Turan Birol
University of Minnesota
“Group Representations and Tensorial Response in Crystals”
Abstract
What defines and distinguishes a phase is its symmetry group, which contains the list of all symmetry operations that leave it invariant. It is also symmetry that determines what macroscopic response is allowed. In solids, macroscopic response to external fields is commonly used to identify novel phases and order parameters, which can be as easy as measuring the magnetic susceptibility to detect a magnetic transition. In certain cases, however, the order parameter can be ‘hidden’ with no obvious macroscopic response property to probe it – such as the case in the hidden-order in URu2Si2, or the loop current phases in AV3Sb5 Kagome metals. In this talk, I will present our recent work on classifying and predicting different types of response tensors using group representations to develop a systematic approach to solve this problem. First, through a review of basic group theoretical concepts, such as representations and their products, I will introduce how one can determine the order parameters that can be probed by an arbitrary response tensor of arbitrary rank. I will then consider real materials examples, and illustrate the power of this approach by brandishing first principles results on a variety of phenomena. In particular, I will show that (1) elastoresistivity, commonly used to probe nematic orders, can also conveniently probe ferro-rotational (or ferro-axial) order, (2) piezoelectricity is not a necessary property of all noncentrosymmetric insulators (and in fact a pure realization of chirality wouldn’t cause piezoelectricity), (3) flexoelectricity (a linear relationship between polarization and strain gradients) is necessarily nonzero in all solids, and (4) altermagnetism has a unique signature in piezomagnetic response, and (4) loop current orders in Kagome charge density wave systems can also give rise to piezomagnetic response.
Turan Birol received his BS, MS, and PhD 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 before joining the University of Minnesota in 2016. His research interests lie in the intersection of condensed matter physics, structural chemistry, and materials science, where he uses first-principles computational tools to study and discover novel realizations of exotic phenomena in crystalline compounds..
Date: January 8, 2025 Wednesday
Time: 15:30
Place: SA-240
All interested are cordially invited.