University of Connecticut

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PhD Dissertation Defense

Friday, September 25, 2020
2:00pm – 4:00pm

Storrs Campus
Video meeting

Graduate Student Benjamin Commeau, Department of Physics, University of Connecticut

Two Studies on Topological Properties of Organic Superconductors and a new Quantum Dynamical Simulation Algorithm

In this thesis I investigate two research topics in topological organic superconductors and one research topic in variational algorithms for quantum computation.

In the first chapter we investigate the structural and electronic properties of the three structural phases alpha, beta and kappa of (BEDT-TTF)2I3, by performing state of the art ab initio calculations in the framework of density functional theory. We furthermore report about the irreducible representations of the corresponding electronic band structures, symmetry of their crystal structure, and discuss the origin of band crossings.

Additionally, we discuss the chemically induced strain in kappa-(BEDT-TTF)2I3 achieved by replacing the iodine layer with other Halogens: Fluorine, Bromine and Chlorine. In the case of kappa-(BEDT-TTF)2F3, we identify topologically protected crossings within the band structure. These crossings are forced to occur due to the non-symmorphic nature of the crystal. In the second chapter, we performed structural optimization and electronic structure calculations in the framework of density functional theory, incorporating, first, the recently developed strongly constrained and appropriately normed semilocal density functional SCAN, and, second, van der Waals corrections to the PBE exchange correlation functional by means of the dDsC dispersion correction method.

In the case of alpha-(BEDT-TTF)2F3 the formation of over-tilted Dirac-type-II nodes within the quasi 2-dimensional Brillouin zone can be found.

For kappa-(BEDT-TTF)2F3, the recently reported topological transition within the electronic band structure cannot be revealed. In the third chapter, we propose a new algorithm called Variational Hamiltonian Diagonalization (VHD), which approximately transforms a given Hamiltonian into a diagonal form that can be easily exponentiated. VHD allows for fast forwarding, and removes Trotterization error and allows simulation of the entire Hilbert space. We prove an operational meaning for the VHD cost function in terms of the average simulation fidelity. Moreover, we prove that the VHD cost function does not exhibit a shallow-depth barren plateau. Our numerical simulations verify that VHD can be used for fast-forwarding dynamics.

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Prof. S. Yelin

Physics Department (primary), College of Liberal Arts and Sciences, UConn Master Calendar

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