Awards Database

Cottrell Scholar Awards - 2016

Di Xiao

Assistant Professor of Physics, Carnegie Mellon University

Topological Excitonics in Gapped Dirac Materials

Di Xiao, an assistant professor of physics at Carnegie Mellon University, is working to understand and predict how a type of quasiparticle called an exciton moves as a wave through certain types of semiconductor materials that may one day make up advanced computer chips and photovoltaic panels.

An exciton is composed of an electron, which is negatively charged, and its “hole,” which is positively charged. An exciton is created when an electron is knocked out of its orbit by, say, an incoming photon, the smallest unit of light. At that point the electron, and the state it left behind (its hole), are excited into a bound state due to energy absorbed from the photon. This newly minted exciton moves around the crystal-lattice molecular structure of a semiconductor while carrying the photon energy and polarization, therefore it could be used to collect solar energy and even process information. 

Xiao is attempting to understand this process in “two-dimensional Dirac materials.” The term refers to matter that is only a molecule or several atoms thick, a characteristic of the recently discovered form of carbon known as graphene; there are also graphene-like two-dimensional materials composed of silicon or germanium. Furthermore, electrons and holes in these materials behave like relativistic particles normally found with much higher energy, as those generated in particle accelerators or naturally occurring in cosmic radiation. All of these two-dimensional Dirac materials have crystalline molecular structures conducive to the movement of electrons and excitons. (This crystalline molecular structure is the chief distinguishing characteristic of all semiconductor materials, including those of much greater thickness used in today’s computer chips and photovoltaic solar panels.)

Specifically, Xiao and his research associates are attempting to understand how the quantum mechanical phase (in quantum mechanics, particles behave like waves, which are described by amplitude and phase) of excitons affects their behavior, as well as how the presence of many excitons in ultrathin materials affects their overall magnetic and optical properties.

“These phenomena are often characterized by novel electromagnetic responses, which may be useful for applications in quantum electronics and quantum computing,” Xiao says.

For the educational component of the Cottrell Scholar Award, Xiao intends to promote a “hypothesis-driven learning process” for solid-state physics through computer simulation, and integrate advanced research-oriented topics into undergraduate physics courses. He hopes this work will eventually be available free through the Internet.

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