Daniel Finkenstadt

Finding the Stable Structures of N1-xWx with an Ab Initio High-Throughput Approach

Abstract : One of the major goals of electronic-structure calculations is the prediction of crystal structures as a function of composition [1 3]. Determining the possible configurations of a compound as a function of composition is the first step in determining its material properties at equilibrium. It is also likely that any pressure- or temperature-driven phase transitions will be from the equilibrium structure to another structure which is close to it in energy and composition. Such calculations are of particular interest when there is little known about the system theoretically. There are a variety of mechanisms for this: searching over a wide range of known [4] and likely [5] structures for the material in question, searches starting from randomly positioned atoms [6], and even structures predicted from apparently out of the blue [7]. In the end these techniques produce a set of metastable structures, all of which have zero force on the atoms in the crystal, zero stress, and no imaginary phonon modes. Some structures will be stable, that is, it is not energetically favorable for them to decompose into into other structures.

A general framework for numerical simulation of improvised explosive device (IED)-detection scenarios using density functional theory (DFT) and terahertz (THz) spectra.

We have developed a general framework for numerical simulation of various types of scenarios that can occur for the detection of improvised explosive devices (IEDs) through the use of excitation using incident electromagnetic waves. A central component model of this framework is an S-matrix representation of a multilayered composite material system. Each layer of the system is characterized by an average thickness and an effective electric permittivity function. The outputs of this component are the reflectivity and the transmissivity as functions of frequency and angle of the incident electromagnetic wave. The input of the component is a parameterized analytic-function representation of the electric permittivity as a function of frequency, which is provided by another component model of the framework. The permittivity function is constructed by fitting response spectra calculated using density functional theory (DFT) and parameter adjustment according to any additional information that may be available, e.g., experimentally measured spectra or theory-based assumptions concerning spectral features. A prototype simulation is described that considers response characteristics for THz excitation of the high explosive β-HMX. This prototype simulation includes a description of a procedure for calculating response spectra using DFT as input to the S-matrix model. For this purpose, the DFT software NRLMOL was adopted.

Dielectric Response of High Explosives at THz Frequencies Calculated Using Density Functional Theory

We present in this study calculations of the ground-state resonance structures associated with the high explosives β-HMX, PETN, RDX, TNT1, and TNT2 using density functional theory (DFT). Our objective is the construction of parameterized dielectric-response functions for excitation by electromagnetic waves at compatible frequencies. These dielectric-response functions provide the basis for analyses pertaining to the dielectric properties of explosives. In particular, these dielectric-response functions provide quantitative initial estimates of spectral-response features for subsequent adjustment with knowledge of additional information, such as laboratory measurements and other types of theory-based calculations. With respect to qualitative analyses, these spectra provide for the molecular-level interpretation of response structure. The DFT software GAUSSIAN was used for the calculations of the ground-state resonance structures presented here.

Active bialkali photocathodes on free-standing graphene substrates

The hexagonal structure of graphene gives rise to the property of gas impermeability, motivating its investigation for a new application: protection of semiconductor photocathodes in electron accelerators. These materials are extremely susceptible to degradation in efficiency through multiple mechanisms related to contamination from the local imperfect vacuum environment of the host photoinjector. Few-layer graphene has been predicted to permit a modified photoemission response of protected photocathode surfaces, and recent experiments of single-layer graphene on copper have begun to confirm these predictions for single crystal metallic photocathodes. Unlike metallic photoemitters, the integration of an ultra-thin graphene barrier film with conventional semiconductor photocathode growth processes is not straightforward. A first step toward addressing this challenge is the growth and characterization of technologically relevant, high quantum efficiency bialkali photocathodes on ultra-thin free-standing graphene substrates. Photocathode growth on free-standing graphene provides the opportunity to integrate these two materials and study their interaction. Specifically, spectral response features and photoemission stability of cathodes grown on graphene substrates are compared to those deposited on established substrates. In addition, we observed an increase of work function for the graphene encapsulated bialkali photocathode surfaces, which is predicted by our calculations. The results provide a unique demonstration of bialkali photocathodes on free-standing substrates, and indicate promise towards our goal of fabricating high-performance graphene encapsulated photocathodes with enhanced lifetime for accelerator applications.Graphene in accelerator technology: A new material for enhanced photocathode performance and lifetimeGraphene has shown potential to unlock new capabilities of electron sources and other aspects of accelerator technology. This report focuses on integrating graphene with high performance photocathodes with the goal of extending lifetime by thousands of hours.Scientists at Los Alamos National Laboratory, USA, and colleagues succeeded in growth of chemically susceptible photocathodes on free-standing graphene substrates while maintaining state-of-the-art performance. Successful growth on graphene is a critical step toward a material-centric approach to photocathode design: enhancing lifetime without compromising efficiency or other performance metrics. Graphene, an atomically thin sheet of carbon, is an emerging material that has inspired new cathode design capabilities, including heterostructuring, resonant tunneling, and impermeable gas barriers. Conventional photocathode materials have no performance regimes. The next step is complete graphene encapsulation of photocathode films and demonstration of lifetime enhancement in the operating environment of accelerator facilities.

