Papers by Andrei Postnikov
Physical Review B, 2011
We present a systematic and comparative study of the structural and electronic properties of Cu-b... more We present a systematic and comparative study of the structural and electronic properties of Cu-based ternary and quaternary semiconductors using first-principles electronic structure approaches. The important role that Cu d electrons play in determining their properties is illustrated by comparing results calculated with different exchange correlation energy functionals. We show that systematic improvement of the calculated anion displacement can be achieved by using the Heyd-Scuseria-Ernzerhof (HSE06) functional compared with the Perdew-Burke-Ernzerhof (PBE) functional. Quasiparticle band structures are then calculated within the G 0 W 0 approximation using the crystal structures optimized within the HSE06 functional and starting from the PBE+U mean-field solution. Both the calculated quasiparticle band gaps and their systematic variation with chemical constituents agree very well with experiments. We also predict that the quasiparticle band gaps of the prototypical semiconductor Cu 2 ZnSnS 4 in the kesterite (KS) phase is 1.65 eV and that of the stannite (ST) phase is 1.40 eV. These results are also consistent with available experimental values which vary from 1.45 to 1.6 eV.
Physical Review B, 2001
We derive orbital symmetry character from the study of peak intensities in angle-resolved photoem... more We derive orbital symmetry character from the study of peak intensities in angle-resolved photoemission spectra taken at different incidence angles of p-polarized light. Our method of analysis is tested at the oxygenderived antibonding band on the Cu͑110͒͑2ϫ1͒O surface. This band has an initial state energy E i ϭϪ1.37 eV (Tϭ300 K) at the Ȳ point and shows even symmetry with respect to the ⌫LUX mirror plane ͑along ⌫Y ͒. The intensity analysis reveals that orbitals both of z-like ͑along surface normal͒ and y-like symmetry ͑y along the Cu-O chains͒ are mixed into this band. This interpretation is supported by several calculations. The ground-state symmetry character was studied by a full-potential linearized augmented planewave method using a seven-layer supercell geometry. For a direct comparison with the experimental intensities, photoemission spectra at the Ȳ point were calculated using the one-step model of photoemission. Our results contribute to a long-standing discussion about the orbital character of the even antibonding band. In particular we demonstrate that this kind of intensity analysis can supply valuable information about symmetry properties.
Current Applied Physics, 2016
Inorganica Chimica Acta, 1972
Abstract The Kβ and L2,3 X-ray emission bands of the disulphide anion are discussed in terms of a... more Abstract The Kβ and L2,3 X-ray emission bands of the disulphide anion are discussed in terms of a molecular orbital model. The bonding between the sulphur atoms, that is consistent with the spectra, is essentially 3p in character, but 3d orbitals help to stabilise the antibonding π orbitals. 3s orbitals play no significant part in bonding. The weakness of high energy emission bands is consistent with antibonding π-orbital participation in covalent FeS2 bonding. Extensive covalent bonding to the iron atom is also indicated by Mossbauer spectroscopy.
Scientific Reports, 2019
Volume-phonon-polaritons (VPP’s) propagating at a light-in-vacuum-like speed are identified in th... more Volume-phonon-polaritons (VPP’s) propagating at a light-in-vacuum-like speed are identified in the wurtzite-type Zn0.74Mg0.26Se mixed crystal by near-forward Raman scattering. Their detection is selective to both the laser energy and the laser polarization, depending on whether the ordinary (n0) or extraordinary (ne) refractive index is addressed. Yet, no significant linear birefringence (n0 $${\boldsymbol{\simeq }}$$ ≃ ne) is observed by ellipsometry. The current access to ultrafast VPP’s is attributed to the quasi-resonant Raman probing of an anomalous dispersion of n0 due to impurity levels created deep in the optical band gap by oriented structural defects. The resonance conditions are evidenced by a dramatic enhancement of the Raman signals due to the polar modes. Hence, this work reveals a capacity for the lattice defects’ engineering to “accelerate” the VPP’s of a mixed crystal up to light-in-vacuum-like speeds. This is attractive for ultrafast signal processing in the terah...
