Archiv 2018

Charge density wave (CDW) ordering and the mechanism for it remains a live issue, particularly in transition-metal dichalcogenide (TMD) systems. These are attractive because of both the variety of material properties they are known to host, and their conveniently layered van der Waals structures. The evolution of properties and interactions of these materials when thinned to a monolayer limit remains an area for exploration. In this talk, I present some background on the study of CDW order in bulk and monolayer TMDs. I then discuss some recent results, as well as my own upcoming investigations of TiTe2, utilising molecular beam epitaxy (MBE) growth and photon energy-dependent angle resolved photoemission spectroscopy (ARPES). [mehr]

Experiments in the time domain allow to determine the electron-boson coupling strength by analyzing the second moment of the Eliashberg function α2∙F(ω) using the relaxation time constant of thermalized, hot electrons after optical excitation. [1] While this approach works well for conventional superconducting materials, it is under discussion for unconventional superconductors due to competing electron and boson dynamics on similar time scales. [2,3] [mehr]

Nano science and technology offer a vast and fascinating playground
to explore the novel physiochemical properties of nanomaterials with the development for various applications including energy conversion and electronics. [mehr]

Coupled-cluster theory has become a key tool in quantum chemistry, providing gold-standard accuracy for ground- and excited-state energetics, and other properties. [mehr]

Terahertz interaction with matter has become one of the hottest topics in ultrafast community. Indeed, intense terahertz pulses have recently proved to be a pivotal tool to manipulate and control the properties of materials and especially complex condensed matter systems. The recent development of terahertz sources driven by lasers and accelerators has led to pioneering experiments demonstrating the access to new metastable phases of matter and nonlinear processes hindered to conventional laser excitation.In this talk, I will give an overview on the high-field terahertz generation and present the capabilities of the nonlinear terahertz spectroscopy. Recent experiments, including terahertz driven insulator-to-metal transition, nonlinear optical processes and coherent phonon control, are discussed. [mehr]

In recent years, electron diffraction has arisen as an alternative to X-ray diffraction for structural studies on threedimensional crystals. Promising features of the technique include complete data sets from a few or even singlenanocrystals, sensitivity to the charged state of ions and the relatively low expense of the apparatus. Experimentalprotocols and detector technologies are improving, so that data collection using the rotation method, dominant inX-ray crystallography, is now feasible in a cryoTEM. This convergence of experimental techniques has beenaccompanied by repurposing of analysis tools: robust and sophisticated algorithms developed over decades forX-ray diffraction integration software can be now employed to tackle electron diffraction data. Nevertheless, thegeometry of the electron diffraction experiment incurs specific challenges to address in the analysis. Here, thediffraction integration package DIALS is discussed, highlighting particular adaptations that were made to thesoftware for various example cases of electron diffraction, particularly on protein crystals. The experience gainedindicates that integration of good quality ED data can be straightforward, but the bottleneck remains with collectionof such good data, which relies on careful calibration and understanding of the instrument. Improved apparatus willameliorate this issue and future studies to develop improved models for details including dynamic diffraction andabsorption are anticipated. [mehr]

Elementary steps of proton transfer between acids and bases occur on ultrafast time scales. To elucidate the microscopic mechanisms of proton transfer many research groups have applied time-resolved spectroscopy utilizing a class of organic molecules called photoacids. In this talk I will provide an overview of results obtained by my research team on photoacid molecules with ultrafast infrared spectroscopy as a local probing technique. Whereas profound insight in aqueous proton transfer pathways in acid-base neutralization have been achieved in recent years, the underlying reasons for photoacidity have remained unsolved. Recent developments will be discussed how to tackle this unsolved question using the technique of soft-x-ray spectroscopy as an alternative local probe. For this liquid flatjet technology appears to be a highly promisingmethodological approach. [mehr]

In recent years, due to the maturity of micro-nano fabrication technology, artificial nano-structures are widely studied, people actively explore the mechanical, thermal, optical and electromagnetic properties in a variety of nano-materials, among which the nano-mechanical resonator draws great attention for its potential use to study the light-matter interactions. In this presentation, I will first report our experimental results of two carbon nanotube mechanical resonators that are strongly coupled when the frequencies of them are tuned very close to each other. The vibrations of the two resonators superpose to produce a frequency splitting phenomenon, indicating a strong coupling regime. Second, a parametric strong coupling between two different modes within single mechanical resonator in linear regime of small amplitudes will be presented, which is different from the strong modes coupling in nonlinear regime. Finally, I will show that a back-and-forth coherent exchange of phonon energy in two different vibration modes, to realize a classical Rabi oscillations. [mehr]

