Oriana Diessel: Polarons and phase transitions in light-matter systems

The theorist showed how the interactions of polarons can be precisely controlled and what new phase transitions exist in light-matter systems.

May 28, 2024

Oriana Diessel completed her doctorate in Richard Schmidt's independent research group. Her theoretical work focusses on quantum many-body systems  complex systems consisting of a large number of interacting particles that are subject to the laws of quantum mechanics. The physicist investigated two special features of many-body systems in more detail: so-called "polarons" and previously unknown phase transitions in light-matter systems. Oriana Diessel developed models to theoretically describe the two phenomena, thereby providing a further building block for our understanding of quantum many-body theory.

Many phenomena that occur in quantum many-body systems have not yet been fully explained. These include so-called “polarons”, which Oriana Diessel investigated in the first part of her doctoral thesis: polarons are quasiparticles that can also be described as "mobile impurities". Mobile impurities can look very different - in a gas consisting of lithium atoms, for example, a single potassium atom represents such an impurity. A polaron is created when the impurity interacts with the surrounding gas.

The theorist was particularly interested in the extent to which the polaron density affects other measurable quantities - such as energy, spin or charge - and how individual impurities interact with each other. To investigate these questions, Oriana Diessel worked closely with experimental groups in the field of ultracold atoms and solids. As part of such a theory-experiment collaboration, Oriana and her colleagues discovered that the interaction between polarons in two-dimensional semiconductor materials so-called "transition metal dichalcogenides" (TMDs) – can be changed both in terms of its strength and its sign: Polarons, which actually interact repulsively with each other, begin to attract each other when their density is increased. The density of the bath particles - i.e. the particles surrounding the impurities - can therefore also be used to specifically control their interactions.

The second part of Oriana Diessel's work deals with phase transitions in light-matter systems, some of which demonstrate unusual properties. Among other things, the photons cause the interaction between the matter particles to have a long range. In addition, light-matter systems are naturally in a so-called "non-equilibrium" - i.e. there is a continuous interaction with the environment as photons only have a short lifetime and have to be constantly pumped into the system. This means that previously unknown phase transitions must be postulated, which differ greatly from the phases in standard systems. Oriana Diessel classified these phases and identified new systems in which they can be visualised.

Oriana Diessel fondly remembers her time at MPQ: "During my PhD, I had the opportunity to work with many exceptional scientists," she says, "and I am particularly grateful for the good balance between supervision and collaboration within the working group and the freedom to work and research independently." Oriana Diessel will remember her participation in international conferences just as favourably as a research stay at the University of Berkeley, which was made possible by IMPRS. "The MPQ offers a great research environment and the theory group is characterised by good cohesion, with regular group excursions and social events contributing significantly to this," adds the theorist.

What's next?

As a recipient of the independent ITAMP Postdoctoral Fellowship, Oriana moved to Harvard University and the Harvard-Smithsonian Center for Astrophysics in Cambridge, USA, in October 2023.

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