Journal of Medical Imaging
HAL (Le Centre pour la Communication Scientifique Directe), Sep 5, 2016
Surfaces and Interfaces, 2021
Abstract The adsorption of CO 2 , CO, and O 2 on the (001) surfaces of the CsPbX 3 (X = Cl, Br, a... more Abstract The adsorption of CO 2 , CO, and O 2 on the (001) surfaces of the CsPbX 3 (X = Cl, Br, and I) perovskites have been investigated at the first-principles level, using a hybrid exchange-correlation functional optimized to yield description of their structural, electronic, and dynamic properties, in good agreement with experiment. The CsX- and PbX 2 -terminated slabs without defects have been considered, and their dynamic properties determined, revealing zone-boundary phonon instabilities for q = π a ( 1 , 0 ) and π a ( 1 , 1 ) . Further on, the relaxation patterns at the surfaces due to adsorption of molecules have been studied for different docking sites, and the corresponding adsorption energies estimated. On the CsX-terminated surfaces, CO 2 was found to be more reactive than CO or O 2 . On the PbX 2 -terminated surfaces, the CO 2 reactivity is two times weaker than on the CsX-terminated ones, becoming in fact comparable to that of CO and O 2 , the latter having the lowest adsorption energy. The adsorption does not seem to have any significant impact on the electronic structure of the surfaces, whatever the perovskites, types of surfaces or molecules. The obtained adsorption energies and electronic structures permit to characterize attachment of the molecules studied on the surfaces in question as physisorption.
Beilstein Journal of Nanotechnology, 2016
Background: The need for practical and adaptable terahertz sources is apparent in the areas of ap... more Background: The need for practical and adaptable terahertz sources is apparent in the areas of application such as early cancer diagnostics, nondestructive inspection of pharmaceutical tablets, visualization of concealed objects. We outline the operation principle and suggest the design of a simple appliance for generating terahertz radiation by a system of nanoobjects – gold nanobars (GNBs) or nanorings (GNRs) – irradiated by microwaves. Results: Our estimations confirm a feasibility of the idea that GNBs and GNRs irradiated by microwaves could become terahertz emitters with photon energies within the full width at half maximum of the longitudinal acoustic phononic DOS of gold (ca. 16–19 meV, i.e., 3.9–4.6 THz). A scheme of the terahertz radiation source is suggested based on the domestic microwave oven irradiating a substrate with multiple deposited GNBs or GNRs. Conclusion: The size of a nanoobject for optimal conversion is estimated to be approx. 3 nm (thickness) by approx. 100 ...
Journal of Nanophotonics, 2012
Physical Review B, 1997
In extension of our previous study of KNbO3 by the semiempirical Hartree-Fock method we present p... more In extension of our previous study of KNbO3 by the semiempirical Hartree-Fock method we present parameterization and total-energy results for non-ferroelectric KTaO3 as a pure crystal (concentrating on the frozen phonon calculations) and that with Li impurities. The magnitudes of off-center Li displacements and the relaxation energies related to reorientation of Li are calculated and compared with experimental estimates and earlier calculation results. The spatial extent of lattice relaxation around Li impurities and contributions from different neighbors to the relaxation energy are discussed.
Physical Review B, 2005
Vibration properties of Zn 1−x Be x Se, a mixed II-VI semiconductor characterized by a high contr... more Vibration properties of Zn 1−x Be x Se, a mixed II-VI semiconductor characterized by a high contrast in elastic properties of its pure constituents ZnSe and BeSe, are simulated by first-principles calculations of electronic structure, lattice relaxation, and frozen phonons. The calculations within the local density approximation have been done with the SIESTA method, using norm-conserving pseudopotentials and localized basis functions; the benchmark calculations for pure end systems were, moreover, done also by the all-electron WIEN2k code. An immediate motivation for the study was to analyze, at the microscopic level, the appearance of anomalous phonon modes detected early in Raman spectra in the intermediate region ͑20%-80%͒ of ZnBe concentration. This was discussed early on the basis of a percolation phenomenon-i.e., the result of the formation of wall-to-wall uBeuSeu chains throughout the crystal. The presence of such chains was explicitly allowed in our simulation and indeed brought about a softening and splitting off of particular modes, in accordance with experimental observation, due to a relative elongation of BeuSe bonds along the chain as compared to those involving isolated Be atoms. The variation of force constants with interatomic distances shows common trends in relative independence of the short-range order.
MRS Proceedings, 2001
The properties of small clusters Fe3 and Fe5, in which the non-collinearity of magnetic density i... more The properties of small clusters Fe3 and Fe5, in which the non-collinearity of magnetic density is expected to be important, and of larger nanoparticles (consisting of up to 62 atoms) are studied from first principles making use of density functional theory, norm-conserving pseudopotential and numerical local orbitals method, as implemented in the SIESTA code. We concentrate on the interplay of lattice relaxation, mostly pronounced near the surface of particles, and the particles' magnetic characteristics. Previously obtained theoretical findings of enhanced magnetic moments in outer shells of nanoparticles are confirmed. These results are refined by taking structure relaxation into account and by considering more representative bcc- and fcc-related particles; moreover, we allowed antiferromagnetic ordering along with ferromagnetic one.
