"Shining Coherent X-rays on Quantum Materials"
This event is part of the Biophysics/Condensed Matter Seminar Series.
Symmetry breaking and recovery in condensed matter systems are closely related with exotic properties of these materials – superconductivity, magnetism, ferroelectricity, etc. Recent and upcoming implementations of coherent X-ray techniques at synchrotrons and free electron lasers open the door to novel experimental opportunities to directly visualize the symmetry changes in quantum materials. In this talk I will discuss two such efforts to study doped Mott insulators with layered perovskite structures.
We have pushed Bragg Coherent Diffractive Imaging (BCDI) into the cryogenic regime where most phase transitions in quantum materials reside. Utilizing coherent photons at 34-ID-C beamline of the Advanced Photon Source, we studied the low-temperature phases in the original cuprate superconductor La2-xBaxCuO4. These phases break the local four-fold rotational symmetry of the high temperature tetragonal (HTT) structure. Our preliminary data suggests the glassy nature of the low-temperature-orthorhombic (LTO) phase . These observations put stringent constraint on the theoretical descriptions regarding the nature of charge order  and its relation with superconductivity.
Recently we demonstrated time-resolved Resonant Inelastic X-ray Scattering (tr-RIXS) for the first time at LCLS to observe the temporal evolution of collective excitations in Sr2IrO4 . At the destruction of long-range antiferromagnetic order under controlled conditions, tr-RIXS provides definitive evidence of transient antiferromagnetic correlations recovering over a few picoseconds, while the long-range order is not restored over a pump-fluence-dependent time scale over hundreds of picoseconds.
Looking forward, many quantum materials have been discovered to possess desired properties in the material science and engineering. Meanwhile quantum effects have been found important in many functional materials. I will briefly discuss how these new X-ray methods will be applied to interrogate the structural and electronic properties in the materials.
 Y. Cao, T. Assefa, I. K. Robinson et al., in preparation (2017).  X. M. Chen et al., Phys. Rev. Lett. 117, 167001 (2016); V. Thampy et al., Phys. Rev. B 95, 241111(R) (2017).  M. P. M. Dean*, Y. Cao* et al., Nat. Mater. 15, 601–605 (2016).