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LASSP & AEP Seminar by Prof. Andrew Rappe (University of Pennsylvania)

Tuesday, March 26, 2024 at 12:20pm to 1:30pm

Clark Hall, 700
Central Campus

Andrew M. Rappe
Professor, University of Pennsylvania

Optoelectronic and Structural Consequences of Spontaneous Symmetry Breaking in Ferroelectrics
In this lecture I will focus on materials that break inversion symmetry, moving from fundamental phenomena enabled by broken symmetry to two classes of applications for future energy-efficient systems. 

As the need for clean and sustainable energy increases, renewed focus on alternative energy sources such as photovoltaics has become vital. This motivates study of the bulk photovoltaic effect (BPVE), a nonlinear optoelectronic property that can generate electricity without a p-n junction. To demonstrate the capability of first principles BPVE theories to guide materials design, we outline an automated method to design distortions that enhance the shift current of monolayer MoS2 and use it to uncover a polar distortion that increases the integrated shift current more than ten-fold. Calculating the shift current contribution to the BPVE only explains part of the experimentally observed photocurrent. We present a method that enables the ballistic current—a current resulting from asymmetric scattering—from first principles. We calculate the ballistic current for BaTiO3 from first principles. The current due to electron-phonon scattering is comparable to the shift current, and is therefore experimentally relevant, while the current due to electron-hole scattering is much smaller in magnitude. This methodological development enables closer agreement between theory and experiments and lays the groundwork for further prediction and design of materials with large BPVE.

Ferroelectric hafnia exhibits excellent size scalability and silicon compatibility, which makes it promising for future ferroelectric on-chip nanoscale computing devices. Unlike conventional ferroelectrics such as perovskites, whose ferroelectric phase transitions are described by a single polar order parameter, hafnia has a wide variety of phases, with complicated phase transitions involving multiple order parameters. From a theoretical perspective, this makes it difficult to understand the underlying reason(s) for hafnia’s unique properties and behavior. From an experimental perspective, this variety also complicates phase identification, characterization, and development of prototype devices. Here, we describe first-principles simulations and Landau-Ginzburg-Devonshire modeling to show that the multi-order parameter nature of hafnia is the key to understanding its unique phase transitions, domain wall structures and polarization switching behaviors. Due to the presence of multiple order parameters, domain walls of different configurations could be formed, and each of them shows distinct switching mechanisms. Moreover, the existence of non-polar order parameters also suggests the existence of special domain walls where the sign of polarization is preserved across it. These domain walls are topological defects that could commonly exists in hafnia thin-film and have distinct properties.

Hosted by Craig Fennie

Pizza served starting at 12:10 p.m.
Please bring your own beverage

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LASSP, Applied and Engineering Physics


engineering, lassp_and_aep_seminar, lassp, quantum_site, applied and engineering physics

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Ann Owens

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Prof. Andrew Rappe

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University of Pennsylvania

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