A photoemission moments model using density functional and transfer matrix methods applied to coating layers on surfaces: Theory

Recent experimental measurements of a bulk material covered with a small number of graphene layers reported by Yamaguchi et al. [NPJ 2D Mater. Appl. 1, 12 (2017)] (on bialkali) and Liu et al. [Appl. Phys. Lett. 110, 041607 (2017)] (on copper) and the needs of emission models in beam optics codes have lead to substantial changes in a Moments model of photoemission. The changes account for (i) a barrier profile and density of states factor based on density functional theory (DFT) evaluations, (ii) a Drude-Lorentz model of the optical constants and laser penetration depth, and (iii) a transmission probability evaluated by an Airy Transfer Matrix Approach. Importantly, the DFT results lead to a surface barrier profile of a shape similar to both resonant barriers and reflectionless wells: the associated quantum mechanical transmission probabilities are shown to be comparable to those recently required to enable the Moments (and Three Step) model to match experimental data but for reasons very different than th...

Density of states of Cs 3 Sb calculated using density-functional theory for modeling photoemission

An analysis is presented that provides a density of states (DOS or D(E)) factor for Cs3Sb in the calculation of its quantum efficiency QE and emittance εn;rms using a Moments Approach. The analysis is based on density functional theory (DFT) adapted for the practical application of treating photoemission from bulk metal and semiconductor materials, and the interfaces between them. The Moments approach treats the processes of absorption, transmission and emission separately, for which DFT affects parameters and processes associated with each step, of which D, the optical constants n and k, and materials parameters such as effective mass mn and band gap Eg are paramount. Such factors are required to provide the components of an evaluation similar to the Tsu-Esaki formula for calculating current density over and through and over barriers, and will become more important when a proper quantum mechanical treatment of the emission barrier is considered beyond the simplistic thermal model (transmission probability is unity only for energy levels in excess of the barrier height and zero otherwise). Such features are expected to be far more consequential if the barrier supports resonant levels, e.g., heterostructures.

Calculation of density of states for modeling photoemission using method of moments

Modeling photoemission using the Moments Approach (akin to Spicer’s “Three Step Model”) is often presumed to follow simple models for the prediction of two critical properties of photocathodes: the yield or “Quantum Efficiency” (QE), and the intrinsic spreading of the beam or “emittance” εn;rms. The simple models, however, tend to obscure properties of electrons in materials, the understanding of which is necessary for a proper prediction of a semiconductor or metal’s QE and εn;rms. This structure is characterized by localized resonance features as well as a universal trend at high energy. Presented in this study is a prototype analysis concerning the density of states (DOS) factor D(E) for Copper in bulk to replace the simple three-dimensional form of D(E) = (m/π2 h3)p2mE currently used in the Moments approach. This analysis demonstrates that excited state spectra of atoms, molecules and solids based on density-functional theory can be adapted as useful information for practical applications, as well as providing theoretical interpretation of density-of-states structure, e.g., qualitatively good descriptions of optical transitions in matter, in addition to DFT’s utility in providing the optical constants and material parameters also required in the Moments Approach.

Ground state resonance structure calculated by density functional theory for estimating the dielectric response of the high explosive PETN

We present calculations of ground state resonance structure associated with the high explosive PETN using density functional theory (DFT), which is for the construction of parameterized dielectric response functions for excitation by electromagnetic waves at compatible frequencies. These dielectric functions provide for different of types of analyses concerning the dielectric response of explosives. In particular, these dielectric response functions provide quantitative initial estimates of spectral response features for subsequent adjustment with respect to additional information such as laboratory measurements and other types of theory based calculations. With respect to qualitative analysis, these spectra provide for the molecular level interpretation of response structure. The DFT software NRLMOL was used for the calculations of ground state resonance structure presented here.

Analytical models of transmission probabilities for electron sources

Electron emission from coated surfaces as a result of thermal, field, and photoemission effects is often described theoretically using models dependent on the Kemble approximation for the transmission probability D(k). The validity of the approximation for the simple potential profiles (rectangular, triangular, and parabolic) is examined, and generalizations with respect to the exponential of the Gamow tunneling factor and the coefficients of D(k), which are generally ignored, are examined and extended to when the barriers become wells. As a result, unity transmission probabilities ( D(k)→1) with regard to both resonant tunneling barrier and reflectionless well behavior are contrasted. The adaptation of the findings to a general thermal-field-photoemission equation is considered. Consequences for the usage of general emission equations in beam optics code [e.g., Particle-in-Cell (PIC)] such as MICHELLE are discussed.

Construction of permittivity functions for high-explosives using density functional theory

We review a framework for the prediction of explosive molecular spectra, namely, for the common explosives found in improvised explosive devices, e.g., β-HMX. Through the use of excitation by incident electromagnetic waves in the THz frequency range, molecular signatures of these explosives may be detected, identified and perhaps neutralised remotely. A central component of this framework is an S-matrix representation of multilayered composite materials. The individual molecules are first simulated using first-principles density functional theory (DFT). An effective electric permittivity function is then constructed, which yields reflectivity and transmissivity functions of frequency and of angle of incident radiation. The input for this component would be a parameterised analytic-function representation of the electric permittivity as a function of frequency, which is provided by another component model of the framework. The permittivity function is constructed by fitting response spectra calculated usin...