MPSD Seminar

18951 1559824357

Theoretical study on solid-state high harmonic generation: from a one-dimensional model to an ab-initio three-dimensional approach

High harmonic generation (HHG) from crystalline solids has become a playground in ultrafast phenomena. In contrast to noble gases, crystalline solids have rich physical properties, e.g. anharmonic energy dispersion, anisotropy depending on crystalline axis, strong electron-hole correlation, and so on. While the three-step model for HHG and its generalizations are successfully applicable to several situations, a deviation from the theoretical prediction is one of the most interesting physics in this field. To understand such deviations in solid-state HHG experiments, we need to go beyond the three-step model or along different directions. I will mainly talk about our recent trials to understand solid-state HHG, electron-hole attraction inclusion based on Hartree-Fock theory for 1D model crystal, and an ab-initio approach based on density-functional theory for 3D bulk solid comparing with experiments. [mehr]

MPSD Seminar

18927 1559648179

Tuning quantum materials out of equilibrium: A FIB-microstructuring approach

“Quantum materials” loosely defines a broad collection of materials whose ground states are defined by unusual quantum properties. This research largely focuses on macroscopic single crystals, yet naturally interesting quantum phenomena lie beyond their equilibrium state. My group works towards reducing the sample size onto the sub-mm length scale, following the general idea that small samples can be driven more strongly and react faster than on the macro scale. Our main tool is Focused Ion Beam machining capable of cutting single crystals into high quality quantum devices. I will present two concrete research projects showcasing how new quantum states out of equilibrium can be accessed and investigated in FIB-prepared microcrystal structures. The first concerns the heavy fermion superconductor, CeIrIn5 (Tc~400mK). When a mm-sized structure is firmly coupled to a mm-sized substrate of different thermal expansion, the microstructure is under significant strain at low temperatures. By precisely controlling its shape, the emergent strain field can be controlled. The key difference to other approaches, such as uniaxial strain, is that complex, yet well-controlled, spatially varying strain fields can be achieved. In collaboration with Katja Nowack (Cornell), we have experimentally mapped out the resulting superconducting landscape in the devices using scanning-SQUID microscopy, and show that this spatial modulation can be well captured by finite element simulations. [1] Second, I will present our ongoing efforts to experimentally identify pseudo-magnetic fields in 3D Dirac semi-metals [2,3]. Owing to their Dirac dispersion, deformation of the crystal structure does not open a gap at the nodes, but shifts the location of the nodes in k-space and hence playing the role of a “pseudo-magnetic field”, B5. I will show how microstructuring gives us unprecedented control of such a process, and discuss how future. [mehr]

MPSD Seminar

18727 1558002303

Fractional Excitonic Insulator

We argue that a correlated fluid of electrons and holes can exhibit a fractional quantum Hall effect at zero magnetic field analogous to the Laughlin state at filling 1/m. We introduce a variant of the Laughlin wavefunction for electrons and holes and show that for m=1 it describes a Chern insulator that is the exact ground state of a free fermion model with p_x + i p_y excitonic pairing. [mehr]

MPSD Seminar

18814 1558428743

Unconventional Charge Density Wave Transitions

Historically charge density waves have been associated with the notions of Fermi surface nesting and, at the transition temperature, a soft phonon mode. In this talk, I will present two cases that defy this common theme. First, I will show that TiSe2 undergoes a transition due to exciton condensation, which exhibits a soft mode of a different, electronic variety. Second, when driving the system away from equilibrium, the phase transition is mediated by topological defects. These defects allow for the formation of a charge density wave that does not occur in equilibrium. This light-induced charge density wave shows some unique properties that suggest that it is not just a trivial extension of an equilibrium one. [mehr]

MPSD Seminar

18709 1558002599

Ab initio few-mode theories for quantum potential scattering problems

The concept of a single mode of the electromagnetic field interacting with matter has been a paradigm in the field of light-matter interactions. For example, the single mode Jaynes-Cummings model and its many generalizations have been indispensable tools in studying the quantum dynamics of various systems. In particular in cavity and circuit QED, where strong light-matter coupling is routinely achieved in experiment, such models have been tremendously successful [1]. [mehr]

MPSD Seminar

18197 1554453272

Shedding New Light on Dirac Materials with Nonlinear Optics

Nonlinear optics has recently emerged as an attractive approach for both probing topological properties and driving Dirac materials into new states. Here, I will describe our use of ultrafast nonlinear optics to study three representative Dirac materials: graphene micro-ribbons, topological insulators, and Weyl semimetals. [mehr]

MPSD Seminar

18171 1553782985

Quantum enhanced super-resolution microscopy

Although the principles of quantum optics have yielded multiple ideas to surpass the classical limitations in optical microscopy, their application in life science imaging has remained extremely challenging. In this talk, I will present two works that apply measurements of photon correlations for the benefit of localization microscopy and image scanning microscopy (ISM). The first uses photon antibunching measurement to estimate the number of emitters in a fluctuating scene and can potentially speed-up super-resolution techniques based on localization microscopy [1]. In the second work, we employ photon antibunching as the imaging contrast itself. Measuring the spatial distribution of ‘missing’ photon pairs in an ISM architecture may enhance lateral resolution four time beyond the diffraction limit [2]. The robustness of the antibunching signal enabled super-resolved imaging of fixed cells, relying solely on a quantum contrast. [mehr]

MPSD Seminar

18169 1553782920

Non-equilibrium control of the effective free energy landscape in a frustrated magnet

