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Title:Resonant soft X-ray studies of charge orders in high-temperature cuprates with energy-resolving transition edge sensors
Author(s):Fang, Yizhi
Director of Research:Abbamonte, Peter
Doctoral Committee Chair(s):MacDougall, Gregory
Doctoral Committee Member(s):Fradkin, Eduardo H; Vieira, Joaquin
Department / Program:Physics
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Resonant Soft X-ray Scattering
high-temperature cuprates
charge orders
Transition Edge Sensors
fluorescence background
quantum efficiency
collecting efficiency
Abstract:The high-temperature superconductivity in cuprates has been intensely studied since 1986; however, many questions still remain unclear even with decades of efforts. The relation between emergent orders, such as charge (stripe) orders, in "pseudogap" regime and superconductivity is one of the most interesting topics. In addition, charge orders also behave variously across different families of cuprates thus there does not exist an universal picture to describe the mechanism of charge orders. Resonant Soft (elastic) X-ray Scattering (RSXS), one of the modern technology for structural analysis, takes the benefits of both X-ray Absorption Spectroscopy (XAS) and x-ray scattering, in other words, it can detect electronic structure of target element and provide spatial modulations of interesting orders. RSXS enhances the signal of charge orders by tuning the incident photon energy close to absorption edge of target element, for example Cu in cuprates; however, the inelastic fluorescence (usually referred as background in RSXS experiments) is also the most intense at resonance making short-ranged charge orders harder to be defined. The fluorescence background can only be separated by introducing energy-resolving detectors because fluorescence differs from elastic peak on energy scale, which is the motivation of this thesis. We therefore propose an energy-resolving detector - Transition Edge Sensors (TESs) that can measure a single photon energy by measuring the thermal fluctuations of superconducting islands. TESs consist of 3 main components: TES array, multiplexing SQUIDs readout system and Adiabatic Demagnetization Refrigerator (ADR). Compared to other modern spectrometers, TESs are more compact and efficient in collecting data with sufficient energy resolution (~ 1 eV on average). Both energy and theta scans are carried out to measure charge orders of 1/8 doped La_{2-x}Ba_xCuO_4 (LBCO) and components in the resulting emission spectroscopy include elastic peak (no energy loss), "dd" excitation, fluorescence emission from both fundamental and second harmonic due to harmonic contamination at beamline. Applying suitable model to each component, i.e. Gaussian for elastic peak, Voigt for "dd" excitation and Eisenberger for emission, one can reconstruct the emission spectroscopy with fitting functions. Errors of elastic peak in this experiment contains (a) random error from Poisson distribution and (b) systematic error from fitting and they are propagated with quadratic rules; in contrast, the error of energy integrated peak (sum of number of detected photons on TESs) only consists of random error from Poisson. By comparing the two, one claims that TESs improve the accuracy of detecting charge orders (elastic peak) by 4 times compared to that of conventional RSXS where energy integrated peak is used instead. They can help to study short-ranged charge orders that cannot be distinguished previously, which plays a vital role in understanding the relationship between charge orders and high-temperature superconductivity.
Issue Date:2019-11-06
Rights Information:Copyright 2019 Yizhi Fang
Date Available in IDEALS:2020-03-02
Date Deposited:2019-12

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