Veranstaltungen

Water and ice surfaces and interfaces are ubiquitous, not just in nature, but also in many technological applications. Water is a rather unique liquid, owing to its strong intermolecular interactions: strong hydrogen bonds hold water molecules together. At the surface of water and ice, the water hydrogen-bonded network is abruptly interrupted, conferring distinct properties on the interface, compared to bulk. I will present some challenges (“how can we study the ~1 monolayer of water molecules that is in direct contact with the other phase, and distinguish this ~Angstrom-thin layer from the bulk?”) and progress in the study of interfacial water. I will specifically address the interaction of water with charged interfaces, and attempt to explain why ice is slippery. [mehr]

Most biological functions and many chemical processes are driven by chiral nanoscale molecular machines in solution, whose structures evolve on multiple time and length scales: from the ultrafast rotations of photo-driven synthetic molecular motors to the global conformational changes of proteins on the microsecond time scale. Yet capturing the associated conformational transitions in real-time continues to be a formidable experimental challenge, as prominent established methods come with their own limitations: solution nuclear magnetic resonance is limited to millisecond real-time resolution, whilst solution X-Ray scattering requires large-scale X-Ray facilities. A promising laboratory-based alternative is circular dichroism (CD), the absorption difference of left- and right-handed circularly polarized light, which is sensitive to the chiral geometrical arrangement of light-absorbing chemical groups within a molecular system. Steady-state CD is already a well-established tool in the far and middle ultraviolet (UV) < 300 nm, where equilibrium structures of proteins, DNA and functional chiral organic complexes are routinely characterized. However, pushing this technique into the time-domain has remained a challenge for over three decades, with only few isolated reports with sub-nanosecond resolution [1]. In this talk, I will present a technological breakthrough with the first time-resolved CD (TRCD) spectrometer that combines highly sensitive broadband UV-detection (250-370 nm) with pulsed laser sources and sub-picosecond time-resolution [2]. With this instrument, it is now possible to extract broadband CD spectra of photo-excited molecular states and follow their transient chirality changes with femtosecond resolution. This is opening a new avenue for capturing solution-phase structural dynamics in chemical and biological systems that I will illustrate with two examples: the coupling of electronic and structural dynamics in a chiral supramolecular metal-complex [3], and the application of a site-specific CD-label to track conformational changes of the peptide backbone [4]. On this basis I will present future developments that will establish TRCD as a complementary method for research in protein dynamics and chiral photochemistry, where the chirality of excited electronic states is the key design feature of chiral organic light-emitting diode materials and unidirectional molecular motors, for example. [mehr]

The normal state of unconventional superconductors often exhibits anomalous transport properties and it is commonly referred to as a “bad” or “strange” metal. Understanding its collective charge dynamics, which defies the standard quasiparticle description of a Fermi liquid, is an outstanding challenge of modern condensed matter physics.In this talk, I will present a direct measurement of the collective charge dynamics of the strange metal using inelastic electron scattering. First, I will discuss how normal-state Bi2Sr2CaCu2O8+d is defined by a featureless, localized continuum, undergoing a low-temperature massive spectral weight redistribution. I will then describe how such a phase is found to coexist with a low-energy Fermi liquid in Sr2RuO4.These results indicate that strange metals are highly localized in space and dissipate on ultrafast timescales, seemingly bound only by quantum limits. Implications for the occurrence of high-temperature superconductivity will be discussed. [mehr]