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Tantalum diselenide (TaSe2) is a metallic transition metal dichalcogenide whose structure and vibrational behavior strongly depend on temperature and thickness, and this behavior includes the emergence of charge density wave (CDW) states at very low temperatures. In this work, observed Raman modes for mono- and bilayer are described across several spectral regions and compared to those seen in the bulk case. These modes, which include an experimentally observed forbidden Raman mode and low-frequency CDWs, are then matched to corresponding vibrations predicted by density functional theory (DFT). The reported match between experimental and computational results supports the presented vibrational visualizations of these modes. Support is also provided by experimental phonons observed in additional Raman spectra as a function of temperature and thickness. These results highlight the importance of understanding CDWs since they are likely to play a fundamental role in the future realization of solid-state quantum information platforms based on nonequilibrium phenomena.

Tantalum diselenide (TaSe(2)) is a metallic transition metal dichalcogenide whose structure and vibrational behavior strongly depend on temperature and thickness, and this behavior includes the emergence of charge density wave (CDW) states at very low temperatures. In this work, observed Raman modes for mono- and bilayer are described across several spectral regions and compared to those seen in the bulk case. These modes, which include an experimentally observed forbidden Raman mode and low-frequency CDWs, are then matched to corresponding vibrations predicted by density functional theory (DFT). The reported match between experimental and computational results supports the presented vibrational visualizations of these modes. Support is also provided by experimental phonons observed in additional Raman spectra as a function of temperature and thickness. These results highlight the importance of understanding CDWs since they are likely to play a fundamental role in the future realization of solid-state quantum information platforms based on nonequilibrium phenomena.

$\mathrm{Mn}{\mathrm{Bi}}_{2}{\mathrm{Te}}_{4}$ has recently been the subject of intensive study, due to the prediction of axion insulator, Weyl semimetal, and quantum anomalous Hall insulator phases, depending on the structure and magnetic ordering. Experimental results have confirmed some aspects of this picture, but several experiments have seen zero-gap surfaces states at low temperature, in conflict with expectations. In this work, we develop a first-principles-based tight-binding model that allows for arbitrary control of the local spin direction and spin-orbit coupling, enabling us to accurately treat large unit-cells. Using this model, we examine the behavior of the topological surface state as a function of temperature, finding a gap closure only above the N\'eel temperature. In addition, we examine the effect of magnetic domains on the electronic structure, and we find that the domain wall zero-gap states extend over many unit-cells. These domain wall states can appear similar to the high-temperature topological surface state when many domain sizes are averaged, potentially reconciling theoretical results with experiments.

First-principles calculations are used to probe the effects of mechanical strain on the magnetic and optical properties of monolayer (ML) $2\mathrm{H}\text{\ensuremath{-}}\mathrm{TaS}{\mathrm{e}}_{2}$. A complex dependence of these physical properties on strain results in unexpected spin behavior, such as ferromagnetism under uniaxial, in-plane, tensile strain and a lifting of the Raman-active ${E}^{\ensuremath{'}}$ phonon degeneracy. We predict the noncollinear magnetic phase below 6% strain and a ferromagnet above 6% strain for some types of strain. While ferromagnetism is observed under compression along $\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{x}$ and expansion along $\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{y}$, no magnetic order occurs when interchanging the strain direction. The calculations show that the magnetic behavior of the system depends on the exchange within the $5d$ orbitals of the Ta atoms. The magnetic moment per Ta atom persists even when an additional compressive strain along the $\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{z}$ axis is added to a biaxially strained ML, which suggests stability of the magnetic order. Exploring the effects of this mechanical strain on the Raman-active phonon modes, we find that the ${E}^{\ensuremath{''}}$ and ${E}^{\ensuremath{'}}$ modes are red shifted due to Ta-Se bond elongation, and that strain lifts the ${E}^{\ensuremath{'}}$ mode degeneracy, except for the symmetrical biaxial tensile case. The electron-phonon interactions depend on both the amount of applied strain and the direction. Additionally, we note that the charge density wave (CDW) phase weakens the magnetism due to symmetry breaking and atomic displacements (Ta atoms) depending on the type and amount of applied strain. Phonon dispersion calculations confirm that using mechanical strain we can manipulate the unique CDW phase of ML-$2\mathrm{H}\text{\ensuremath{-}}\mathrm{TaS}{\mathrm{e}}_{2}$. Our results support the possibility of tuning the properties of two-dimensional transition metal dichalcogenide materials for nanoelectronic applications through strain.

