A. I. Chumakov

European Synchrotron Radiation Facility, Grenoble, Rhône-Alpes, France

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Publications (256)634.79 Total impact

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    ABSTRACT: The evolution of a phonon spectrum in a narrow-gap semiconductor FeSi was investigated in a wide range of temperatures (46 K ≤T≤297 K, P=0.1 MPa) and pressures (0.1 MPa ≤P≤43 GPa, T=297 K) using nuclear inelastic scattering of synchrotron radiation with the energy resolution 0.53 meV. Decreasing temperature as well as increasing pressure causes a strong reorganization of the phonon spectrum manifested in splitting and shifts of the phonon peaks. The phonon spectra measured under the temperature and pressure conditions corresponding to the same unit cell volume reveal nearly complete matching. On the contrary, the spectra measured under the conditions of the equal mean-square atomic displacements differ drastically. These observations suggest that the transformation of the electronic spectrum of FeSi is controlled predominantly by the change of the unit cell volume. The corresponding changes of the interatomic forces and the resulting modification of the phonon spectrum appear as the direct consequence of this transformation.
    Full-text · Article · Feb 2016
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    ABSTRACT: The physical properties of disordered systems are in the focus of a large research effort. In particular, one of the open problems related to glasses is their excess of modes in the THz frequency range over the vibrational contribution predicted by the Debye model. In fact, one could expect the continuum approximation underlying the Debye model to hold in glasses at long wavelengths at least as well as in crystalline solids. Experiments, instead, seem to indicate that the specific heat at low temperature (a few Kelvin) and the density of vibrational states at low frequency (a few terahertz) are very different from the Debye prediction. We present here a detailed analysis of specific heat measurements of vitreous GeO2, a prototype of strong glasses, and of permanently densified vitreous GeO2. Our data give experimental evidence that glasses do not show any excess of vibrational modes when compared to their crystalline counterparts of similar mass density.
    Full-text · Article · Jan 2016 · Philosophical Magazine
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    ABSTRACT: We analysed the relation between the boson peak in glasses and the van Hove singularity in crystals. The comparison of the experimental results obtained for amorphous iron to the published data for two different iron crystals suggests that the appearance of the pseudo-Brillouin zone with corresponding flattening of the dispersion relations and the resulting boson peak in glasses are the fundamental effects caused by a finite and well-defined value of interatomic distance, i.e. simply by an atomistic character of nature. Accordingly, the Brillouin zone and the van Hove singularities in crystals can be considered as the crystalline counterparts, or ‘derivatives’, of these ‘glassy’ quantities. In addition, we analyse the options for observing the excess states in glasses caused by another effect, i.e. by phonon scattering. Possibly, this is indeed seen in the reduced DOS of ambient silica glass and amorphous iron in the 1–2 meV energy range.
    Full-text · Article · Nov 2015 · Philosophical Magazine
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    ABSTRACT: Developments in pulsed laser heating applied to nuclear resonance techniques are presented together with their applications to studies of geophysically relevant materials. Continuous laser heating in diamond anvil cells is a widely used method to generate extreme temperatures at static high pressure conditions in order to study the structure and properties of materials found in deep planetary interiors. The pulsed laser heating technique has advantages over continuous heating, including prevention of the spreading of heated sample and/or the pressure medium and, thus, a better stability of the heating process. Time differentiated data acquisition coupled with pulsed laser heating in diamond anvil cells was successfully tested at the Nuclear Resonance beamline (ID18) of the European Synchrotron Radiation Facility. We show examples applying the method to investigation of an assemblage containing ε-Fe, FeO, and Fe3C using synchrotron Mössbauer source spectroscopy, FeCO3 using nuclear inelastic scattering, and Fe2O3 using nuclear forward scattering. These examples demonstrate the applicability of pulsed laser heating in diamond anvil cells to spectroscopic techniques with long data acquisition times, because it enables stable pulsed heating with data collection at specific time intervals that are synchronized with laser pulses.
