Archive 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). [more]

Cell membranes are formed of lipid bilayers, and separate the interiors of all living cells from the surroundings. They have an integral role in maintaining the internal environment of cells. Anaesthetics have been shown to have a potency directly proportional to their affinity for lipid substances, strongly implying that the effect of anaesthesia is due to the action on cell membranes of the anaesthetic molecules. Previous studies have demonstrated the effects of anaesthetics on lipid melting points, but no experiments have looked at changes in mechanical properties. [more]

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] [more]

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. [more]

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. [more]

The key to designing modern optoelectronic and magnetic functional materials lies in understanding how the charge carriers respond to the excitation on ultrafast timescales – tens or hundreds of femtoseconds. In this talk, I will introduce two projects which attempt to shed light on the fundamental processes behind the remarkable physics of two-dimensional materials and molecular magnets, and how we can control them. [more]

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

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. [more]

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. [more]

Resilience and burnout both are not well defined concepts. But whereas theories about the development of a burnout-syndrom stress pathological pathways, theories about resilience are based on the concept of salutogenesis: two sides of thesame coin? [more]

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. [more]

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The theoretical description of nonlinear optical spectroscopy has traditionally been laid in the framework of perturbation theory. Within this formalism, an intuitive approach to the understanding of the dynamics of a molecular system excited by several external laser pulses is based on the concept of nonlinear response functions. However, as the system complexity increases or nontrivial dynamic effects have to be taken into account (nonadiabatic interstate couplings, bath-induced relaxation) the perturbative approach becomes computationally expensive. To tackle this scenario nonperturbative approaches based on the numerically exact solution of quantum equations of motions have been developed. [more]

The 2018 European Physical Society Award Lecture for Research in Laser Science and Applications: The Picosecond Infrared Laser (PIRL) Scalpel: Achieving Fundamental (Single Cell) Limits to Minimally Invasive Surgery and Biodiagnostics. Followed by: The Future of PIRL Technology: Presentations and Round Table Discussion [more]

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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. [more]

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. [more]

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. [more]

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. [more]

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). [more]

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. [more]

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Higgs mode (collective amplitude mode) in superconductors, recently detected and analysed in a conventional, s-wave superconductor, opens a novel avenue for probing the U(1) symmetry broken state. Now we have extended the notion to an unconventional, d-wave high-Tc cuprate, where a characteristic third-harmonic generation hallmarks the d-wave superconductor in a space-group resolved manner[1]. We can also predict unique features in Higgs and Leggett (phase) modes if we turn to multiband superconductors[2].[1] K. Katsumi et al, PRL 120, 117001 (2018).[2] Y. Murotani et al, PRB 95, 104503 (2017). [more]

We can capture various unconventional high-Tc superconductors basically either with single-band models or multiband ones. We can theoretically explore how we can optimise them for higher Tc's. There, "multiband" should not be confused with "multiorbital" systems, for which I shall compare merits and demerits of the two classes from both quantum many-body algorithms and materials-science points of view. For the former, I shall mention the dynamical vertex approximation to fathom the correlation between the electronic structure and the superconductivity and to search for enhanced Tc's. For the latter, I shall present various ideas that include "flat-band" superconductivity. [more]

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. [more]

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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. [more]

The scanning tunneling microscope is an amazing tool because of its atomic-scale spatial resolution. This can be combined with the use of low temperatures, culminating in precise atom manipulation and spectroscopy with microvolt energy resolution. In this talk we will apply these techniques to the investigation of the quantum spin properties of magnetic atoms sitting on thin insulating films. [more]

How can you publish via Open Access journals and boost your citations? What are GOLD and GREEN Open Access? Which science journals have Open Access or reduced fee agreements with the Max Planck Society? These and many other questions will be answered at this two-hour workshop. Open to all. To register, please mail jenny.witt@mpsd.mpg.de. [more]

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. [more]

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. [more]

Lecture VI Abstract will follow. [more]

