arXiv subjects

Maikel C. Rheinstädter

Publications and source records attributed to Maikel C. Rheinstädter.

7 recordsLinked to original sources

arXiv2006

Nanosecond molecular relaxations in lipid bilayers studied by high energy resolution neutron scattering and in-situ diffraction

We report a high energy-resolution neutron backscattering study to investigate slow motions on nanosecond time scales in highly oriented solid supported phospholipid bilayers of the model system DMPC -d54 (deuterated 1,2-dimyristoyl-sn-glycero-3-phoshatidylcholine), hydrated with heavy water. Wave vector resolved quasi-elastic neutron scattering (QENS) is used to determine relaxation times $τ$, which can be associated with different molecular components, i.e., the lipid acyl chains and the interstitial water molecules in the different phases of the model membrane system. The inelastic data are complemented both by energy resolved and energy integrated in-situ diffraction. From a combined analysis of the inelastic data in the energy and time domain, the respective character of the relaxation, i.e., the exponent of the exponential decay is also determined. From this analysis we quantify two relaxation processes. We associate the fast relaxation with translational diffusion of lipid and water molecules while the slow process likely stems from collective dynamics.

cond-mat.soft↗

arXiv2005

Molecular motions in lipid bilayers studied by the neutron backscattering technique

We report a high energy-resolution neutron backscattering study to investigate slow motions on nanosecond time scales in highly oriented solid supported phospholipid bilayers of the model system DMPC -d54 (deuterated 1,2-dimyristoyl-sn-glycero-3-phoshatidylcholine), hydrated with heavy water. This technique allows to discriminate the onset of mobility at different length scales for the different molecular components, as e.g.\@ the lipid acyl-chains and the hydration water in between the membrane stacks, respectively, and provides a benchmark test regarding the feasibility of neutron backscattering investigations on these sample systems. We discuss freezing of the lipid acyl-chains, as observed by this technique, and observe a second freezing transition which we attribute to the hydration water.

cond-mat.soft↗

arXiv2004

Probing Dynamics at Interfaces: Options for Neutron and X-ray Spectroscopy

Inelastic neutron and X-ray scattering experiments on surfaces and interfaces are a challenging topic in modern physics. Particular interest arises regarding surfaces and interfaces of soft matter and biological systems. We review both neutron and x-ray spectroscopic techniques with view at their applicability to these samples. We discuss the different methods, namely neutron triple-axis, backscattering and spin-echo spectroscopy as well as x-ray photon correlation spectroscopy, in the context of planar lipid membrane models as an example. By the combination of the different methods, a large range in momentum and energy transfer is accessible.

physics.bio-ph↗

arXiv2004

Probing Dynamics at Interfaces: Molecular Motions in Lipid Bilayers studied by Neutron Backscattering

Lipid membranes in a physiological context cannot be understood without taking into account their mobile environment. Here, we report on a high energy-resolution neutron backscattering study to investigate slow motions on nanosecond time scales in highly oriented solid supported phospholipid bilayers of the model system DMPC -d54 (deuterated 1,2-dimyristoyl-sn-glycero-3-phoshatidylcholine). This technique allows discriminating the Q-dependent onset of mobility and provides a benchmark test regarding the feasibility of dynamical neutron scattering investigations on these sample systems. Apart from freezing of the lipid acyl-chains, we could observe a second freezing temperature that we attribute to the hydration water in between the membrane stacks. The freezing is lowered several degrees as compared to (heavy) bulk water.

physics.bio-ph↗

arXiv2004

Quenched chirality in RbNiCl$_3$

The critical behaviour of stacked-triangular antiferromagnets has been intensely studied since Kawamura predicted new universality classes for triangular and helical antiferromagnets. The new universality classes are linked to an additional discrete degree of freedom, chirality, which is not present on rectangular lattices, nor in ferromagnets. However, the theoretical as well as experimental situation is discussed controversially, and generic scaling without universality has been proposed as an alternative scenario. Here we present a careful investigation of the zero-field critical behaviour of RbNiCl$_3$, a stacked-triangular Heisenberg antiferromagnet with very small Ising anisotropy. From linear birefringence experiments we determine the specific heat exponent $α$ as well as the critical amplitude ratio $A^+/A^-$. Our high-resolution measurements point to a single second order phase transition with standard Heisenberg critical behaviour, contrary to all theoretical predictions. From a supplementary neutron diffraction study we can exclude a structural phase transition at T$_N$. We discuss our results in the context of other available experimental results on RbNiCl$_3$ and related compounds. We arrive at a simple intuitive explanation which may be relevant for other discrepancies observed in the critical behaviour of stacked-triangular antiferromagnets. In RbNiCl$_3$ the ordering of the chirality is suppressed by strong spin fluctuations, yielding to a different phase diagram, as compared to e.g.\@ CsNiCl$_3$, where the Ising anisotropy prevents these fluctuations.

cond-mat.other↗

arXiv2004

Low dimensional ordering and fluctuations in methanol-$β$-hydroquinone-clathrate studied by X-ray and neutron diffraction

Methanol-$β$-hydroquinone-clathrate has been established as a model system for dielectric ordering and fluctuations and is conceptually close to magnetic spin systems. In X-ray and neutron diffraction experiments, we investigated the ordered structure, the one-dimensional (1D) and the three-dimensional (3D) critical scattering in the paraelectric phase, and the temperature dependence of the lattice constants. Our results can be explained by microscopic models of the methanol pseudospin in the hydroquinone cage network, in consistency with previous dielectric investigations.

cond-mat.other↗

arXiv2004

Aging and scaling laws in $β$-hydroquinone-clathrate

The dielectric permittivity of the orientational glass methanol(x=0.73)-$β$-hydroquinone-clathrate has been studied as function of temperature and waiting time using different temperature-time-protocols. We study aging, rejuvenation and memory effects in the glassy phase and discuss similarities and differences to aging in spin-glasses. We argue that the diluted methanol-clathrate, although conceptually close to its magnetic pendants, takes an intermediate character between a true spin-glass and a pure random field system.

cond-mat.dis-nn↗