    No preview · Article · Nov 2015 · Review of Scientific Instruments
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    ABSTRACT: Fe-bearing carbonates have been proposed as possible candidate host minerals for carbon inside the Earth's interior and hence their spectroscopic properties can provide constraints on the deep carbon cycle. Here we investigate high-pressure spin crossover in synthetic FeCO3 (siderite) using a combination of Mössbauer, Raman, and X-ray absorption near edge structure spectroscopy in diamond-anvil cells. These techniques sensitive to the short-range atomic environment show that at room temperature and under quasi-hydrostatic conditions, spin crossover in siderite takes place over a broad pressure range, between 40 and 47 GPa, in contrast to previous X-ray diffraction data that described the transition as a sharp volume collapse at approximately 43 GPa. Based on these observations we consider electron spin pairing in siderite to be a dynamic process, where Fe atoms can be either high spin or low spin in the crossover region. Mode Grüneisen parameters extracted from Raman spectra collected at pressures below and above spin crossover show a drastic change in stiffness of the Fe-O octahedra after the transition, where they become more compact and hence less compressible. Mössbauer experiments performed on siderite single crystals as well as powder samples demonstrate the effect of differential stress on the local structure of siderite Fe atoms in a diamond-anvil cell. Differences in quadrupole splitting values between powder and single crystals show that local distortions of the Fe site in powder samples cause spin crossover to start at higher pressure and broaden the spin crossover pressure range.
    No preview · Article · Nov 2015 · American Mineralogist
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    ABSTRACT: Skiagite-rich garnet was synthesized as single crystals at 9.5 GPa and 1100 °C using a multi-anvil apparatus. The crystal structure [cubic, space group Ia3̄d, a = 11.7511(2) Å, V = 1622.69(5) Å3, Dcalc = 4.4931 g/cm3] was investigated using single-crystal synchrotron X-ray diffraction. Synchrotron Mössbauer source spectroscopy revealed that Fe2+ and Fe3+ predominantly occupy dodecahedral (X) and octahedral (Y) sites, respectively, as expected for the garnet structure, and confirmed independently using nuclear forward scattering. Single-crystal X-ray diffraction suggests the structural formula of the skiagite-rich garnet to be Fe32+(Fe2+0.234(2)Fe3+1.532(1)Si4+0.234(2))(SiO4)3, in agreement with electron microprobe chemical analysis. The formula is consistent with X-ray absorption near-edge structure spectra. The occurrence of Si and Fe2+ in the octahedral Y-site indicates the synthesized garnet to be a solid solution of end-member skiagite with ~23 mol% of the Fe-majorite end-member Fe32+(Fe2+Si4+)(SiO4)3.
    Full-text · Article · Nov 2015 · American Mineralogist
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    ABSTRACT: The structural and magnetic properties of ultrathin FeO(111) films on Pt(111) with thicknesses from 1 to 16 monolayers (MLs) were studied using the nuclear inelastic scattering of synchrotron radiation. A distinct evolution of vibrational characteristics with thickness, revealed in the phonon density of states (PDOS), shows a textbook transition from 2D to 3D lattice dynamics. For the thinnest films of 1 and 2 ML, the low-energy part of the PDOS followed a linear ∝E dependence in energy that is characteristic for two-dimensional systems. This dependence gradually transforms with thickness to the bulk ∝E^{2} relationship. Density-functional theory phonon calculations perfectly reproduced the measured 1-ML PDOS within a simple model of a pseudomorphic FeO/Pt(111) interface. The calculations show that the 2D PDOS character is due to a weak coupling of the FeO film to the Pt(111) substrate. The evolution of the vibrational properties with an increasing thickness is closely related to a transient long-range magnetic order and stabilization of an unusual structural phase.
    Full-text · Article · Oct 2015 · Physical Review Letters
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    ABSTRACT: We have investigated the electronic configuration of iron in Fe-, Al-containing magnesium silicate perovskite, i.e., bridgmanite, the main component of the lower mantle, at conditions of the deep Earth's interior using the energy domain Synchrotron Mössbauer Source technique. We show that the high ferric iron content observed previously in quenched samples is preserved at high temperatures and high pressures. Our data are consistent with high-spin to intermediate-spin (HS-IS) crossover in Fe2+ at high pressures and ambient temperature. We see no evidence of spin crossover in Fe3+ occupying the A-position of bridgmanite. On laser heating at pressures above ∼40 GPa we observe a new doublet with relative area below 5% which is assigned to Fe3+ in the octahedral (B-site) position in bridgmanite. We conclude that at lower mantle conditions Fe3+ remains predominantly in the HS state, while Fe2+ occurs solely in the IS state.
