The Microscopic Electronic Structure of Iron Superconductors Under Strain: New Frontiers in Scanning Probe Microscopy

Columbia University, New York NY

Investigators

Abstract

Non-technical Abstract: When a material is mechanically stretched, its properties can undergo dramatic changes. For example, we are familiar with how the elasticity of rubber changes as it is stretched. Less familiar but equally important are changes to the electronic properties of materials. When stretched, materials can change their electrical resistance, light absorption and a host of other electronic properties. The causes for such changes in the electronic properties are often poorly understood at the microscopic level. In this project, the research team will use scanning tunneling microscopy (STM) to study the electronic properties of materials as they are stretched. STM is a technique to measure the electronic properties of materials with the precision of single atoms. In this research, a specially designed apparatus is used to mount and stretch crystal materials in the STM while their response is monitored. This equipment is designed, manufactured, assembled and run by the principal investigator's research team. It is used to impart training to students at the high school, undergraduate and graduate levels, and is used to liaison with industrial partners. The goal of the project will be to develop a microscopic understanding of the changes brought about by strain in materials, especially those belonging to the iron pnictide class of superconductors. Technical abstract: External strain when applied to materials can cause electronic changes via modifying band structure, interaction strengths and even changes in phase. Understanding the microscopic changes brought about by the application of strain is the key scientific problem of interest in this project. The main scope of the research is to study the electronic nematic phase of the iron pnictides. In this phase, electronic properties display nematicity, the spontaneous breaking of the underlying discrete rotational symmetry of the lattice. Developing an understanding of the nematic electronic phase and its coupling to the other phases of these materials is the key scientific goal of this project. The main experimental technique used will be atomic-resolution, cryogenic STM measurements. The project is enabled by a technical breakthrough that allows for the very first time the application of uniaxial or biaxial in-plane strains to a sample, while continuously performing microscopic measurements on the same atomically resolved area of the surface. Using this apparatus, STM is used to measure (a) gaps from spectroscopy (b) domain structure from topography and (c) scattering rates from quasiparticle interference, all under the influence of strain. The apparatus is used across the phase diagram in three different pnictide families of NaFe(Co)As, Fe(Te)Se and LiFe(Co)As systems that display very different nematic behaviors. The final goal of the project is to determine the driving forces for nematicity and its connection to superconductivity in the pnictides.