$2H\text{\ensuremath{-}}\mathrm{Ta}{\mathrm{S}}_{2}$ undergoes a charge density wave (CDW) transition at ${T}_{\mathrm{CDW}}\ensuremath{\sim}75$ K, however key questions regarding the onset of CDW order remain under debate. In this study, we explore the CDW transition through a combination of temperature and excitation-dependent Raman spectroscopy, angle resolved photoemission spectroscopy (ARPES), and density functional theory (DFT). Below ${T}_{\mathrm{CDW}}$ we identify two CDW amplitude modes that redshift and broaden with increasing temperature and one zone-folded mode that disappears above ${T}_{\mathrm{CDW}}$. Above ${T}_{\mathrm{CDW}}$, we observe a strong two-phonon mode that softens substantially upon cooling, which suggests the presence of substantial lattice distortions at temperatures as high as 250 K. This correlates with the ARPES observation of the persistence of a CDW energy gap above ${T}_{\mathrm{CDW}}$ and finite-temperature DFT calculations of the phonon band structure that indicate an instability occurring well above the CDW transition temperature. DFT also provides the atomic displacements of the CDW amplitude modes and reproduces their temperature dependence. From these observations we suggest that short range CDW order exists well above ${T}_{\mathrm{CDW}}$, which poses new questions regarding the interplay between electronic structure and vibrational modes in layered CDW materials.

Three-dimensional materials with strong spin–orbit coupling and magnetic interactions represent an opportunity to realize a variety of rare and potentially useful topological phases with broken time-reversal symmetry. In this work, we use first principles calculations to show that the recently synthesized material Bi2MnSe4 displays a combination of spin–orbit-induced band inversion, also observed in non-magnetic topological insulator Bi2PbSe4, with magnetic interactions, leading to several topological phases. In bulk form, the ferromagnetic phase of Bi2MnSe4 has symmetry protected band crossings at the Fermi level, leading to either a nodal line or Weyl semimetal, depending on the direction of the spins. Due to the combination of time reversal symmetry plus a partial translation, the ground state layered antiferromagnetic phase is instead an antiferromagnetic topological insulator. The surface of this phase intrinsically breaks time-reversal symmetry, allowing the observation of the half-integer quantum anomalous Hall effect. Furthermore, we show that in thin film form, for sufficiently thick slabs, Bi2MnSe4 becomes a Chern insulator with a band gap of up to 58 meV. This combination of properties in a stoichiometric magnetic material makes Bi2MnSe4 an excellent candidate for displaying robust topological behavior.

Spintronics is an exciting field which exploits the magnetic effect of electrons in motion for transferring information across devices. In this paper, we report transport properties of the LaSrMn-CNT-LaSrMn magnetic tunnel junction (MTJ) using Density Function Theory (DFT) in combination with Non-Equilibrium Green’s Function (NEGF) formalism. The lanthanum strontium manganite (LaSrMn) material is chosen as the electrode, keeping in mind its high spin polarization and enhanced spin filtration properties. The non-magnetic CNT channel in the device is homogeneously doped with boron (B) and nitrogen (N) atoms using the Atomistix ToolKit (ATK) software and its graphical interface, virtual nanolab. The magnetic tunnel junctions (MTJs) were analyzed in terms of their I–V characteristics, transmission spectra, tunnel magnetoresistance (TMR) and spin injection factor attributes. Among the simulated devices studied, the boron doped MTJ showed TMR of 100%. Furthermore, we found that the I–V characteristics, TMR and spin injection factor, of the modeled devices depend on the type of the dopant used. In addition, we found that the nitrogen doping transformed the device to ‘always on’ device. The findings of this study are important from an applied point of view and will provide a new dimension to spintronic device applications.

