Energy Density Of States Research Articles

Bounds on approximating Max kXOR with quantum and classical local algorithms

We consider the power of local algorithms for approximately solving Max kXOR, a generalization of two constraint satisfaction problems previously studied with classical and quantum algorithms (MaxCut and Max E3LIN2). In Max kXOR each constraint is the XOR of exactly k variables and a parity bit. On instances with either random signs (parities) or no overlapping clauses and D+1 clauses per variable, we calculate the expected satisfying fraction of the depth-1 QAOA from Farhi et al [arXiv:1411.4028] and compare with a generalization of the local threshold algorithm from Hirvonen et al [arXiv:1402.2543]. Notably, the quantum algorithm outperforms the threshold algorithm for k>4.On the other hand, we highlight potential difficulties for the QAOA to achieve computational quantum advantage on this problem. We first compute a tight upper bound on the maximum satisfying fraction of nearly all large random regular Max kXOR instances by numerically calculating the ground state energy density P(k) of a mean-field k-spin glass [arXiv:1606.02365]. The upper bound grows with k much faster than the performance of both one-local algorithms. We also identify a new obstruction result for low-depth quantum circuits (including the QAOA) when k=3, generalizing a result of Bravyi et al [arXiv:1910.08980] when k=2. We conjecture that a similar obstruction exists for all k.

Metastable ferroelectricity driven by depolarization fields in ultrathin Hf0.5Zr0.5O2

As ferroelectric Hf0.5Zr0.5O2 (HZO) thickness scales below 10 nm, the switching characteristics are severely distorted typically showing an antiferroelectric-like behavior (pinched hysteresis) with reduced remanent polarization. Using Landau-Ginsburg-Devonshire (LGD) theory for the analysis of the experimental results, it is shown here that, in thin (5 nm) HZO, depolarization fields drive the system in a stable paraelectric phase coexisting with a metastable ferroelectric one, which explains the pinched hysteresis. This state of matter resembles a first order ferroelectric above the Curie temperature which is known to result in similar double-loop behavior. Here, based on the analysis of experimental data in the framework of LGD theory, it is reported that charge injection and trapping at pre-existing interface defects during field cycling (“wake-up”) screens the depolarization field stabilizing ferroelectricity. It is found in particular that a sufficiently large energy density of interface states is beneficial for the recovery of fully open ferroelectric loops.

Deformation-contributed negative triboelectric property of polytetrafluoroethylene: A density functional theory calculation

Polytetrafluoroethylene (PTFE) is a promising negative triboelectric material used in high-performance triboelectric nanogenerators (TENGs). To improve the triboelectric output of PTFE, an understanding of the triboelectric properties and electron transfer factor in PTFE is highly required. However, the triboelectric properties of PTFE have not been analyzed by considering the side effects during TENG operations (e.g., deformation). Here, we investigated the change in the triboelectric properties of PTFE due to molecular structure deformation, which is driven by contact force, using density functional theory (DFT). The deformation of the molecular structure induces modification of the electronic structure and triboelectric properties. Using the linear and deformed PTFE models (80°, 70°, 60°), we determined that the energy of the lowest unoccupied molecular orbital (LUMO) is decreased under deformation using the energy band diagram and density of states (DOS) (linear: 5.831 eV, 80°: 5.358 eV, 70°: 4.028 eV, 60°: 1.729 eV). This implies that the deformation due to the contact force enhances its negative triboelectric property (i.e., electron-accepting property). We analyzed this phenomenon because carbon in the deformation region has a strongly electron-deficient state, and the positive local dipole due to that state is enhanced. In addition, we investigated the LUMO changes, in part, from anti-bonding orbital to a bonding orbital. Because the bonding orbital has a more stable energy state than the anti-bonding orbital, the energy level of the LUMO could be lowered.

Vibrational analysis of harmonic oscillator chains with random topological constraints

The effect of topological disorder on the dynamic behaviour of topologically constrained oscillator chains is investigated through a normal mode analysis. A measure for mode localization based on the skewness of the density of energy states of normal modes is proposed, which correlates well with main configuration characteristics. Appropriate configuration sampling is achieved by employing the Ising model together with the Metropolis algorithm.

