Quantum Criticality and Dimensionality in Quasi-2D Spin-Dimer Systems

Diplôme Thèse Quantum Criticality and Dimensionality in Quasi-2D Spin-Dimer Systems eng 2021-07-28 author Allenspach Stéphan Daniel 884832 director Ruegg Christian 790594 170 10.13097/archive-ouverte/unige:156658 urn:nbn:ch:unige-1566586 Sc. 5594 Phase transitions and their accompanying concepts of criticality and universality are a foundation stone of statistical physics. Gapped spin-dimer systems are an ideal testbed to study criticality because a quantum phase transition can be induced by a magnetic field resulting in a phase equivalent to the XY spin model. It was proposed that BaCuSi2O6, a quasi-2D spin-dimer system consisting of stacked bilayers, undergoes a dimensional reduction, from 3D to 2D, thereby realizing an effective 2D-XY spin model. In this thesis, we investigate the magnetic excitations and order in BaCuSi2O6 and Ba0.9Sr0.1CuSi2O6 using a combination of experimental techniques and analyze the collected data by modern statistical methods. We find that BaCuSi2O6 contains three different bilayer types, which cause an anomalous scaling regime previously misinterpreted as dimensional reduction. Furthermore, we verify that the bilayers in Ba0.9Sr0.1CuSi2O6 are equivalent and that this system therefore displays 3D scaling without any hallmarks of 2D physics. Quantum Criticality Quantum Magnetism Neutron Scattering Bayesian Inference Dewey 500.2 33 Département de physique de la matière quantique 131 false 88f7eafb-fd83-43b5-9ef2-24b2d2542900 Thesis Public Thesis_Stephan_Allenspach.pdf 41181202 application/pdf Restricted 2022-07-28 e14156ba144ecf201525b00c69e40e258475d66ca146636008a1d3108cb0b8e3 openAccess Faculté des sciences / Section de physique / Département de physique de la matière quantique

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Quantum Criticality and Dimensionality in Quasi-2D Spin-Dimer Systems				2021	479	77

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Allenspach, Stéphan Daniel