Electronic collective modes in two dilute conductors with momentum-resolved electron energy-loss spectroscopy

Rubeck, Samantha Ilene

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https://hdl.handle.net/2142/113898 Copy

Description

Title

Electronic collective modes in two dilute conductors with momentum-resolved electron energy-loss spectroscopy

Author(s)

Rubeck, Samantha Ilene

Issue Date

2021-12-03

Director of Research (if dissertation) or Advisor (if thesis)

Abbamonte, Peter

Doctoral Committee Chair(s)

Eckstein, Jim

Committee Member(s)

• Wagner, Lucas
• Gadway, Bryce

Department of Study

Physics

Discipline

Physics

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• momentum-resolved electron energy-loss spectroscopy (M-EELS)
• CuxTiSe2, SrTi(1−x)NbxO3, collective modes, exciton condensation, dilute superconductivity, plasmon

Abstract

Much of modern condensed matter research tries to understand strongly correlated electron systems. These materials exhibit a variety of interesting quantum phenomena that stem from strong electron interactions and the emergent quasiparticles that define the system. Understanding the collective mode dynamics of these particles is key to understanding the macroscopic behavior of these materials. One quantity that contains fundamental information about the about these boson collective modes is the charge susceptibility χ(q,ω) which contains information about the propagation of density fluctuations that are mediated by bosonic excitations. Until recently though, it was not possible to measure χ(q, ω) at low energy (< 100 meV) and with the momentum resolution and accuracy needed to see interesting phenomena. With the development of momentum-resolved electron energy-loss spectroscopy (M-EELS) in the Abbamonte Group at UIUC, χ(q, ω) can now be probed at low energy scales of interest. Here we use M-EELS to study the collective modes of several strongly correlated materials with interesting low energy physics. Bose condensed phases of excitons have the potential to realize macroscopic quantum phenomena at unprecedented high temperatures [1,2]. When excitons Bose condense in a real material, however, some rearrangement of the charge density inevitably results, leading to a distortion of the crystal lattice [3–6]. This raises the question of whether there can ever be a distinction between a Bose condensate of excitons and a conventional, structural phase transition that breaks the same symmetry. Here, we use inelastic electron scattering (M-EELS) to study copper-intercalated TiSe2, in which exciton condensation can be directly observed as a soft electronic mode at the exciton condensation temperature, T_XC [7]. While the lattice distortion in CuxTiSe2 persists to x > 0.10 [8], we find the exciton condensate is fully suppressed by x = 0.014, which is short of the semimetal-metal transition we identify at x = 0.025. Our observations indicate that the excitonic and lattice instabilities split as x is increased, showing that structural and excitonic subsystems can exhibit separate transitions and may be distinct subsystems that break different symmetries. In addition we found that although exciton condensation is suppressed, exciton fluctuations remain for most of the doping phase diagram. These fluctuations have been theorized to aid in superconducting pairing in this system [9,10]. 
SrTi(1−x)NbxO3, is an electron doped ionic semiconductor that exhibits aborted ferroelectricity due to quantum fluctuations [11], dilute (unconventional) superconductivity that survives even when the plasmon energy (ω_p) is lower than the Fermi energy (E_F ) [12], and charge transport properties that suggests bad metal behavior [13]. In general, the polar nature of ionic semiconductors leads to coupling of the longitudinal optical (LO) phonons to collective charge modes, such as plasmons, due to long range polarization fields. Understanding the dynamics of these collective modes, which have been implicated in superconducting pairing [14] [15] [16], can provide insight into the nature of these unusual properties. Here, we use inelastic electron scattering (M-EELS) with a fracturing surface preparation technique to study the doping, temperature, and momentum dependence of the charge collective modes in SrTi(1−x)NbxO3. We measure propagating and diffusive acoustic phonons, Fuchs-Kliewer optical phonons and overtones that get suppressed with doping due to metallic screening, a 93 meV phonon mode that becomes highly damped and asymmetric at x=0.002, and a very broad plasmon that blue-shifts with decreasing temperature. We find that the plasmon in SrTi(1−x)NbxO3 remains dispersion-less at all temperatures and dopings even with other propagating collective modes (acoustic phonon), contrary to RPA predictions and dispersions in other polar doped semiconductors [17]. The width of the plasmon deviates from RPA predictions by an order of magnitude. In addition, the energy of the plasmon measured in M-EELS is less than that of other techniques like infrared spectroscopy for samples of comparable carrier densities.

Graduation Semester

2021-12

Type of Resource

Thesis

Permalink

http://hdl.handle.net/2142/113898

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Copyright 2021 Samantha Rubeck

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