Synchrotron radiation is regularly generated at large-scale facilities where GeV electrons move along kilometer-long circular paths. Here, we use a metasurface to bend light and demonstrate synchrotron radiation produced by a sub-picosecond pulse, which moves along a circular arc of radius 100 μm inside a nonlinear crystal. The emitted radiation, in the THz frequency range, results from the nonlinear polarization induced by the pulse. The generation of synchrotron radiation from a pulse revolving continuously about a circular trajectory has profound implications for on-chip THz sources. We present the first step towards this realization. [more]

Lecture V Abstract will follow. [more]

Lecture IV Abstract will follow. [more]

Time-resolved electron imaging, diffraction and spectroscopy are exceptional laboratory-based tools to trace non-equilibrium dynamics in materials with a sensitivity to structural, electronic and electromagnetic degrees of freedom. The capabilities of these approaches are largely governed by the quality of the beam of electrons used.This talk will discuss recent advances made by employing high-coherence ultrashort electron pulses from nanoscale field emitters, which substantially enhance the achievable image resolution in both real and reciprocal space. Two complementary developments with ultimate surface sensitivity and spatial resolution, respectively, will be presented, namely Ultrafast Low-Energy Electron Diffraction (ULEED) and Ultrafast Transmission Electron Microscopy (UTEM). Several recent examples of applying these methods to the observation of phase-ordering kinetics, the excitation of strongly-coupled fluctuation modes and the control of metastable states will be given. [more]

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Over the recent years, high-harmonic generation has established itself as a promising spectroscopic technique. Notable examples include the tomographic imaging of molecular wave functions [1], the tracking of nuclear dynamics [2], and the reconstruction of the attosecond time-scale electron dynamics in molecules [3, 4].All these applications, however, have been limited to laser fields with linear polarization. High harmonic generation has only recently been extended to circularly-polarized drivers by utilizing a technique known as bi-circular high-harmonic generation (BHHG) [5,6,7]. In this talk, I will demonstrate the spectroscopic applications of this technique to the study of structure and dynamics of gas-phase atoms and molecules in a self-probing manner. I will start with an analysis of the helicity asymmetry of BHHG in noble-gas atoms and then proceed by illustrating how BHHG can be applied to study dynamical symmetry-breaking in a time-dependent manner in the context of rotational and vibrational molecular motion.Extension of high-harmonic generation to the regime of highly-elliptical fields opens up the way towards the study of chiral phenomena in high-harmonic generation. In this work, circular dichroism in the range of 3-8 % is observed on randomly oriented methyl oxirane (C3H6O) molecules in the gas phase. This chiral sensitivity is attributed to the sub-cycle chiral electron dynamics involving excited states of the cation that take place during the electron continuum propagation. Finally, I will present a study of time-dependent chirality based on following the temporal evolution of circular dichroism during the course of an ultrafast photodissociation reaction. [more]

Lecture notes see 'more'. [more]

Lecture notes see 'more'. [more]

Multiferroic materials, which exhibiting simultaneously two or more coupled ferroic orders, are attracting extensive research interest mainly due to their promising device applications in low power electronics and the exotic physics involved. Despite the fact that an increasing number of multiferroics have been identified through two decades of diligent and creative studies, their low transition temperature or small magnetoelectric coupling strength has hindered direct device applications. Searching for materials with room-temperature ferroelectricity, a large enough magnetization and coupling between these two is still the Holy Grail for fundamental condensed matter research. In this talk, I will report a novel room temperature multiferroic in the Bi-Fe-O system. The material is comprised of a hitherto-unexplored, hexagonal crystalline structure with Bi2O3 layers separated by γ-Fe₂O₃ blocks, which blends the excellent ferroelectric and ferrimagnetic performances of BiFeO₃ and γ-Fe₂O₃ together to form an exotic multiferroic material with ferroelectric and ferrimagnetic transition temperature of above 760 K and 600 K, respectively. A combination of experimental and theoretical studies reveals that the polar state stems from the two dissimilar Bi sites and extends further into oxygen ions with the exquisitely modulated structure. This finding identifies a novel strategy for the realization of high-temperature multiferroicity and paves the pathway for the realization of practical room temperature magnetoelectric devices. [more]

