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Physics department colloquia

Here is the current colloquium schedule for semester 2 2022/23, and a list of previous talks.

The hybrid colloquia take place at 13:15 UK time on a Friday (occasionally on other days) during the semester and are open to anyone from the university, students are encouraged to attend.

For any questions about the colloquia, please contact Dr Anton Souslov

The next colloquia

Prof Philip Moriarty, University of Nottingham Friday 23 June, 13:15 UK time How quickly does an encaged electron tunnel free?

Abstract: Remarkable advances in synthetic chemistry mean that fullerenes can now be partially ‘unzipped’, an atom or molecule placed inside, and the carbon framework subsequently zipped up again to restore the original cage architecture [1]. The resulting endofullerene represents both an exotic state of matter (essentially a gas phase-solid state hybrid) and a unique platform for the controlled study of intra- and intermolecular charge transfer. I will discuss unpublished results from our recent beamtime experiments at the Diamond Light Source (Beamline I09) in which we have used the core-hole clock technique[2], coupled with normal incidence X-ray standing wave (NIXSW) analysis [3], to time how long it takes for an electron excited to the 4s state of an argon atom in Ar@C60 to tunnel free. Time permitting, application of the core-hole clock technique to an encapsulated molecule – namely, nitrogen in N2@C60 – will also be covered, highlighting the key role of vibrational excitation.

Previous talks - semester 2 2022-23

Fri 16 June Dr Gareth Alexander, University of Warwick Active Liquid Crystals and Chiral Mechanics

Active liquid crystals are materials with liquid crystalline order – the stuff that’s in our flat screen displays – but that are driven out of equilibrium by energy uptake at the microscopic level. In recent years it has become established that active liquid crystals provide a framework for modelling a range of biological materials, including individual cells and entire tissues, giving insights into motility, cell apoptosis, and morphogenesis. The majority of these advances have come from analysis of the simplest liquid crystal phase, the nematic, and systems that are effectively two-dimensional. However, there are a great variety of different liquid crystalline phases, including in biological materials both in vitro and in vivo. After summarising the theoretical description of active liquid crystals using non-equilibrium statistical physics, I will describe our recent work on three-dimensional materials with focus on spatially modulated states, including cholesteric and columnar phases. These phases exhibit distinctly new phenomenology associated with active chiral, or ‘odd’, mechanics, including the formation of controllable vortex lattices and oscillations in a purely overdamped Stokesian fluid.

Fri 9 June Prof Nathan Lepora, University of Bristol
How will AI transform robotics?

Over the last decade, AI has made huge advances in interpreting complex information learnt from lots of data. First, this transformed image processing, e.g. recognizing photos, then it transformed speech/language processing, e.g. Siri/Alexa, and more recently it has transformed text generation, e.g. ChatGPT. However, to affect the real world directly, the AI needs to be embodied in robots. In this talk, I describe how this may be the next major advance in AI and how it could transform future robotics.

Fri 26 May
Dr Sophie Nedelec, University of Exeter Coral reef soundscapes and boat noises

Anthropogenic noise is a pollutant of international concern, with mounting evidence of impacts on animal behaviour and physiology that are pervasive across taxa, ecosystems and the world. Recent work from Dr Nedelec and her team shows that underwater noise affects all stages of the life cycles of fishes that inhabit fragile coral reef habitats. Stressed and badly behaved fishes are pushed to their limits, with impacts on survival.

But the tide could turn on noise pollution. Dr Nedelec’s team experimentally tested the hypothesis that protecting vulnerable habitats from noise pollution can improve animal reproductive success. Using a season-long field manipulation with an established model system on the Great Barrier Reef, the spiny chromis, they demonstrated that limiting motorboat activity on reefs leads to the survival of more fish offspring compared to reefs experiencing busy motorboat traffic. A complementary laboratory experiment isolated the importance of noise and, in combination with the field study, showed that the enhanced reproductive success on protected reefs is likely due to improvements in parental care and offspring growth. Noise mitigation and abatement offer simple wins in protecting coral reefs from human impacts, and present a valuable opportunity for enhancing ecosystem resilience.

