Department of Physics John Anderson Research Colloquia

Wednesday's at 3.00pm (unless otherwise stated) 

Colloquia will usually be held in JA3.14
John Anderson Building 
107 Rottenrow, Glasgow

Coffee and Tea served at 4.00 pm.

All Welcome

Coordinated with the Colloquia at the Department of Physics and Astronomy of the University of Glasgow. (They may have donuts but we have free chocolate covered biscuits and coffee!)

Colloquia Schedule 2017-2018

Semester I

  • 20/09/17 - Oliver Henrich (Dept of Physics, University of Strathclyde)
  • 01/11/17 - Sebastian Van De Linde (Dept of Physics, University of Strathclyde)
  • 08/11/17 - Nick Kelly (Dept of Mechanical and Aerospace Engineering, University of Strathclyde) *
  • 14/11/17 - Peter Mulser (TQE: Theoretical Quantum Electronics, Tech. Univ. Darmstadt) *
  • 15/11/17 - Stephane Coen (The University of Auckland, New Zealand)
  • 29/11/17 - Ralph Tatam (Cranfield)

Semester II

 

* Note: Outside of regular schedule.

Mesoscopic Simulation of Soft Condensed Matter

Oliver Henrich (Dept of Physics, University of Strathclyde) 20 September 2017, JA3.14, 3pm

Soft condensed matter is an interdisciplinary field at the interface of physics, chemistry and biology. It comprises a huge variety of materials that we know from our everyday lives, for instance colloidal suspensions, glasses, amphiphilic mixtures, polymeric liquids, foams, gels, granular matter, liquid crystals, and a vast number of biologically relevant substances.

All these materials share the common feature that their generic physical behaviour occurs at energy scales that are comparable to thermal energies at room temperature, which is why we call them ‘soft’. As deformability often entails flow, hydrodynamics plays a particularly important part in their dynamical behaviour.

Another characteristic feature is their propensity to self-assemble into more complex structures on intermediate time and length scales that are well above the molecular level, but also much smaller than the macroscopic lab scale. Both aspects have led to the development of specialised, mesoscopic simulation methods for soft matter.

Starting from Landau’s phenomenological theory of a binary, phase-separating fluid, I will outline a thermodynamically consistent description of complex fluids that is based on so-called free energy models. I will introduce into the lattice Boltzmann method, which solves a discretised version of the Boltzmann equation from statistical mechanics, and show how these concepts can be applied to study novel composite materials for tuneable optical filters and encryption devices.

TBA

Sebastian Van De Linde (University of Strathclyde) 1 November 2017, JA3.14, 3pm

TBA

Nick Kelly (University of Strathclyde) 8 November 2017, JA3.14, 3pm

Adiabatic invariants and generalized ponderomotive force

Peter Mulser (TQE: Theoretical Quantum Electronics, Tech. Univ. Darmstadt) 14 November 2017

The ponderomotive force embraces a realm of physics: collisionless shock wave generation, novel schemes of particle acceleration, stimulated Brillouin and Raman scattering in solids, liquids, gases and plasmas, ac Stark effect in atoms, and the Lamb shift. In the introduction the standard expressions of the ponderomotive force, or wave pressure, are presented and their derivations and physical interpretations are indicated.

In the central part of the talk its connection with the adiabatic invariants is established. Conservation of action is illustrated by various, also historical examples to begin with Lord Rayleigh, and are verified by simple arguments. The adiabatic theorem is formulated and its connection with the Poincaré-Cartan invariant is shown. On this Hamiltonian basis we are prepared for generalized expressions of the ponderomotive potential and force. A fundamental difference between the forces from a longitudinal and a transverse wave will appear. Adiabatic particle trapping, detrapping and acceleration in the electron plasma wave will result as a byproduct. The results will be illustrated by simple case studies.

In the conclusion a survey on conservation of action in various disciplines of physics and applied mathematics: in optics, thermodynamics, quantum physics, time scale methods and linearization, will be briefly discussed. The principle unifying them lies in one property of alternating series of mathematics.

TBA

Stephane Coen (The University of Auckland, New Zealand) 15 November 2017, JA3.14, 3pm

 

TBA

Ralph Tatam (Cranfield) 29 November 2017, JA3.14, 3pm

 

Correlative Imaging: From Cells to Stars

Lucy M. Collinson (The Francis Crick Institute) 31 January 2018, JA3.14, 3pm, 

Correlative light and electron microscopy (CLEM) combines the benefits of fluorescence and electron imaging, revealing protein localisation against the backdrop of cellular architecture. The correlative imaging field is expanding rapidly, and encompasses workflows that link many different imaging modalities, to answer scientific questions in the biological and physical sciences. We link fluorescence microscopes (widefield, confocal, super-resolution and light-sheet) with electron microscopes (scanning, transmission, serial block face and focused ion beam) and X-ray microscopes (microCT and soft X-ray) to analyse a range of biological samples, from single cells to whole model organisms.

