M. M. Koza

Institut Laue-Langevin, Grenoble, Rhône-Alpes, France

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Publications (76)214.39 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: The dynamics of light hydrocarbons, including ethane, propane, and n-butane, is explored in the highly flexible metal–organic framework MIL-53(Cr) by combining quasi-elastic neutron scattering measurements and molecular dynamics simulations. The loading dependence of the self-diffusivity shows peculiarities, including (i) a relatively rapid decrease of Ds at low loading for ethane and propane and (ii) an unusual increase of Ds for n-butane at high loading, following a decreasing profile up to intermediate loading. These diffusion behaviors are analyzed in light of the structural flexibility of the solids upon alkane adsorption characterized by the neutron measurements. A 1D-type diffusion is evidenced for all alkanes with a jump sequence mainly ruled by the hydroxyl groups present at the surface of the MOF pore wall. This global translational motion is associated with a rotational dynamics that differs according to the nature of the alkane: whereas n-butane follows uniaxial displacements, ethane shows random rotational reorientation.
    The Journal of Physical Chemistry C. 06/2014; 118(26):14471–14477.
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    ABSTRACT: The elastic (diffraction) component of the neutron scattering cross section, which carries information on the atomic structure of solid helium confined in silica aerogel, has been studied. Analysis of the crystalline structure of solid helium in a porous medium, which is determined from the existing neutron diffraction data, indicates that the superfluid phase is localized inside a hexagonal close-packed phase and is not present in a body-centered cubic crystal. It has also been revealed that the addition of the 3He isotope changes the structure of solid helium and hardly affects the formation of a superfluid phase.
    JETP Letters 10/2013; 98(4):233-236. · 1.52 Impact Factor
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    ABSTRACT: The magnetic states of the non-centrosymmetric, pressure induced superconductor CeCoGe3 have been studied with magnetic susceptibility, muon spin relaxation(muSR), single crystal neutron diffraction and inelastic neutron scattering (INS). CeCoGe3 exhibits three magnetic phase transitions at T_N1 = 21 K, T_N2 = 12 K and T_N3 = 8 K. The presence of long range magnetic order below T_N1 is revealed by the observation of oscillations of the asymmetry in the muSR spectra between 13 K and 20 K and a sharp increase in the muon depolarization rate. Single crystal neutron diffraction measurements reveal magnetic Bragg peaks consistent with propagation vectors of k = 2/3 between T_N1 and T_N2, k = 5/8between T_N2 and T_N3 and k = 1/2 below T_N3. An increase in intensity of the (1 1 0) reflection between T_N1 and T_N3 also indicates a ferromagnetic component in these phases. These measurements are consistent with an equal moment, two-up, two-down magnetic structure below T_N3, with a magnetic moment of 0.405(5) mu_B/Ce. Above T_N2, the results are consistent with an equal moment, two-up, one-down structure with a moment of 0.360(6) mu_B/Ce. INS studies reveal two crystal-field (CEF) excitations at 19 and 27 meV. From an analysis with a CEF model, the wave-functions of the J = 5/2 multiplet are evaluated along with a prediction for the magnitude and direction of the ground state magnetic moment. Our model correctly predicts that the moments order along the c axis but the observed magnetic moment of 0.405(5) mu_B is reduced compared to the predicted moment of 1.01 mu_B. This is ascribed to hybridization between the localized Ce^3+ f-electrons and the conduction band. This suggests that CeCoGe3 has a degree of hybridization between that of CeRhGe3 and the non-centrosymmetric superconductor CeRhSi3.