Fri 31 March
Dr Claire Cisowski, University of Glasgow
Optical skyrmions: novel topological fields of light

Skyrmions are topologically protected field configurations that have been observed in condensed matter systems, chiral magnetic systems and nematic liquid crystals, and most recently in optics. Since their initial conception in 2018, optical skyrmions have attracted considerable attention due to their highly reconfigurable generation. Unlike their magnetic counterparts, optical skyrmions are free from energy constraints and therefore allow the investigation of topological stability in its purest form. In this talk, I will provide a definition of optical skyrmions based on rational maps, I will show how we generate such beams in the laboratory and I will demonstrate how we can characterize them using topological considerations.

Fri 24 March
Dr Habib Rostami, University of Bath
Straintronics and Phononics in 2D Quantum Materials

Previous talks - semester 1 2022-23

Fri 16 December 2022
Prof Tannie Liverpool, University of Bristol
Mathematics, Physics, Biology and the 2nd law of Thermodynamics (or the Statistical Mechanics of Wound Healing)

Abstract: I will discuss some recent work looking quantitatively at the process of wound healing using ideas from thermodynamics and statistical mechanics. Wound healing is a highly conserved process required for survival of an animal after tissue damage. The wound repair process is not only of great interest in its own right but is also a laboratory to study complex tissue dynamics and regeneration.

Many wounds involve damage to an epithelial (barrier) tissue (like skin) that separates different regions of the body of a living organism. I will describe some recent work on studying wound healing in two dimensional epithelial tissues of a fruit fly pupal wing. This epithelium was chosen because it is transparent and accessible to sophisticated imaging techniques. We use live confocal time-lapse microscopy to follow the behaviour of cells in a tissue before and after wounding.

I will focus on three cell-behaviours that are generally accepted to contribute to wound re-epithelialisation: cell shape deformation, cell division, and cell migration.

I will describe how we are beginning to use a combination of theory and experiment to disentangle some of the organising principles behind the complex orchestrated dynamics that lead to wound healing.

Fri 2 December 2022
Prof Euan Hendry, University of Exeter
Single pixel THz imaging using near field photomodulation

Abstract: Many materials respond in interesting ways to terahertz frequency radiation, and due its non-ionizing photon energies there is great interest and potential in imaging in the THz region. However, working in this region of the EM spectrum is notoriously difficult, with limited options for sources and focal lane detectors. Moreover, with the long (~mm) wavelengths, the diffraction limit places very restrictive limits on resolution.

In this talk, I will describe a relatively new approach to THz imaging that uses optically modulated, spatially patterned THz beams, created via a photomodulator. Using this approach, we can leverage the power of single pixel imaging techniques such as Hadmard imaging, with excellent signal to noise and high resolution (down to 8um), breaking the THz diffraction limit by almost two orders of magnitude. I will present our most recent work in this area, which targets specific applications ranging from cancer detection to security screening.

Fri 4 November 2022
Prof Paul Williams, University of Reading
Forecasting atmospheric turbulence from hours to decades ahead

Turbulence was called “the most important unsolved problem of classical physics” by Richard Feynman. It has also been called “the last major weather-related safety challenge facing large commercial aircraft” by Delta Air Lines. Operational turbulence forecasts for the aviation sector have historically had limited skill, to the extent that many pilots simply ignored them. In this colloquium, I will explain how we developed an improved atmospheric turbulence forecasting algorithm, which is based on a new physical mechanism for turbulence generation. The algorithm is now used operationally to forecast turbulence for the aviation sector up to 18 hours ahead. It has helped make billions of passenger journeys smoother, safer, and greener. I will also explain how climate change is steadily increasing the amount of wind shear in the atmosphere. This effect is projected to lead to hundreds of per cent more turbulence in the coming decades, causing bumpier flights on some of the world’s busiest flight routes.