Geometrically frustrated magnets often possess accidentally degenerate ground states at zero temperature. At low temperature, thermal fluctuations lift the accidental degeneracy and tend to stabilize ground states with maximal entropy. This phenomenon, known as “order by disorder”, underlines the fluctuation contribution to the free energy landscape in frustrated magnets.In this talk, I show that one can control such free energy landscape in a non-equilibrium setting. In a frustrated magnet with precessional dynamics, the system’s slow drift motion within the degenerate ground state manifold is governed by the fast modes out of the manifold. Exciting these fast modes generates a tuneable effective free energy landscape with minima located at thermodynamically unstable portions of the ground state manifold. I demonstrate this phenomenon on pyrochlore XY antiferromagnet, where a magnetic field pulse is sufficient for controlling the effective free energy landscape at nonequilibrium. [mehr]

MPSD Seminar

17971 1553598659

Coherent states of light and ordered states of matter in cavity QED

Collective phenomena originating from interactions between light and matter have become a major focus of interest spanning different fields of research. [mehr]

MPSD Seminar

18034 1552472212

Single-shot optical probing of laser-generated plasmas

Lasers have captured scientific interest since their inception and increase in the on-target intensity has resulted in powerful petawatt (≈1015W) laser systems across the globe [1]. Such a laser gives the possibility to study and optimize processes such as electron [2] or ion [3] acceleration resulting from interaction of extreme electric fields (E ≥ 0.5 TV/m) with matter 0. In this talk, I would outline the current efforts of POLARIS (a Petawatt laser system) in Jena to study the effects of such laser-plasma interaction. A single-shot all optical probing was performed with Aluminum targets to fully characterize the plasma evolution. The basic motivation of the work, the experimental setup used and some results would be presented in the talk. [mehr]

MPSD Seminar

17647 1549554641

Manipulating quantum materials with cavity fields

We investigate ground state properties of electronic materials strongly coupled to cavity fields. In a two-dimensional electron gas, we explore electron paring mediated by vacuum fluctuations of the transverse electromagnetic field. To date, these interactions have only been discussed in free space, where their impact is restricted to extremely low temperatures. We argue that the sub-wavelength confinement of the light field in nanoplasmonic cavities can enhance the induced interaction to an experimentally accessible regime. In a one-dimensional Hubbard model, the cavity further enhances magnetic couplings at half-filling, and introduces next-nearest-neighbor hopping. References: F. Schlawin, A. Cavalleri, and D. Jaksch, arXiv:1804.07142. M. Kiffner, J. Coulthard, F. Schlawin, A. Ardavan and D. Jaksch, arXiv: 1806.06752. [mehr]

MPSD Seminar

17608 1549376870

Nonequilibrium dynamics in strongly correlated systems: spin-charge coupling in a photodoped Mott insulator and possible induced superconductivity

Nonequilibrium pump-probe time-domain spectroscopy opens new perspectives in studying the dynamical properties of the strongly correlated electron systems. In particular, the interplay between different degrees of freedom in strongly correlated materials can be studied by their temporal evolution [1] and also the optical switching to some novel phases is possible [2]. [mehr]

MPSD Seminar

16937 1544611408

Charge density wave (CDW) order in monolayer TMDs

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]

MPSD Seminar

16884 1543579363

A time domain perspective on electron-boson coupling in superconducting materials

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]

MPSD Seminar

16665 1543492206

Nano Surface Science and Engineering for Energy Conversion and Diamond Transistors

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]

MPSD Seminar

16198 1542719197

An introduction to coupled-cluster theory, and recent developments in quantum embedding

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]

MPSD Seminar

16108 1542207560

Ultrafast control of matter by high-field terahertz pulses

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]

MPSD Seminar

15877 1540388639

DIALS for ED: Adapting X-ray software for electron diffraction integration

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]

MPSD Seminar

15808 1539873705

Exploring and Exploiting Photoacids to Reveal Ultrafast Hydrogen Bond and Proton Transfer Dynamics in Solution: How to Move from the Mid-IR to the Soft-X-Ray Regime

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]

MPSD Seminar

15344 1536838877

Experimental study of carbon nanotube resonator

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]

MPSD Seminar

15343 1536838669

Study of pressure-induced abnormal ice growth and multipath water-ice transition with dynamic diamond anvil cell

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]

MPSD Seminar

15167 1536071247

Strain tuning of quantum materials

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]

MPSD Seminar

15168 1536071358

California New Age Physics: Sunshine, Crystals, and Quantum Geometry of Bands

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]

MPSD Seminar

14402 1529494756

Ultrafast pump-probe spectroscopy in the ultra violet wavelength region

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]

MPSD Seminar

14350 1528971302

Higgs mode in the d-wave cuprate superconductor Bi2Sr2CaCu2O8+δ induced by an intense THz pulse

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 Nb1-xTixN [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]

MPSD Seminar

13671 1523535751

Short Course on: Ultrafast Spectroscopy of Phonons and Spin Excitations in Solids - Lecture VI

Lecture VI Abstract will follow. [mehr]

MPSD Seminar

13670 1523535570

Short Course on: Ultrafast Spectroscopy of Phonons and Spin Excitations in Solids - Lecture V

Lecture V Abstract will follow. [mehr]

MPSD ARD Seminar

MPSD Seminar

13799 1523963295

Resonant Thermalization of periodically driven strongly correlated electrons


MPSD Seminar

13387 1522938521

Numerically exact full counting statistics of the Anderson impurity model

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]

MPSD Seminar

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