Neutron reflectometry (NR) is a technique used for probing the structure of buried interfaces and is particularly useful for studying the structure of surfaces and thin films within condensed matter systems. In the context of soft condensed matter lipid bilayers deposited on the surface of a solid substrate, are heavily investigated as they can be designed to mimic different kinds of biological membranes. NR can be used to obtain structural properties such as thickness, solvent penetration or roughness of the adsorbed layers at interfaces. Moreover, by fitting the neutron reflectivity data to a model of neutron scattering length density (SLD) it is possible to determine the chemical composition of the films. In addition, due to the neutron’s magnetic moment, it is possible to obtain the magnetic properties of a material by using polarised neutron beams and analysing the magnetic SLD depth profile.

When fitting model parameters to experimental NR data, it is often challenging to decouple material related parameters, such as real and imaginary parts of the SLD, and structural parameters like layer thicknesses and interface roughness. In optical (photonic) analysis, many methods have been developed to solve such correlation problems. One approach is referred to as multiple sample analysis (MSA), where two or more similar samples, but with some parameters varied, are measured. In the subsequent analysis, two or more corresponding models are fitted simultaneously to the measurements. In NR there is an analogous standard technique of contrast variation, where the problem to decouple parameters is even more challenging since only intensities are measured with the loss of phase information – often termed the "phase problem". Furthermore, an additional possibility to find unique solutions of the SLD from reflectivity data is to use switchable magnetic reference layers (MRL). In the layered thin film structure, a MRL is deposited, whose characteristics can be controlled and, therefore, known beforehand. By applying an external magnetic field this layer is magnetised in a specific direction and probed with neutrons of different spin states. The MRL thus provides additional measurement data and a possibility to decouple the model parameters.

Since NR experiments are both extremely expensive to run, as well as difficult to access, it is important to make the best possible use of the experimental time. Reducing measurement time while maintaining high precision is key to expanding the applicability of neutron scattering techniques. To improve the effectiveness in extracting useful information from neutron reflectivity experiments we have designed substrate assemblies comprising a Si slab, a switchable MRL, and an inert top layer, specifically for modelling and characterisation of thin coatings with unknown properties with lipid bilayers or polymers being prime examples. An optimised substrate stack yields significantly different SLD profiles for polarised neutrons upon opposite magnetisations, effectively increasing the available data for obtaining the SLD profile for the unknown coating. The substrate assemblies are designed using the Holistic Optimization for Gaining Better Evidence from Neutrons HOGBEN software employing a sensitivity analysis based on Fisher information FI and correlation matrices, enabling systematic evaluation of the information gain for different configurations. The importance of this research lies in the potential to address the problem with limited beamtime access at neutron facilities by reducing the total measurement time required per sample without compromising obtained data quality.

The present study aims at optimising the design of solid substrates for polarised neutron reflectometry (PNR) experiments at the solid/liquid interface for the structural investigation of soft matter/biology samples. The substrate assembly in this work consisted of a Si single crystal with its native oxide, a ferromagnetic Fe reference layer, and a SiO₂ capping layer. By exploiting the magnetic contrast provided by the Fe layer and performing measurements in different ambients (H₂O, D₂O, and SMW), we obtained several reflectivity datasets from a single sample. The measurements, carried out with the POLREF instrument at the ISIS Neutron and Muon Source, STFC Rutherford Appleton Laboratory, UK provided detailed information on the SLD profile of a head-tail-head bilayer lipid structure. Building on these results, we initiated sensitivity studies using parameter correlation and FI to find the optimal substrate assembly designs that minimise measurement time while preserving data quality. Our results demonstrate that sensitivity is significantly improved by jointly optimising the thicknesses of the Fe and the capping layer. In particular, we find that well-chosen configurations of the MRL and capping layers can yield equivalent experimental information with up to a fivefold reduction in measurement time. 

Hypothesis: Sponge phase (L₃) lipid nanoparticles (L₃-NPs) have been shown to have large potential for the encapsulation of biomolecules, such as enzymes, with applications in food and pharmaceutical science. In this study, we introduce new formulations of L₃-NPs including the phospholipids dioleoylphosphatidylcholine (DOPC) and dioleoyltrimethylammonium propane (DOTAP). The interaction of these new L₃-NPs with myoglobin is of interest for the development of iron supplements which can be incorporated during food processing. Experiments: We characterized the sample structure by small-angle X-ray scattering (SAXS) measurements with and without the addition of myoglobin. We also tested the myoglobin-lipid interaction in an experimental setup that mimicked the interface between the bilayer and water channels within the bicontinuous sponge structure. This included spreading the L₃-NPs onto a hydrophilic surface to form supported lipid bilayers and characterizing their interaction with myoglobin by means of quartz crystal microbalance with dissipation monitoring and polarized neutron reflectometry. Findings: SAXS data indicate that the new formulations containing DOPC and DOTAP formed a sponge phase in the bulk. The data from the surface techniques showed that deposited bilayers containing DOPC were largely unaffected by the addition of myoglobin, whereas those without DOPC were destabilized and partially removed.