    No preview · Article · Aug 2015 · Earth and Planetary Science Letters
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    ABSTRACT: The structural and magnetic properties of ultrathin FeO(111) films on Pt(111) with thicknesses from 1 to 16 monolayers (ML) were studied using the nuclear inelastic scattering (NIS) of synchrotron radiation. Distinct evolution of vibrational characteristics with thickness that is revealed in the phonon density of states (PDOS) witnesses a textbook transition from 2D to 3D lattice dynamics. For the thinnest films of 1 and 2 ML, the low energy part of the PDOS followed a linear dependence in energy that is characteristic for 2-dimensional systems. This dependence gradually transforms with thickness to the bulk ~E-square relation. Density functional theory phonon calculations perfectly reproduced the measured 1 ML PDOS within a simple model of a pseudomorphic FeO/Pt(111) interface. In this model, a weak coupling to the Pt(111) substrates results in a quasi-free-standing FeO film behavior. The evolution of the vibrational properties with an increasing thickness is closely related to a transient long range magnetic order and stabilization of an unusual structural phase.
    Full-text · Article · Jul 2015
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    ABSTRACT: To design custom magnetic nanostructures, it is indispensable to acquire precise knowledge about the systems in the nanoscale range where the magnetism forms. In this paper we present the effect of a curved surface on the evolution of magnetism in ultrathin iron films. Nominally 70 Å thick iron films were deposited in 9 steps on 3 different types of templates: (a) a monolayer of silica spheres with 25 nm diameter, (b) a monolayer of silica spheres with 400 nm diameter and (c) for comparison a flat silicon substrate. In situ iron evaporation took place in an ultrahigh vacuum chamber using the molecular beam epitaxy technique. After the evaporation steps, time differential nuclear forward scattering spectra, grazing incidence small angle X-ray scattering images and X-ray reflectivity curves were recorded. In order to reconstruct and visualize the magnetic moment configuration in the iron cap formed on top of the silica spheres, micromagnetic simulations were performed for all iron thicknesses. We found a great influence of the template topography on the onset of magnetism and on the developed magnetic nanostructure. We observed an individual magnetic behaviour for the 400 nm spheres which was modelled by vortex formation and a collective magnetic structure for the 25 nm spheres where magnetic domains spread over several particles. Depth selective nuclear forward scattering measurements showed that the formation of magnetism begins at the top region of the 400 nm spheres in contrast to the 25 nm particles where the magnetism first appears in the region where the spheres are in contact with each other.
    Full-text · Article · Jul 2015 · Nanoscale
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    ABSTRACT: Das Protein LytB/ISPH katalysiert den letzten Schritt des Methylerythritolphosphat(MEP)-Wegs, der zur Terpenoidbiosynthese in vielen pathogenen Bakterien dient. Deswegen gilt der MEP-Weg als Angriffspunkt für neue potentielle Antibiotika, da er essenziell für Mikroorganismen ist, beim Menschen jedoch nicht vorkommt. Substratfreies LytB hat einen besonderen [4Fe-4S]2+-Cluster mit einer bisher unbekannten Struktur. Experimente mit nuklearer inelastischer Streuung (NIS, oder “nuclear resonance vibrational spectroscopy”, NRVS) in Kombination mit quantenchemischen-molekülmechanischen (QM/MM) Rechnungen zeigen, dass das apikale Eisen des Clusters eindeutig mit drei Wassermolekülen koordiniert ist. Zusätzlich präsentieren wir NIS-Experimente von LytB gebunden mit dem natürlichen Substrat (E)-4-Hydroxy-3-methylbut-2-en-1-yl-diphosphat (HMBPP) sowie mit den Inhibitoren (E)-4-Amino-3-methylbut-2-en-1-yl-diphosphat und (E)-4-Mercapto-3-methylbut-2-en-1-yl-diphosphat.