    Physical Review B 09/2013; 88(13). · 3.66 Impact Factor
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    ABSTRACT: The dynamics of CO2 and CH4 in a mixture of different compositions has been explored in two metal–organic frameworks, namely, MIL-47(V) and MIL-53(Cr), by combining molecular dynamics (MD) simulations and quasi-elastic neutron scattering (QENS) measurements. The experimental and simulated self-diffusion coefficient (Ds) values for CH4 are in very good agreement in the whole range of the CO2 explored loadings. It is clearly stated that CH4 which shows a fast diffusivity at low loading becomes significantly slower in both metal–organic frameworks (MOFs) when CO2 molecules are introduced within the porosities of these materials. Further, compared to its behavior in a single component, CH4 tends to diffuse slightly faster in the presence of CO2. The MD simulations revealed that this speeding up is concomitant with a mutual speeding up or a slowing down of the slower CO2 molecules in MIL-47(V) and MIL-53(Cr), respectively. Analysis of the MD trajectories emphasizes that both gases in the mixture follow individually a 1D-type diffusion mechanism in both MOFs, where the CO2 molecules diffuse close to the pore wall while the motions of CH4 are restricted in the central region of the tunnel.
    The Journal of Physical Chemistry C. 05/2013; 117(21):11275–11284.
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    ABSTRACT: We present results on neutron scattering in solid 4He in the range of parameters where supersolidity is observed. The measurements address, among other questions, the viability of one possible mechanism of supersolidity: via a metastable amorphous phase. We have attempted to observe a glassy phase by neutron scattering. We have found that it is impossible to do this by total scattering, as it would be common in a classical solid, due to an extremely large inelastic diffuse signal related to the anomalously strong zero-point motion of helium atoms. This raises a general question on the interpretation of such scattering as the signature of an amorphous phase. Results from energy-resolved elastic scattering are heavily influenced by multiple scattering of neutrons which may be the major contribution to the measured elastic signal, but allow to put the limit on the concentration of an amorphous phase to 5% in a polycrystal with millimeter-size crystallites and to 2% in a single crystal. The values of NCRIf, expected from these limits should be much lower, although exact values depend strongly on a particular model of glass-related supersolidity.
    EPL (Europhysics Letters) 01/2013; 101(2):26002. · 2.26 Impact Factor
  • U. D. Wdowik, M. M. Koza, T. Chatterji
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    ABSTRACT: Dynamical properties of the lanthanum manganite lattice are examined by inelastic neutron scattering experiments and density functional theory calculations. Densities of vibrational states are measured close to the Jahn-Teller transition temperature of 750 K. Substantial changes observed in the phonon spectra above the phase transformation are due to residual orthorhombic distortions that persist in the high-temperature structure of lanthanum manganite. Results of the present theoretical investigations supply additional information useful for both Raman and infrared spectroscopies. In addition, they indicate that typical static phonon calculations are insufficient to reproduce accurately experimental magnitudes of these vibrational quantities of lanthanum manganite that are determined to a large extent by dynamical effects.
    Physical review. B, Condensed matter 11/2012; 86(17). · 3.77 Impact Factor
  • S.M. Chathoth, M.M. Koza, A. Meyer
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    ABSTRACT: Quasielastic neutron scattering (QENS) was used to explore relaxation processes in a deep-eutectic Ce80Ni20 melt over a temperature range of 750 K, i.e., from 775 K to 1525 K. At low temperatures (T ≤ 890 K), the self-correlation function shows a fast β-relaxation process. The α-relaxation process displays stretching of the self-correlation and obeys a time–temperature superposition principle that extends over the entire measured temperature range. Even though the decay of the self-correlation functions of the Ce80Ni20 melt exhibit that of glass-forming melts, the temperature dependence of self-diffusivity displays an Arrhenius behavior, as observed for many simple, non-glass forming liquids.
    Materials Chemistry and Physics. 10/2012; 136(s 2–3):296–299.
