Archive 2019

Location: CFEL (Bldg. 99)

Valley Jahn-Teller effect in Twisted Bilayer Graphene

MPSD Seminar
The surprising insulating and superconducting states of narrow-band graphene twisted bilayers have been mostly discussed so far in terms of strong electron correlation, with little or no attention to phonons and electron-phonon effects. We found that, among the 33492 phonons of a fully relaxed 1.08° twisted bilayer, there are few special, hard, and nearly dispersionless modes that resemble global vibrations of the moiré supercell ('moirè phonons'). [more]

Electronic dynamics of strange metals

MPSD Seminar
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. [more]

Non-linear Optics (IMPRS-UFAST Core Course)

IMPRS-UFAST core course
Nonlinear optics (NLO) is one of the most fascinating fields of modern physics. It deals with light-matter interactions at extreme electro-magnetic field strengths. Such fields are today routinely available thanks to laser technology. NLO started with the observation of second harmonic generation from a ruby laser in 1961, just 1 year after the first laser was operated. It allows producing optical pulses with durations in the femtosecond (fs, 10-15 s) and even attosecond (as, 10-18 s) order. With such sources, one can observe chemical reactions, physical and biological phenomena in real time. During the lectures, I will give a short overview of NLO. I will discuss the main physical phenomena (second harmonic generation, optical parametric amplification, difference and sum frequency generation, white light generation, third harmonic generation, high harmonic generation…) and some of their applications, and conclude with the newest trends of research like coherent pulse synthesis. [more]

Gauge issues in the description of solids with strong light-matter coupling

MPSD Seminar
The rich physics of complex condensed matter systems is largely understood in terms of minimal tight-binding models, which describe interacting electron systems on a lattice with only few valence orbitals per site. To incorporate a strong light-matter coupling into such models, one can project the continuum theory on a given set of valence bands. [more]

Aqueous Nanoscale Systems

MPSD Seminar
  • Date: Nov 7, 2019
  • Time: 03:00 PM - 04:00 PM (Local Time Germany)
  • Speaker: Sylvie Roke
  • Laboratory for fundamental BioPhotonics (LBP), Institute of Bioengineering (IBI), and Institute of Materials Science (IMX), School of Engineering (STI), and Lausanne Centre for Ultrafast Science (LACUS), École Polytechnique Fédérale de Lausanne (EPFL), CH 1015 Lausanne, Switzerland
  • Location: CFEL (Bldg. 99)
  • Room: Seminar Room III, EG.080
  • Host: Andrea Cavalleri
Water is the most important liquid for life. It is intimately linked to our well-being. Without water, cell membranes cannot function. Charges and charged groups cannot be dissolved, self-assembly cannot occur, and proteins cannot fold. Apart from the intimate link with life, water also shapes the earth and our climate. Our landscape is formed by slow eroding/dissolving processes of rocks in river and sea water; aerosols and rain drops provide a means of transport of water. Because of the complexity of liquid water and aqueous interfaces, the relationship between the unique properties of water and its molecular structure has not been solved. [more]

Strongly Correlated Electrons from the Perspective of Dynamical Mean-Field Theory (IMPRS-UFAST Focus Course)

IMPRS-UFAST focus course
Strong electronic correlations are a main driver behind many exciting phenomena in quantum many-body systems, ranging from correlated quantum materials (Mott transition, high-temperature superconductivity) to cold atoms in optical lattices. However, the strong-correlation problem still poses many challenges when it comes to a quantitative and even qualitative understanding of the relevant degrees of freedom and microscopic interactions that drive phase transitions in solids. Dynamical mean-field theory (DMFT), first developed in the late 1980s and 1990s, provides one key limit in which the correlation problem becomes tractable, namely the one of large spatial dimensions, or local self-energies. In this focus course we will discuss the basics behind DMFT and learn how this allows one to understand the paradigmatic Mott metal-to-insulator transition. [more]

Two-particle correlation functions for the theoretical description of strongly correlated electrons systems

MPSD Seminar
Green's functions represent one of the most useful tools for the theoretical description of correlated lattice electrons. In particular, the one-particle Green's function contains information about the spectral properties of the system and can be directly compared to (angular resolved) photoemission spectroscopy experiments. However, also two-particle correlations functions provide very interesting insights into the properties of correlated electron systems as they contain crucial information on response functions such as the magnetic susceptibility or the optical conductivity. In my talk, I will present an overview about the physical content as well as the applications of two-particle Green's and vertex functions in frontier condensed matter research. In particular, I will demonstrate how local frequency-dependent vertices can be used to include non-local correlations effects in interacting many-electron systems on top of the local ones of dynamical mean-field theory (DMFT). While these so-called diagrammatic extensions [1] of DMFT have been successfully exploited to describe collective phenomena such as magnetism and superconductivity, their predictive power is still limited by specific inconsistencies between the one- and the two-particle level [2]. In the final part of my talk, I will present possible solutions to these problems [3] which I will address in the framework of my Emmy Noether project at the University of Hamburg. [more]

