Speaker: Turan Birol (Univ of Minnesota)

Title: The Many Phases of AV3Sb5 Kagome Metals (A case study for first principles methods and group theory) 

Abstract: First principles computational methods such as Density Functional Theory (DFT) provide a reliable means to both reproduce and predict the properties of crystalline materials. These approaches also allow performing thought experiments to elucidate the microscopic mechanisms of macroscopic phenomena, and build structure-property relationships. In this talk, I am going to present results from our ongoing work on the vanadium based Kagome metals with chemical formula AV3Sb5 (A=Cs, Rb, K). These materials undergo a series of crystal structural phase transitions with multiple complex phases stabilized by charge density wave instabilities. After showing that DFT can capture and explain the structural transitions in these compounds, I will also introduce the idea of a imaginary charge density waves (also known as loop currents), which are beyond-DFT phenomena that are proposed to be realized in the Kagome metals. I will then discuss how group theory, a powerful analytical tool, can be used in combination with DFT to map out the energy landscape and the structural phase diagram of these compounds, as well as to classify and phenomenologically study the different possible loop current orders. I will conclude by speculating about the experimental signatures of the phases these materials host, including nonreciprocal optical response that emerges from the way time-reversal symmetry is broken by the charge density waves.  

Short Bio: Turan Birol received his BS, MS, and PhD degrees degrees in Physics from METU, Koc, and Cornell Universities respectively, and worked as a postdoctoral research associate at the Condensed Matter Theory group in Rutgers University until he joined the department of Chemical Engineering and Materials Science at the University of Minnesota in 2016. His research interests lie on the intersection of condensed matter physics, structural chemistry, and materials science; and he uses first-principles computational tools (including density functional theory and dynamical mean field theory) to study and discover novel realizations of exotic phenomena in crystalline compounds, with a focus on transition metal oxides. He is currently a member of the University of Minnesota Materials Research Science and Engineering Center (MRSEC) and the  Center for Quantum Materials (CQM), and received the ONR Young Investigator and NSF CAREER awards, as well as the University of Minnesota McKnight Assistant Professorship.