CAREER: Visualizing the Formation of the Charge Density Wave Phase at the Atomic Scale

****NON-TECHNICAL ABSTRACT****

A simple metal such as gold or copper can be imagined as an empty box with electrons bouncing around freely inside. In some solids, however, the electrons form waves in space with alternating regions of higher and lower charge. This state of matter is known as a 'charge density wave' (CDW). In such solids, the formation of the charge density wave happens at a critical temperature, above which the electrons are once again free to move around. Why does this happen? How exactly do these waves of electrons form in space as the sample goes through the critical temperature? This project aims to answer this question by performing temperature-dependent scanning tunneling microscopy (STM) measurements of CDW materials to directly visualize the onset of charge density waves at the critical temperature. An STM is an instrument with which we can probe the electrons at the surface of a material with sub-atomic precision. These advanced instruments will be custom-built for this project, and the new STM measurements will give us vital information on how electrons with different energies behave in these materials as they go through the CDW transition. This project will support the education of undergraduate and graduate students in the advanced technologies required to perform STM experiments including electronics, computer-aided design, vacuum technology and cryogenics. This project seeks to answer questions about the collective motion of electrons in solids, one of the fundamental challenges in modern physics research.

****TECHNICAL ABSTRACT****

The aim of this project is to visualize the onset of the charge density wave (CDW) phase in real space using scanning tunneling microscopy (STM). In a simple second-order phase transition in a uniform system, the amplitude of the order parameter goes to zero at the phase transition temperature. When defects or other spatial inhomogeneity is present, the situation can be dramatically different.

Using variable-temperature atomic resolution STM, recent experiments have shown that nanoscale CDW order can be stabilized above the bulk transition temperature in the transition metal dichalcogenides. How do these nanoscale patches of CDW transition to bulk CDW order? What is the local electronic spectrum in a nanoscale patch? What is the electronic spectroscopic difference between the CDW state and the normal state in these materials? What is the nature of scattering from defects and CDW patches above the transition temperature? During this project, state of the art, homebuilt STM instruments will be used to answer these questions. Graduate and undergraduate students will learn how to build and operate these instruments, and new designs for improved stability and cryogenic efficiency will be implemented. The nature of spatially ordered collective electronic phases is a topical question that arises in many modern materials, and the dichalcogenides present a clean material system where the onset of such a phase can be measured with atomic spatial precision and millivolt energy resolution.