The Nanoscale Effects of Intrinsic and Externally-Applied Strain on Charge Density Wave States

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Non-technical abstract:Charge density wave states occur in numerous materials where, below a certain "transition" temperature, there is a periodic lattice distortion, and a periodic variation in electron density is established across the material. The electronic properties of the material are altered from the material's original, undistorted state. Charge density wave-hosting materials have been targeted for potential use in electronic devices (such as switches, ion field-effect transistors, and logic circuits). However, there is currently an incomplete understanding as to 1) the origin of charge density wave states, as well as 2) how these states coexist/interact with other technologically-important material properties such as superconductivity or magnetism. Strain engineering is an emerging field in which the application of strain to materials is used to manipulate their optical, electronic, and structural properties. In this project, controlled strain is applied to materials hosting charge density wave states, so as to compress, stretch, or shear the materials, leading to changes in their properties. Scanning tunneling microscopy, a technique which can detail electronic and structural changes in a material on the atomic scale, is used to, simultaneously, detail the evolution of charge density wave states under, both, varying strain and temperature conditions. An overarching goal of the project is to develop a fundamental understanding of the physics governing these compounds. An understanding of how to effectively manipulate their properties in a controlled fashion, is essential for optimizing their performance in devices. This project enhances the education of undergraduate- and graduate-student participants who are developing essential skills with applications in research, industry, and beyond. In addition, mini-classes using experimental techniques such as scanning tunneling microscopy, atomic force microscopy, and scanning electron microscopy, are being developed and incorporated into established, STEM-focused local outreach programs. In particular, these classes will expose precollege students to cutting-edge experimental techniques, providing a unique experience and opportunity for these students to become interested and engaged in STEM fieldsTechnical abstract:Charge density wave (CDW) states are prevalent in condensed matter systems where they are often found to coexist with other orders. The nature of the interplay of CDW states with quantum orders, such as superconductivity and magnetism, particularly on the nanoscale, is complex and not well-understood. In addition, CDW states can have differing and not always well-established origins, further complicating this understanding. Studies of CDW-hosting compounds demonstrate that strain can alter an array of CDW properties, including changing the transition temperature, altering electronic and structural periodicities of the CDW state, and changing the associated electronic band gap. The overarching goals of this NSF project are to understand 1) how strain drives these CDW changes on the atomic-scale, and 2) how to manipulate these changes in a controlled fashion using externally-applied strain. To do this, temperature-dependent scanning tunneling microscopy is used to probe the nanoscale structural and electronic properties of charge density wave states, their formation, their manipulation, and their interplay with coexisting quantum orders, under quantifiable and varied strained conditions. In order to study fundamental differences strain has on CDW states arising from differing origins (for example, Fermi surface nesting versus momentum-dependent electron-phonon coupling), compounds from three well-known, distinct, CDW-hosting families are studied: the blue bronzes, transition metal dichalcogenides, and the rare-earth tellurides. This project supports the research education of graduate and undergraduate students through their participation in all aspects of the project. Furthermore, in an effort to extend science education to precollege students who are historically underrepresented in the sciences (precollege girls and students from the many lower-income households in the local community), the principal investigator is developing mini-classes on "probing material properties" which utilizes Clark facilities to expose area students to the basics of materials research. These classes will be held in connection with established, STEM-focused outreach collaborations involving local area middle and high school students.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.