Mon 28 December 2022
Prof Ruth Oulton, University of Bristol Nanolightbulbs: a bright future for quantum technology

Previous talks - semester 2 2021/22

Fri 4 March 2022
Prof Hendrik Ulbricht, University of Southampton
Testing quantum mechanics and gravity with levitated mechanical systems

Abstract: I will introduce the general context of macroscopic quantum physics, discuss a few concrete proposals to measure gravity of quantum systems, to test wavefunction collapse models as well as effects on quantum systems in non-inertial reference frames. I will then report on our focused experimental progress to probe into macroscopic quantum mechanics and gravity, as well as the theoretically predicted interplay of both theories in the low-energy regime. The illustration of quantum optomechanics and magnetomechanics experiments, as performed at Southampton, will be the core topic of my talk.

Fri 18 March 2022
Prof Peter A. Bobbert, Technische Universiteit Eindhoven
Thermodynamic theory for light-induced halide segregation in mixed halide perovskites

Abstract: Mixed halide perovskites that are thermodynamically stable in the dark demix under illumination. This is problematic for their application in solar cells. We present a unified thermodynamic theory for this light-induced halide segregation that is based on a free energy lowering of photocarriers funnelling to a nucleated phase with different halide composition and lower band gap than the parent phase. We apply the theory to a sequence of mixed iodine-bromine perovskites. The spinodals separating metastable and unstable regions in the composition-temperature phase diagrams only slightly change under illumination, while light-induced binodals separating stable and metastable regions appear signalling the nucleation of a low-band gap very iodine-rich phase. We find that the threshold photocarrier density for halide segregation is governed by the band gap difference of the parent and very iodine-rich phase. Partial replacement of organic cations by cesium reduces this difference and therefore has a stabilizing effect. We predict that three-phase coexistence should occur of an I-rich phase, a Br-rich phase, and a light-induced very I-rich phase around the critical point for halide demixing in the dark. Experimental verification of this three-phase coexistence is of fundamental interest and will help unravel the mechanism behind light-induced halide segregation in mixed halide perovskites.

Fri 25 March 2022
Prof Jonathan Reid, University of Bristol
SARS-COV-2: Respiratory aerosols, droplets and airborne transmission

Abstract: Aerosols of respirable size (<10um diameter) are exhaled when breathing, speaking and coughing and can transmit respiratory pathogens. Improved quantification of number and mass exhalation rates could support estimates of viral shedding and assessments of transmission risk. Previous studies report aerosol concentrations in an exhaled plume; we will present absolute exhalation rates from measurements of minute ventilation using cardiopulmonary exercise testing with exhaled particle concentrations. Measurements are made in a zero-background space to ensure all aerosols detected originate from the participant, reporting mass exhalation rates for children (aged 12-14) and adults (19-72) when breathing, speaking, singing and exercising. We will also report measurements of hygroscopic response of exhaled aerosol along with studies of evaporation rates and phase behaviour. Aerosol and droplet size, composition and moisture content are dynamic with both drying rapidly once exhaled from the highly humid respiratory tract and we will review the interplay of dynamic and equilibrium properties in governing the distance of transmission of droplets. We will also report new measurements of the survival of SARS-COV-2 in aerosol using a novel single particle approach in a CL-3 laboratory, examining the dependence of infectivity on relative humidity and temperature.

Previous talks - semester 1 2021/22

Fri 10 December 2021
Prof Corentin Coulais, University of Amsterdam
Non-orientable order and non-Abelian response in frustrated metamaterials

Abstract: From atomic crystals to bird flocks, most forms of order are captured by the concept of spontaneous symmetry breaking. This paradigm was challenged by the discovery of topological order, in materials where the number of accessible states is not solely determined by the number of broken symmetries, but also by spacetopology. Until now however, the concept of topological order has been linked to quantum entanglement and has therefore remained out of reach in classical systems. Here, we show that classical systems whose global geometry frustrates the emergence of homogeneous order realise an unanticipated form of topological order defined by non-orientable order-parameter bundles: non-orientable order. We validate experimentally and theoretically this concept by designing frustrated mechanical metamaterials that spontaneously break a discrete symmetry underhomogeneous load. While conventional order leads to a discrete ground-state degeneracy, we show that non-orientable order implies an extensive ground-state degeneracy–in the form of topologically protected zero-nodes and zero-lines. Our metamaterials escape the traditional classifi-cation of order by symmetry breaking. Considering more general stress distributions, we leverage non-orientable order to engineer robust mechanical memory and achieve non-Abelian mechanical responses that carry an imprint of thebraiding of local loads. We envision this principle to open the way to designer materials that can robustly process information across multiple areas of physics, from mechanics to photonics and magnetism.