Exploring the effect of hadron cascade time on particle production in Xe+Xe collisions at sNN=5.44 TeV using a multiphase transport model

Heavy-ion collisions at ultrarelativistic energies provide extreme conditions of energy density and temperature to produce a deconfined state of quarks and gluons. Xenon (Xe), being a deformed nucleus, further gives access to the effect of initial geometry on final state particle production. This study focuses on the effect of nuclear deformation and hadron cascade-time on the particle production and elliptic flow using a multiphase transport (AMPT) model in $\mathrm{Xe}+\mathrm{Xe}$ collisions at $\sqrt{{s}_{NN}}=5.44\phantom{\rule{0.16em}{0ex}}\mathrm{TeV}$. We explore the effect of hadronic cascade time on identified particle production through the study of ${p}_{\mathrm{T}}$-differential particle ratios. The effect of hadronic cascade time on the generation of elliptic flow is studied by varying the cascade time between 5 and $25\phantom{\rule{0.16em}{0ex}}\mathrm{fm}/c$. This study shows the final state interactions among particles generate additional anisotropic flow with increasing hadron cascade time, especially at very low and high ${p}_{\mathrm{T}}$.

Ancient Mesopotamian Stone Bridge: Numerical Modeling and Structural Assessment

This study aimed to investigate the stress-strain and strain energy density (SED) states of Dalal stone arch bridge in Mesopotamia. Structural modeling of ancient bridge made of natural stone has been proven reliable, and accurate results have been obtained using 3D finite elements. Based on the more applicable theories of failure, a general methodology is presented for evaluating the ringstone of the largest ellipse-shaped arch of the Dalal Bridge. The elliptical arch was built in the COMSOL Multiphysics complex using 70 3D elements to represent the number of stones used along the length of the arch in the Dalal Bridge. Therefore, to create an accurate model, the coordinates of the four nodes of each stone were entered. Then, all domains were extruded for 0.8 m in the y-axis direction, i.e., 0.8 m of the bridge width was selected for investigation. That is, tapered fields were used to represent the stones of the arch ring. Using Rankine’s, St. Venant’s, and Haigh’s theories, the qualitative and quantitative characteristics of all components of the stresses and SED states are investigated. The maximum positive values of the principal stresses, σ1, σ2, and σ3, in the 3D model reach 1.4, 0.51, and 0.09 MPa, respectively, and their maximum negative values were 13, 6.8, and 3.4 MPa, respectively. The equivalent principal stresses determined via a 2D investigation did not exceed these values. Evaluating the ringstone against the maximum principal strain theory (i.e., St. Venant’s theory) reveals a safety factor of four in the existing state. Also, application of Haigh’s theory confirms the results of the previously applied approaches. Even though the safety of the arch, according to the total strain energy theory (i.e., Haigh’s approach), has been verified, a significant variation in the nonuniformity of the distribution of the SED (0.0011 J/m3–4416 J/m3) confirmed that the geometry of the investigated arch is not optimal for applied loading. The maximum value of the vertical component of the displacement is 3.4 mm, significantly lower than the allowable deflection for such an arch span.

Investigations on electronic properties and optical properties of Zn9-xAgxO/G based on first-principles

In order to enhance the interaction between ZnO monolayer （ZnO-ML） and graphene monolayer (G), the photoelectric properties of G, ZnO-ML, Ag-doped ZnO-ML (Zn9-xAgxO) (X = 1, 2, 3, 4), and Zn9-xAgxO composite G (Zn9-xAgxO/G) were investigated based on density functional theory (DFT). Then, Band structure, partial density of state (PDOS), differential charge density, and absorption spectrum were analyzed. Compared with ZnO-ML/G where no charge transition exists between G and ZnO-ML, the graphene layer has varying degrees of charge depletion and charges mainly accumulate around the oxygen atom for Zn9-xAgxO/G with the addition of Ag atom. The Dirac points characteristics of graphene are still retained and the Dirac points are all above the impurity band and the Fermi level enters the valence band which leads to the form of p-type doping for Zn9-xAgxO/G. PDOS shows that Ag doping causes the redistribution of energy state density of each atom, promoting the coupling of C-p and O-p orbitals. Meanwhile, Ag-d also has a strong coupling with C-p. In addition, for ZnO-ML/G, the absorption peak intensity at 8 eV corresponding ZnO and 14.3 eV corresponding graphene decreases significantly. Moreover, for Zn9-xAgxO/G, the absorption strength is much greater than that of G, ZnO-ML and Zn9-xAgxO, entirely. The results can provide more theoretical support for the photoelectric properties for graphene-reinforced ZnO matrix composite.

