M. M. Koza

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

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Publications (85)232.43 Total impact

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    ABSTRACT: The ability of some materials with a perfectly ordered crystal structure to mimic the heat conduction of amorphous solids is a remarkable physical property that finds applications in numerous areas of materials science, for example, in the search for more efficient thermoelectric materials that enable to directly convert heat into electricity. Here, we unveil the mechanism in which glass-like thermal conductivity emerges in tetrahedrites, a family of natural minerals extensively studied in geology and, more recently, in thermoelectricity. By investigating the lattice dynamics of two tetrahedrites of very close compositions (Cu12Sb2Te2S13 and Cu10Te4S13) but with opposite glasslike and crystal thermal transport by means of powder and single-crystal inelastic neutron scattering, we demonstrate that the former originates from the peculiar chemical environment of the copper atoms giving rise to a strongly anharmonic excess of vibrational states.
    Physical Chemistry Chemical Physics 06/2015; DOI:10.1039/c5cp02900g · 4.20 Impact Factor
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    ABSTRACT: We have investigated the non-centrosymmetric tetragonal heavy-fermion compound CeAuAl3 using muon spin rotation (muSR), neutron diffraction (ND) and inelastic neutron scattering (INS) measurements. We have also revisited the magnetic, transport and thermal properties. The magnetic susceptibility reveals an antiferromagnetic transition at 1.1 K with a possibility of another magnetic transition near 0.18 K. The heat capacity shows a sharp lambda-type anomaly at 1.1 K in zero-filed, which broadens and moves to higher temperature in applied magnetic field. Our zero-field muSR and ND measurements confirm the existence of a long-range magnetic ground state below 1.2 K. Further the ND study reveals an incommensurate magnetic ordering with a magnetic propagation vector k = (0, 0, 0.52) and a spiral structure of Ce moments coupled ferromagnetically within the ab-plane. Our INS study reveals the presence of two well-defined crystal electric field (CEF) excitations at 5.1 meV and 24.6 meV in the paramagnetic phase of CeAuAl3 which can be explained on the basis of the CEF theory. Furthermore, low energy quasi-elastic excitations show a Gaussian line shape below 30 K compared to a Lorentzian line shape above 30 K, indicating a slowdown of spin fluctuation below 30 K. We have estimated a Kondo temperature of TK=3.5 K from the quasi-elastic linewidth, which is in good agreement with that estimated from the heat capacity. This study also indicates the absence of any CEF-phonon coupling unlike that observed in isostructural CeCuAl3. The CEF parameters, energy level scheme and their wave functions obtained from the analysis of INS data explain satisfactorily the single crystal susceptibility in the presence of two-ion anisotropic exchange interaction in CeAuAl3.
    Physical Review B 04/2015; 91(13). DOI:10.1103/PhysRevB.91.134425 · 3.74 Impact Factor
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    ABSTRACT: YbNi4P2 is one of the very few heavy-fermion systems which allow the study of ferromagnetic quantum criticality. The Curie temperature TC=0.17 K can be suppressed by substituting arsenic on the phosphorus site, without changing the ferromagnetic nature of the ordered state. The ordered moment, even of the unsubstituted compound, is only around 0.05 μB, which hinders elastic neutron scattering experiments. To gain microscopic insight into the nature of the interactions, we have studied the magnetic excitations of polycrystalline YbNi4P2 by time-of-flight neutron spectroscopy. For momentum transfers larger than about 0.6 Å−1 we find a quasi-elastic response whose width at low temperatures is limited by the Kondo effect. In contrast, the low-energy magnetic response is distinctly different for Q approaching zero: At low temperatures, but still in the paramagnetic phase, susceptibility and lifetime of the spin fluctuations are strongly enhanced, indicating the proximity of ferromagnetism.
    Journal of Physics Conference Series 03/2015; 592(1):012083. DOI:10.1088/1742-6596/592/1/012083
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    ABSTRACT: The opening of a spin gap in the orthorhombic compounds CeT$_2$Al$_{10}$ (T = Ru and Os) is followed by antiferromagnetic ordering at $T_N$ = 27 K and 28.5 K, respectively, with a small ordered moment (0.29$-$0.34$\mu_B$) along the $c-$axis, which is not an easy axis of the crystal field (CEF). In order to investigate how the moment direction and the spin gap energy change with 10\% La doping in Ce$_{1-x}$La$_x$T$_2$Al$_{10}$ (T = Ru and Os) and also to understand the microscopic nature of the magnetic ground state, we here report on magnetic, transport, and thermal properties, neutron diffraction (ND) and inelastic neutron scattering (INS) investigations on these compounds. Our INS study reveals the persistence of spin gaps of 7 meV and 10 meV in the 10\% La-doped T = Ru and Os compounds, respectively. More interestingly our ND study shows a very small ordered moment of 0.18 $\mu_B$ along the $b-$axis (moment direction changed compared with the undoped compound), in Ce$_{0.9}$La$_{0.1}$Ru$_2$Al$_{10}$, however a moment of 0.23 $\mu_B$ still along the $c-$axis in Ce$_{0.9}$La$_{0.1}$Os$_2$Al$_{10}$. This contrasting behavior can be explained by a different degree of hybridization in CeRu$_2$Al$_{10}$ and CeOs$_2$Al$_{10}$, being stronger in the latter than in the former. Muon spin rotation ($\mu$SR) studies on Ce$_{1-x}$La$_x$Ru$_2$Al$_{10}$ ($x$ = 0, 0.3, 0.5 and 0.7), reveal the presence of coherent frequency oscillations indicating a long$-$range magnetically ordered ground state for $x$ = 0 to 0.5, but an almost temperature independent Kubo$-$Toyabe response between 45 mK and 4 K for $x$ = 0.7. We will compare the results of the present investigations with those reported on the electron and hole$-$doping in CeT$_2$Al$_{10}$.