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    ABSTRACT: The magnetic ground state of CeRhGe3 has been investigated using magnetic susceptibility, heat capacity, neutron diffraction, muon spin relaxation (μSR), and inelastic neutron scattering (INS) techniques. Our μSR study clearly reveals the presence of two frequencies below TN2 = 7 K and three frequencies between 7 K and TN1 = 14.5 K, indicating long-range magnetic ordering of the Ce3+ moment. The temperature dependence of the highest frequency follows a mean-field order parameter. Our powder neutron diffraction study at 1.5 K reveals the presence of magnetic Bragg peaks, indexed by the propagation vector k = (0, 0, 3/4) with the Ce3+ magnetic moment ∼0.45(9) μB along the c axis. INS studies at 18 K (i.e., above TN1) show the presence of two well-defined crystal-field (CEF) excitations at 7.5 and 18 meV. At 10 and 4.5 K, a very small increase has been observed in the CEF excitation energies. At 100 K, both CEF excitations broaden and a broad quasielastic component has also been observed. Further, the low-energy INS study reveals the presence of a nearly temperature-independent quasielastic linewidth between 16 and 60 K, which indicates a Kondo temperature TK = 12.6(3) K. The presence of well-defined CEF excitations in CeRhGe3 suggests local moment magnetism and may explain the absence of pressure-induced superconductivity. Analyzing the INS data based on a CEF model, we have evaluated the CEF ground-state wave functions and ground-state moment. The observed small value of the ordered moment along the c axis, deduced from the neutron diffraction data, contrasts with the ab-plane moment direction predicted by the single-ion CEF anisotropy and indicates the presence of two-ion anisotropic magnetic exchange interactions, which govern the direction of the moment.
    Physical review. B, Condensed matter 04/2012; 85(13). · 3.77 Impact Factor
  • German Neutron Scattering Conference 2012; 01/2012
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    ABSTRACT: We demonstrate by neutron scattering that a localized superfluid component exists at high pressures within solid helium in aerogel. Its existence is deduced from the observation of two sharp phonon-roton spectra which are clearly distinguishable from modes in bulk superfluid helium. These roton excitations exhibit different roton gap parameters than the roton observed in the bulk fluid at freezing pressure. One of the roton modes disappears after annealing the samples. Comparison with theoretical calculations suggests that the model that reproduces the observed data best is that of superfluid double layers within the solid and at the helium-substrate interface.
    Physical Review Letters 12/2011; 107(26):265301. · 7.73 Impact Factor
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    ABSTRACT: The temperature dependence of the generalized phonon density of states (GDOS) of polycrystalline Mo3Sb7 and Mo3Sb5.4Te1.6 was studied from 300 K down to 2 K using inelastic neutron scattering. Even though Mo3Sb7 undergoes a magnetic as well as a structural phase transition at T* = 53 K, no appreciable change in the recorded spectra has been revealed. The generalized density of states of the nonmagnetic Mo3Sb5.4Te1.6 compound shows similar main characteristics, though substituting Sb by Te leads to a shift of the entire spectrum towards higher energy. The temperature dependence of the GDOS reveals an anomalous softening of phonons with decreasing temperature in Mo3Sb7 which cannot be captured by the harmonic approximation while Mo3Sb5.4Te1.6 exhibits a normal behavior. This feature might be related to a strong interplay between magnetic excitations and phonons at the core of the anomalous thermal transport displayed by Mo3Sb7. In general, the main characteristics of the experimentally derived GDOS of Mo3Sb7 can be reproduced by lattice dynamical calculations.
    Physical review. B, Condensed matter 12/2011; 84(22). · 3.77 Impact Factor
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    ABSTRACT: Lattice dynamics ab-initio calculations of the new thermoelectric compound La4Sb3 with anti- Th4P3 are reported. Hybridization of the lower optic mode with the acoustic modes is observed between 5.9 and 7.2 meV. As similar observations were done in other thermoelectric compounds such as skutterudites, it is suggested that this is an important effect in order to achieve low lattice thermal conductivity not only for the compounds La4Sb3 and La3Te4 with Th4P3 structure but also for other thermoelectric compounds, whose structures are not formed by cages.
    Fuel and Energy Abstracts 12/2011;
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    ABSTRACT: The heavy fermion system CeNi9Ge4 exhibits a paramagnetic ground state with remarkable features such as: a record value of the electronic specific heat coefficient in systems with a paramagnetic ground state, \gamma = C/T \simeq 5.5 J/molK^2 at 80 mK, a temperature-dependent Sommerfeld-Wilson ratio, R=\chi/\gamma, below 1 K and an approximate single ion scaling of the 4f-magnetic specific heat and susceptibility. These features are related to a rather small Kondo energy scale of a few Kelvin in combination with a quasi-quartet crystal field ground state. Tuning the system towards long range magnetic order is accomplished by replacing a few at.% of Ni by Cu or Co. Specific heat, susceptibility and resistivity studies reveal T_N \sim 0.2 K for CeNi8CuGe4 and T_N \sim 1 K for CeNi8CoGe4. To gain insight whether the transition from the paramagnetic NFL state to the magnetically ordered ground state is connected with a heavy fermion quantum critical point we performed specific heat and ac susceptibility studies and utilized the \mu SR technique and quasi-elastic neutron scattering.