The Structural, Lattice Dynamic, Thermodynamic, and Mechanical Characterization of Na2Mo2O7 Crystal by First‐Principles

The structural, electronic, lattice dynamic, thermodynamic, and mechanical properties of disodium dimolybdate (Na2Mo2O7) are studied using first‐principles. First, for the crystal structure, the calculated structural parameters have a standard deviation of less than 1.7% compared with the experimental results. Then, the calculated electronic energy band and density of states reveal that Na2Mo2O7 is an insulator and its bandgap is about 2.87 eV. In addition, the linear response method is used to study the lattice dynamics, and the complete phonon dispersion curve, infrared spectra, Raman shift, Born effective charge, and longitudinal optical–transverse optical (LO–TO) splitting characteristic for Na2Mo2O7 are presented for the first time; its thermodynamic properties are worked out based on the phonon density of states (PDOS). Furthermore, the result of elastic anisotropy shows the title compound is a highly anisotropic material. The calculation results in this article are conducive to the development of Na2Mo2O7 and provide guidelines for further experimental research to a certain extent.

Studying the Richtmyer–Meshkov instability in convergent geometry under high energy density conditions using the Decel platform

The “Decel” platform at Sandia National Laboratories investigates the Richtmyer–Meshkov instability (RMI) in converging geometry under high energy density conditions [Knapp et al., Phys. Plasmas 27, 092707 (2020)]. In Decel, the Z machine magnetically implodes a cylindrical beryllium liner filled with liquid deuterium, launching a converging shock toward an on-axis beryllium rod machined with sinusoidal perturbations. The passage of the shock deposits vorticity along the Be/D2 interface, causing the perturbations to grow. In this paper, we present platform improvements along with recent experimental results. To improve the stability of the imploding liner to the magneto Rayleigh–Taylor instability, we modified its acceleration history by shortening the Z electrical current pulse. Next, we introduce a “split rod” configuration that allows two axial modes to be fielded simultaneously in different axial locations along the rod, doubling our data per experiment. We then demonstrate that asymmetric slots in the return current structure modify the magnetic drive pressure on the surface of the liner, advancing the evolution on one side of the rod by multiple ns compared to its 180° counterpart. This effectively enables two snapshots of the instability at different stages of evolution per radiograph with small deviations of the cross-sectional profile of the rod from the circular. Using this platform, we acquired RMI data at 272 and 157 μm wavelengths during the single shock stage. Finally, we demonstrate the utility of these data for benchmarking simulations by comparing calculations using ALEGRA MHD and RageRunner.

Super-strong magnetic field-dominated ion beam dynamics in focusing plasma devices

High energy density physics is the field of physics dedicated to the study of matter and plasmas in extreme conditions of temperature, densities and pressures. It encompasses multiple disciplines such as material science, planetary science, laboratory and astrophysical plasma science. For the latter, high energy density states can be accompanied by extreme radiation environments and super-strong magnetic fields. The creation of high energy density states in the laboratory consists in concentrating/depositing large amounts of energy in a reduced mass, typically solid material sample or dense plasma, over a time shorter than the typical timescales of heat conduction and hydrodynamic expansion. Laser-generated, high current–density ion beams constitute an important tool for the creation of high energy density states in the laboratory. Focusing plasma devices, such as cone-targets are necessary in order to focus and direct these intense beams towards the heating sample or dense plasma, while protecting the proton generation foil from the harsh environments typical of an integrated high-power laser experiment. A full understanding of the ion beam dynamics in focusing devices is therefore necessary in order to properly design and interpret the numerous experiments in the field. In this work, we report a detailed investigation of large-scale, kilojoule-class laser-generated ion beam dynamics in focusing devices and we demonstrate that high-brilliance ion beams compress magnetic fields to amplitudes exceeding tens of kilo-Tesla, which in turn play a dominant role in the focusing process, resulting either in a worsening or enhancement of focusing capabilities depending on the target geometry.

