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## Researcher

# Matthew Houtput

- Research Expertise:Most of my research experience is in the field of theoretical condensed matter physics. I have started my research career by theoretically modelling a high-pressure experiment on metallic hydrogen, and most of my further research has been at least partially inspired by metallic hydrogen or high-pressure hydrides. For example, I have investigated the optical response of metallic nanospheres with a smooth electron density on the edge, which can be used as pressure sensors in high-pressure metallic hydrogen experiments. Since 2019 I have been studying the large polaron, a quasiparticle that is formed when an electron in a solid interacts weakly with the ions (or equivalently, the phonons) of that solid. Specifically, I investigate the effect of anharmonicity on such polarons. In my PhD thesis, an additional anharmonic interaction term in the Fröhlich Hamiltonian is derived, which represents the interaction of the electron with 2 longitudinal optical phonons. The model is simplified enough so that this interaction depends on only 1 additional material parameter. Several of the single polaron properties were investigated analytically in terms of this new anharmonic material parameter: the binding energy, the effective mass, the optical absorption spectrum, and the possibility of bipolaron formation.

- Keywords:POLARONS, MANY BODY THEORY, OPTICAL ABSORPTION, BIPOLARONS, QUANTUM FIELD THEORY, ELECTRON PHONON INTERACTION, CONDENSED MATTER PHYSICS, Physics (incl. astronomy)

- Disciplines:Classical physics, Condensed matter physics and nanophysics, Materials physics, Mathematical physics, Optical physics, Quantum physics, Electronic (transport) properties, Magnetism and superconductivity, Optical properties and interactions with radiation, Semiconductors and semimetals, Condensed matter physics and nanophysics not elsewhere classified, Classical and physical optics

- Research techniques:As a theoretical physicist, I rely mostly on pen-and-paper calculations to obtain my results. Usually, the theory leads to equations or integrals which must be solved numerically. I do most of my numerical work in Matlab, and some of it in Mathematica and Python. My preferred methods to tackle problems are the path integral method and Green’s function expansions. As such, I have plenty of expertise and insight in both of these techniques. I am currently also learning density functional theory, which will allow me to transfer my currently theoretical results to the field of computational solid state physics.

- Users of research expertise:My research is aimed at other condensed matter physicists, who can turn my generic qualitative predictions into concrete applications for materials. This is best illustrated by two examples. Firstly, I have investigated a one-dimensional model of a diamond anvil cell, which is often used in high-pressure experiments on metallic hydrogen. The main result was a lower bound on the unknown latent heat of the insulator-to-metal transition in hydrogen. This lower bound was be used by the experimental physicists doing these experiments to correctly interpret their experimental results. Secondly, my research on anharmonic polarons shows that large bipolarons can likely form in materials with strong electron-phonon coupling and significant anharmonicity. This is an important result, since there is currently no experimental evidence for large bipolaron formation, and theoretical calculations which do not include anharmonicity find that bipolarons only form at unrealistically strong electron-phonon coupling. My result provides a qualitative guideline for selecting a material that may show bipolarons (strontium titanate, potassium tanatalate, ...): these materials can then be investigated in more detail by experimental or computational experts.