    No preview · Article · Jun 2015 · Angewandte Chemie
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    ABSTRACT: The LytB/IspH protein catalyzes the last step of the methylerythritol phosphate (MEP) pathway which is used for the biosynthesis of essential terpenoids in most pathogenic bacteria. Therefore, the MEP pathway is a target for the development of new antimicrobial agents as it is essential for microorganisms, yet absent in humans. Substrate-free LytB has a special [4Fe-4S](2+) cluster with a yet unsolved structure. This motivated us to use synchrotron-based nuclear resonance vibrational spectroscopy (NRVS) in combination with quantum chemical-molecular mechanical (QM/MM) calculations to gain more insight into the structure of substrate-free LytB. The apical iron atom of the [4Fe-4S](2+) is clearly linked to three water molecules. We additionally present NRVS data of LytB bound to its natural substrate, (E)-4-hydroxy-3-methylbut-2-en-1-yl diphosphate (HMBPP) and to the inhibitors (E)-4-amino-3-methylbut-2-en-1-yl diphosphate and (E)-4-mercapto-3-methylbut-2-en-1-yl diphosphate. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    Full-text · Article · Jun 2015 · Angewandte Chemie International Edition
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    ABSTRACT: A high-pressure study of the lattice dynamics in the filled skutterudite Eu0.84Fe4Sb12 was carried out by means of x-ray powder diffraction and nuclear inelastic scattering. The anharmonicity of particular phonon modes was characterized by mode and element specific Grüneisen parameters. The large anharmonicity of the rattling optical mode that is hybridized with the acoustical phonons at ambient pressure is reduced at high pressure as the phonon modes decouple. This result suggests that anharmonic coupling between acoustic and optical phonon modes plays a central role in the reduced thermal conductivity.
    No preview · Article · Jun 2015 · Physical Review B
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    ABSTRACT: We performed nuclear forward and inelastic scattering of synchrotron radiation by elemental Os utilizing the nuclear excited state of Os187 which is otherwise inaccessible using any practical radioactive decay process. The lifetime of the excited state, 3.06(8)ns, and the energy of the transition, 9.778(3)keV, are refined. The nuclear quadrupole moment of the excited state, Q3/2=1.46(10)b, is determined. The density of phonon states for elemental Os, which is herein experimentally determined, suggests that the Os lattice is a model for the lattice dynamics of hcp-Fe. The combination of the low energy of the nuclear transition and the large nuclear mass leads to a high recoil free fraction, fLM=0.95(1), at room temperature, a large value that strongly supports the viability of nuclear resonance scattering as a reliable method to study electronic, magnetic, and elastic properties of Os compounds, including Os organometallics.
    Full-text · Article · Jun 2015 · Physical Review B
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    Full-text · Dataset · May 2015
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    ABSTRACT: Geochemical, cosmochemical, geophysical, and mineral physics data suggest that iron (or iron–nickel alloy) is the main component of the Earth’s core1–3 . The inconsistency between the density of pure iron at pressure and temperature conditions of the Earth’s core and seismological observations can be explained by the presence of light elements1,4 .However, the low shear wave velocity and high Poisson’s ratio of the Earth’s core remain enigmatic2 . Here we experimentally investigate the effect of carbon on the elastic properties of iron at high pressures and temperatures and report a high-pressure orthorhombic phase of iron carbide, Fe7C3 . We determined the crystal structure of the material at ambient conditions and investigated its stability and behaviour at pressures up to 205GPa and temperatures above 3,700K using single-crystal and powder X-ray diffraction, Mössbauer spectroscopy, and nuclear inelastic scattering. Estimated shear wave and compressional wave velocities show that Fe7C3 exhibits a lower shear wave velocity than pure iron and a Poisson’s ratio similar to that of the Earth’s inner core. We suggest that carbon alloying significantly modifies the properties of iron at extreme conditions to approach the elastic behaviour of rubber. Thus, the presence of carbon may explain the anomalous elastic properties of the Earth’s core.