First-principles insights into the mechanical, optoelectronic, thermophysical, and lattice dynamical properties of binary topological semimetal BaGa2

In the present study we have investigated the structural properties, electronic band dispersion, elastic constants, acoustic behavior, phonon spectrum, optical properties, and a number of thermophysical parameters of binary topological semimetal BaGa2 in details via first-principles calculations using the density functional theory (DFT) based formalisms. The electronic band structure and density of states calculations with spin–orbit coupling reveal semimetallic nature with clear topological signatures. The minimum thermal conductivities and anisotropies of the compound are calculated. The elastic constants and phonon dispersion calculations show that the compound under study is both mechanically and dynamically stable. Comprehensive study of elastic parameters shows that BaGa2 possesses fairly isotropic mechanical properties, reasonably good machinability, low Debye temperature and melting point. The chemical bonding in BaGa2 is interpreted via the electronic energy density of states, electron density distribution, elastic properties and Mulliken bond population analysis. The compound possesses both ionic and covalent bondings. The reflectivity spectra show strong anisotropy with respect to polarization of the incident electric field in the visible to mid-ultraviolet regions. High reflectivity over wide spectral range makes BaGa2 suitable as a reflecting material. BaGa2 is also an efficient absorber of ultraviolet radiation. Furthermore, the refractive index is quite high in the infrared to visible range. All the energy dependent optical parameters show metallic features and are in complete accord with the underlying bulk electronic density of states calculations. Most of the results presented in this study are novel and should serve as useful reference for future study.

Melt pool feature analysis using a high-speed coaxial monitoring system for laser powder bed fusion of Ti-6Al-4 V grade 23

In laser powder bed fusion (LPBF), defects such as pores or cracks can seriously affect the final part quality and lifetime. Keyhole porosity, being one type of porosity defects in LPBF, results from excessive energy density which may be due to changes in process parameters (namely, laser power and scan speed) and/or result from the part’s geometry and/or hatching strategies. To study the possible occurrence of keyhole pores, experimental work and simulations were carried out for optimum and high volumetric energy density conditions in Ti-6Al-4 V grade 23. By decreasing the scanning speed from 1000 to 500 mm/s for a fixed laser power of 170 W, keyhole porosities are formed and later observed by X-ray computed tomography. Melt pool images are recorded in real-time during the LPBF process by using a high-speed coaxial Near-Infrared (NIR) camera monitoring system. The recorded images are then pre-processed using a set of image processing steps to generate binary images. From the binary images, geometrical features of the melt pool and features that characterize the spatter particle formation and ejection from the melt pool are calculated. The experimental data clearly show spatter patterns in case of keyhole porosity formation at low scan speed. A correlation between number of pores and amount of spatter is observed. Besides the experimental work, a previously developed high fidelity finite volume numerical model was used to simulate the melt pool dynamics with similar process parameters as used during the experiments. Simulation results illustrate and confirm the keyhole porosity formation by decreasing laser scan speed.

Possible applications of Mo2C in the orthorhombic and hexagonal phases explored via ab-initio investigations of elastic, bonding, optoelectronic and thermophysical properties

Binary carbides demonstrate an attractive set of physical properties that are suitable for numerous and diverse applications. In the present study, we have explored the structural properties, electronic structures, elastic constants, acoustic behaviors, phonon dispersions, optical properties, and various thermophysical properties of binary orthorhombic and hexagonal Mo2C (O-MC and H-MC, respectively) compounds in detail via first-principles calculations using the density functional theory (DFT). The calculated ground state lattice parameters in both the symmetries are in excellent agreement with available experimental results. The calculated electronic band structure, density of states, and optical properties of Mo2C in both structures reveal metallic features. The orthorhombic crystal shows a higher level of mechanical and thermal anisotropy compared to that in the hexagonal phase. The elastic constants and phonon dispersion calculations show that, in both structures, Mo2C is mechanically and dynamically stable. A comprehensive mechanical and thermophysical study shows that both phases possess high structural stability, reasonably good machinability, ductile nature, high hardness, low compressibility, high Debye temperature and high melting temperature. Moreover, the electronic energy density of states, electron density distribution, elastic properties, and Mulliken bond population analyses indicate that the structures under consideration consist of mixed bonding characteristics with ionic and covalent contributions. Investigation of the optical properties reveals that the reflectivity spectra are anisotropic with respect to the polarization directions of the electric field in the visible to mid‑ultraviolet regions. High reflectivity over a wide spectral range makes the compound suitable as reflecting coating. Both the structures are efficient absorbers of ultraviolet radiation. The refractive indices are quite high in the infrared to the visible range. Both structures show directional optical anisotropy. Though, H-MC exhibits stronger optical anisotropy than the O-MC.

