Discovering new mechanisms for ferroelectricity in hexagonal half Heuslers
Abstract
The proposed work will to use molecular beam epitaxy to stabilize the polar phase and demonstrate switching in a new class of ferroelectric materials, the intermetallic hexagonal half Heuslers (composition ABC). Intermetallic compounds are an unusual family in which to search for ferroelectricity. But due to their covalent bonding character with small Born effective charges (in contrast with the ionically bonded oxides), these materials are proposed to host a number of unusual properties, including hyperferroelectricity (proper ferroelectrics that can polarize in the presence of an unscreened depolarizing field) and polar metallicity. Moreover, these materials also exhibit a diverse range of electronic and magnetic properties, making them strong candidates for coupling ferroelectricity to other phenomena in layered heterostructures. However, due to challenges in stabilizing the correct phase and fabricating samples with sufficiently high quality, ferroelectricity in these materials remains yet to be demonstrated experimentally. The proposed work will use epitaxial strain, chemical pressure, and adsorption-controlled growth to stabilize, tune the structure, and control the stoichiometry and interfaces of hexagonal half Heusler films and superlattices. Overcoming these materials growth challenges is critical to the demonstration and understanding of ferroelectricity in this new class of materials. The target materials are (Li,Na)ZnSb, RAuGe, and RPdSb (R= rare earth metal). Here the high volatility of group I, II, and V elements will be used to (1) identify a temperature/flux ratio window for adsorption-controlled growth during codeposition and (2) determine the window for self-limiting shuttered layer-by-layer growth (atomic layer epitaxy). These studies will be coupled with in-situ measurements of composition, bonding, electronic, and surface atomic structure by photoemission spectroscopy (UPS/XPS) and scanning tunneling microscopy (STM), for rapid feedback on growth conditions without exposing samples to air. The anticipated outcomes are: (1) epitaxial stabilization and minimization of electronic defects in (Li,Na)ZnSb, RAuGe, and RPdSb films in the polar phase, (2) new insights into the tuning of metal-insulator transitions, bandgap, polarization, switching barriers, and Tc as functions of BC layer buckling, strain, and superlattice dimensionality, (3) demonstration of polar metallic behavior in RAuGe and RPdSb, and (4) fundamental insights into the mechanisms that stabilize ferroelectricity in hexagonal Heusler semiconductors and semimetals. If fully successful, the proposed work would open the door to ferroelectric switching and hyperferroelectricity in semiconducting hexagonal Heuslers. But even if it falls short of demonstrating switching, the fundamental insights gained from tuning lattice-electronic structure coupling would be significant advances for the field of functional intermetallic compounds. The long term vision is to establish hexagonal Heuslers as a new class of multifunctional, multiferroic materials that rival the complex oxides and can be integrated directly onto III-V semiconductor substrates
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- Oct 15, 2018
- Source ID
- W911NF1710254
Entities
People
- Jason K Kawasaki
Organizations
- Army Contracting Command
- United States Army
- University of Wisconsin–Madison