46 research outputs found
Effects of filling in CoSb3: Local structure, band gap, and phonons from first principles
No full textWe use ab initio computations to investigate the effect of filler ions on the properties of CoSb3 skutterudites. We analyze global and local structural effects of filling, using the Ba-filled system as an example. We show that the deformation of Sb network induced by the filler affects primarily nearest neighboring Sb sites around the filler site as the soft Sb rings accommodate the distortion. Rearrangement of Sb atoms affects the electronic band structure and we clarify the effect of this local strain on the band gap. We compute the phonon dispersions and identify the filler-dominated modes from the lowest-frequency optical modes at Gamma. Their weak dispersion across the Brillouin zone indicates that they are localized and a force-constant analysis shows that the filler vibration is strongly coupled with nearby Sb atoms.Massachusetts Institute of Technology. Energy InitiativeRobert Bosch Gmb
Electronic, vibrational, and transport properties of pnictogen-substituted ternary skutterudites
No full textFirst principles calculations are used to investigate electronic band structure and vibrational spectra of pnictogen-substituted ternary skutterudites. We compare the results with the prototypical binary composition CoSb3 to identify the effects of substitutions on the Sb site, and evaluate the potential of ternary skutterudites for thermoelectric applications. Electronic transport coefficients are computed within the Boltzmann transport formalism assuming a constant relaxation time, using a methodology based on maximally localized Wannier function interpolation. Our results point to a large sensitivity of the electronic transport coefficients to carrier concentration and to scattering mechanisms associated with the enhanced polarity. The ionic character of the bonds is used to explain the detrimental effect on the thermoelectric properties.National Science Foundation (U.S.), United States. Dept. of Energy Partnership in Thermoelectrics (CBET-0853350
Screening for high-performance piezoelectrics using high-throughput density functional theory
No full textWe present a large-scale density functional theory (DFT) investigation of the ABO3 chemical space in the perovskite crystal structure, with the aim of identifying those that are relevant for forming piezoelectric materials. Screening criteria on the DFT results are used to select 49 compositions, which can be seen as the fundamental building blocks from which to create alloys with potentially good piezoelectric performance. This screening finds all the alloy end points used in three well-known high-performance piezoelectrics. The energy differences between different structural distortions, deformation, coupling between the displacement of the A and B sites, spontaneous polarization, Born effective charges, and stability is analyzed in each composition. We discuss the features that cause the high piezoelectric performance of the well-known piezoelectric lead zirconate titanate (PZT), and investigate to what extent these features occur in other compositions. We demonstrate how our results can be useful in the design of isovalent alloys with high piezoelectric performance
Erratum: Electrostatics in periodic boundary conditions and real-space corrections [Phys. Rev. B 77, 115139 (2008)]
No full textInternational audienc
Static dielectric properties of carbon nanotubes from first principles
No full textWe characterize the response of isolated single-wall (SWNT) and multiwall (MWNT) carbon nanotubes and nanotube bundles to static electric fields using first-principles calculations and density-functional theory. The longitudinal polarizability of SWNTs scales as the inverse square of the band gap, while in MWNTs and bundles it is given by the sum of the polarizabilities of the constituent tubes. The transverse polarizability of SWNTs is insensitive to band gaps and chiralities and is proportional to the square of the effective radius; in MWNTs, the outer layers dominate the response. The transverse response is intermediate between metallic and insulating, and a simple electrostatic model based on a scale-invariance relation captures accurately the first-principles results. The dielectric response of nonchiral SWNTs in both directions remains linear up to very high values of applied field.THEO
Role of Solvent-Anion Charge Transfer in Oxidative Degradation of Battery Electrolytes
No full textElectrochemical stability windows of electrolytes largely determine the limitations of operating regimes of lithium-ion batteries, but the degradation mechanisms are difficult to characterize and poorly understood. Using computational quantum chemistry to investigate the oxidative decomposition that govern voltage stability of multi-component organic electrolytes, we find that electrolyte decomposition is a process involving the solvent and the salt anion and requires explicit treatment of their coupling. We find that the ionization potential of the solvent-anion system is often lower than that of the isolated solvent or the anion. This mutual weakening effect is explained by the formation of the anion-solvent charge-transfer complex, which we study for 16 anion-solvent combinations. This understanding of the oxidation mechanism allows to formulate a simple predictive model that explains experimentally observed trends in the onset voltages of degradation of electrolytes near the cathode. This model opens opportunities for rapid rational design of stable electrolytes for high-energy batteries.<br
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Nanoscale Investigations of Monolayer Thin Films and Heavy Element Materials
No full textDuring my Ph.D, my work were divided into two parts. First, I spent a lot of time and effort using
molecular beam epitaxy (MBE) to grow monolayer thin films that have novel quantum properties.
Second, I used the scanning tunneling microscope (STM) to study cleavable single crystals, including materials with a strong surface Rashba effect and Kondo effect.