Metallic transition metal dichalcogenides, such as tantalum diselenide $(\mathrm{TaS}{\mathrm{e}}_{2})$, display quantum correlated phenomena of superconductivity and charge density waves (CDWs) at low temperatures. Here, the photophysics of 2H-$\mathrm{TaS}{\mathrm{e}}_{2}$ during CDW transitions is revealed by combining temperature-dependent, low-frequency Raman spectroscopy and density functional theory (DFT). The spectra contain amplitude, phase, and zone-folded modes that are assigned to specific phonons and lattice restructuring predicted by DFT calculations with superb agreement. The noninvasive and efficient optical methodology detailed here demonstrates an essential link between atomic-scale and microscopic quantum phenomena.

We perform high-throughput density functional theory (DFT) calculations for optoelectronic properties (electronic bandgap and frequency dependent dielectric function) using the OptB88vdW functional (OPT) and the Tran-Blaha modified Becke Johnson potential (MBJ). This data is distributed publicly through JARVIS-DFT database. We used this data to evaluate the differences between these two formalisms and quantify their accuracy, comparing to experimental data whenever applicable. At present, we have 17,805 OPT and 7,358 MBJ bandgaps and dielectric functions. MBJ is found to predict better bandgaps and dielectric functions than OPT, so it can be used to improve the well-known bandgap problem of DFT in a relatively inexpensive way. The peak positions in dielectric functions obtained with OPT and MBJ are in comparable agreement with experiments. The data is available on our websites http://www.ctcms.nist.gov/~knc6/JVASP.html and https://jarvis.nist.gov.

Monolayer epitaxial graphene (EG) is a suitable candidate for a variety of electronic applications. One advantage of EG growth on the Si face of SiC is that it develops as a single crystal, as does the layer below, referred to as the interfacial buffer layer (IBL), whose properties include an electronic band gap. Though much research has been conducted to learn about the electrical properties of the IBL, not nearly as much work has been reported on the optical properties of the IBL. In this work, we combine measurements from Mueller matrix ellipsometry, differential reflectance contrast, atomic force microscopy, and Raman spectroscopy, as well as calculations from Kramers-Kronig analyses and density functional theory (DFT), to determine the dielectric function of the IBL within the energy range of 1 eV to 8.5 eV.

Data Security is very much important in every aspect of data collection as well as data storage. Hardware device like USB is very much useful worldwide in order to store real time confidential data. So it is necessary to provide hardware level security to every small storage device like pen drive, external hard disk etc. In this context cryptography is one of the most promising technique to encrypt the actual data in other form. In this paper, we propose a robust encryption technique in hardware level which will keep the data safe from external theft. Our proposed algorithm is associated with hardware storage like pen drive and external hard disk.

In this letter, we report the visualization of topologically protected spin textures, in the form of magnetic skyrmions, in recently discovered monoatomic-thin two-dimensional CrI$_3$. By combining density functional theory and atomistic spin dynamic simulation, we demonstrate that an application of out-of-plane electric field to CrI$_3$ lattice favors the formation of sub-10 nm skyrmions at 0 K temperature. The spin texture arises due to a strong correlation between magneto-crystalline anisotropy, Dzyaloshinskii-Moriya interaction and the vertical electric field, whose shape and size could be tuned with the magnetic field. Such finding will open new avenues for atomic-scale quantum engineering and precision sensing., Comment: 18 pages; 4 figures

First-principles calculations are used to investigate the electronic properties of negatively charged n-pyrrole oligomers with n ¼ 2-18. Chains of neutral oligomers are bent, whereas the negatively charged oligomers become almost planar due to accumulation of negative charge at the end monomers. Isomers of short oligomers (n < 6) display negative electron affinity although the corresponding anions are energetically stable. For longer oligomers with n � 6, the electron affinity is small and positive, slowly increasing with oligopyrrole length. Doping of 12-pyrrole with lithium atoms shows that negative oxidation states are possible due to electron transfer from dopant to oligomer at locations close to dopant. These 12-pyrrole regions support extra negative charge and exhibit a local structural change from benzenoid to quinoid structure in the CAC backbone conjugation. Comparison between neutral and doped polypyrrole (PPy) indicates that doped polymers displays a substantial depletion of the band gap energy and the appearance of dopant-based bands in the gap for a 50% per monomer doping level. It is predicted that Li-doped PPy is not metallic. V C 2010 Wiley Periodicals, Inc. Int J Quantum Chem 111: 2295-2305, 2011

