Local structural excitations in model glasses

Structural excitations of model Lennard-Jones glass systems are investigated using the Activation-Relaxation-Technique (ART), which explores the potential energy landscape of a local minimum energy configuration by converging to a nearby saddle-point configuration. Performing ART results in a distribution of barrier energies that is single-peaked for well relaxed samples. The present work characterises such atomic scale excitations in terms of their local structure and environment. It is found that, at zero applied stress, many of the identified events consist of chain-like excitations that can either be extended or ring-like in their geometry. The location and activation energy of these saddle-point structures are found to correlate with the type of atom involved, and with spatial regions that have low shear moduli and are close to the excess free volume within the configuration. Such correlations are however weak and more generally the identified local structural excitations are seen to exist throughout the model glass sample. The work concludes with a discussion within the framework of $\alpha$ and $\beta$ relaxation processes that are known to occur in the under-cooled liquid regime.Comment: 34 Pages, 13 Figure

Four-photon orbital angular momentum entanglement

Quantum entanglement shared between more than two particles is essential to foundational questions in quantum mechanics, and upcoming quantum information technologies. So far, up to 14 two-dimensional qubits have been entangled, and an open question remains if one can also demonstrate entanglement of higher-dimensional discrete properties of more than two particles. A promising route is the use of the photon orbital angular momentum (OAM), which enables implementation of novel quantum information protocols, and the study of fundamentally new quantum states. To date, only two of such multidimensional particles have been entangled albeit with ever increasing dimensionality. Here we use pulsed spontaneous parametric downconversion (SPDC) to produce photon quadruplets that are entangled in their OAM, or transverse-mode degrees of freedom; and witness genuine multipartite Dicke-type entanglement. Apart from addressing foundational questions, this could find applications in quantum metrology, imaging, and secret sharing.Comment: 5 pages, 4 figure

Sub-nanometer free electrons with topological charge

The holographic mask technique is used to create freely moving electrons with quantized angular momentum. With electron optical elements they can be focused to vortices with diameters below the nanometer range. The understanding of these vortex beams is important for many applications. Here we present a theory of focused free electron vortices. The agreement with experimental data is excellent. As an immediate application, fundamental experimental parameters like spherical aberration and partial coherence are determined.Comment: 4 pages, 5 figure

Magnetic thermodynamics as proxy for chemical inhomogeneity in hemo-ilmenite solid solutions: A dynamic ac susceptibility study

In this study, we present ac susceptibility measurements for a synthetic and a natural hemo-ilmenite (HI) solid solution (x)FeTiO3-(1−x)Fe2O3 with compositions x=0.87(1) and 0.88(8), respectively. The focus of the investigation is the magnetic ordering at the Curie temperature T C and the spin-glass-like freezing at the freezing temperature T f. The sharpness of T C for the synthetic solid solution with well-defined structure indicates the chemical homogeneity of the solution, whereas the disperse magnetic ordering of the natural solid solution reveals inhomogeneities described as spin glass system ofvariations in composition x. The frequency dispersion of T f was determined between 10Hz and 10kHz and was found to obey a dynamic scaling power law. The relaxation rates deviate by five orders of magnitude where the synthetic solid solution exhibits ω0=3(1)×104Hz and the natural one 5.5×109Hz. The strong deviation is attributed to the difference in the ordered state above T f. These findings provide an insight into the cooling-rate effects of natural solid solutions and how magnetic thermodynamics can be used to probe the chemical homogeneity of such system

Circular dichroism of cholesteric polymers and the orbital angular momentum of light

We explore experimentally if the light's orbital angular momentum (OAM) interacts with chiral nematic polymer films. Specifically, we measure the circular dichroism of such a material using light beams with different OAM. We investigate the case of strongly focussed, non-paraxial light beams, where the spatial and polarization degrees of freedom are coupled. Within the experimental accuracy, we cannot find any influence of the OAM on the circular dichroism of the cholesteric polymer.Comment: 3 pages, 4 figure

An experiment on the shifts of reflected C-lines

An experiment is described that tests theoretical predictions on how C-lines incident obliquely on a surface behave on reflection. C-lines in a polarised wave are the analogues of the optical vortices carried by a complex scalar wave, which is the usual model for describing light and other electromagnetic waves. The centre of a laser beam that carries a (degenerate) C-line is shifted on reflection by the well-known Goos-H\"anchen and Imbert-Fedorov effects, but the C-line itself splits into two, both of which are shifted longitudinally and laterally; their shifts are different from that of the beam centre. To maximise the effect to be measured, internal reflection in a glass prism close to the critical angle was used. In a simple situation like this two recently published independent theories of C-line reflection overlap and it is shown that their predictions are identical. The measured differences in the lateral shifts of the two reflected C-lines are compared with theoretical expectations over a range of incidence angles.Comment: 9 pages, 2 figure