Fri 26 November 2021
Prof Mark Dennis, University of Birmingham
How to tie an optical field into knots and links

Abstract: Tying a knot in a piece of string can be a hard practical problem. It seems even harder to tie a field into a knot – say a function from real 3-dimensional space to the complex numbers such that the function is zero on a curve which is a given knot or link. Nevertheless, several ways of doing this have been proposed in recent years, linking several areas in modern optics such as optical vortices, position-dependent polarisation, optical helicity and tightly-focused beams. I will discuss recent progress in this area, including creating laser beams containing a variety of different knots and links, detecting knottedness in random speckle fields and relations with knots in other systems such as fluids, nuclear physics and quantum chaos. I will conclude with some comparisons with 3D topological textures and skyrmions.

Previous talks - semester 2 2020/21

Fri 30 April 2021
Prof Chris Bowen, University of Bath Piezoelectric and pyroelectric materials and structures for energy harvesting

Abstract: The continuing need for reduced power requirements for small electronic components, such as wireless sensor networks, has prompted renewed interest in recent years for energy harvesting technologies capable of capturing energy from ambient vibrations and heat. This presentation provides an overview of piezoelectric harvesting system along with the closely related sub-classes of pyroelectrics and ferroelectrics. These properties are, in many cases, present in the same material, providing the intriguing prospect of a material that can harvest energy from multiple sources including vibration and thermal fluctuations . Examples of modeling and manufacture of porous materials and pyroelectric harvesting are discussed where the harvesting generates power from temperature fluctuations using piezoelectric materials such as lead zirconate titanate (PZT) and polyvinylidenedifluoride (PVDF). The potential of novel porous and sandwich structures are also described. Water-splitting using pyroelectric materials are examined analytically and experimentally.

Fri 26 March 2021
Prof Andrea Liu, University of Pennsylvania
How Materials Can Learn How to Function

Abstract: How does learning occur? In the context of neural networks, learning occurs via optimization, where a loss function is minimized to achieve the desired result. But physical networks such as mechanical spring networks or flow networks cannot minimize such a loss function by themselves—they need the help of a computer. An alternative is to encode local rules into those networks so that they can evolve under external driving to develop function. For example, if the springs in a mechanical network have equilibrium lengths that grow if the springs are stretched, and shrink when the springs are compressed, the network will naturally evolve under applied stresses. I will describe how both of these strategies—global minimization of a loss function as well as training by local rules–can be used to teach materials how to perform functions inspired by biology, such as the ability of proteins (e.g. hemoglobin) to change their conformations upon binding of an atom (oxygen) or molecule, or the ability of the brain’s vascular network to send enhanced blood flow and oxygen to specific areas of the brain associated with a given task.

Fri 12 March 2021
Prof. Jan Anton Koster, University of Groningen
Towards efficient organic thermoelectrics

