[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] 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; · 4.84 Impact Factor
[Show abstract][Hide abstract] 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). · 3.86 Impact Factor
[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. · 4.84 Impact Factor
[Show abstract][Hide abstract] 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
[Show abstract][Hide abstract] 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
[Show abstract][Hide abstract] 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. · 4.84 Impact Factor
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] 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).
[Show abstract][Hide abstract] 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. · 1.68 Impact Factor
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] 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
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
Journal of Experimental and Theoretical Physics 08/2010; 111(2):215-219. · 0.93 Impact Factor
[Show abstract][Hide abstract] 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.