Archiv 2018

In this talk I will discuss how the classical theory of x-ray diffraction can be used to simulate the intensity pattern produced by a powdered sample, generalised to the case where the finite size of the crystallites in the sample is accounted for. I will then explain how we are using this theory to develop new methods of measuring the shape and size of crystallites in a fractured crystal sample from x-ray diffraction data. [mehr]

Distinctive superconducting behaviors between bulk and monolayer FeSe make it challenging to obtain a unified picture of all FeSe-based superconductors. Here, we investigate the ultrafast quasiparticle dynamics of an intercalated superconductor (Li_1-xFe_x)OHFe_1-ySe, which is a bulk crystal but shares a similar electronic structure with single-layer FeSe on SrTiO₃. We obtain the electronphonon coupling (EPC) constant λ (0.24 ± 0.03), which well bridges that of bulk FeSe crystal and single-layer FeSe/SrTiO3 [1]. Moreover, we find that such a positive correlation between λ and superconducting T_c holds among all knownFeSe-based superconductors, even in line with reported FeAs-based superconductors. Our observation indicates possible universal role of EPC in the superconductivity of all known categories of iron-based superconductors, which is a critical step towards achieving a unified superconducting mechanism for all iron-based superconductors.References:[1] Y. C. Tian, W. H. Zhang, F. S. Li, Y. L. Wu, Q. Wu, F. Sun, G. Y. Zhou, L. L. Wang, X. C. Ma, Q. K. Xue, Jimin Zhao, Ultrafast dynamics evidence of high temperature superconductivity in single unit cell FeSe on SrTiO₃. PRL 116, 107001 (2016). [mehr]

I will talk about our findings on the theory of ultrafast and ultrastrong optical field interacting with two-dimensional materials with an emphasis on honeycomb-shaped structures such as graphene. In fact, this talk is the emergence of two important and growing branches of science: ultrafast optics and 2D nanocrystals. Ultrafast optics or more specifically, attosecond science, is developed to study the quantum mechanical dynamics of electrons, both collective and individual, on atomic and molecular scales and in high-density mesoscopic systems. Recent advances in attosecond metrology and generation of ultrashort optical pulses with just a few oscillations of the electric field have provided real-time access to the motion of electrons on atomic and sub-atomic scales and opened a unique possibility for the coherent control of electron dynamics at sub-femtosecond time scale. Hence, the research and exploration of ultrafast dynamics of electrons in novel graphene-like materials under the illumination of few-cycle optical fields should prove useful to a growing community of scientists and hold promises for the future technologies especially high-speed memories, ultrafast imaging, and petahertz signal processing. [mehr]

1T-TaS2 is a layered transition metal dichalcogenide with a very rich phase diagram, which was investigated since the early 1970s. At T=180K it undergoes a metal to Mott insulator transition. Mott insulators usually display anti-ferromagnetic ordering in the insulating phase but 1T-TaS2 was never shown to order magnetically. We have shown that 1T-TaS2 has a large paramagnetic contribution to the magnetic susceptibility but it does not show any sign of magnetic ordering or freezing down to 20mK, as probed by muSR, possibly indicating a quantum spin liquid ground state. Although 1T-TaS2 exhibits a strong resistive behavior both in and out of plane at low temperatures we find a linear term in the heat capacity suggesting the existence of a Fermi-surface, which has an anomalously strong magnetic field dependence. [mehr]

More than fifty years ago, it was the invention of the laser that revolutionized atomic physics and laid the foundations for quantum optics and coherent control. With only optical frequencies available, the interaction of coherent light with matter was for a long time mainly restricted to atomic transitions. Only recently have novelhigh-frequency light sources rendered x-ray quantum optics possible. In this higher frequency regime, atomic nuclei rise as natural candidates for the interaction with coherent light creating a new bridge between atomic physics, quantum optics and nuclear condensed matter physics. Nuclei are very clean quantum systems, well isolated from the environment and benefiting from long coherence times. Combining the advantages of x-rays and nuclei, a prominent incentive is to exploit x-rays as the future quantum information carriers or for novel probing technologies based on quantum effects. Furthermore, the control of nuclear transitions would open the possibility to use long-lived nuclear excited states as a compact and clean energy storage solution. The lecture will follow the developments on the emerging field of x-ray quantum optics and focus on the mutual control of coherent x-ray radiation and nuclear transitions in this new regime of laser-matter interactions. [mehr]