The surface response of metallic superlattices

Boyd, Christian

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https://hdl.handle.net/2142/115318 Copy

Description

Title

The surface response of metallic superlattices

Author(s)

Boyd, Christian

Issue Date

2021-12-21

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

Phillips, Philip

Doctoral Committee Chair(s)

Abbamonte, Peter

Committee Member(s)

• Stone, Michael
• Clark, Bryan

Department of Study

Physics

Discipline

Physics

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• Surface plasmon
• superlattice
• reflection EELS
• reflection electron energy loss spectroscopy
• plasmon continuum
• acoustic plasmon
• surface loss function
• surface response

Abstract

The study of plasmons — the quantized charge density oscillations of a system — is often through a (bulk) framework which treats the material as infinite in extent and translationally-invariant. When a surface is introduced, the material response changes due to the decreased symmetry and the analogous excitation is a surface plasmon: a charge density wave that oscillates parallel to the vacuum-material interface and decays into the bulk, rather than propagating. While bulk plasmons are routinely modeled for inhomogeneous materials (e.g., crystals), surface plasmons are excitations of materials that lack sufficient symmetry to be analyzed with the same methods. As a result, the nature of surface plasmons is mostly understood through models that presume material homogeneity — at least in the direction perpendicular to the surface plane. In order to clarify the behavior of surface plasmons and, more generally, the effect of a surface on the material response, this thesis examines a model of extreme conduction anisotropy: a (semi-infinite) system of periodically-spaced conducting planes — i.e., a metallic superlattice. The study of metallic superlattices provides a concrete description of how both bulk and surface modes contribute to the surface response. Oddly, the surface plasmon in metallic superlattices is predicted to not exist below a critical wavevector; specifically, the long wavelength limit lacks any excitations characterized by a single frequency. This is a statement derived from studying the self-sustained resonances within a metallic superlattice; however, the surface response has to measure something. We present the fate of surface plasmons in metallic superlattices concretely: as the in-plane wavevector is lowered beyond a critical value, the surface plasmon merges with the bulk plasmon continuum. The surface plasmon smoothly evolves from a sharp peak — localized at the surface plasma frequency — to a broad spectral feature whose width and shape are determined solely by the bulk plasmons within the system. Curiously, this leads to the conclusion that the long wavelength surface response of a metallic superlattice is a probe of the bulk plasmon.

Graduation Semester

2022-05

Type of Resource

Thesis

Copyright and License Information

Copyright 2021 Christian Boyd

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