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All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Tutorial: Defects in topological semimetals

Three-dimensional topological semimetals are a class of electronic materials in which their bulk and surface states contain linear band touching nodes near the Fermi level. Like semiconductors, their properties will be affected by point and extended defects in their crystal structures, although the extent to which defects and disorders influence topological semimetals may differ in key ways due to their unique electronic structures. In this Tutorial, we provide an overview of the defects in topological semimetals, covering both computational and experimental methods for exploring defect-property relationships. We also include a discussion on open questions that still need to be explored further.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Unidirectional amplification in the frozen mode regime enabled by a nonlinear defect

A stationary inflection point (SIP) is a spectral singularity of the Bloch dispersion relation ω(k) of a periodic structure where the first and the second derivatives of ω with respect to k vanish. An SIP is associated with a third-order exceptional point degeneracy in the spectrum of the unit-cell transfer matrix, where there is a collapse of one propagating and two evanescent Bloch modes. At the SIP frequency, the incident wave can be efficiently converted into the frozen mode with greatly enhanced amplitude and vanishing group velocity. This can be very attractive for applications, including light amplification. Due to its non-resonant nature, the frozen mode regime (FMR) has fundamental advantages over common cavity resonances. Furthermore, we propose, a novel, to the best of our knowledge, scheme for FMR-based unidirectional amplifiers by leveraging a tailored amplification/attenuation mechanism and a single nonlinear defect. The defect breaks the directional symmetry of the periodic structure and enables nonlinearity-related unidirectional amplification/attenuation in the vicinity of the SIP frequency. We demonstrate the robustness of the amplification mechanism to local impurities and parasitic nonlinearity.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2012

Solid State Lighting Program (Falcon)

Over the past two years, KLA-Tencor and partners successfully developed and deployed software and hardware tools that increase product yield for High Brightness LED (HBLED) manufacturing and reduce product development and factory ramp times. This report summarizes our development effort and details of how the results of the Solid State Light Program (Falcon) have started to help HBLED manufacturers optimize process control by enabling them to flag and correct identified killer defect conditions at any point of origin in the process manufacturing flow. This constitutes a quantum leap in yield management over current practice. Current practice consists of die dispositioning which is just rejection of bad die at end of process based upon probe tests, loosely assisted by optical in-line monitoring for gross process deficiencies. For the first time, and as a result of our Solid State Lighting Program, our LED manufacturing partners have obtained the software and hardware tools that optimize individual process steps to control killer defects at the point in the processes where they originate. Products developed during our two year program enable optimized inspection strategies for many product lines to minimize cost and maximize yield. The Solid State Lighting Program was structured in three phases: i) the development of advanced imaging modes that achieve clear separation between LED defect types, improves signal to noise and scan rates, and minimizes nuisance defects for both front end and back end inspection tools, ii) the creation of defect source analysis (DSA) software that connect the defect maps from back-end and front-end HBLED manufacturing tools to permit the automatic overlay and traceability of defects between tools and process steps, suppress nuisance defects, and identify the origin of killer defects with process step and conditions, and iii) working with partners (Philips Lumileds) on product wafers, obtain a detailed statistical correlation of automated defect and DSA map overlay to failed die identified using end product probe test results. Results from our two year effort have led to “automated end-to-end defect detection” with full defect traceability and the ability to unambiguously correlate device killer defects to optically detected features and their point of origin within the process. Success of the program can be measured by yield improvements at our partner’s facilities and new product orders.

Meeks, Steven↗
DOE OSTI2015

Topological defects from the multiverse

Many theories of the early universe predict the existence of a multiverse where bubbles continuously nucleate giving rise to observers in their interior. In this paper, we point out that topological defects of several dimensionalities will also be produced in de Sitter like regions of the multiverse. In particular, defects could be spontaneously nucleated in our parent vacuum. We study the evolution of these defects as they collide with and propagate inside of our bubble. We estimate the present distribution of defects in the observable part of the universe. The expected number of such nearby defects turns out to be quite small, even for the highest nucleation rate. We also study collisions of strings and domain walls with our bubble in our past light cone. We obtain simulated full-sky maps of the loci of such collisions, and find their angular size distribution. Similarly to what happens in the case of bubble collisions, the prospect of detecting any collisions of our bubble with ambient defects is greatly enhanced in the case where the cosmological constant of our parent vacuum is much higher than the vacuum energy density during inflation in our bubble.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2023