Abstract: Organic semiconductors are light-weight, flexible, and tuneable. This makes them excellent materials for a wide range of applications that rely on the ability to conduct electricity, such as solar cells, light-emitting diodes, thin-film transistors, and thermoelectrics. The ‘phonon-glass electron-crystal’ (PGEC) concept has triggered most of the progress that has been achieved in inorganic thermoelectrics in the past two decades. Organic thermoelectric materials, unlike their inorganic counterparts, exhibit molecular diversity, flexible mechanical properties and easy fabrication, and are mostly ‘phonon glasses’. However, the thermoelectric performances of these organic materials are largely limited by low molecular order and they are therefore far from being ‘electron crystals’. In this talk, I will discuss our recent efforts to improve the performance of n-type organic thermoelectrics through the use of polar side groups. Through meticulous design of the side chain we have been able to introduce a fullerene derivative that approaches an organic ‘PGEC’ thermoelectric material. This thermoelectric material exhibits an excellent electrical conductivity of >10 S/cm and an ultralow thermal conductivity of <0.1 W/mK, leading to the best figure of merit ZT = 0.34 (at 120 C) among all reported single-host n-type organic thermoelectric materials. The key factor to achieving the record performance is to use ‘arm-shaped’ double-triethylene-glycol-type side chains, which not only offer excellent doping efficiency (~60%) but also induce a disorder-to-order transition upon thermal annealing. This study illustrates the potential of organic semiconductors as thermoelectric materials.

Previous talks - semester 1 2020/21

Fri 19 February 2021
Prof Stefan Maier, University of Munich, LMU/Imperial College London Nanoantennas for light harvesting and energy conversion

Abstract: Metallic and dielectric nanostructures provide distinct and unique means for shaping the electromagnetic near field, and for channelling radiation from the far field to the nanoscale. The associated electromagnetic field hot spots can be exploited for the enhancement of interactions between light and matter, most prominently for surface-enhanced spectroscopy and sensing, the boosting of non-linear interactions, and also for nanoscale spatial control over chemical reactions. In my lecture I will approach plasmonic and dielectric nanoantennas from the viewpoint of being a means for energy conversion at the nanoscale. With example materials systems such as gold and silver (plasmonic) and gallium phosphide (dielectric) I will highlight applications such as non-linear optics, photon-phonon interactions for the launching of acoustic surface waves, and the plasmon-assisted triggering of redox reactions

Fri 4 December 2020
Prof Chiara Daraio, Caltech
Organic temperature and IR sensors

Abstract: Organic electronic materials, including conductive and semiconductive polymers, are emerging as competitive alternatives to conventional, silicon-based microelectronics, due to their low-cost manufacturing and wider range of functionalities, such as stretchability, degradability and self-healing. The ability to synthetically tailor properties of organic compounds at the molecular levels makes them particularly appealing for sensing applications in wearable and implantable devices, which benefit from materials that are biocompatible and flexible, features that are difficult to achieve with inorganic materials and silicon-based manufacturing. Organic temperature sensing layers, for example, have been realized exploiting the variation of the polymers' electrical resistance with thermal expansion. Temperature sensing films offer exciting opportunities for wearable thermometer, environmental and industrial monitoring and for robotic surfaces to augment human-machine interactions. Our group demonstrated that pectin, a structurally and functionally complex, acid-rich polysaccharide found in plant cell walls, presents a record temperature response, linked to its ionic conductivity. In this talk, I will detail pectin’s temperature responsivity and I will describe the design and synthesis of a new synthetic polymer that mimics the structure of pectin and exhibits superior thermal sensitivity, while also being mechanically robust and flexible. With it, we realize temperature mapping systems and IR absorbing devices for consumer electronics.

Fri 13 November 2020
Prof Wilson Poon FRSE FInstP, University of Edinburgh
Soft matter physics and the COVID-19 pandemic

Abstract: Much of the science underpinning the global response to the COVID-19 pandemic lies in the soft matter domain. Coronaviruses are composite particles with a core of nucleic acids complexed to proteins surrounded by a protein-studded lipid bilayer shell. A dominant route for transmission is via air-borne aerosols and droplets. Viral interaction with polymeric body fluids, particularly mucus, and cell membranes controls their infectivity, while their interaction with skin and artificial surfaces underpins cleaning and disinfection and the efficacy of masks and other personal protective equipment. The global response to COVID-19 has highlighted gaps in the soft matter knowledge base. I will survey these gaps, especially as pertaining to the transmission of the disease, and suggest questions that can (and need to) be tackled, both in response to COVID-19 and to better prepare for future viral pandemics.