    Journal of Physics Conference Series 10/2011; 344(1).
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    ABSTRACT: We present a combined experimental and theoretical study of thermoelectric properties and lattice dynamics of the R4Sb3 (R = La, Yb) compounds with the anti-Th3P4 structure. Bi alloying of Yb4Sb3 was found to improve the thermoelectric properties but not as well as La or Sm alloying. From the band structure, we are able to explain the origin of the relatively large and positive thermopower in Yb4Sb3 and of the small and negative thermopower found in La4Sb3 at high temperatures. Since the contribution of the 4f level of Yb atoms must be negative, as in other intermediate valence Yb-based compounds, the positive thermopower in Yb4Sb3 is due to the conduction electrons. We conjecture that the origin of the low thermal conductivity is similar to that of skutterudites, i.e., due to hybridization between acoustic and low-energy optical modes of the same symmetry.
    Journal of Electronic Materials 01/2011; 40(5):1171-1175. · 1.64 Impact Factor
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    ABSTRACT: We report powder inelastic neutron-scattering experiments on the type-I clathrates Ba8ZnxGe46−x−y◻y with x=0,2,4,6,8, y=3−3x/8, and ◻ characterizing a vacancy. The focus of this study is set on the modulation of the eigenfrequency distribution of the clathrate compounds by the progressive substitution of germanium by zinc and its response to temperature variation between 2 and 300 K. A shift of a number of peaks toward higher energies by about 1–2 meV can be confirmed as a result of the substitution. An identification of the partial contribution of the barium cations to the eigenmode spectrum has been determined by ab initio lattice-dynamics calculation of the compounds Ba8Zn6Ge40 and Ba8Ge43◻3. The Ba-weighted eigenmodes are located predominantly at energies lower than 14 meV. Within the resolution of the experiment the density of vibrational states associated with the Ba2 site, e.g., modes at about 4.8 meV, are not affected by the substitution. The entire suite of our temperature-dependent data on Ba8ZnxGe46−x−y◻y is in agreement with a softening of the mode at 4.5–4.8 meV upon cooling by at most 5%. No clear temperature effect on the Ba-weighted modes can be confirmed in the binary Ba8Ge43◻3 and Ba8Zn2Ge41.2◻2.8 compounds. Higher-frequency Ge and Zn weighted modes display a redshift upon heating.
    Physical review. B, Condensed matter 12/2010; 82(21). · 3.77 Impact Factor
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    ABSTRACT: Linear-polarized soft-x-ray absorption (XAS) and inelastic neutron scattering (INS) experiments have been performed on CeMIn5 with M=Rh , Ir, and Co to determine the crystal-field scheme and characteristic Kondo temperatures T∗ for the hybridization between 4f and conduction electrons. The ground-state wave functions are determined from the polarization-dependent soft-XAS data at the cerium M4,5 edge and the crystal-field splittings from INS. The characteristic temperature T∗ has been determined from the line widths of the neutron scattering data. We find that the quasielastic linewidths of the superconducting compounds CeCoIn5 and CeIrIn5 are comparable with the low-energy crystal-field splitting.
    Physical review. B, Condensed matter 05/2010; 81:195114-. · 3.77 Impact Factor
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    ABSTRACT: We investigate the magnetic properties of three Mn6 single-molecule magnets by means of inelastic neutron scattering and frequency domain magnetic resonance spectroscopy. The experimental data reveal that small structural distortions of the molecular geometry produce a significant effect on the energy-level diagram and therefore on the magnetic properties of the molecule. We show that the giant spin model completely fails to describe the spin-level structure of the ground spin multiplets. We analyze theoretically the spin Hamiltonian for the low-spin Mn6 molecule (S=4) and we show that the excited S multiplets play a key role in determining the effective energy barrier for the magnetization reversal, in analogy to what was previously found for the two high spin Mn6 (S=12) molecules [ S. Carretta et al. Phys. Rev. Lett. 100 157203 (2008)].