Mental Health Awareness Week

Details to be found in the intranet soon. [more]

Full Quantum Nature of Water on Salt Surface

MPSD Seminar
Despite water being a ubiquitous substance, it is surprising that some basic questions are still debated. Here using a combination of experimental (cryogenic STM) and theoretical (first-principle electronic structures and molecular dynamics) methods, we systematically studied the unusual structure and dynamics of water molecules on NaCl surface. More interestingly, for the first time, we observe the full quantum effect and magic number hydrates in water system. These results shed light on our understanding of water at atomic scale. [more]

Growth Dynamics of Graphene on molten copper

MPSD Seminar
Since it’s discovery in 2006, Graphene has known no rivals in terms of number of applications that scientists from all over the globe have thought for him, ranging from spintronics to energy storage, from transistors to bio-compatible devices. However, what’s still hindering his big step from laboratories to industry is a cost-effective method to synthesize large-scale good-quality crystals. Over the past decade, great improvements have been made in this direction, and CVD consolidated as an excellent candidate for this arduous task. Among other methods, a novel technique consisting in the synthesis of crystals on transition metals in the liquid phase has proven to overcome many difficulties related to defect-inducing dislocations and low-diffusivity of solid substrates. Nevertheless, a clear physical insight over the processes involved during graphene nucleation and growth is still lacking, and many of its parameters are derived by post-process analyses, neglecting those crucial intermediate steps that may conceal key-factors involved in the process. The reason for this trend is that it’s technically difficult to combine different experimental set-ups, and an ad-hoc design is more than ever needed to conduct a complete and satisfying investigation. This is the reason behind the LMCat project, that developed a reactor suitable both for CVD growth at high temperature by hydrocarbon decomposition and for in-situ Raman and optical studies, in order to follow in real time the growth of graphene flakes and, at the same time, determine its physical properties. Additionally, it aims to prove X-ray techniques, such as GID and XRR, as an efficient tool for high temperature characterization, a feat never achieved before. This is the framework of this thesis work, which can of course cover it only partially and at a rather early stage. The focus has been put on the surprising high contrast showed by radiative optical microscopy at high temperatures (∼ 1100 C°) and on the first, surprising results coming from X-ray analysis. The former has been proven as an effective tool for following the growth and derive kinematical parameters, the latter as a potential tool for quantitatively estimate its crystal structure at conditions prohibitive for standard probes. [more]

Ultrafast single-molecule videography and choreography

MPSD Seminar
To understand the function of condensed matter, it would be desirable to directly watch its atomistic building blocks dynamically interact on their intrinsic length and time scales. Recently, lightwave electronics has made this long-standing dream come true. The idea is to exploit the carrier wave of light as an ultrafast, contact-free bias to interrogate and control the nanocosm. I will first review how lightwaves can drive electrons in solids into surprising sub-cycle quantum motion. By combining this idea with the sub-angstrom spatial resolution of scanning tunnelling microscopy we can set an ultrashort time window for single-electron tunnelling into a single orbital and record first atom-scale slow-motion movies of individual vibrating molecules. Finally, I will show how to directly exert femtosecond atomic forces, which can selectively choreograph a coherent structural motion of a single-molecule switch in its electronic ground state. This stunningly direct access to the atomistic world may tailor key elementary dynamics in nature and steer (bio)chemical reactions or ultrafast phase transitions, on their intrinsic spatio-temporal scales. [more]

Correlated driven-dissipative systems

MPSD Seminar
Driven-dissipative systems represent natural platforms to study non-equilibrium phases. In the first part of the talk, I will present some physical results for which both non-equilibrium conditions and interactions are crucial. I will argue that a prototype model of correlated driven-dissipative lattice bosons, relevant for upcoming generation of circuit QED arrays experiments, exhibits a phase transition where a finite frequency mode becomes unstable, as an effect of quantum interactions and non-equilibrium conditions. In the broken-symmetry phase the corresponding macroscopic order parameter becomes non-stationary and oscillates in time without damping, thus breaking continuous time-translational symmetry. To get some more insights on this transition, I studied the spectral properties of Markovian driven-dissipative quantum systems using a Lehmann representation. Focusing on the nonlinear quantum Van der Pol oscillator as a paradigmatic example, I showed that a sign constraint of spectral functions, which is mathematically exact for closed systems, gets relaxed for open systems; it is eventually replaced by an interplay between dissipation and interactions. In the last part of the talk, I will finally discuss a new method to solve quantum impurity models, small interacting quantum systems coupled to a non-Markovian environment, in presence of additional Markovian dissipation. I will derive a Dyson equation for the time-evolution operator of the reduced density matrix and approximate its self-energy resuming only non-crossing diagrams. I will test this approach on a simple problem of a fermionic impurity. [more]

Fractional Excitonic Insulator

MPSD Seminar
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. [more]