Modeling of fission gas diffusion and release for Gd 2 O 3 doped UO 2

Uranium dioxide (UO 2 ) is the primary nuclear fuel in light water reactors, and its excess neutronic reactivity can be controlled by adding burnable absorbers, such as Gd 2 O 3 . This burnable absorber has a large neutron absorption cross-section, lowering the high reactivity of the reactor's initial fuel load. However, there needs to be more understanding of how added Gd 2 O 3 influences the properties of UO 2 under irradiation. To understand the behavior of defects and fission gas in the UO 2 /Gd 2 O 3 system under irradiation, we use cluster dynamics modeling supported by density functional theory calculations. First, we calculate the formation energies of Gd point and cluster defects, and evaluate the temperature-dependent defect concentrations using the defect formation energies and entropies. We show that Gd is soluble in UO 2 , introducing a negative charge in the system. Using this information, we adapted the cluster dynamics code Centipede to model the influence of Gd on U self-diffusion and Xe diffusion in UO 2 with 10 wt% Gd 2 O 3 . Also, we analyzed the Xe diffusion as a function of Gd 2 O 3 concentration, showing that the Xe diffusivity is decreased, which means that the athermal diffusivity due to electronic stopping persists at higher temperatures. In conclusion, the decrease in Xe diffusion means that more Xe stays in the matrix, decreasing the Xe release, and lowering its influence of fission gas release on the thermomechanical properties of UO 2 .

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2019

Flux Pinning Mechanism in BaSnO{sub 3}-Added GdBa{sub 2}Cu{sub 3}O{sub 7-x} Films with Different Thickness

Types of flux pinning mechanism in pure and BaSnO{sub 3} (BSO)-added GdBa{sub 2}Cu{sub 3}O{sub 7-x} (GdBCO) superconducting films have been determined by using the Dew-Hughes model. We have found that as the BSO content increases up to 4 wt%, dominant pinning mechanism of GdBCO films has changed from normal surface pinning due to intrinsic linear defects to normal point pinning by BSO addition. Careful analysis on the scaling of the flux pinning for the 4 wt% BSO-doped GdBCO films with various film thicknesses revealed that the pinning mechanism came from the hybrid pinning of two kinds of pinning types. In addition to the normal point pinning by BSO addition, another type of Δκ-pinning was observed to be prominent in the thinner films in low magnetic field region as normal point pinning was deteriorated. Through the EXAFS analysis, it was found that the Δκ-pinning is possibly originated from the oxygen deficiency in localized position. These results suggested that film thickness directly affects the flux pinning mechanism of the BSO-doped GdBCO films, and in the application point of view, an optimization of the film thickness may be crucial to operate normal point pinning induced by BSO addition.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2011

2010 Defects in Semiconductors GRC

Continuing its tradition of excellence, this Gordon Conference will focus on research at the forefront of the field of defects in semiconductors. The conference will have a strong emphasis on the control of defects during growth and processing, as well as an emphasis on the development of novel defect detection methods and first-principles defect theories. Electronic, magnetic, and optical properties of bulk, thin film, and nanoscale semiconductors will be discussed in detail. In contrast to many conferences, which tend to focus on specific semiconductors, this conference will deal with point and extended defects in a broad range of electronic materials. This approach has proved to be extremely fruitful for advancing fundamental understanding in emerging materials such as wide-band-gap semiconductors, oxides, sp{sup 2} carbon based-materials, and photovoltaic/solar cell materials, and in understanding important defect phenomena such as doping bottleneck in nanostructures and the diffusion of defects and impurities. The program consists of about twenty invited talks and a number of contributed poster sessions. The emphasis should be on work which has yet to be published. The large amount of discussion time provides an ideal forum for dealing with topics that are new and/or controversial.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2015

