Date of Award

August 2024

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Department

Physics

First Advisor

Daniel F Agterberg

Committee Members

Peter Schwander, Jolien Crieghton, Michael Weinert, Prasenjit Guptasarma

Abstract

In this work, we consider the interplay of different magnetic orders and superconductivity.We consider both phenomenological and microscopic models to capture these interactions. The models are built using group theoretic arguments to ensure the constraints provided by the crystal symmetry are satisfied. We first consider a phenomenological theory for two transitions in superfluid 3He which involves coupling superfluidity and spin fluctuations through an approach called spin fluctuation feedback effect. We then apply this phenomenological approach to UPt3, PrOs4Sb12 and U1−xThxBe13 to obtain a common underlying mechanism for two superconducting transitions. We then consider odd-parity multipolar antiferromagnetic order i.e. Kramers’ degenerate magnetic order and its interplay with superconductivity. We show that the presence of such magnetic states generically suppresses superconductivity, whether it be spin-singlet or spin-triplet, unless the magnetic state drives a symmetry-required pair density wave superconducting order. We apply our results to CeRh2As2 where no pair density wave order appears, and to the loop current order in the Cuprates, where such pair density wave superconductivity must appear together with Bogoliubov Fermi surfaces. In the former case, we explain why superconductivity is not suppressed. We finally apply this approach to stabilize Bogoliubov Fermi surfaces in FeSe1−xSx which have been observed for x > 0.17. These Bogoliubov Fermi surfaces appear together with broken time-reversal symmetry and surprisingly demonstrate nematic behavior in a structurally tetragonal phase. Through a symmetry-based analysis of Bogoliubov Fermi surfaces that can arise from broken time-reversal symmetry, we argue that the likely origin of time-reversal symmetry breaking is due to magnetic toroidal order. We show that this magnetic toroidal order naturally appears as a consequence of either static N ́eel antiferromagnetic order or due to the formation of a spontaneous pair density wave superconducting order.

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