Density Functional Theory (DFT) has been applied to a comprehensive mechanistic study of the conversion reaction of the Pd(II)-hydride complex, (IMe)2(RCO2)PdH (R=CH3, Ph, and p-O 2NC6H4), to the corresponding Pd(II)-hydroperoxide in the presence of molecular oxygen. The calculations have evaluated the two mechanistic proposed alternatives, that are both considered viable on the basis of current data, of slow RCO2H reductive elimination followed by oxygenation (Path A) and direct O2 insertion (Path B). Results suggest that the mechanism of direct insertion of molecular oxygen into the Pd-H bond of the initial complex is energetically preferred. The activation energy relative to the rate-determining step of Path A, indeed, is calculated to be lower than the activation energy of the rate determining step of the alternative Path B, whatever ligand (CH3CO2, Ph, CO2, p-O2NC6H4CO2) is coordinated to the Pd center. The calculated free activation energy of the rate-determining hydrogen abstraction step (ΔG* = 24.8 kcal/mol) in the case of the oxygenation reaction of the benzoate-ligated Pd(II)-hydride complex is in very good agreement with the experimentally determined value of 24.4 kcal/mol. In addition, according to the experimentally detected enhancement of the reaction rate due to the presence of a nitro group on the benzoate ligand, our calculations show that the transition state for the hydrogen atom abstraction by molecular oxygen along the pathway for the oxygenation reaction of (IMe)2(p-O2NC6H4CO 2)PdH lies lower in energy with respect to the analogous transition state calculated for R=Ph. © 2008 American Chemical Society.

Significant recent developments in homogeneous palladium catalysis for selective aerobic oxidation have emphasized the importance of developing a thorough comprehension of reactions of palladium complexes with O2. Density functional theory (DFT) calculations, employing the B3LYP exchange-correlation functional, have been performed to probe the mechanism for the insertion reaction of molecular oxygen into the palladium(II) hydride bond of the trans-[PdH(O2Ac)(IMes)2] complex to form the corresponding Pd(II)-hydroperoxide. A crossing between triplet and singlet surfaces occurs before the formation of a stable singlet intermediate from which the reaction straightforwardly proceeds to yield the singlet hydroperoxo complex. Results appear to be different with respect to previous analogous theoretical investigations providing a new minimum energy pathway. © 2008 Elsevier B.V. All rights reserved.

Density functional theory (DFT) in its hybrid B3LYP formulation was applied to the investigation of the mechanistic details of the insertion of molecular oxygen into the palladium(II) hydride bond of the (PCP)Pd-H complex (PCP = 1,3-(CH2PtBu2)2C6H3-) to form the corresponding Pd(II) hydroperoxide. The calculations reveal that minima and transition state through which formation of the final product takes place are different along the singlet and triplet pathways. A triplet-singlet crossing occurs before the formation of a stable singlet intermediate from which the reaction straightforwardly proceeds to yield the singlet hydroperoxo complex. Differences and improvements with respect to analogous theoretical investigations are highlighted. © 2007 Elsevier B.V. All rights reserved.

In bulk form, TaSe2 exhibits transitions between commensurate and incommensurate charge-density wave (CDW) phases, and is attracting interest for advanced device applications. In order to explore the evolution of the groundstate CDW phase with layer number, mechanical exfoliation of bulk crystals provides few- to single-layer flakes. In the present work, we extend our opto-thermal Raman measurements1 on MoS2 to include other TMDs, specifically TaSe2 in both 1T and 2H crystallographic phases. A novel, magneto-Raman microscope system affords measurement of low-frequency (down to 10 cm-1) vibrational modes as a function of: temperature (100 K to 400 K), magnetic field (0 T to 9T) and laser excitation. The dependence of the observed Raman-active phonons on temperature and magnetic field will be discussed and compared with earlier results on MoS2. Specifically, we observe the appearance of low-frequency, zone-folded modes in the CDW state, which soften with temperature similar to the higher frequency, in-plane E2g mode. Additionally, we compare the measured magneto-Raman results to calculations using ab initio, density functional theory. 1ACS Nano 2014 8, 986