Defects evolution and their impacts on conductivity of indium tin oxide thin films upon thermal treatment

Indium tin oxide (ITO) thin films were deposited on silicon substrates by radio-frequency magnetron sputtering. The influence of annealing temperature on the crystallite, surface morphology, defects evolution, and electrical property of the thin films was studied. The conductivity of the ITO films was significantly enhanced by two orders of magnitude by increasing the annealing temperature up to 600 °C, which was interpreted in point view of defects evolution in ITO films as revealed by positron annihilation. It was interesting to find that positron diffusion length was amazingly comparable to crystallite size in ITO films annealed below 300 °C, indicating positrons were preferentially localized and annihilated in defects around crystallite boundaries. By further increasing the temperature, positron diffusion length was far beyond the grain size with little increment. This demonstrated that defects were effectively removed around grain boundaries. The results indicated defect structure around crystallite/grain boundaries played an important role on carrier transportation in nanocrystal ITO films.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Inflaton fluctuations in the presence of cosmological defects

We study quantum fluctuations of a free massless scalar field during inflation in the presence of a point, line, or plane defect such as a black hole, cosmic string, or domain wall, using a perturbative expansion in powers of small defect parameters. We provide results for the scalar two-point correlation functions that show explicitly a small violation of translational invariance during inflation.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Scale-dependent CMB asymmetry from primordial configuration

We demonstrate that a topological defect can explain the hemispherical power asymmetry of the CMB. The first point is that a defect configuration, which already exists prior to inflation, can source asymmetry of the CMB. The second point is that modulation mechanisms, such as the curvaton and other modulation mechanisms, can explain scale-dependence of the asymmetry. Using a simple analysis of the δ N formalism, we show models in which scale-dependent hemispherical power asymmetry is explained by primordial configuration of a defect.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2018

Charge trapping properties of Ge nanocrystals grown via solid-state dewetting

In the present work, we report on the charge trapping properties of Germanium Nanocrystals (Ge NCs) self assembled on SiO 2 thin layer for promising applications in next-generation non volatile memory by the means of Deep Level Transient Spectroscopy (DLTS) and high frequency C-V method. The Ge NCs were grown via dewetting phenomenon at solid state by Ultra-High Vacuum (UHV) annealing and passivated with silicon before SiO 2 capping. The role of the surface passivation is to reduce the electrical defect density at the Ge NCs-SiO 2 interface. The presence of the Ge NCs in the oxide of the MOS capacitors strongly affects the C-V characteristics and increases the accumulation capacitance, causes a negative flat band voltage (VFB) shift. The DLTS has been used to study the individual Ge NCs as a single point deep level defect in the oxide. DLTS reveals two main features: the first electron traps around 255 K could correspond to dangling bonds at the Si/SiO2 interface and the second, at high-temperature (>300 K) response, could be originated from minority carrier generation in Ge NCs.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Influence of defects on the thermal conductivity of compressed LiF

We report defect formation in LiF, which is used as an observation window in ramp and shock experiments, has significant effects on its transmission properties. Given the extreme conditions of the experiments it is hard to measure the change in transmission directly. Using molecular dynamics, we estimate the change in conductivity as a function of the concentration of likely point and extended defects using a Green-Kubo technique with careful treatment of size effects. With this data, we form a model of the mean behavior and its estimated error; then, we use this model to predict the conductivity of a large sample of defective LiF resulting from a direct simulation of ramp compression as a demonstration of the accuracy of its predictions. Given estimates of defect densities in a LiF window used in an experiment, the model can be used to correct the observations of thermal energy through the window. Also, the methodology we develop is extensible to modeling, with quantified uncertainty, the effects of a variety of defects on the thermal conductivity of solid materials.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Effect of activation volume on the defect-induced superconductivity in semiconducting superlattices

