Michael Marek Koza

Liquid 1-propanol studied by neutron scattering, near-infrared, and dielectric spectroscopy

Liquid monohydroxy alcohols exhibit unusual dynamics related to their hydrogen bonding induced structures. The connection between structure and dynamics is studied for liquid 1-propanol using quasi-elastic neutron scattering, combining time-of-flight and neutron spin-echo techniques, with a focus on the dynamics at length scales corresponding to the main peak and the pre-peak of the structure factor. At the main peak, the structural relaxation times are probed. These correspond well to mechanical relaxation times calculated from literature data. At the pre-peak, corresponding to length scales related to H-bonded structures, the relaxation times are almost an order of magnitude longer. According to previous work [C. Gainaru, R. Meier, S. Schildmann, C. Lederle, W. Hiller, E. Rössler, and R. Böhmer, Phys. Rev. Lett. 105, 258303 (2010)] this time scale difference is connected to the average size of H-bonded clusters. The relation between the relaxation times from neutron scattering and those determined from dielectric spectroscopy is discussed on the basis of broad-band permittivity data of 1-propanol. Moreover, in 1-propanol the dielectric relaxation strength as well as the near-infrared absorbance reveal anomalous behavior below ambient temperature. A corresponding feature could not be found in the polyalcohols propylene glycol and glycerol.

Dynamics of heparan sulfate explored by neutron scattering

The temperature dependence of atomic fluctuations in heparan sulfate was measured for different time-scales between the picosecond and the nanosecond. The data established the role of hydration for the emergence of high-amplitude motions at 200-240 K, and the higher resilience of the polysaccharide compared to proteins measured under similar conditions.

Anharmonicity and guest–host coupling in clathrate hydrates

The dynamic response of Ar, Xe, O2 and N2-containing clathrate hydrates has been studied by neutron spectroscopy. Type I and type II clathrate hydrates show some small but significant differences in the low-frequency host contribution to the density of states. Both differ markedly from that of ice Ih and depend only weakly on the guest. While the vibrational modes associated with Xe and Ar are found to be close-to-harmonic, strong temperature dependent shifts are encountered for the O2 and N2 low-frequency vibrations. A coupling of the guest and host vibrations is confirmed by our experiments. Its strength depends strongly on the type of included guest. In particular Xe-hydrate cannot be considered representative as due to the large mass of the guest atom all its vibrational bands fall into the acoustic range of the host lattice.

On the heterogeneous character of water's amorphous polymorphism

In this paper we report in situ small-angle neutron scattering results on the high-density amorphous (HDA) and low-density amorphous (LDA) ice structures and on intermediate structures as found during the temperature-induced transformation of HDA into LDA. We show that the small-angle signal is characterized by two Q regimes featuring different properties [Q is the modulus of the scattering vector defined as Q = (4\pi/\lambda_{\rm i})\sin(\theta) with \theta being half the scattering angle and \lambda_{\rm i} the incident neutron wavelength]. The very low Q regime (\lt\, \sim \!5\times 10^{-2} A−1) is dominated by a Porod-limit scattering. Its intensity reduces during the course of the HDA-to-LDA transformation following kinetics reminiscent of those observed in wide-angle diffraction experiments. The small-angle neutron scattering form factor in the intermediate regime of 5 \times 10^{-2} \,\lt\, Q \,\lt\, 0.5 A−1 for HDA and LDA features a rather flat plateau. However, the HDA signal shows an ascending intensity towards smaller Q marking this amorphous structure as heterogeneous. When following the HDA-to-LDA transition, the form factor shows a pronounced transient excess in intensity marking all intermediate structures as strongly heterogeneous on a length scale of some nanometres.

Phonon density of states, anharmonicity, electron-phonon coupling, and possible multigap superconductivity in the clathrate superconductors Ba 8 Si 46 and Ba 24 Si 100 : Factors behind large difference in T c

We report a detailed study of specific heat, electrical resistivity, and thermal expansion in combination with inelastic neutron and inelastic x-ray scattering to investigate the origin of superconductivity in the two silicon clathrate superconductors Ba8Si46 and Ba24Si100. Both compounds have a similar structure based on encaged barium atoms in oversized silicon cages. However, the transition temperatures are rather different: 8 and 1.5 K, respectively. By extracting the superconducting properties, phonon density of states, electron-phonon coupling function, and phonon anharmonicity from these measurements, we discuss the important factors governing Tc and explain the difference between the two compounds.

Absence of molecular mobility on nano-second time scales in amorphous ice phases

High-resolution neutron backscattering techniques are exploited to study the elastic and quasi-elastic response of the high-density amorphous (HDA), the low-density amorphous (LDA) and the crystalline ice Ic upon temperature changes. Within the temperature ranges of their structural stability (HDA at T < or = 80 K, LDA at T < or = 135 K, ice Ic at T < or = 200 K) the Debye-Waller factors and mean-square displacements characterise all states as harmonic solids. During the transformations HDA --> LDA (T approximately 100 K), LDA --> Ic (T approximately 150 K) and the supposed glass transition with Tg approximately 135 K no relaxation processes can be detected on a time scale t < 4 ns. It can be concluded from coherent scattering measurements (D2O) that LDA starts to recrystallise into ice Ic at T approximately 135 K, i.e. at the supposed Tg. In the framework of the Debye model of harmonic solids HDA reveals the highest Debye temperature among the studied ice phases, which is in full agreement with the lowest Debye level in the generalised density of states derived from time-of-flight neutron scattering experiments. The elastic results at low T indicate the presence of an excess of modes in HDA, which do not obey the Bose statistics.