    No preview · Article · Feb 2015 · Nature Geoscience
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    ABSTRACT: The lattice dynamics of the silica polymorph {\alpha}-cristobalite has been investigated by a combination of diffuse and inelastic x-ray scattering and ab initio lattice dynamics calculations. Phonon dispersion relations and vibrational density of states are reported and the phonon eigenvectors analysed by a detailed comparison of scattering intensities. The experimentally validated calculation is used to identify the vibration contributing most to the first peak in the density of vibrational states. The comparison of its displacement pattern to the silica polymorphs {\alpha}-quartz and coesite and to vitreous silica reveals a distinct similarity and allows for decisive conclusions on the vibrations causing the so-called Boson peak in silica glass.
    Full-text · Article · Jan 2015 · Journal of Physics Condensed Matter
  • Aleksandr I. Chumakov · Giulio Monaco
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    ABSTRACT: The heat capacity of glasses at temperatures of about ~ 10 K for a long time was considered to be anomalously higher than that of the corresponding crystals. The related excess of the low-energy vibrational states, the so-called ‘boson’ peak, was similarly considered to be anomaly distinguishing glasses from crystals and related to their disordered state. Recent results reveal that (i) the difference in the discussed properties occurs not because the glass is structurally disordered, but because it (usually) has lower density than that of the corresponding crystal, (ii) the heat capacity of glasses and crystals with the same densities is quite similar, and (iii) the boson peak is the glassy counterpart of the van Hove singularity of the corresponding crystal. We analyze the generality of the new results and discuss the compatibility of the suggested interpretation of the boson peak with available experimental data. Analyzing the relation of the new results to various theoretical models, we discuss a possible experimental approach to explore further the nature of the low-frequency vibrational excitations in glasses.
    No preview · Article · Jan 2015 · Journal of Non-Crystalline Solids
  • D Bessas · D G Merkel · A I Chumakov · R Rüffer · R P Hermann · I Sergueev · A Mahmoud · B Klobes · M A McGuire · M T Sougrati · L Stievano
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    ABSTRACT: We measured nuclear forward scattering spectra utilizing the ^{99}Ru transition, 89.571(3) keV, with a notably mixed E2/M1 multipolarity. The extension of the standard evaluation routines to include mixed multipolarity allows us to extract electric and magnetic hyperfine interactions from ^{99}Ru-containing compounds. This paves the way for several other high-energy Mössbauer transitions, E∼90 keV. The high energy of such transitions allows for operando nuclear forward scattering studies in real devices.
    No preview · Article · Oct 2014 · Physical Review Letters
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    ABSTRACT: We measured nuclear forward scattering spectra utilizing the Ru-99 transition, 89.571(3) keV, with a notably mixed E2/M1 multipolarity. The extension of the standard evaluation routines to include mixed multipolarity allows us to extract electric and magnetic hyperfine interactions from Ru-99-containing compounds. This paves the way for several other high-energy Mossbauer transitions, E similar to 90 keV. The high energy of such transitions allows for operando nuclear forward scattering studies in real devices.
    Full-text · Article · Oct 2014 · Physical Review Letters

Publication Stats

3k Citations
634.79 Total Impact Points

Institutions

  • 1995-2015
    • European Synchrotron Radiation Facility
      • Division of Experiments
      Grenoble, Rhône-Alpes, France
    • Universität Paderborn
      • Department of Physics
      Paderborn, North Rhine-Westphalia, Germany
  • 2014
    • PANalytical B.V.
      Almeloo, Overijssel, Netherlands
  • 1986-2014
    • Kurchatov Institute
      Moskva, Moscow, Russia
  • 2010
    • Universität zu Lübeck
      • Institute of Physics
      Lübeck Hansestadt, Schleswig-Holstein, Germany
  • 2009
    • Universität Heidelberg
      Heidelburg, Baden-Württemberg, Germany
  • 2002
    • University of Bologna
      Bolonia, Emilia-Romagna, Italy
  • 2001
    • Uppsala University
      Uppsala, Uppsala, Sweden
  • 1996
    • Freie Universität Berlin
      Berlín, Berlin, Germany