We firstly verify our transition-state approaches by comparing our preliminary calculations for electronic transport resistance with previous available measurements in the heterostructure of semiconducting materials (PbTe/PbS). After the intensive computations, we illustrate the effect of activation volume and energy on the possible superconducting transition temperatures for superconducting interfaces in semiconducting superlattices (PbTe/PbS). The quantum chemistry approach we adopt has been successfully treated the defect-induced critical transport of many condensed electrons. Our results suggest that tuning activation volume is the efficient way to enhance the superconducting temperature. - Graphical abstract: Particles on the surface experience atomic forces equal to the negative gradient of the surface. B is the activated state and a transition is from A to C. Highlights: Black-Right-Pointing-Pointer Use quantum chemistry to capture the transition of PbTe/PbS-superconductors. Black-Right-Pointing-Pointer Activation energy and volume are crucial to the superconducting temperature. Black-Right-Pointing-Pointer Defect-induced interface superconductivity can be studied via our approach.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

The influence of lattice defects, recombination, and clustering on thermal transport in single crystal thorium dioxide

Thermal transport is a key controlling property for thorium dioxide in many applications where defect-generating radiation fields are present. An understanding of the effect of nanoscale lattice defects on thermal transport in this material is currently unavailable due to a lack of high-quality, single crystal material from which unit processes may be investigated. In this work, a series of thorium dioxide single crystals are exposed to 2 MeV proton irradiation at room temperature and 600°C to create microscale regions with varying densities and types of point and extended defects. Defected regions are investigated using spatial domain thermoreflectance to quantify the change in thermal conductivity as a function of ion fluence as well as transmission electron microscopy and Raman spectroscopy to interrogate the structure of the generated defects. Together, this com- bination of methods provides important initial insight into defect formation, recombination, and clustering in thorium dioxide and the effect of those defects on thermal transport. These methods also provide a promising pathway for the quantification of the smallest-scale defects which cannot be captured using traditional microscopy techniques and play and outsized role in degrading thermal performance.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2015

On the density of states of disordered epitaxial graphene

The study is concerned with two types of disordered epitaxial graphene: (i) graphene with randomly located carbon vacancies and (ii) structurally amorphous graphene. The former type is considered in the coherent potential approximation, and for the latter type, a model of the density of states is proposed. The effects of two types of substrates, specifically, metal and semiconductor substrates are taken into account. The specific features of the density of states of epitaxial graphene at the Dirac point and the edges of the continuous spectrum are analyzed. It is shown that vacancies in epitaxial graphene formed on the metal substrate bring about logarithmic nulling of the density of states of graphene at the Dirac point and the edges of the continuous spectrum. If the Dirac point corresponds to the middle of the band gap of the semiconductor substrate, the linear trend of the density of states to zero in the vicinity of the Dirac point in defect-free graphene transforms into a logarithmic decrease in the presence of vacancies. In both cases, the graphene-substrate interaction is assumed to be weak (quasi-free graphene). In the study of amorphous epitaxial graphene, a simple model of free amorphous graphene is proposed as the initial model, in which account is taken of the nonzero density of states at the Dirac point, and then the interaction of the graphene sheet with the substrate is taken into consideration. It is shown that, near the Dirac point, the quadratic behavior of the density of states of free amorphous graphene transforms into a linear dependence for amorphous epitaxial graphene. In the study, the density of states of free graphene corresponds to the low-energy approximation of the electron spectrum.