The dependence of the energy density states on the substitution of chemical elements in the Se6Te4-xSbx thin film

The energy density state are the powerful factor for evaluate the validity of a material in any application. This research focused on examining the electrical properties of the Se6Te4- xSbx glass semiconductor with x=1, 2 and 3, using the thermal evaporation technique. D.C electrical conductivity was used by determine the current, voltage and temperatures, where the electrical conductivity was studied as a function of temperature and the mechanical electrical conduction were determined in the different conduction regions (the extended and localized area and at the Fermi level). In addition, the density of the energy states in these regions is calculated using the mathematical equations. The constants of energy density states are determined, namely the electron hopping distance, the width of the tails, and pre - exponential factor. The densities of the energetic states (extended N (Eext), localize N (Eloc) and at the Fermi states N (Ef) will be calculated in each of the regions. Moreover, the effect of partial substitution of Se with antimony on energy states and degree of randomness, results observed that the energy densities changing with an increase antimony Sb concentration.

Theoretical study on adsorption of SF6 decomposition gas in GIS gas cell based on intrinsic and Ni-doped MoTe2 monolayer

It is significant to probe SF6 decomposition gas product to determine the type of faults in Gas-insulated substations (GIS). In the text, the adsorption features of intrinsic and Ni doped MoTe2 (Ni-MoTe2) on SF6 decomposition gas products H2S, SO2, SOF2 and SO2F2 in GIS gas cell were investigated according to density functional theory (DFT). The adsorption parameters, energy band structure (BS) and density of states (DOS) of the adsorption structures were obtained as well as analyzed. The findings show that the Ni-MoTe2 monolayer has better adsorption performance than intrinsic MoTe2 for SF6 decomposition gases (H2S, SO2, SOF2 and SO2F2). The larger adsorption energy and more charge transfer amount were noticed by the comparative analysis, demonstrating that the chemisorption between these four gas molecules and Ni-MoTe2 produced strong interactions that could enhance the adsorption performance of gas detection. As a result, Ni-MoTe2 monolayer gas sensor may be recommended for SF6 decomposition gas detection in GIS gas cell.

Jet Transport Coefficient at the Large Hadron Collider Energies in a Color String Percolation Approach

Within the color string percolation model (CSPM), jet transport coefficient, q^, is calculated for various multiplicity classes in proton-proton and centrality classes in nucleus-nucleus collisions at the Large Hadron Collider energies for a better understanding of the matter formed in ultra-relativistic collisions. q^ is studied as a function of final state charged particle multiplicity (pseudorapidity density at midrapidity), initial state percolation temperature and energy density. The CSPM results are then compared with different theoretical calculations from the JET Collaboration those incorporate particle energy loss in the medium.

Validation of hybrid WC1LYP functional for ferroelectric LiNbO3 and LiTaO3 using Compton spectroscopy and first-principles computations

Electronic structure and theoretical momentum densities (EMDs) of LiNbO3 and LiTaO3 using linear combination of atomic orbitals (LCAO) scheme are deduced. Present energy bands and density of states using local density approximation, generalised gradient approximation and hybrid (HF + DFT) PBE0 and WC1LYP schemes show a direct band gap (varying between 3.13- 5.13 eV and 3.11–5.18 eV for LiNbO3 and LiTaO3, respectively) at Γ point of Brillouin zones. The WC1LYP hybrid functional based EMDs depict reasonable agreement with present experimental Compton lines (using 100 mCi Am-241 γ-ray Compton spectrometer) than other investigated exchange-correlation potentials, thereby suggesting the applicability of hybrid model like WC1LYP in such ferroelectrics. Accuracy of energy bands is also ensured via electronic and optical properties using modified Becke–Johnson potential as facilitated in the full-potential linearized augmented plane wave method. Presently reported UV–vis measurements for optical band gaps are in consonance with the hybrid potential (WC1LYP) for the both ferroelectrics, which further confirms usefulness of hybrid scheme. Moreover, equal–valence–electron–density scaled experimental data reveal more covalent character of LiNbO3 than that in LiTaO3, which is in agreement with the present LCAO based Mulliken's population analysis.

