2019
Many congratulations to Ming-Min Yang, Eduardo Mendive-Tapia, Petra Kohutova and Odette Toloza Castillo for their success in the 2019 Thesis Prize competition!
SEM Faculty Thesis Prize-Ming-Min Yang
Developing a cheap and efficient way to convert the solar energy into electricity has been at the forefront of research for decades spanning physical sciences to engineering. However, the efficiency of the conventional solar cells almost reaches their theoretical limit. My thesis studied a distinctive photovoltaic effect termed the bulk photovoltaic effect, which only exists in materials with non-centrosymmetric structures. It can directly convert solar energy into electricity in the bulk of semiconductors without the assistance of any junctions which are prerequisite in conventional solar cells. The bulk photovoltaic effect is free from the thermodynamical limitation and can generate photovoltages way larger than the semiconductor bandgap (i.e. the energy required to generate free carriers). We studied the origin of the bulk photovoltaic effect in thin film materials prepared in our lab and researched the its correlation with films’ microstructures. We also found that the current density generated by the bulk photovoltaic effect can be dramatically enhanced by reducing the electrode size, offering a way to enhance the output efficiency. At last, we developed a fundamentally new photovoltaic effect named the flexo-photovoltaic effect, which can generate electricity once the illuminated semiconductor is subjected to an inhomogeneous deformation. The flexo-photovoltaic effect provides a new strategy to boost the ultimate efficiency of solar cells.
Figure: Inhomogeneous deformation can squeeze more solar energy out of solar cells
Springer Thesis Prize-Eduardo Mendive-Tapia
We live in an age greatly dependent on technology exploiting and manipulating a rich diversity of magnetic structures, i.e. phases, to produce device functionality. Most theoretical and experimental research in this area, however, is restricted to a limited number of phases owing to computational and resource constraints. Here we present a first-principles theory capable of predicting efficiently most stable complex magnetic phases of a material. It also characterizes their temperature-dependent properties, competition, and control by application of external stimuli, such as mechanical stresses and magnetic fields.
Magnetism arises from the cooperative behaviour of septillions of fast interacting electrons, which form a glue binding together the nuclei in materials. By describing how this electronic glue produces and, in turn, is affected by magnetic order, we have devised a general framework. Potential magnetic phases are determined from correlations connecting pairs of atomic sites which we extract from studying the high temperature regime. How the glue transforms at lower temperatures and/or under the application of external stimuli, determines the competition and evolution of the magnetic phases generally and, at a fundamental level, leads to multispin interactions over groups of atoms.
Application of this theoretical approach has shown that multispin correlations are fundamentally behind transitions and temperature evolution among a wide variety of long-period complex phases in the heavy rare earth elements (Fig 1a). Studying the effect of strain on a Mn-based materials class (Fig 1b), which promises high efficiency in magnetic cooling technology, led to the demonstration that multispin interactions are a new source to enhance cooling performance, which has been recently shown in real materials in a collaboration with experimentalists.
Winton Prize-Petra Kohutova joint winner
Solar corona, very much like the Earth’s atmosphere, undergoes cycles of evaporation and condensation which lead to so-called coronal rain. As the name suggests, it shares many parallels with its terrestrial counterpart; it consists of cool blobs of dense plasma that fall from coronal heights towards the solar surface. Coronal rain is formed as a direct consequence of thermal instability in coronal loops. Magnetic field lines in these loops act similar to strings on a guitar and can support a variety of wave modes. My PhD work was focused on thermal instability cycles in coronal loops leading to coronal rain formation and on interplay between coronal rain and magnetohydrodynamic oscillations. To do this, I combined analysis of high-resolution observations from a number of space-based observatories with numerical simulations I carried out using Warwick’s high performance computing facilities. I have shown that coronal loops in which rain is observed show signatures of heating-condensation cycles, that various types of oscillations can be present in coronal rain and that the coronal rain formation can lead to excitation of such oscillations.
Winton Prize-Odette Toloza Castillo joint winner
My PhD work focused on white dwarfs, which are the cores surviving the final phases of the evolution of more than 90 per cent of the stars in the Universe. Therefore, the study of white dwarfs is important across many areas of astrophysics, and I studied white dwarfs within three very different contexts.
Small rocky bodies that venture too close to white dwarfs, are disrupted, and eventually accreted onto white dwarfs, providing the opportunity to measure the chemical composition of extra-solar planetesimals (See Figure).I tested the validity of one ofthe ``silent’’ assumptions commonly used in this method, i.e. that the material spreads homogeneously across the white dwarf surface. Using the quakes that one of the brightest member among these debris-polluted white dwarfs experiences, I was able to measure the chemical composition in small, localised areas. I concluded that it is likely that accretion is nothomogeneous, which will require revision of all published works, since the assumption of a homogenous distribution led to overestimates of the sizes of these planetesimals.
The star quakes I used in this study are found among a considerable number of white dwarfs, but it is not possible to determine the behaviour of these oscillations only on the basis of the physical properties of the atmospheres of white dwarfs. However, if a white dwarf atmosphere is perturbed by a large amount of material that is suddenly dumped into it from a close stellar companion, thetemperature of white dwarf is suddenly and drastically increased. The oscillations are damped out by this event, and evenmore than ten years later, only one of the three oscillations seen in the past has recovered. In addition, I found a new, unexpected long-period modulation in the brightness of this star, and developed a tentative explanation, which has in the meantime led to an intense follow-up study of this star.
Finally, white dwarfs are the source of one of the most energeticevents in the Universe: the Type Ia Supernovae.These explosive events are used to calibrate distances that are far beyond the Milky Way, and led to the discovery of the accelerating expansion of the Universe. However, yet it is unclear how some white dwarfs get to the ignition state. My work exploredthe evolutionary channel where white dwarfs accrete from stars more massive than the Sun, which have characteristic features that can be identified from ultraviolet spectroscopy taken with Hubble Space Telescope. My numerical simulations of these systems constrained the initial configurations that the progenitors must have had to undergo the high accretion rates necessary to grow the white dwarf mass, and eventually explode.