After considering several potential areas, a committee from Brookhaven and UD selected two projects - one on rice soil chemistry and the other on quantum materials - for the new initiative. For each project, one graduate student based at Brookhaven and one graduate student from UD will work with and be supervised by a principal investigator from each respective institution. The research, to start in October 2019, is funded separately by the two institutions. Brookhaven funding is provided through its Laboratory-Directed Research and Development programme, which promotes highly innovative and exploratory research that supports the Lab's mission and areas for growth.
The quantum materials project, "Growth and characterization of quantized antimony-based topological insulators", is co-led by Peter Johnson, group leader of the Electron Spectroscopy Group in Brookhaven's Condensed Matter Physics and Materials Science Division, and Stephanie Law, the Clare Boothe Luce Assistant Professor of Materials Science in UD's College of Engineering.
"Our existing collaborations with UD are producing exceptional results, and we hope that we can expand this success to other strategic areas of research through the joint initiative", stated Priscilla Antunez, assistant director for strategic partnerships at the Center for Functional Nanomaterials (CFN) - another DOE Office of Science User Facility at Brookhaven - and coordinator of the Brookhaven-UD relationship.
"The University of Delaware is pleased and excited to expand research collaborations with Brookhaven National Laboratory", stated Charlie Riordan, vice president for research, scholarship and innovation, and professor of chemistry and biochemistry. "Our work together is destined to have positive and far-reaching impacts, of benefit to our students and to society."
When materials have nanoscale (billionths of a meter) dimensions, their electrons can only occupy specific (discrete) energy levels. This phenomenon, called quantum confinement, occurs when the nanostructures - for example, wires or dots - are smaller than a critical length scale. The unique properties of such nanostructures are of interest for many applications, including quantum computers.
To date, much of the research on quantum confinement has focused on semiconductor materials, which have an electrical conductivity in between that of conductors (high conductivity) and insulators (low conductivity). But recently, scientists have been turning their attention toward a new class of materials that behave as insulators internally but conductors on the surface - i.e., electrons can only move along the surface. The surfaces of these topological insulators are special because they are protected from backscattering, which occurs when electrons hit atomic defects or other imperfections in a crystal structure or move in response to vibrations of the atoms. The scattering of electrons is problematic because it interferes with the flow of electric current, causing energy dissipation and thus loss.
Some scientists have theoretically proposed that the surfaces of topological insulators should also have electrons of discrete energy levels that are "topologically" protected. Law and Johnson will test this theory by growing self-assembled quantum dots from a topological insulator made of the elements bismuth, antimony, and telluride and measuring the dots' energy level spectrum.
"Topological insulators are already known to be interesting materials from both a physics and an engineering perspective", stated Stephanie Law. "What isn't known is how they behave when confined to extremely small scales. The collaboration between UD and Brookhaven will allow us to address these questions directly, and in so doing, uncover new physics and discover new applications for these materials."
At UD, Stephanie Law will grow samples of varying size and chemical composition with a thin-film deposition technique called molecular beam epitaxy and characterize the structure of the samples using x-ray diffraction and microscopy techniques. Then, at Brookhaven, Peter Johnson and team member Dario Stacchiola - group leader of the CFN Interface Science and Catalysis Group - will measure the samples' energy spectra through scanning tunneling spectroscopy (STS) and angle-resolved photo-emission spectroscopy (ARPES). In STS, a voltage is applied between a sharp metallic tip and a sample, allowing electrons to tunnel between the two. The amount of electrical current is proportional to the density of states - the number of electrons per unit volume over a given energy level. In ARPES, x-ray or ultraviolet light is directed on the sample, and the energy and momentum of the emitted electrons are measured.
"This study will allow us to understand the electronic structure of topological insulators at the nanoscale, by measuring their quantized energy level spectrum", stated Peter Johnson. "Our research could help us answer fundamental questions about the physics of topological systems. It could also be used as the foundation to develop qubits--the counterpart to the binary bits used in today's computers - that operate at room temperature over extended distances for more efficient computing."