Dynamic Charge-Density Waves and Electronic Anomalies of Inorganic Solids

University Of Houston, Houston TX

Investigators

Abstract

Vassiliy Lubchenko of the University of Houston is supported by an award from the Chemical Theory, Models and Computational Methods program in the Division of Chemistry for theoretical research focused on electronic phenomena in complex, disordered systems. The astonishing diversity of structures and properties of solids underlie their use in everyday applications ranging from information technology to heat storage to metallurgy. Yet making materials with tailored properties is still somewhat of a trial and error enterprise. It is difficult to predict how the atoms will arrange in a specific compound, how well this compound will conduct electricity or respond to an electromagnetic field. The reason for these predictive difficulties is that quantum-mechanical equations governing the motion of electrons in atoms (and molecules and materials) are difficult to solve. Their solution, even if known, are hard to survey. Professor Lubchenko’s research aims to reduce this complexity by treating electrons not as separate particles, but as a fluid. In contrast with ordinary liquids, the electrons can flow even at very low temperatures, as they do in metals. Lubchenko hypothesizes that slow, wavelike motions of this quantum fluid account for a puzzling feature of photoemission in sodium and potassium. Experiments reveal an apparent excess of electrons that should contribute to electric conduction but, mysteriously, do not do so. The electrons can be forced to stay put, but only if there is enough pull from the positively charged nuclei. The electrons thus become localized around the nuclei, while the material becomes an insulator or semiconductor. Lubchenko and coworkers explore this localized-electron regime to predict properties of amorphous alloys that may be used in making the next generation of computer memory and smart optics, among many other things. A major component of the Lubchenko group's educational and outreach activities is direct involvement of high school and undergraduate students in the process of scientific discovery. Much progress has been recently achieved in rationalizing the structures and electronic spectrum of simple inorganic compounds whose Born-Oppenheimer vibrational ground state is unique. There has been much less success understanding the very important class of solids that exhibits a vast degeneracy of nearly equivalent metastable Born-Oppenheimer configurations. The microscopic hypothesis of this research is that multi-electron excitations--such as those giving rise to plasma oscillations at long wavelengths, can make solids unstable toward the formation of long-lived charged-density waves (CDW) on short wavelengths. Notably, these density waves can be aperiodic. If coupled sufficiently strongly to the underlying atomic lattice, the aperiodic CDWs then lead to the formation of glassy amorphous semiconductors. Domain walls separating distinct aperiodic low-energy charge patterns are expected to host special midgap electronic states, which play the role of electronically active defects. When sufficiently close to each other, these midgap states contribute to the exponential tail of localized states near the mobility edge and dominate the electrical conductivity in glassy semiconductors. In the interesting case of where the CDW-lattice coupling is intermediate in strength between that found in metallic and intermetallic compounds, the instability is expected to lead to dynamic disorder so that the lattice remains ordered but only on the average. If filled, the midgap states residing on dynamic domain walls could provide a route to high-temperature superconductivity; no additional effective attraction between electrons is necessary. To test this hypothesis, Lubchenko and his research group implement a novel methodology in which one applies carefully chosen spatially varying external fields to controllably induce aperiodic charge distributions in order to quantify their stability, degeneracy, and kinetics of their mutual interconversion. In testing the proposed methodology on alkali metals, they attempt to resolve a decades old controversy regarding the apparent excess density of electron states near the Fermi surface revealed by photoemission experiments. 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.