MRS Proceedings, 2001
The results of first-principles simulations of relaxed ground-state structure and vibrational mod... more The results of first-principles simulations of relaxed ground-state structure and vibrational modes are presented for titanium carbide and titanium nitride clusters of nearly stoichiometric composition and compared to frozen phonon and molecular dynamics calculations for crystalline TiC and TiN. The calculations have been done with the SIESTA method, using norm-conserving pseudopotentials and the basis of strictly localized numerical pseudoatomic orbitals. The dominant vibration mode corresponding to the zone-center TO phonon (14 THz) persists and gets hardened (21 THz) in the small Ti4C4 cluster. The increase of the cluster size to Ti14C13 leads to an enhancement of vibrational density of states in the intermediate range of frequencies, including the phonon band gap of pure crystalline TiC (near 15 THz). Similar trends can be noted for the Ti-N system, with the vibration spectrum slightly scaled upwards but otherwise very close to that of TiC. The clusters studied are yet too small...
Physical Review B, 2003
The electronic properties of the ferroelectric compound Sn 2 P 2 S 6 are investigated by x-ray ph... more The electronic properties of the ferroelectric compound Sn 2 P 2 S 6 are investigated by x-ray photoelectron spectroscopy and soft x-ray fluorescence spectroscopy. Excellent agreement between theoretical calculations and experimental data for the electronic structure of the investigated compound is achieved. With help of the Sn core level spectra it is confirmed that the compound contains Sn 2ϩ ions. The valence band mainly consists of five resolvable bands between 3.3 eV and 14.5 eV. Consistent with the results of band-structure calculation and the soft x-ray fluorescence spectra, Sn 2 P 2 S 6 can be viewed as an ionic crystal, built of Sn 2ϩ and the (P 2 S 6) 4Ϫ fragments. Within the latter, P-P and P-S bonds are largely covalent and characterized by sp hybridization.
Physical Review B, 2001
The four A1-TO Γ phonon frequencies in lithium tantalate are calculated in the frozen-phonon appr... more The four A1-TO Γ phonon frequencies in lithium tantalate are calculated in the frozen-phonon approach from first principles using the full-potential linearized augmented plane wave method. A good agreement with the experimental data available is found for all modes; reliable displacement pattern of different modes becomes available from the calculated eigenvectors. The Raman spectra recorded for A1 modes in LiNbO3 exhibit a counter-intuitive softening of the A1-TO3 mode frequency with respect to that measured in LiTaO3. We explain this behavior by a comparatively harder oxygen rotation in LiTaO3 and discuss other differences in lattice dynamics of two materials, notably delocalization of Ta and Li contributions over more that one corresponding mode in LiTaO3, differently from the situation in lithium niobate. The Li isotope shift is predicted in the calculation.
Surface and Interface Analysis, 2005
Ion channeling is used to investigate the electronic density corrugation at surfaces by analysing... more Ion channeling is used to investigate the electronic density corrugation at surfaces by analysing the electronic stopping behaviour of ions scattering grazingly off a clean single crystalline Pt(110)(1 × 2) surface. We use the fact that under these conditions the elastic contribution can be separated from the inelastic energy‐loss processes, which are closely related to the electronic density sampled along the projectiles trajectories. Therefore, we have a direct probe of the surface electronic density structure. Here, experiments and their theoretical interpretation for N+ ions at primary ion energies of 1–10 keV are presented. Multi‐peak features of the energy spectra, depending on the azimuthal surface orientation, are attributed to different particle trajectories. A detailed analysis of trajectory calculations affords an unambiguous assignment of the observed peak structures to three trajectory classes, each sampling specific energy losses. The comparison of the experimental energy‐loss values with results obtained from a surface‐adapted energy‐loss model for ion scattering that includes the electron density allows us to investigate the electronic surface of the sample. The results are compared with calculated surface electron density contours. Reasonable agreement is obtained, showing that the presented method is suitable to characterize the electronic density structure at crystalline surfaces. Copyright © 2005 John Wiley & Sons, Ltd.
Polyhedron, 2009
The periodic DFT method has been used to analyze the energetics and the structure of the spin cro... more The periodic DFT method has been used to analyze the energetics and the structure of the spin crossover coordination polymer [Fe(pyim) 2 (bpy)](ClO 4) 2 Á 2C 2 H 5 OH (pyim = 2-(2-pyridyl)imidazole). It is shown that quantum chemical calculations are able to predict correctly the ground state and internal geometry of the zigzag chain polymer, whereas the description of the interchain space is less satisfactory. The calculations display the role of intermolecular interactions in tuning the energy gap between high-spin and low-spin states.
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Papers by Andrei Postnikov