Reimar Lüst Lecture -Prof. Charles Kane: Symmetry, topology and electronic phases of matter

Reimar Lüst Lecture
Symmetry and topology are two of the conceptual pillars that underlie our understanding of matter. While both ideas are old, over the past several years a new appreciation of their interplay has led to dramatic progress in our understanding of topological electronic phases. A paradigm that has emerged is that insulating electronic states with an energy gap fall into distinct topological classes. [more]

Engineering with vacuum fields

MPSD Seminar
When a collection of electronic excitations are strongly coupled to a single mode cavity, mixed light-matter excitations called polaritons are created. The situation is especiallyinteresting when the strength of the light-matter coupling Ωr is such that the coupling energy becomes close to the one of the bare matter resonance ω0. For this value of parameters, the system enters the so-called ultra-strong coupling regime, in which a number of very interesting physical effects were predicted. Using metamaterial coupled to two-dimensional electron gases[1], we have demonstrated that a ratio Ωr/ω0 close to[2] or above unity can be reached. [more]

Ab initio few-mode theories for quantum potential scattering problems

MPSD Seminar
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]. [more]

Solid State Physics

IMPRS-UFAST core course
From a microscopic point of view, a solid is just a regular arrangement of atoms, embedded in a soup of electrons. Yet, a remarkably rich manifold of phenomena emerges from this simple starting point, ranging from simple metals and semiconductors to multiple kinds of magnetic order or superconductivity. In this course we will discuss basic properties of solids and their microscopic understanding.Topics include:- band theory- screening- phonons - ordered phases [more]

Ab-initio description for propagation of extreme light pulse in solids: recent progresses

MPSD Seminar
When we theoretically investigate interaction of an intense and ultrashort laser pulse with solids, there are two aspects that should be considered: the strong electric field of the light pulse induces nonlinear electron dynamics in solids, and the nonlinear polarization that arises from the electron dynamics affects the propagation of the light pulse. [more]

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

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

Cooperative valence dynamics in Anderson Lattices observed by resonant inelastic x-ray scattering

MPSD Seminar
In rare earth intermetallics with weakly bound f-electrons and a Kondo energy scale much larger than magnetic exchange interactions or crystal field splittings, the screening of local moments may result in a non-magnetic Fermi liquid ground state [1]. At low temperatures, the quantum fluctuations between magnetic and non-magnetic valence configurations can then acquire a cooperative (lattice) character. On a phenomenological basis, a sound understanding of this Anderson Lattice phenomenon has been achieved. On the other hand, the microscopic description of the coherent coupling between Kondo-screened sites remains an outstanding theoretical challenge [2]. In experiment, the cooperative character of Anderson Lattices has only recently become directly accessible. Momentum-resolved spectroscopies, such as angle-resolved photoemission and inelastic neutron scattering, reveal the emergence of characteristic low-energy quasiparticle dynamics at low temperatures [3]. These methods probe single-particle excitations in the charge and magnetic channels, respectively. By contrast, high-resolution resonant inelastic x-ray scattering (RIXS) experiments couple to both charge and spin degrees of freedom in a non-trivial way and thus provide a more subtle point of view. If calculations of the underlying Kramers-Heisenberg term on a basis of strongly correlated f-electronic bands are achieved, RIXS may unlock unprecedented microscopic insights into the entanglement of local and itinerant charge and magnetic degrees of freedom. This would address a fundamental mechanism of quantum matter, with relevance far beyond lanthanides and actinides. I will review previous spectroscopic investigations of intermediate valence materials, present our recent RIXS results on the archetypal Anderson Lattice compound CePd3, and highlight some ideas for future x-ray scattering studies at 3rd and 4th generation light sources. [more]

Single-shot optical probing of laser-generated plasmas

MPSD Seminar
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. [more]

Manipulating quantum materials with cavity fields

MPSD Seminar
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. [more]

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

MPSD Seminar
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]. [more]

Strain, lattice distortions and the metal-insulator transition in correlated electron materials

MPSD Seminar
Correlation-driven metal-insulator transitions are typically coupled strongly both to local (octahedral distortion) and long wavelength (strain) lattice distortions. I present a theory of the intertwined electronic and lattice transitions in correlated materials, and show how it accounts for phenomena ranging from the interplay between nematic and magnetic ordering in pnictide superconductors, to the strain and current dependence of the metal insulator transitions in Ca2RuO4 and Ca3Ru2O7 and superlattice effects in the rare earth nickelates. [more]

Probing Topological Matter by «Heating»: From Quantized Circular Dichroism to Tensor Monopoles

MPSD Seminar
The intimate connection between topology and quantum physics has been widely explored in high-energy and solid-state physics, revealing a plethora of remarkable physical phenomena over the years. Building on their universal nature, topological properties are currently studied in an even broader context, ranging from ultracold atomic gases to photonics, where distinct observables and probes offer a novel view on topological quantum matter. [more]
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