Water, due to its unique hydrogen bonds, is one of the materials exhibiting diverse phases, crystal morphologies, and phase transformation. As a result of interplay between environmental conditions and molecular kinetics, more than 25 crystalline and amorphous ices have been reported with various growing morphologies from polyhedron to needle. Dynamic diamond anvil cell (dDAC), which may simply resolve the interference of macroscopic driving force and microscopic kinetics by changing compression rate, is essential to study complex phase transition behaviour of water. In the present study, we haveinvestigated the effect of compression rate on crystal growth and the multiple freezing-melting pathways of H2O under far-from-equilibrium condition by using dDAC at room temperature. First, we reveal the origin of shock growth of ice VI single crystal. Under rapid compression (strain rate > ~0.1 /s), we observed a morphological transition in ice VI growth from three-dimension (3-d) to twodimension (2-d) with one-order higher growth speed. It is found that local growth condition and interface kinetics can be affected by compression rate, which facilitate the 2-d shock growth. Secondly, we explore five different pathways of freezing and melting of deeply supercompressed water and ice via metastable phases. We will discuss the mechanism of freezing and melting by calculating driving force and interfacial free energy based on the classical nucleation theory, and comparing structure of supercompressed water and stable and metastable ices obtained from Raman spectroscopy. [mehr]

In this talk I will discuss the development of novel methods of applying uniaxialpressure to single crystals of quantum materials. Much of our work so far hasbeen on ruthenates, but I will also mention projects on other materials. I willshow that it is now possible to strain single crystals, reversibly, to change latticeparameters by at least 1%, and that this provides a ‘tuning energy scale’equivalent to the Zeeman energy of magnetic fields of well over 1000T. Uniaxial techniques are particularly suited to controlled tuning throughLifshitz transitions, and are also a useful complement to epitaxial strain in thenfilms, which is usually biaxial. [mehr]

Nonlinear optical properties of materials are important as tools in basicresearch and optical technology. Recently there has been a tremendousupsurge of interest in optical nonlinear effects, especially in crystals with curvedbandstructure geometry. Such materials are candidates for applications basedon the conversion of light to dc current. In this talk I describe our discovery thata family of Weyl semimetals has by far the largest second-order susceptibility ofany previously known crystal. In puzzling over this result, we uncovered asurprising theorem relating the strength of optical nonlinearity to a quantuminvariant property of the bandstructure that unites nonlinear optics with thecelebrated “modern theory of polarization.” This quantum invariant provides anew strategy for algorithmic computational searches for nonlinear materialswith optimal response functions. [mehr]

Explanation of the Pump-Probe spectroscopic technique with ultrashort pulses in the ultra-violet region.Firstly, it reveals the two fundamental requirements for our experiments, which are the generation of sub-20 fs monochromatic pulses, the pump pulses, in the UV and the generation of a second broadband pulse, the probe, explaining in detail the method that we used to overcome issues and problems. Experimental setup and components are described carefully, focusing on the physics principles behind. Then some experimental data are shown for Triptophan and Azzurin, in order to try to better understand the fast processes which happen in organic molecules. [mehr]

The nonequilibrium dynamicis of the superconductors has been intensively studied over decades. Among the viriety of nonequlibrium phenomena, the study of the collective dynamics of superconducting order parameter is of essential importance, as it provides deep insights into the properties of the order parameter. Recent developments of generating intense electromagnetic pulses in the terahertz (THz) frequency range have enabled the access to low energy collective modes without giving excess energy to the system. Particularly the amplitude fluctuation of the order parameter, referred to as the Higgs mode, has been observed in a conventional s-wave superconductor Nb_1-xTi_xN [1,2]. Its extension to unconventional d-wave superconductors is intriguing, whereas it has been nontrivial whether the Higgs mode in d-wave superconductors is observable or not. [mehr]

Lecture VI Abstract will follow. [mehr]

Lecture V Abstract will follow. [mehr]

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The full characterization of charge transfer processes in molecular junctions requires techniques for evaluating not only the first and second moments of charge currents, but also higher-order statistical cumulants of the charge transfer process. The complete set of cumulants gives access to the full counting statistics (FCS) through the so-called generating function [1]. [mehr]

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