Quantum wires and quantum point contacts (QPCs) have been realized in GaAs/AlGaAs heterostructures, where a two-dimensional electron gas (2DEG) resides at the interface between the GaAs and AlGaAs layered semiconductors. This paper presents a theoretical study of electron transport in a device composed of two QPCs connected via a wider two-dimensional (2D) region. The QPCs, defined by split gates, serve as injector and detector of electrons, while the intermediate 2D region the two QPCs can be electrostatically tuned by a top gate. Electron transport is modeled using density functional theory. Experimental observations suggest that, when the injector QPC is asymmetrically biased, the broadening of the current peak detected at the detector QPC may be associated with spin polarization in the injector. Our simulations for both symmetric and asymmetric injector QPCs indicate that the shape of detector current profile does not strongly depend on injector’s asymmetry, however the width of the current distribution varies with the current through injector QPC. This variation is consistent with the presence of spin-related effects, such as the 0.7(2e²/h) conductance anomaly observed in the injector QPC. These results provide insight into the electronic property of 2DEG in the proposed device and may be useful for the future design of semiconductor structures for spintronics and quantum device applications.

Quantum wires (QWs) and quantum point contacts (QPCs) have been realized in GaAs/AlGaAs heterostructures in which a two-dimensional electron gas resides at the interface between GaAs and AlGaAs layered semiconductors. The electron transport in these structures has previously been studied experimentally and theoretically, and a 0.7 conductance anomaly has been discovered. The present paper is motivated by experiments with a QW in shallow symmetric and asymmetric confinements that have shown additional conductance anomalies at zero magnetic field. The proposed device consists of a QPC that is formed by split gates and a top gate between two large electron reservoirs. This paper is focussed on the theoretical study of electron transport through a wide top-gated QPC in a low-density regime and is based on density functional theory. The electron–electron interaction and shallow confinement make the splitting of the conduction channel into two channels possible. Each of them becomes spin-polarized at certain split and top gates voltages and may contribute to conductance giving rise to additional conductance anomalies. For symmetrically loaded split gates two conduction channels contribute equally to conductance. For the case of asymmetrically applied voltage between split gates conductance anomalies may occur between values of 0.25(2e²/h) and 0.7(2e²/h) depending on the increased asymmetry in split gates voltages. This corresponds to different degrees of spin-polarization in the two conduction channels that contribute differently to conductance. In the case of a strong asymmetry in split gates voltages one channel of conduction is pinched off and just the one remaining channel contributes to conductance. We have found that on the perimeter of the anti-dot there are spin-polarized states. These states may also contribute to conductance if the radius of the anti-dot is small enough and tunneling between these states may occur. The spin-polarized states in the QPC with shallow confinement tuned by electric means may be used for the purposes of quantum technology.

We investigate the phase transition of Bose-Einstein particles with the hard-core repulsion in the grand canonical ensemble within the Van der Waals approximation. It is shown that the pressure of non-relativistic Bose-Einstein particles is mathematically equivalent to the pressure of simplified version of the statistical multifragmentation model of nuclei with the vanishing surface tension coefficient and the Fisher exponent , which for such parameters has the 1-st order phase transition. The found similarity of these equations of state allows us to show that within the present approach the high density phase of Bose-Einstein particles is a classical macro-cluster with vanishing entropy at any temperature which, similarly to the system of classical hard spheres, is a kind of solid state. To show this we establish new relations which allow us to identically represent the pressure of Fermi-Dirac particles in terms of pressures of Bose-Einstein particles of two sorts.