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Transmission electron microscopy (TEM) has traditionally been associated with the study of thin solid samples in vacuum. With the availability of reliable thin membranes of silicon nitride, TEM of liquid specimens has become accessible with nanoscale resolution in the past decade [1]. The usage of scanning transmission electron microscopy (STEM) presents a novel concept to study membrane proteins within whole mammalian cells in their native liquid environment [2]. The cells in liquid are placed in a microfluidic chamber enclosing the sample in the vacuum of the electron microscope, and are then imaged with STEM. It is not always necessary to enclose the cells in the microfluidic chamber. For many studies, it is sufficient to obtain information from the thin outer regions of the cells, and those can be imaged with high resolution using environmental scanning electron microscopy (ESEM) with STEM detector [3]. A third option is to cover a liquid specimen under a thin membrane of graphene providing the thinnest possible layer [4]. Liquid STEM was used to explore the formation of HER2 homodimers at the single-molecule level in intact SKBR3 breast cancer cells in liquid state [3]. HER2 is a membrane protein and plays an important role in breast cancer aggressiveness and progression. Data analysis based on calculating the pair correlation function from individual HER2 positions revealed remarkable differences its functional state between rare- and bulk cancer cells with relevance for studying the role of cancer cell heterogeneity in drug response. We discovered a small sub-populations of cancer cells with a different response to a prescription drug [5]. Liquid STEM was also used to explore the behavior of nanoparticles in liquid in time-lapse experiments. It was discovered that nanoparticle movement in close proximity of the supporting silicon nitride membrane was three orders of magnitude slower than what was expected on the basis of Brownian motion for a bulk liquid [6], pointing to the existence of a layer of highly ordered liquid at the membrane. References [1] de Jonge, N. and Ross, F.M. Nat. Nanotechnol., 6, 695-704 (2011) [2] de Jonge, N., et al. Proc. Natl. Acad. Sci., 106, 2159-2164 (2009) [3] Peckys, D.B., et al. Sci. Adv., 1, e1500165 (2015) [4] Dahmke, I.N., et al. ACS Nano, 11, 11108-11117 (2017) [5] Peckys, D.B., et al. Mol. Biol. Cell, 28, 3193-3202 (2017) [6] Verch, A., et al. Langmuir, 31, 6956–6964 (2015) [more]

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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]. [more]

Organic molecules interact strongly with confined electromagnetic fields in plasmonicarrays or optical microcavities owing to their bright transition dipole moments. Thisinteraction gives rise to molecular polaritons, hybrid light-matter quasiparticles.Molecular polaritonics opens new room-temperature opportunities for the nontrivialcontrol of energy transport in the nano and mesoscales and modification of physicochemicalproperties of molecular assemblies. In this talk, I’ll showcase some of theseopportunities that we have been theoretically exploring in the past few years within thecontext of physical chemistry. I’ll start by briefly mentioning our work on topologicallynontrivial phases in excitonic and polaritonic systems of organic dye molecules [1,2].Next, I will discuss recent work on how polaritons can enhance singlet-fissionprocesses [3] or how excitation energy can be transferred across mesoscopicdistances of hundreds of nanometers to micron lengthscales [4]. If time permits, I’llconclude by explaining what we can learn about molecular polaritons using twodimensionalspectroscopy [5,6].[1] J. Yuen-Zhou et al., Nature Mater. 13, 1026 (2014).[2] J. Yuen-Zhou et al., Plexcitons: Dirac points andtopological modes, Nat. Commun. 7, 11783 (2016).[3] L. A. Martínez-Martínez, et al., Polariton-assistedsinglet fission in acene aggregates, under review in J.Phys. Chem. Lett., arXiV:1711.11264.[4] M. Du et al., Polariton-assisted remote energy transfer(PARET), under review in Chem. Sci., arXiv:1711.11576.[5] B. Xiang et al., Revealing hidden vibration polaritoninteractions by 2D IR spectroscopy, under review in Proc.Nat. Acad. Sci., arXiv:1711.11222.[6] R. F. Ribeiro et al., Theory for nonlinear spectroscopyof vibrational polaritons, submitted to J. Phys. Chem.Lett., arXiv:1711.11576. [more]

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