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    ABSTRACT: The concentration dependence of the self-diffusivity of short-chain linear alkanes in the narrow window type metal–organic framework (MOF) UiO-66(Zr) has been studied by means of quasi-elastic neutron scattering (QENS) measurements combined with molecular dynamics (MD) simulations. These computations employ a force field to describe the host/guest interactions which was preliminarily validated on the adsorption data obtained for the system of interest via gravimetry and microcalorimetry measurements. The QENS-measured self-diffusivity profile presents a nonmonotonic tendency as the alkane loading increases, with the existence of a maximum that depends on the size of the alkane. The comparison with the simulated results obtained using either a flexible or a rigid framework highlights that the consideration of the flexibility is of prime importance when exploring the diffusion of ethane molecules in porous materials. The self-diffusivities subsequently calculated for propane and n-butane corroborate the results obtained for ethane, leading to a similar form for the plots of self-diffusion coefficient vs loading. The global microscopic diffusion mechanism is further shown to involve a combination of intracage motions and jump sequences between the tetrahedral and octahedral cages of the framework. The self-diffusion coefficients which decrease with increasing molecular size, and thus increasing confinement, are further compared to the values previously reported for MOFs with pore networks of different dimensions.
    The Journal of Physical Chemistry C 10/2014; DOI:10.1021/jp509672c · 4.77 Impact Factor
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    ABSTRACT: It is a long debated question whether catalytic activities of enzymes, which lie on the millisecond timescale, are possibly already reflected in variations in atomic thermal fluctuations on the pico- to nanosecond timescale. To shed light on this puzzle, the enzyme human acetylcholinesterase in its wild-type form and complexed with the inhibitor huperzine A were investigated by various neutron scattering techniques and molecular dynamics simulations. Previous results on elastic neutron scattering at various timescales and simulations suggest that dynamical processes are not affected on average by the presence of the ligand within the considered time ranges between 10 ps and 1 ns. In the work presented here, the focus was laid on quasi-elastic (QENS) and inelastic neutron scattering (INS). These techniques give access to different kinds of individual diffusive motions and to the density of states of collective motions at the sub-picoseconds timescale. Hence, they permit going beyond the first approach of looking at mean square displacements. For both samples, the autocorrelation function was well described by a stretched-exponential function indicating a linkage between the timescales of fast and slow functional relaxation dynamics. The findings of the QENS and INS investigation are discussed in relation to the results of our earlier elastic incoherent neutron scattering and molecular dynamics simulations.
    Journal of The Royal Society Interface 08/2014; 11(97). DOI:10.1098/rsif.2014.0372 · 3.86 Impact Factor
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    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. DOI:10.1021/jp504900q · 4.77 Impact Factor
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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. DOI:10.1134/S0021364013170062 · 1.36 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). DOI:10.1103/PhysRevB.88.134416 · 3.74 Impact Factor
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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. DOI:10.1209/0295-5075/101/26002 · 2.27 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). DOI:10.1103/PhysRevB.86.174305 · 3.66 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. DOI:10.1016/j.matchemphys.2012.07.011 · 2.13 Impact Factor
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    ABSTRACT: By means of quasielastic neutron scattering we have investigated the hydrogen dynamics in poly(alkylene oxide)s (PAOs) with different side-chain lengths at temperatures below as well as above the glass-transition. The combination of results from three different spectrometers (a time-of-flight and two backscattering instruments) has allowed covering almost 4 orders of magnitude in time—from the ps to ns range—with spatial resolution. The results evidence the simultaneous occurrence of vibrations and localized side-group motions at low temperatures and additional diffusive-like (segmental) dynamics at high temperatures. The localized processes of the side groups show (i) stretching of the scattering function, (ii) associated activation energies similar to those found for single and cooperative bond rotations of polyethylene, and (iii) spatial extents that increase with increasing temperature. Compared with poly(ethylene oxide) (PEO), the diffusive segmental process in PAOs presents (i) the same spectral shape, (ii) slower characteristic times—antiplasticization—(iii) similar deviations from Gaussian behavior. For comparison, we also report on backscattering results on the side-group dynamics of poly(n-hexyl methacrylate) in the same temperature range, that show evidence for confinement effects. We suggest that the dynamic asymmetry in systems with intrinsic dynamic heterogeneities between constituent parts is the key ingredient leading to both plasticization and confinement effects.
    Macromolecules 05/2012; 45(10):4394-4405. DOI:10.1021/ma3003399 · 5.93 Impact Factor
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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). DOI:10.1103/PhysRevB.85.134405 · 3.66 Impact Factor
  • F Kargl · H Weis · E Sondermann · T Unruh · G Simeoni · M.M. Koza · A Meyer
    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. DOI:10.1103/PhysRevLett.107.265301 · 7.51 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). DOI:10.1103/PhysRevB.84.224306 · 3.66 Impact Factor
  • R. Viennois · M. M. Koza · P. Jund · J.-C. Tedenac
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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; DOI:10.1016/j.calphad.2011.03.004
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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). DOI:10.1088/1742-6596/344/1/012001