High-mobility two-dimensional electron gas (2DEG) which resides at the interface between GaAs and AlGaAs layered semiconductors has been used experimentally and theoretically to study ballistic electron transport. The present paper is motivated by recent experiments in magnetic electron focusing. The proposed device consists of two quantum point contacts (QPCs) serving as electron injector and collector which are placed in the same semiconductor GaAs/AlGaAs heterostructure. Here we focus on a theoretical study of the injection of electrons via a quantum wire/QPC into an open two-dimensional (2D) reservoir. The transport is considered for non-interacting electrons at different transmission regimes using the mode-matching technique. The proposed mode-matching technique has been implemented numerically. Electron flow through the quantum wire with hard-wall rectangular, conical and rounded openings has been studied. We have found for these three cases that the geometry of the opening does not play a crucial role for the electron propagation. When a perpendicular magnetic field is applied the electron paths in the 2D reservoir are curved. We analyse this case both classically and quantum-mechanically. The effect of spin-splitting due to exchange interactions on the electron flow is also considered. The effect is clearly present for realistic choices of device parameters and consistent with observations. The results of this study may be applied in designing magnetic focusing devices and spin separation.

We develop a novel formulation of the hadron resonance gas model which, besides a hard-core repulsion, explicitly accounts for the surface tension induced by the interaction between the particles. Such an equation of state allows us to go beyond the Van der Waals approximation for any number of different hard-core radii. A comparison with the Carnahan Starling equation of state shows that the new model is valid for packing fractions 0.2-0.22, while the usual Van der Waals model is inapplicable at packing fractions above 0.1-0.11. Moreover, it is shown that the equation of state with induced surface tension is softer than the one of hard spheres and remains causal at higher particle densities. The great advantage of our model is that there are only two equations to be solved and neither their number nor their form depend on the values of the hard-core radii used for different hadronic resonances. Such an advantage leads to a significant mathematical simplification compared to other versions of truly multi-component hadron resonance gas models. Using this equation of state we obtain a high-quality fit of the ALICE hadron multiplicities measured at the center-of-mass energy 2.76 TeV per nucleon and we find that the dependence of chi(2)/ndf on the temperature has a single global minimum in the traditional hadron resonance gas model with the multi-component hard-core repulsion. Also we find two local minima of chi(2)/ndf in the model in which the proper volume of each hadron is proportional to its mass. However, it is shown that in the latter model a second local minimum located at higher temperatures always appears far above the limit of its applicability. (C) 2017 Elsevier B.V. All rights reserved.

Here we present a generalization of the multicomponent Van der Waals equation of state in the grand canonical ensemble. For the one-component case the third and fourth virial coefficients are calculated analytically. It is shown that the adjustment of a single model parameter allows us to reproduce the third and fourth virial coefficients of the gas of hard spheres with small deviations from their exact values. A thorough comparison of the compressibility factor and speed of sound of this model with the one- and two-component Carnahan-Starling equation of state is made. We show that the model with the induced surface tension can reproduce the results of the Carnahan-Starling equation of state up to the packing fractions 0.2-0.22 at which the Van der Waals equation of state is inapplicable. Using this approach we develop an entirely new hadron resonance gas model and apply it to a description of the hadron yield ratios measured at AGS, SPS, RHIC and ALICE energies of nuclear collisions. We confirm that the strangeness enhancement factor has a peak at low AGS energies and that there is a jump of chemical freeze-out temperature between the two highest AGS energies. Also we argue that the chemical equilibrium of strangeness, i.e gamma(s) similar or equal to 1, observed above the center of mass collision energy 8.7 GeV, may be related to a hadronization of quark gluon bags which have a Hagedorn mass spectrum, and, hence, it may be a new signal for the onset of deconfinement.

High-mobility two-dimensional electron gas (2DEG) which resides at the interface between GaAs and AlGaAs layered semiconductors has been used experimentally and theoretically to study ballistic electron transport. The present project is motivated by recent experiments in magnetic electron focusing. The proposed device consists of two quantum point contacts (QPCs) serving as electron injector and detector which are placed in the same semiconductor GaAs/AlGaAs heterostructure. This thesis is focused on the theoretical study of electron flow coming from the injector QPC (a short quantum wire) and going into an open two-dimensional (2D) reservoir. The transport is considered for non-interacting electrons at different transmission regimes using the mode-matching technique. The proposed mode-matching technique has been implemented numerically using Matlab software. Electron flow through the quantum wire with rectangular, conical and rounded openings has been studied with and without an applied electric bias. We have found that the geometry of the opening does not play a crucial role for the electron flow propagation while the conical opening allows the electrons to travel longer distances into the 2D reservoir. When electric bias is applied, the electron flow also penetrates farther into the 2D region. The results of this study can be applied in designing magnetic focusing devices.