Determination of Conformational Entropy of Fully and Partially Folded Conformations of Holo- and Apomyoglobin

Holo- and apomyoglobin can be stabilized in native folded, partially folded molten globules (MGs) and denatured states depending on the solvent composition. Although the protein has been studied as a model system in the field of protein folding, little is known about the internal dynamics of the different structural conformations on the picosecond time scale. In a comparative experimental study we investigated the correlation between protein folding and dynamics on the picosecond time scale using incoherent quasielastic neutron scattering (QENS). The measured mean square displacements (MSDs) of conformational motions depend significantly on the secondary structure content of the protein, whereas the correlation times of the observed internal dynamics were found to be similar irrespective of the degree of folding. The conformational entropy difference ΔSconf between the folded conformations and the acid denatured state could be determined from the measured MSDs and was compared to the entropy difference ΔS obtained from thermodynamic parameters reported in the literature. The observed difference between ΔS and ΔSconf was attributed to the entropy difference ΔShydr of dynamically disordered water molecules of the hydration shell. The entropy content of the hydration water is significantly larger in the native folded proteins than in the partially folded MGs. We demonstrate the potential of incoherent neutron scattering for the investigation of the role of conformational dynamics in protein folding.

Experimental determination of the phonon density of states in filled skutterudites: evidence for a localized mode of the filling atom

The generalized density of states of LaFe4Sb12 and CeFe4Sb12 has been determined by inelastic neutron scattering and its main features are found to be in agreement with recently published calculations (J. L. Feldman, D. L. Singh, C. Kendziora, D. Mandrus and B. C. Sales, Phys. Rev. B, 2003, 68, 094301). In both compounds a localized vibrational contribution appears superposed on the low-energy Debye response. The distinct inelastic response of La in LaFe4Sb12 is obtained by subtraction of the data for the Ce filled compound and it shows even more clearly the resolution limited peak at 7 meV, attributed to the localized mode of La-atoms.

Direct comparison of elastic incoherent neutron scattering experiments with molecular dynamics simulations of DMPC phase transitions.

Abstract.Neutron scattering techniques have been employed to investigate 1,2-dimyristoyl-sn -glycero-3-phosphocholine (DMPC) membranes in the form of multilamellar vesicles (MLVs) and deposited, stacked multilamellar-bilayers (MLBs), covering transitions from the gel to the liquid phase. Neutron diffraction was used to characterise the samples in terms of transition temperatures, whereas elastic incoherent neutron scattering (EINS) demonstrates that the dynamics on the sub-macromolecular length-scale and pico- to nano-second time-scale are correlated with the structural transitions through a discontinuity in the observed elastic intensities and the derived mean square displacements. Molecular dynamics simulations have been performed in parallel focussing on the length-, time- and temperature-scales of the neutron experiments. They correctly reproduce the structural features of the main gel-liquid phase transition. Particular emphasis is placed on the dynamical amplitudes derived from experiment and simulations. Two methods are used to analyse the experimental data and mean square displacements. They agree within a factor of 2 irrespective of the probed time-scale, i.e. the instrument utilized. Mean square displacements computed from simulations show a comparable level of agreement with the experimental values, albeit, the best match with the two methods varies for the two instruments. Consequently, experiments and simulations together give a consistent picture of the structural and dynamical aspects of the main lipid transition and provide a basis for future, theoretical modelling of dynamics and phase behaviour in membranes. The need for more detailed analytical models is pointed out by the remaining variation of the dynamical amplitudes derived in two different ways from experiments on the one hand and simulations on the other.Graphical abstract

Effect of the electropositive elements A = Sc, La, and Ce on the microscopic dynamics of AV2Al20

We report on the inelastic response of AV2Al20 (with A = Sc, La and Ce) probed by high-resolution inelastic neutron scattering experiments. Intense signals associated with the dynamics of Sc, La and Ce are identified in the low-energy range at 6-14 meV in ScV2Al20 and at 8-16 meV in LaV2Al20 and CeV2Al20. Their response to temperature changes between 2 and 300 K reveals a very weak softening of the modes upon heating in LaV2Al20 and CeV2Al20 and a distinguished blue shift by about 2 meV in ScV2Al20. By means of density functional theory (DFT) and lattice dynamics calculations (LDC) we show that the unusual anharmonicity of the Sc-dominated modes is due to the local potential of Sc featured by a strong quartic term. The vibrational dynamics of ScV2Al20 as well as of LaV2Al20 and CeV2Al20 is reproduced by a set of eigenmodes. To screen the validity of the DFT and LDC results they are confronted with data from X-ray diffraction measurements. The effect of the strong phonon renormalization in ScV2Al20 on thermodynamic observables is computed on grounds of the LDC derived inelastic response. To set the data in a general context of AV2Al20 compounds and their physical properties we report in addition computer and experimental results of the binary V2Al20 compound.