due to elemental tin leakage in varying quantities during heat treatment from each coil. Robust splicing techniques were developed with measured resistivites consistent with nΩ joints. Stress management has not been incorporated by any other high field dipole research laboratory and has not yet been put to a definitive high-field test. The TAMU Physics Accelerator Research Laboratory has constructed a Nb_(3)Sn dipole, TAMU3, that is specially designed to provide a test bed for high-field stress management