Person: Benjamin, David
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Benjamin
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Benjamin, David
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Publication Microscopic Theory of Resonant Soft-X-Ray Scattering in Materials with Charge Order: The Example of Charge Stripes in High-Temperature Cuprate Superconductors(American Physical Society (APS), 2013) Benjamin, David; Abanin, Dmitry; Abbamonte, Peter; Demler, EugeneWe present a microscopic theory of resonant soft-x-ray scattering that accounts for the delocalized character of valence electrons. Unlike past approaches based on local form factors, our functional determinant method treats realistic band structures. This method builds upon earlier theoretical work in mesoscopic physics and accounts for excitonic effects as well as the orthogonality catastrophe arising from interaction between the core hole and the valence band electrons. We show that the two-peak structure observed near the O K edge of stripe-ordered \(La_{1.875}Ba_{0.125}CuO_{4}\) is due to dynamical nesting within the canonical cuprate band structure. Our results provide evidence for reasonably well-defined, high-energy quasiparticles in cuprates and establish resonant soft-x-ray scattering as a bulk-sensitive probe of the electron quasiparticles.Publication Impurity Physics in Resonant X-Ray Scattering and Ultracold Atomic Gases(2014-10-21) Benjamin, David; Demler, Eugene A.; Hoffman, Jennifer; Sachdev, SubirThis thesis presents work on theoretical tools used to study transient and quantum-fluctuating impurity potentials that arise in resonant x-ray scattering and ultracold atomic gases. These tools fall under two main classes, functional determinants for exact evaluation of many-fermion matrix elements, and the variational polaron transformation. The following work carefully introduces both approaches and compares theoretical predictions to known experimental and computational results. In several cases this thesis presents arguments that experiments on high-temperature superconducting cup rates must be reinterpreted in terms of a quasiparticle picture. Where no experimental data exist, predictions are made and suggestions given for new uses for simple experimental techniques. For example, indirect resonant inelastic x-ray scattering turns out to be a versatile pseudo gap probe, and radio frequency absorption of a fermi gas with an impurity can detect a repulsively-bound state.Publication Single-Band Model of Resonant Inelastic X-Ray Scattering by Quasiparticles in High-Tc Cuprate Superconductors(American Physical Society (APS), 2014) Benjamin, David; Klich, Israel; Demler, EugeneWe show that a simple model of noninteracting quasiparticles accurately describes resonant inelastic x-ray scattering (RIXS) experiments in the hole-doped cuprate superconductors. Band structure alone yields signatures previously attributed to collective magnetic modes, such as the dispersing peaks and nontrivial polarization dependence found in several experiments. We conclude that RIXS data can be explained without positing the existence of magnetic excitations that persist with increasing doping. In so doing we develop a formalism for RIXS in itinerant electron systems that accounts for the positively charged core hole exactly and discover a mechanism by which the core hole produces polarization dependence mimicking that of a magnetic system.Publication Variational polaron method for Bose-Bose mixtures(American Physical Society (APS), 2014) Benjamin, David; Demler, EugeneWe study degenerate mixtures of heavy bosons and light superfluid bosons using a variational polaron transformation. We consider the Mott-insulator–superfluid transition of the heavy species and find that at T=0 interaction favors the superfluid phase of the heavy species. Our analytic results agree well with numerically exact quantum Monte Carlo simulations in two dimensions. We then show that in three dimensions the variational polaron transformation can be combined with a Gutzwiller approximation to give good results.