Collective modes of semiconductors and unconventional metals with intertwined orders

Often in the field of condensed matter, materials are studied by looking to one particular property at a time, while either neglecting the others or treating them as weak perturbations. However, some materials do not fall into this paradigm. These materials with intertwined orders cannot be understood by treating their physical properties, i.e., electronic, lattice, magnetic, etc. properties separately or as weak perturbations on each other. Instead, the ground states of these materials appear when multiple ordered phases coexist collaboratively. Here we study collective modes in materials with intertwined orders through probing the both the static structure factor, S(q), and the dynamic structure factor, S(q,ω). First, we begin with measurements of SrTi1−xNbxO3 using momentum-resolved electron energy loss spectroscopy (M-EELS), which is sensitive to the dynamic structure factor of the material, S(q, ω). Our study of the plasmon and phonon collective modes in SrTi1−xNbxO3, x = 0, 0.02%, 1%, and 1.4% highlights the intimate coupling between the lattice and electronic properties in this material. We compare our results to calculations of the collective excitations of SrTi1−xNbxO3 using the random phase approximation (RPA), to assess whether the behavior of the collective modes conforms to established explanations. Our measurements reveal that the plasmon energy and linewidth are momentum independent and that the phonon frequencies do not shift with q in the expected way. Our results highlight that the lattice and the electronic behaviors cannot be treated separately or as having only weakly-perturbative effects on one another. Instead, we conclude that a radically different starting point, perhaps based on lattice anharmonicity, may be needed to explain the collective charge excitations of SrTi1−xNbxO3. Next, we turn to measurements of EuGa2Al2 and UTe2 using X-ray scattering, sensitive to S(q). After a brief discussion of the interplay between the charge density wave (CDW) and magnetic ordering in EuGa2Al2, we turn to a study of the CDW and superconducting orders in unconventional heavy fermion superconductor UTe2. Scanning tunneling microscopy measurements have identified a CDW both above and below the superconducting transition temperature as well as a pair density wave (PDW) below the superconducting transition. Here, we perform hard X-ray (100 keV) diffraction measurements on crystals of UTe2 at T = 1.9 K and resonant X-ray (U M4 and Te L1) scattering at T = 2.2 K.We find no signal at the expected CDW wavevectors, nor at any other reciprocal space positions. From this we determine that the upper bound of the charge density, ρCDW ≤ 2.13 electrons per unit cell. Additionally, we find that the atomic displacements expected from our measurements are smaller than the zero-temperature atomic displacement parameters. Our results suggest that the CDW observed in STM may be either of purely electronic-correlation driven origin or restricted to the surface of UTe2. Finally, we turn our discussion to M-EELS investigation of candidate dark matter detector materials. While the range of possibilities for the mass of dark matter particles extends over 50 orders of magnitude, mass range just below weakly interacting massive particles (WIMPs) is largely unexplored. This range has been historically overlooked, as dark matter particles in this range cannot be understood through classical elastic scattering. Instead, this mass range overlaps with the energy range of many collective modes in quantum materials. Here, we utilize this fact and investigate how “light” dark matter would interact with narrow gap semiconductors. We first discuss how the quantity measured in M-EELS is analogous to dark matter scattering. Then we present M-EELS data taken on two candidate materials, Eu5In2Sb6 and EuZn2P2, for future use in benchmarking them as detector materials.

Name thesis:degree_name

Ph.D.

Level thesis:degree_level

Dissertation

Discipline thesis:degree_discipline

Physics

Grantor

University of Illinois at Urbana-Champaign

Year dc:date

2024

Author dc:creator

• Kengle, Caitlin Sophia

Contributors dc:contributor

• Abbamonte, Peter
• Cooper, S Lance
• Fradkin, Eduardo
• Kahn, Yonatan

• Electron Scattering
• X-ray Scattering
• Intertwined Orders
• Dark Matter Direct Detection

Statement dc:rights

• Copyright 2024 Caitlin S. Kengle

Language dc:language

en, eng

Handle dc:identifier

https://hdl.handle.net/2142/124509

OAI record oai:identifier

https://www.ideals.illinois.edu/oai-pmh?verb=GetRecord&metadataPrefix=etdms&identifier=oai%3Awww.ideals.illinois.edu%3A2142%2F124509