A density functional study of the structural, electronic, optical and lattice dynamical properties of AgGaS2

Based on first-principles density functional theory the electronic structure, optical and lattice dynamical properties of infrared nonlinear optical crystal AgGaS2 (AGS) are calculated with four different hybrid functional (PBE0, HSE03, HSE06, B3LYP) respectively. The results show that the theoretical results obtained by PBE0 method are in good agreement with the experimental values. The geometrical structure optimization results show that the theoretical lattice parameters of AGS are in good agreement with the experimental values. The electronic structure results show that AGS is a direct wide band gap nonlinear optical crystal, and the band gap obtained by PBE0 method is 2.66 eV, which is basically in good agreement with the experimental values. The calculation results of energy band structure and density of states show that the value band edge is mainly contributed by S-3p, Ag-4d and a small amount of Ga-4p orbital electrons, while the bottom of conduction band is mainly contributed by Ag-5 s, Ga-4 s, 4p and a small amount of S-3p orbital electrons. The orbital coupling between Ga and S atoms determines the optical properties of AGS. Ag atoms contribute little to the optical properties. Especially, the results show that the crystal material has strong absorption and reflection characteristics in the ultraviolet region and strong transmittance in the infrared region. The average static dielectric constant and average static refractive index are 1.887 and 2.14, respectively, and the static birefringence is 0.02.Based on the density functional theory of linear response, the phonon dispersion curve of AGS is calculated. The results show that AGS has no virtual frequency and its dynamic performance is relatively stable. Meanwhile, the optical mode frequency of the Γpoint in the center of Brillouin region is analyzed in detail. The above results show that AGS crystal material enjoys large nonlinear optical coefficient and suitable birefringence in a wide infrared band, and AGS is an important infrared nonlinear optical crystal material.

Cosmological Constraints on Nonflat Exponential f(R) Gravity

Abstract We explore the viable f(R) gravity models in FLRW backgrounds with a free spatial curvature parameter Ω K . In our numerical calculation, we concentrate on the exponential f(R) model of f ( R ) = R − λ R ch ( 1 − exp ( − R / R ch ) ) , where R ch is the characteristic curvature scale, which is independent of Ω K , and λ corresponds to the model parameter, while R ch λ = 2Λ with Λ the cosmological constant. In particular, we study the evolutions of the dark energy density and equation of state for exponential f(R) gravity in open, flat, and closed universes, and compare with those for ΛCDM. From the current observational data, we find that λ − 1 = 0.42927 − 0.32927 + 0.39921 at 68% C.L. and Ω K = − 0.00050 − 0.00414 + 0.00420 at 95% C.L. in the exponential f(R) model. By using the Akaike Information Criterion, Bayesian Information Criterion, and Deviance Information Criterion, we conclude that there is no strong preference between the exponential f(R) gravity and ΛCDM models in the nonflat universe.

Ab Initio Calculations of New α-L<sub>5-7a</sub> and β-L<sub>5-7a</sub> Graphyne Polymorphic Varieties

Calculations of the structural and energy parameters, band structure and density of electronic states of new structural varieties of graphyne have been performed by the density functional theory method. The initial structure of the nine polymorphs was theoretically constructed on the basis of the 5-7a graphene layer. As a result of the calculations, the structure of only five graphyne layers was found to be stable: α-L5-7a, β1-L5-7a, β2-L5-7a, β3-L5-7a and β4-L5-7a. The structure of layers γ1-L5-7a, γ2-L5-7a, and γ3-L5-7a is transformed into the structure of graphene layers by geometric optimization, and the graphyne layer γ4-L5-7a is transformed sp+sp2 layer L3-6-13. The sublimation energy of the stable graphyne polymorphs varies from 6.66 to 6.78 eV/atom. The density of electronic states at the Fermi energy level for all α-L5-7a and β-L5-7a layers of graphyne is different from zero, so the new graphyne polymorphs should have metallic properties.