Seminar: Quantifying and extracting multi-scale information from nanoscale heterogeneity using coherent X-ray scattering

The question of fluctuation and population growth at the local level and how that influences global properties has been studied extensively in physical, biological and social sciences. In liquids and colloidal solutions, fluctuations are ubiquitous. In quantum material interplay of fluctuation and phase transitions have traditionally been studied using bulk techniques. For example, ferromagnetic transitions are usually studied using a SQUID magnetometer which measures bulk magnetization. Results are often explained using mean field theory, which can miss important role played by heterogeneity.

In this talk I will show how we can work at the interface of condensed matter and statistical physics using coherent X-ray scattering and obtain information about local events that provides pathway for phase transition. We show that phase transition in amorphous FeGe (a-FeGe) involve existence of nanoscale fluctuation “hot-spots” whose origin lies in local non-equilibrium states. The fluctuating hot spots start over a small fraction of the domains at random length scales, and the fluctuating population gradually grows non-linearly into collective fluctuations. The growth of the fluctuation population resembles dynamic coherence length which forms the basis of phase transition. We also studied the distribution of the fluctuation amplitude and it followed a gaussian distribution implying ergodic dynamics whereas few kelvins below this transition temperature the distribution becomes skewed or asymmetric due to non-ergodic behavior. Our approach provides a new way to evaluate the statistics of the fluctuations in many classes of heterogeneous materials.

Finally, I will show our recent studies on generating soft X-ray orbital angular momentum (OAM) beams. I will show how we have used the OAM beam to distinguish degenerate ground state in an artificial antiferromagnet. OAM beam has the potential to unravel new information about quantum properties in materials.

Work is funded by U.S. DOE.