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Department of Physics

The Cavendish Laboratory

Congratulations to the winners of the Cavendish Annual Thesis Prize. The prizes are awarded in recognition of exceptional computational, experimental and theoretical physics research by PhD students. 2023 has been another record-breaking year with many excellent nominations and the panel of judges have recognised four exceptional candidates for this year’s awards.

From developing new tools and methods to study quantum phenomena in disordered materials, accelerating the calculation of even complex properties of materials that will now take under an hour to run, designing disordered multilayer nanoparticle films which can achieve a record-breaking confinement strengths of IR light to improving the theoretical understanding of dynamics in frustrated magnetic systems, their projects are looking at solving some of the most pressing challenges of the 21st century. Our prize winners tell us more about their research below –

Siyu Chen – Winner of the Abdus Salam Prize in Computational Physics

“Nonuniform grids for Brillouin zone integration and interpolation”

Physical Review B (October 2021)

Siyu is a PhD student in the Theory of Condensed Matter group of the Cavendish Laboratory working with Dr Bartomeu Monserrat.

“Matter is made of electrons and nuclei, which obey the laws of quantum mechanics. This means that, in principle, it should be possible to predict the properties of any material surrounding us by solving the equations of quantum mechanics. However, solving the equations of quantum mechanics is an extremely challenging task, and has only become possible thanks to the use of modern supercomputers. Even with modern hardware, it takes hundreds of CPUs multiple days to predict complex properties such as thermal and electronic transport or superconductivity.”

“In my work, I use a combination of physical insight, mathematical number theory, and computational geometry, to design new algorithms to solve the equations of quantum mechanics more efficiently. As a result, I have been able to accelerate the calculation of even complex properties of materials by one to three orders of magnitude. This means that calculations that took days to run on hundreds of CPUs now take under one hour.”

“These developments have two important implications for our ability to predict material properties using quantum mechanics. First, we can perform calculations much faster, exploring many more materials to identify interesting phenomena. For example, I have been able to predict that a two-dimensional form of bismuth should host an exotic state of matter, the anomalous quantum spin Hall effect, at room temperature, a phenomenon that could be harnessed in spintronics devices. Second, we can now study materials and phenomena that were inaccessible with traditional methods. For example, we are currently exploring the properties of strongly correlated materials, a class of materials that has traditionally eluded calculations, but is now becoming accessible.” 

                                                                                           Figure 1: Symmetry-adapted Voronoi tessellation of the Brillouin zone

Arjun Ashoka – Winner of the Cavendish Prize in Experimental Physics

“Investigating quantum processes in disordered materials”

(Nature Communications, October 2022)

Arjun is a PhD student in the Optoelectronics group of the Cavendish Laboratory working with Prof. Akshay Rao.

“Elucidating the three-dimensional transport of excitations in condensed matter is key to advancements in our understanding and utilisation of functional materials, ranging from novel quantum systems to next-generation optoelectronic materials. Of particular interest are quantum coherent processes in heterogeneous, disordered systems, which require both ultrafast time resolution and local measurements to study and understand their transport characteristics. Here, we present a quantitative ultrafast interferometric pump-probe microscope capable of tracking of photoexcitations with sub-10 nm spatial precision in three dimensions with 15 fs temporal resolution, through retrieval of the full transient photoinduced complex refractive index. We use this methodology to study the spatio-temporal dynamics of the quantum coherent photophysical process of ultrafast singlet exciton fission that has gained relevance in the fields of photovoltaics and quantum computing. We reveal that photogenerated singlet excitons expand along the direction of maximal orbital π-overlap in the crystal a,c-plane to form correlated triplet pairs, which subsequently electronically decouples into free triplets along the crystal b-axis due to molecular sliding motion of neighbouring pentacene molecules.”

Figure 2: Direct Observation of Ultrafast Singlet Exciton Fission in Three Dimensions

Rakesh Arul – Winner of the Cavendish Prize in Experimental Physics

“Bridging the visible and infrared using plasmonic nanocavities to control molecular optoelectronics”

(Light:Science and Applications, September 2022)

Rakesh Arul is doing his PhD in Jeremy Baumberg’s research group at the Cavendish.

“Looking at the world with infrared (IR) light enables medical professionals to identify diseases, environmental scientists to monitor greenhouse gases, and the new James Webb Space Telescope to view the early universe. However, unlike visible light detection, IR detection is not widely applied due to existing technologies being inefficient, prohibitively expensive, and impractical. Hence tools to concentrate IR light and enhance its interaction with molecules are necessary.”

“To do this, I designed disordered multilayer nanoparticle films which can simultaneously squeeze IR light down to the molecular scale and achieved record-breaking confinement strengths of IR light. The key innovation was the use of a precise molecular spacer to assemble these structures from the bottom up and stacking them. This produced IR optical mode volumes >1000-fold smaller than anything previously observed, which is why we see world-record enhanced molecular vibrational absorptions. Careful control of nanogaps and layers still leaves vacancy disorder which turns out to aid detection by improving light in/out coupling and access of analytes to be detected.”

“I developed an easy and scalable fabrication protocol for these films, enabling scientists in the field to quickly adopt the technique without access to state-of-the-art clean room facilities. This advance opens multilayer gold nanoparticle films as a highly useful platform to confine light spanning the entire mid-infrared region (1-11 μm), which has wider implications as a tool for optical control of chemical processes, enhancing IR nonlinear optics, and pushing the limit-of-detection of molecular IR absorption.”

Figure 3: Bridging the visible and infrared using plasmonic nanocavities to control molecular optoelectronics

Jonathan Nilsson Hallén - Winner of the Cavendish Prize in Theoretical Physics

“Dynamics and noise in spin liquids”


Jonathan is doing his PhD jointly in Claudio Castelnovo’s group (TCM group) at the Cavendish and Roderich Moessner’s group at the Max Planck Institute for the Physics of Complex Systems in Dresden, Germany. 

“Frustration occurs in a material when the inter-particle interactions compete with each other and cannot be simultaneously satisfied. Unlike conventional materials, frustrated systems often do not display long-ranged order down to very low or even absolute zero temperature, and can have vast numbers of degenerate groundstates. This makes them particularly conducive for the discovery and study of new physical phenomena, including phenomena that fall under topics of intense current research efforts such as topological order and fractionalized excitation.”

“My research aims to improve the theoretical understanding of dynamics in frustrated magnetic systems.  Understanding the dynamical behaviour is important to enable the use of powerful dynamical experimental techniques, but is also vital for any future practical applications that require control of the physical state of the system.”   


“A particular focus of my research has been on the low-temperature dynamics of spin ice materials. The low energy excitations in spin ice are fractionalized quasi-particles that take the form of emergent magnetic monopoles. My collaborators and I were able to show that the motion of these monopoles is more constrained than previously thought. In fact, despite spin ice being a three dimensional system, the monopoles do not behave as if they are moving in three dimensions, but instead move on clusters described by a smaller, fractal dimension. This realization has explained several experimental observations in spin ice materials that have been puzzling the frustrated magnetism community for some time.”

“The discovery of fractal structures in spin ice illustrates how unexpected phenomena can emerge in relatively simple frustrated systems, and demonstrates that significant work remains to be done if we want to understand fractionalized excitations to the extent we now understand conventional particles.”

                                                         Figure 4: Magnetic moments in a spin ice state with a single magnetic monopole excitation



The winners received their prizes and presented their award-winning work as part of the graduate student conference on Thursday 1st December. Each prize comes with a cheque of 500 pounds.

The Abdus Salam Prize for Postgraduate Student Research has been donated to the Department to be presented to a Research Student at the Cavendish.