[{"publication_status":"published","author":[{"id":"e1a2682f-dc8d-11ea-abe3-81da9ac728f1","first_name":"Asbjørn Bækgaard","orcid":"0000-0003-4476-2288","full_name":"Lauritsen, Asbjørn Bækgaard","last_name":"Lauritsen"}],"OA_place":"publisher","publisher":"Institute of Science and Technology Austria","has_accepted_license":"1","date_updated":"2026-09-17T11:04:10Z","year":"2024","oa_version":"Published Version","corr_author":"1","publication_identifier":{"issn":["2663-337X"],"isbn":["978-3-99078-042-8"]},"date_published":"2024-09-23T00:00:00Z","abstract":[{"text":"This thesis consists of two separate parts. In the first part we consider a dilute Fermi gas interacting through a repulsive interaction in dimensions $d=1,2,3$. Our focus is mostly on the physically most relevant dimension $d=3$ \r\nand the setting of a spin-polarized (equivalently spinless) gas, where the Pauli exclusion principle plays a key role. We show that, at zero temperature, the ground state energy density of the interacting spin-polarized gas differs (to leading order) from that of the free (i.e. non-interacting) gas by a term of order $a_p^d\\rho^{2+2/d}$  with $a_p$ the $p$-wave scattering length of the repulsive interaction and $\\rho$ the density. Further, we extend this to positive temperature and show that the pressure of an interacting spin-polarized gas differs from that of the free gas by a now temperature dependent term, again of order $a_p^d\\rho^{2+2/d}$. Lastly, we consider the setting of a spin-$\\frac{1}{2}$ Fermi gas in $d=3$ dimensions and show that here, as an upper bound, the ground state energy density differs from that of the free system by a term of order $a_s \\rho^2$ with an error smaller than $a_s \\rho^2 (a_s\\rho^{1/3})^{1-\\eps}$ for any $\\eps > 0$, where $a_s$ is the $s$-wave scattering length of the repulsive interaction. \r\n\r\nThese asymptotic formulas complement the similar formulas in the literature for the dilute Bose and spin-$\\frac{1}{2}$ Fermi gas, where the ground state energies or pressures differ from that of the corresponding free systems by a term of order $a_s \\rho^2$ in dimension $d=3$. In the spin-polarized setting, the corrections, of order $a_p^3\\rho^{8/3}$ in dimension $d=3$, are thus much smaller and requires a more delicate analysis.\r\n\r\nIn the second part of the thesis we consider the Bardeen--Cooper--Schrieffer (BCS) theory of superconductivity and in particular its associated critical temperature and energy gap. We prove that the ratio of the zero-temperature energy gap and critical temperature $\\Xi(T=0)/T_c$ approaches a universal constant $\\pi e^{-\\gamma}\\approx 1.76$ in both the limit of high density in dimension $d=3$ and in the limit of weak coupling in dimensions $d=1,2$. This complements the proofs in the literature of this universal behaviour in the limit of weak coupling or low density in dimension $d=3$. Secondly, we prove that the ratio of the energy gap at positive temperature and critical temperature $\\Xi(T)/T_c$ approaches a universal function of the relative temperature $T/T_c$ in the limit of weak coupling in dimensions $d=1,2,3$.","lang":"eng"}],"department":[{"_id":"GradSch"},{"_id":"RoSe"}],"fulldoi":"https://doi.org/10.15479/at:ista:18135","language":[{"iso":"eng"}],"file":[{"creator":"alaurits","file_size":3648831,"relation":"main_file","checksum":"c7bc3b31e430d57c65393051ca439575","date_created":"2024-09-26T13:11:24Z","access_level":"open_access","file_name":"Lauritsen-thesis-final.pdf","content_type":"application/pdf","success":1,"date_updated":"2024-09-26T13:11:24Z","file_id":"18147"},{"creator":"alaurits","file_size":1625888,"file_name":"Lauritsen-thesis-source.zip","checksum":"39f6b1b7f83e25a3bf9f933f1ea0bc06","date_created":"2024-09-26T13:12:55Z","access_level":"closed","relation":"source_file","date_updated":"2024-09-26T13:12:55Z","file_id":"18148","content_type":"application/x-zip-compressed"}],"supervisor":[{"first_name":"Robert","id":"4AFD0470-F248-11E8-B48F-1D18A9856A87","last_name":"Seiringer","full_name":"Seiringer, Robert","orcid":"0000-0002-6781-0521"}],"degree_awarded":"PhD","tmp":{"legal_code_url":"https://creativecommons.org/licenses/by/4.0/legalcode","name":"Creative Commons Attribution 4.0 International Public License (CC-BY 4.0)","short":"CC BY (4.0)","image":"/images/cc_by.png"},"doi":"10.15479/at:ista:18135","ec_funded":1,"page":"353","ddc":["515","539"],"article_processing_charge":"No","citation":{"chicago":"Lauritsen, Asbjørn Bækgaard. “Energies of Dilute Fermi Gases and Universalities in BCS Theory.” Institute of Science and Technology Austria, 2024. <a href=\"https://doi.org/10.15479/at:ista:18135\">https://doi.org/10.15479/at:ista:18135</a>.","short":"A.B. Lauritsen, Energies of Dilute Fermi Gases and Universalities in BCS Theory, Institute of Science and Technology Austria, 2024.","ama":"Lauritsen AB. Energies of dilute Fermi gases and universalities in BCS theory. 2024. doi:<a href=\"https://doi.org/10.15479/at:ista:18135\">10.15479/at:ista:18135</a>","ista":"Lauritsen AB. 2024. Energies of dilute Fermi gases and universalities in BCS theory. Institute of Science and Technology Austria.","mla":"Lauritsen, Asbjørn Bækgaard. <i>Energies of Dilute Fermi Gases and Universalities in BCS Theory</i>. Institute of Science and Technology Austria, 2024, doi:<a href=\"https://doi.org/10.15479/at:ista:18135\">10.15479/at:ista:18135</a>.","ieee":"A. B. Lauritsen, “Energies of dilute Fermi gases and universalities in BCS theory,” Institute of Science and Technology Austria, 2024.","apa":"Lauritsen, A. B. (2024). <i>Energies of dilute Fermi gases and universalities in BCS theory</i>. Institute of Science and Technology Austria. <a href=\"https://doi.org/10.15479/at:ista:18135\">https://doi.org/10.15479/at:ista:18135</a>"},"month":"09","status":"public","alternative_title":["ISTA Thesis"],"type":"dissertation","_id":"18135","file_date_updated":"2024-09-26T13:12:55Z","date_created":"2024-09-24T10:56:25Z","project":[{"name":"Mathematical Challenges in BCS Theory of Superconductivity","_id":"bda63fe5-d553-11ed-ba76-a16e3d2f256b","grant_number":"I06427"},{"name":"Analysis of quantum many-body systems","_id":"25C6DC12-B435-11E9-9278-68D0E5697425","grant_number":"694227","call_identifier":"H2020"}],"oa":1,"related_material":{"record":[{"status":"public","id":"11732","relation":"part_of_dissertation"},{"status":"public","id":"14542","relation":"part_of_dissertation"},{"status":"public","id":"18107","relation":"part_of_dissertation"},{"status":"public","id":"17240","relation":"part_of_dissertation"},{"id":"14931","status":"public","relation":"part_of_dissertation"}]},"day":"23","title":"Energies of dilute Fermi gases and universalities in BCS theory","user_id":"ba8df636-2132-11f1-aed0-ed93e2281fdd"}]