    Physical review. B, Condensed matter 03/2010; 81(17). · 3.77 Impact Factor
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    ABSTRACT: The magnetic properties of three Mn6 molecular magnets have been investigated by means of inelastic neutron scattering. By comparing the obtained data to results from frequency domain magnetic resonance studies it was possible to determine the isotropic exchange and zero-field splitting parameters. The findings enable detailed quantification of the effect of small structural distortions of the molecular geometry on the magnetic properties. We show that the giant spin model completely fails to describe the spin level structure of the ground spin multiplets. A theoretical analysis of the corresponding spin Hamiltonian reveals that the excited S multiplets play a key role in determining the effective energy barrier for magnetization reversal for all three compounds.
    03/2010;
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    ABSTRACT: The results of recent neutron scattering studies of solid helium in silica aerogel are discussed. Previously I.V. Kalinin et al., Pis’ma Zh. Éksp. Teor. Fiz. 87 (1), 743 (2008) [JETP Lett. 87 (1), 645 (2008)], we detected the existence of a superfluid phase in solid helium at a temperature below 0.6 K and a pressure of 51 bar, although, according to the phase diagram, helium should be in the solid state under these conditions. This work is a continuation of the above studies whose main goal was to examine the detected phenomenon and to establish basic parameters of the existence of a superfluid phase. We have determined the temperature of the superfluid transition from solid to superfluid helium, T C = 1.3 K, by analyzing experimental data. The superfluid phase excitation parameters (lifetime, intensity, and energy) have a temperature dependence similar to that of bulk helium. The superfluid phase coexists with the solid phase in the entire measured temperature range from T = 0.05 K to T C and is a nonequilibrium one and disappears at T C.
    Journal of Experimental and Theoretical Physics 01/2010; 111(2):215-219. · 0.92 Impact Factor
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    ABSTRACT: First-principles density-functional theory and lattice-dynamics calculations were performed to study the vibrational dynamics and related observables of the ternary compounds Ca1-xFe4Sb12 , Sr1-xFe4Sb12 , Ba1-xFe4Sb12 . and Yb1-xFe4Sb12 . The calculation results are supported by experimental data, which were obtained from neutron inelastic scattering, neutron-diffraction, and heat-capacity measurements. Within the calculation approach based on the theory of harmonic solids all observables are linked to the phonon density of states Z(omega) . The good agreement with experimental data shows that the vibrational dynamics of the ternary skutterudite structures can be described by a set of normal modes. Features in the experimentally obtained density of states G(omega) reflecting the variation in properties (mass, ionic radius) of the cations Ca, Sr, Ba, and Yb are reproduced by the calculations. The quality of the inelastic neutron experiments enables the detection of at least two mode peaks at 4.9 and 5.7 meV with a pronounced spectral weight of ytterbium.
    Physical Review B 01/2010; · 3.66 Impact Factor

Publication Stats

615 Citations
214.39 Total Impact Points

Institutions

  • 2001–2013
    • Institut Laue-Langevin
      Grenoble, Rhône-Alpes, France
  • 2011
    • Oak Ridge National Laboratory
      Oak Ridge, Florida, United States
  • 2007
    • Technische Universität München
      München, Bavaria, Germany
  • 2005
    • Johannes Gutenberg-Universität Mainz
      • Institute of Physics
      Mainz, Rhineland-Palatinate, Germany
    • Universität Augsburg
      • Institute of Physics
      Augsburg, Bavaria, Germany
    • Université de Montpellier 1
      Montpelhièr, Languedoc-Roussillon, France
  • 2000
    • Technische Universität Dortmund
      Dortmund, North Rhine-Westphalia, Germany