Add to Calendar 2025-07-11T09:00:00 2025-07-11T11:00:00 Electronic structure manipulation by distortions in square-net materials Event Information: Abstract: Commonly used to describe the invariance of properties under transformations, symmetry is an important concept in mathematics and a fundamental principle in chemistry and physics. In crystalline materials, the crystal symmetry can impose strong constraints on the electronic structure. For example, in materials containing square-nets the highly symmetric square-net guarantees the existence of several line-node degeneracies in the electronic structure. In this thesis, we study how breaking crystal symmetries affects a line-node degeneracy in square-net materials. To simulate the symmetry lowering, we exploit the naturally existing structural distortions in materials with antimony square-nets and track the changes in electronic structure by direct and indirect measurements. Combined with a simple tight-binding model and density functional theory calculations, we establish a framework that explains the evolution of the electronic structure upon breaking fourfold and n glide symmetry. Angle-resolved photoemission spectroscopy (ARPES) and scanning tunneling microscopy (STM) measurements on the LaSbxTe2-x system provide evidence that breaking the n glide symmetry by selectively displacing half of the square-net atoms causes the opening of a large gap in the line-node degeneracy. Similar behavior is found in CaSb2 and EuSb2 that share the same broken n glide symmetry. Breaking fourfold symmetry, equivalent to stretching the lattice in one direction, only leads to marginal changes as we demonstrate for the case of YbSb2. The LaSbxTe2-x system is unique in the simplicity of its electronic structure and richness of electronic phase transitions induced by symmetry breaking. The control parameter for these transitions is electron doping, which can be realized via bulk chemical substitution or in situ adatom deposition on the surface. The latter process is fully reversible and mimics electrostatic gating, opening a pathway towards applications of the associated gap opening in devices. The developed framework to explain the electronic structure evolution is not limited to the two material families studied here and should generally apply to all square-net materials because it is based on symmetry rather than material specifics. This thesis therefore provides a good starting point for future investigations of the electronic structure of other square-net materials.  Event Location: Henn 318

Add to Calendar 2025-07-11T12:30:00 2025-07-11T15:00:00 Detecting Antihydrogen Annihilations in ALPHA-g for Measurement of Gravitational Free Fall Event Information: Abstract:Antihydrogen, the bound state of an antiproton and a positron, is an ideal system for testing fundamental symmetries between matter and antimatter. The Antihydrogen Laser PHysics Apparatus (ALPHA) at CERN has a proven history of producing and trapping antihydrogen atoms, with many precision tests of charge-parity-time (CPT) symmetry. The new ALPHA-g apparatus extends this program to a test of gravity, directly probing the applicability of the weak equivalence principle (WEP) to an antimatter system for the first time. This dissertation presents the first ALPHA-g results, which confirm that Earth’s gravity acts downwards on antimatter. The ratio between the gravitational acceleration of antihydrogen and hydrogen is estimated to be g/g = 0.75 ± 0.13 (statistical and systematic) ± 0.16 (simulation). This measurement depends on accurately determining the annihilation positions of antihydrogen atoms released from the ALPHA-g magnetic trap, which is accomplished using a time projection chamber (TPC) detector. To reduce the substantial background due to cosmic rays, a time-of-flight barrel scintillator (BSC) detector was also implemented. The author’s primary contributions include the commission of the BSC, including devising and implementing time-of-flight measurements, leading to a significantly reduced background rate. Additional contributions include improving the TPC tracking algorithms to reach the required annihilation position resolution. This dissertation begins with theoretical motivations for testing the WEP using antihydrogen, followed by an overview of the ALPHA apparatus and antihydrogen production. The TPC and BSC detectors are introduced, including the event reconstruction methods, with care to highlight the author’s work. Finally, the gravity measurement methodology and results, as well as future perspectives, are discussed. This first direct test of gravity on antimatter rules out a large difference in the gravitational behaviour of antimatter and matter, furthering our understanding of the fundamental matter-antimatter symmetry, and laying the foundation for future precision WEP tests using antimatter.  Event Location: TRIUMF MOB Conference Room - 4004 Wesbrook Mall