36 MATERIALS SCIENCE↗
NASA NTRS2009

DNA Repair Defects and Chromosomal Aberrations

Yields of chromosome aberrations were assessed in cells deficient in DNA doublestrand break (DSB) repair, after exposure to acute or to low-dose-rate (0.018 Gy/hr) gamma rays or acute high LET iron nuclei. We studied several cell lines including fibroblasts deficient in ATM (ataxia telangiectasia mutated; product of the gene that is mutated in ataxia telangiectasia patients) or NBS (nibrin; product of the gene mutated in the Nijmegen breakage syndrome), and gliomablastoma cells that are proficient or lacking in DNA-dependent protein kinase (DNA-PK) activity. Chromosomes were analyzed using the fluorescence in situ hybridization (FISH) chromosome painting method in cells at the first division post irradiation, and chromosome aberrations were identified as either simple exchanges (translocations and dicentrics) or complex exchanges (involving >2 breaks in 2 or more chromosomes). Gamma irradiation induced greater yields of both simple and complex exchanges in the DSB repair-defective cells than in the normal cells. The quadratic dose-response terms for both simple and complex chromosome exchanges were significantly higher for the ATM- and NBS-deficient lines than for normal fibroblasts. However, in the NBS cells the linear dose-response term was significantly higher only for simple exchanges. The large increases in the quadratic dose-response terms in these repair-defective cell lines points the importance of the functions of ATM and NBS in chromatin modifications to facilitate correct DSB repair and minimize the formation of aberrations. The differences found between ATM- and NBS-deficient cells at low doses suggest that important questions should with regard to applying observations of radiation sensitivity at high dose to low-dose exposures. For aberrations induced by iron nuclei, regression models preferred purely linear dose responses for simple exchanges and quadratic dose responses for complex exchanges. Relative biological effectiveness (RBE) factors of all of the DNA repair-defective cell lines were smaller than those of normal cells, with the DNA-PK-deficient cells having RBEs near unity. To further investigate the sensitivity differences that were observed in ATM and NBS deficient cells, chromosomal aberrations were analyzed in normal lung fibroblast cells treated with KU-55933 (a specific ATM kinase inhibitor) or Mirin (an Mre11- Rad50-Nbs1 complex inhibitor involved in activation of ATM). We also performed siRNA knockdown of these proteins. Preliminary data indicate that chromosome exchanges increase in cells treated with the specific ATM inhibitor. Possible cytogenetic signatures of acute and low dose-rate gamma irradiation in ATM or nibrin deficient and suppressed cells will be discussed.

Hada, Megumi↗
DOE OSTI2018

Magnetic field-, strain-, and disorder-induced responses in an energy spectrum of graphene

We study numerically a role of the uniaxial tensile strains and disordered defects in their impact on electronic density of states (DOS) in graphene exposed to an external magnetic field. The DOS curves are calculated using the tight-binding Hamiltonian, where the perpendicular magnetic field and uniaxial tension are included via the relevant modifications of the hopping parameters. Observed non-equidistant Landau levels (LLs) in the energy spectrum of a defectless graphene undergo the displacement towards the non-shiftable zero-energy Landau level (LL), thus they get contraction as the uniaxial tension is applied independently on the stretching direction. The presence of both point and extended defects reduces LLs peaks, broadens, smears, and can even suppress the LLs depending on a degree of disorders, their strength, and largely effective ranges. Splitting of the zero-energy LL is observable in case of a short-range disorder. Alteration of the localized electronic states in graphene is sensitive to the axis of strain: the uniaxial tensile strains along armchair- and zigzag-edge directions can result to competing phenomena associated with enhancement and reduction of the DOS, respectively. Mutual action of the perpendicular magnetic field and uniaxial stress along the zigzag-edge direction in graphene contributes to the band gap observed in its energy spectrum: the gap becomes more pronounced and even wider as compared with that emerges in case of a zigzag strain effect only, i.e. beyond magnetic field effect.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2026

Optimizing spin qubit coherence through materials codesign

The evolution of defect-based spin qubit systems is currently transitioning from fundamental studies and proof-of-concept demonstrations into applications in the burgeoning field of quantum technology. Within this context, new challenges emerge, in particular, the need to understand and engineer the fundamental materials that form the hardware building blocks critical for the scalability and wide-scale adoption of such technologies. While earlier discussions have often focused on qubits within idealized systems, major limitations on spin coherence and optical properties arise from effects imposed by the nonideality of the surrounding host matrix. Decoherence can stem from a variety of sources, including other qubits, nuclear spins, and parasitic point- and extended defects, which interact with the qubit via magnetic and electric fields, photons, phonons, and strain. In this article, we focus on the relevant sources and mechanisms through which decoherence occurs and provide potential mitigation strategies via the synergistic integration of first-principles simulations and materials synthesis and engineering. We aim to provide a tangible link between material properties and material functions thereby enabling materials-by-design.

ab initio simulations↗