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Title:Collective modes in strongly correlated electron systems measured with momentum-resolved electron energy-loss spectroscopy
Author(s):Rak, Melinda
Director of Research:Abbamonte, Peter M
Doctoral Committee Chair(s):Madhavan, Vidya
Doctoral Committee Member(s):Fradkin, Eduardo; Stack, John D
Department / Program:Physics
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):momentum-resolved electron energy-loss spectroscopy (M-EELS)
collective modes
exciton condensate
exciton condensation
charge density wave (CDW)
Abstract:Strongly correlated electron systems comprise an exciting area of research in condensed matter physics due to the variety of quantum phenomena that results from the strong interactions between the electrons. Strongly correlated materials can be described in terms of weakly interacting, emergent particles. Studying the dynamics of these emergent particles at low energy scales is key to understanding the quantum phenomena of strongly correlated materials. The dynamic charge susceptibility, χ(q,ω), contains fundamental information about the bosonic emergent particles, or the collective modes, of a material. Momentum-resolved electron energy-loss spectroscopy (M-EELS) can measure χ(q,ω) of strongly correlated materials at the low energy scales of interest. The ability to measure the momentum-dependence of the collective modes often leads to new insight about a material’s ground state. Here we use M-EELS to study the collective modes of several strongly correlated materials with interesting low temperature phases. One material is the unconventional superconductor Sr2RuO4 that has been proposed to be a rare spin-triplet superconductor. The normal state of Sr2RuO4 is thought to have strong interaction effects that may be key to understanding the nature of the superconducting state. The dispersions of the electron bands in Sr2RuO4 do show strong interaction effects in the form of anomalies known as "kinks." Measuring Sr2RuO4 with M-EELS reveals two collective modes at the same energies as the kinks. The momentum-dependence of the modes identifies them as an optical phonon and a surface phonon. These phonons likely couple with the electrons and cause the kinks. Another material of interest is the transition metal dichalcogenide 1T-TiSe2, which has been proposed to contain a Bose condensate of excitons. Using M-EELS, we demonstrate the existence of an exciton condensate in TiSe2. Its experimental signature is a soft electronic collective mode that disperses to zero energy near the charge density wave (CDW) transition temperature TC, signaling the presence of a macroscopic condensate of electron-hole pairs. As TiSe2 is doped with copper to form CuxTiSe2, the CDW transition temperature is suppressed and a superconducting dome emerges around x = 0.04. Using M-EELS, we find that the electronic mode softens near TC for very low doping values (x ≤ 0.004), but does not soften for doping values above x ∼ 0.01. Our results indicate that the exciton condensate is rapidly destroyed in CuxTiSe2 by screening from the additional Cu electrons and that a conventional, structural CDW phase persists for doping values above x ∼ 0.01. We conclude that the excitonic state is not directly related to the superconducting state in CuxTiSe2.
Issue Date:2019-06-27
Rights Information:Copyright 2019 Melinda Rak
Date Available in IDEALS:2019-11-26
Date Deposited:2019-08

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