Our technology development work has focused on improving the speed, accuracy and accessibility of CLEM. During this development work, it became clear that the technical challenges associated with correlative imaging are exaggerated when working in 3D. To increase protein localisation precision, we developed an ‘In-Resin Fluorescence’ (IRF) protocol that preserves the activity of GFP and related fluorophores in resin-embedded cells and tissues. The sample preparation is relatively fast, and also introduces electron contrast so that cell structure can be visualised in the electron microscope. Once the resin blocks have been cut into ultrathin sections, out-of-plane fluorescence is removed resulting in physical ‘super-resolution’ light microscopy in the axial direction, which increases the accuracy of the LM-EM overlays. Localisation precision is further increased by imaging the IRF sections in vacuo in the next generation of commercial integrated light and electron microscopes (ILEM). We were able to further improve accuracy by developing integrated super-resolution light and electron microscopy, using the remarkable blinking properties of GFP and YFP in-resin in vacuo.

With the advent of dual contrast samples comes the potential to locate and track fluorescent cells during sample preparation and automated 3D EM image acquisition. We designed and built two new locator tools – a fluorescence microscope designed to integrate with an ultramicrotome to locate cells during trimming and sectioning (the ultraLM), and an even smaller version that fits into the extremely tight space of the SBF SEM vacuum chamber for on-the-fly tracking of fluorescent cells during long automated imaging runs (the miniLM).

As electron microscopes become more automated, data outputs are increasing astronomically, and so the bottleneck in our work is shifting from data acquisition to data analysis. I will describe the ideas and workflows we are developing to deal with big data, and describe our Citizen Science collaboration with the Zooniverse platform, which is helping us to develop automated detection and segmentation of cell structures in EM images.

Group-III-nitride-based hetero structures for sensor and UV-LED applications: Research activities at Ulm University

Ferdinand Scholz (Ulm) 28 February 2018, JA3.14, 3pm

GaN and related compounds have attracted immense research interest world-wide since the first presentation of very bright blue LEDs in 1992. At Ulm University, we have focused on the epitaxial growth and characterization of GaN-based hetero structures for optoelectronic devices since more than 20 years, contributing in particular to studies of semipolar LED structures over the last 10 years. Currently, we focus our efforts on two other topics which will be taken as main subjects for this talk.

Owing to the excellent chemical stability of GaN, such materials are regarded as bio-compatible and hence they are good candidates to act as chemical and bio-medical sensors. In our current studies, polar GaInN quantum well structures are applied for sensing different molecules adsorbed on the sample surface. Adsorbate-caused changes of the band bending near the surface and hence the photoluminescence signal of near-surface GaInN quantum wells are taken as chemical sensing signal. This optical read-out makes electrical contacts obsolete, which might be problematic in chemically harsh environments. The sensitivity depends on the design of the hetero structures like QW and cap layer thickness, as evaluated by band structure simulations. Besides gases such as oxygen and hydrogen, also biomolecules can be adsorbed on the semiconductor surface and studied by PL. As an example, we have studied the protein “ferritin”. Its concentration in blood is an important indicator for the iron storage state in our body. Ferritins with and without iron-load (the latter corresponds to apoferritin) are immobilized on hydroxylated polar GaInN quantum well surfaces and lead to a distinct spectral shift of the quantum well PL. In order to selectively detect such proteins, we investigate possibilities to functionalize the GaN surface by other specific organic molecules.

Another focus of our work is set on AlGaN-based hetero structures for LED applications in the UV-C spectral range (around 270 nm). Such LEDs still suffer from relatively poor external quantum efficiencies, which is discussed to be caused at least partly by the fairly large lattice mismatch between the various materials forming these hetero structures. Therefore, we investigate the incorporation of boron (B) into AlGaN layers, which reduces the lattice constant and hence may help to manage the strain in such hetero structures. Unfortunately, the miscibility of B in AlGaN is very low. For total boron contents of about 5% in AlBN epitaxially grown by MOVPE, we see strong indications for phase separation between AlN and boron. For lower boron contents, phase separation could not be observed in thin layers. However, strong 3D growth developed for thicker or multi layers (Fig. 1) resulting in tilted facets and surface roughening, certainly a consequence of the low mobility of B on the growing surface. Hence, further optimization of the MOVPE growth conditions has to be done.

These studies are done in cooperation with K. Thonke et al., Inst. of Quantum Matter, T. Weil et al., Inst. of Organic Chemistry III, and U. Kaiser et al., Central Facility of Electron Microscopy, all at Ulm University.

Fig.1: Cross-section Transmission electron micrograph of an AlN/AlBGaN superlattice structure.

TBA

Fred Manby (University of Bristol) 14 March 2018, JA3.14, 3pm