In this paper a typical microgrid is modeled and simulated based on several logical assumptions and the same is studied for steady and transient state analysis in grid connected and islanded mode with critical and non-critical load selectivity in the perspective of integration of distributed energy resources. The authors present the simulation of microgrid by Powergui interfacing of Simulink using Matlab software in such a manner that a study of waveforms of voltage, current and power in selective points in the microgrid system is possible. Since microgrid is an interconnected system of distributed energy resources, utility grid and loads; there are immense chances of fluctuation of any one of those components and the disturbances are to be analyzed through simulation in planning stage for successful on-site operation in future. Among many factors and type of disturbances and response, as a typical case, the present microgrid model is studied for step change in grid voltage and the corresponding response in the line currents are presented with their waveforms showing peak responses, settling time, rise time etc. using LTI viewer toolbox of Simulink, Matlab. The results are discussed and analyzed for microgrid modeling in the perspective of integration of DERs.

The topological insulating phase results from inversion of the band gap due to spin-orbit coupling at an odd number of time-reversal symmetric points. In Bi${}_{2}$Se${}_{3}$, this inversion occurs at the $\ensuremath{\Gamma}$ point. For bulk Bi${}_{2}$Se${}_{3}$, we have analyzed the effect of arbitrary strain on the $\ensuremath{\Gamma}$ point band gap using density functional theory. By computing the band structure both with and without spin-orbit interactions, we consider the effects of strain on the gap via Coulombic interaction and spin-orbit interaction separately. While compressive strain acts to decrease the Coulombic gap, it also increases the strength of the spin-orbit interaction, increasing the inverted gap. Comparison with Bi${}_{2}$Te${}_{3}$ supports the conclusion that effects on both Coulombic and spin-orbit interactions are critical to understanding the behavior of topological insulators under strain, and we propose that the topological insulating phase can be effectively manipulated by inducing strain through chemical substitution.

The mechanistic details of the hydrogenation of molecular oxygen by the 18e amino-hydride Cp*IrH(TsDPEN) (1H(H)) complex to give Cp*Ir(TsDPEN-H) (1) and 1 equiv of H(2)O were investigated by means of hybrid density functional calculations (B3LYP). To comprehensively describe the overall catalytic cycle of the hydrogenation of dioxygen using H(2) catalyzed by the Ir complex 1, the potential energy surfaces for the hydrogenation process of both the catalyst 1 and the corresponding unsaturated iridium(III) amine cation ([1H](+)) were explored at the same level of theory. The results of our computations, in agreement with experimental findings, confirm that the addition of H(2) to the 16e diamido complexes 1 is favorable but is slow and is accelerated by the presence of Bronsted acids, such as HOTf, which convert 1 into the corresponding amine cation [1H](+). By deprotonation of the subsequently hydrogenated [1H(H(2))](+) complex the amine hydride catalyst 1H(H) is generated, which is able to reduce molecular oxygen. Calculations corroborate that the O(2) reduction goes through formation of an intermediate iridium hydroperoxo complex that reacts with 1H(H) to eliminate water, restore 1, and restart the catalytic cycle. From the outcomes of our computational analysis it results that the slow step of the overall O(2) hydrogenation process is the O(2) insertion into the Ir-H bond, and the highest calculated barrier along this pathway to give the hydroperoxo product shows a good agreement with the experimentally estimated value. As a consequence, unreacted 1H(H) approaches 1H(OOH) to give 1H(OH) and water according to the experimentally observed second-order kinetics with respect to [1H(H)]. Calculations were carried out to explore the possibility that H(2)O(2) is released from the hydroperoxo intermediate together with catalyst 1, and the subsequent water elimination reaction occurs by reduction of produced H(2)O(2) with 1H(H) to regenerate catalyst 1. Preliminary results concerning the O(2) reduction in acidic conditions show that the reaction proceeds by intermediate production of H(2)O(2), which reacts with 1H(H) to eliminate water, restore [1H](+), and restart the catalytic cycle. The energetics of the process appear to be definitely more favorable with respect the analogous pathways in neutral conditions.