CEF-phonon interactions in rare earth sesquioxides: pressure-, temperature-, and magnetic field-dependent Raman scattering studies

Slimak II, John Edward

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https://hdl.handle.net/2142/124131 Copy

Description

Title

CEF-phonon interactions in rare earth sesquioxides: pressure-, temperature-, and magnetic field-dependent Raman scattering studies

Author(s)

Slimak II, John Edward

Issue Date

2024-01-16

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

Cooper, S L

Doctoral Committee Chair(s)

MacDougall, Gregory J

Committee Member(s)

• Wagner, Lucas K
• Eckstein, James N

Department of Study

Physics

Discipline

Physics

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• Raman Scattering
• Rare Earth Materials
• Rare Earth Oxides
• High Pressure Materials
• Crystal Field Theory
• Crystal Field Interactions
• Electron-Phonon Interactions

Abstract

In this dissertation, I present our Raman scattering studies that were performed in attempts to explore the consequences of near-resonance of low-lying 4f electronic excitations and phonon modes in lanthanide-based materials. In particular, we present three spectroscopic studies of four materials, Pr2O3, Eu2O3, Yb2O3, and Dy2O3 as functions of pressure, temperature, and/or magnetic field. The Raman-active excitations of these materials exhibit anomalous behavior, which we find are well described by models of coupling between phonon modes and localized 4f electronic states initially developed by Thalmeier and Fulde to describe similar behavior observed in CeAl2 [1], [2]. Our studies of hexagonal A-type Pr2O3 reveal that, unlike in isostructural Ce2O3 [3], the coupling between crystal electric field (CEF) excitations and energetically proximate phonons is too weak to form emergent bound states. Despite this, we observe anomalous softening of two phonon modes with decreasing temperature. The Eg bending phonon mode softens by approximately 1.5 cm−1 from its maximum energy and the A1g stretching phonon mode softens by approximately 30 cm−1 from its maximum energy. We find that pressures up to 30 kbar are insufficient to drive Pr2O3 into a magnetically ordered phase, but are sufficient to lift the degeneracy of the first excited CEF level in this system. Further, we find that high pressures modulate the splitting of this first excited CEF level by magnetic fields, yielding near-zero field-dependence at pressures above 30 kbar. The Raman-active phonon modes in cubic C-type Eu2O3 and Yb2O3 have been reported to occur with energies consider lower than expected given the masses of the rare earth atoms or the lattice constants of these materials. In addition to these soft phonon modes, we report anomalous temperature-dependent softening of several phonon modes in both Eu2O3 and Yb2O3. Though we observe CEF excitations in both materials, only in Yb2O3 does this temperature-dependent phonon mode softening appears to arise from coupling of these phonons to energetically proximate CEF excitations. Instead, in Eu2O3, we observe Raman-forbidden CEF transitions, suggesting that the phonon softening in Eu2O3 is a product of structural symmetry-lowering defects. Lastly, in Dy2O3, we report temperature- and magnetic field-dependent Raman scattering studies, wherein we observe for several anomalous modes consistent with hybrid bound states of CEF excitations and phonon modes. The intensities of these bound states are significantly reduced at high magnetic fields, suggesting that the coupling between the low energy 4f electronic states and phonons is near the critical strength necessary for formation of bound states. Importantly, these bound states are observed at temperatures much higher than in Ce2O3 [3], suggesting that interesting magnetocapacitive effects may be accessible at these higher temperatures.

Graduation Semester

2024-05

Type of Resource

Thesis

Copyright and License Information

Copyright 2024 J. E. Slimak

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