This dissertation focuses on a subset of the work that I have done in my Ph.D. Part I is about film
growth using MBE, which includes three chapters. Chapter 1 gives a general introduction to the
characterization tools I used for film growth, including STM, X-ray photoelectron spectroscopy
(XPS), atomic force microscope (AFM), scanning transmission electron microscope (STEM), and
electron energy loss spectroscopy (EELS). Chapter 2 appears in its entirety in the manuscript:
Samantha O’Sullivan, Ruizhe Kang, Jules A. Gardener, Austin J. Akey, Christian
E. Matt, and Jennifer E. Hoffman ”Imaging Se diffusion across the FeSe/SrTiO3
interface.” Physical Review B 105, 165407 (2022)
There has been a long debate on the exact structure of the FeSe/SrTiO3 interface. Some groups
reported a clean interface between the FeSe and the SrTiO3 surface while others observed an additional Se layer. In this chapter, we provided evidence aiming to put an end to this debate. Even
though we didn’t observe an ordered Se layer between the film and the substrate, we discovered a significant amount of Se diffused across the monolayer FeSe/SrTiO3 interface using EELS. This work shines light for a possible factor that affect the high-temperature superconductivity at FeSe/SrTiO3
interface. Chapter 3 demonstrates my efforts in growing a monolayer honeycomb bismuth film (bismuthene) on hydrogen-passivated SiC substrates. In this chapter, I have demonstrated the importance of the hydrogen passivation of the SiC substrate and provided evidence of the air sensitivity of
the bismuthene film.
The second part of this thesis is about STM studies on two cleavable materials composed of
heavy elements, BiTeI and UTe3. Chapter 4 is adapted from this manuscript:
Ruizhe Kang, Jian-Feng Ge, Yang He, Zhihuai Zhu, Daniel T. Larson, Mohammed
Saghir, Jason D. Hoffman, Geetha Balakrishnan, Jennifer E. Hoffman. ”Nanoscale
variation of the Rashba energy in BiTeI.” arXiv.2402.18779
The strong spin-orbit coupling (SOC) leads to a huge Rashba effect in BiTeI. In this chapter,
we observed ring-like charging states on the iodine surface of BiTeI, which could be used as a probe
of the local electric field. We extracted the local Rashba energies by fitting the van Hove singularities observed in our scanning tunneling spectroscopy. We discovered that the Rashba energies have
nanoscale variations, which positively correlate with the local electric field probed by the charging
ring states.
Chapter 5 reports the first-ever STM measurement on UTe3, where we measured Kondo resonance. In this chapter, we demonstrate how the Kondo holes affect the local Kondo resonance. We
discovered that the Kondo holes in UTe3 will reduce the local q factors and shift the Kondo resonance energies towards the valance band. However, the hybridization factor Γ shows a very weak
correlation with the Kondo hole locations, indicating that the Kondo holes could induce some hybridization disorder. This manuscript is in preparation.Engineering and Applied Sciences - Applied Physic
Electrostatics in Periodic Boundary Conditions and Real-space Corrections
No full textInternational audienceWe address periodic-image errors arising from the use of periodic boundary conditions to describe systems that do not exhibit full three-dimensional periodicity. The difference between the periodic potential, as straightforwardly obtained from a Fourier transform, and the potential satisfying any other boundary conditions can be characterized analytically. In light of this observation, we present an efficient real-space method to correct periodic-image errors, based on a multigrid solver for the potential difference, and demonstrate that exponential convergence of the energy with respect to cell size can be achieved in practical calculations. Additionally, we derive rapidly convergent expansions for determining the Madelung constants of point-charge assemblies in one, two, and three dimensions
Induced diploid gynogenesis and polyploidy in ornamental (koi) carp, Cyprinus carpio L. 2. Timing of heat shock during the first cleavage
No full textRole of Disorder and Anharmonicity in the Thermal Conductivity of Silicon-Germanium Alloys: A First-Principles Study
No full textThe thermal conductivity of disordered silicon-germanium alloys is computed from density-functional perturbation theory and with relaxation times that include both harmonic and anharmonic scattering terms. We show that this approach yields an excellent agreement at all compositions with experimental results and provides clear design rules for the engineering of nanostructured thermoelectrics. For SixGe1-x, more than 50% of the heat is carried at room temperature by phonons of mean free path greater than 1 μm, and an addition of as little as 12% Ge is sufficient to reduce the thermal conductivity to the minimum value achievable through alloying. Intriguingly, mass disorder is found to increase the anharmonic scattering of phonons through a modification of their vibration eigenmodes, resulting in an increase of 15% in thermal resistivity.Massachusetts Institute of Technology. Institute for Soldier Nanotechnologies (ISN-ARO)Massachusetts Institute of Technology. Institute for Soldier Nanotechnologies ((ISN-ARO) Grant No. W911NF-07-D-0004)
