Abstracts - MPSD Workshop on Non-equilibrium phenomena at short length scales and in low dimensions

- authors in alphabetical order -

Tracking Photoinitiated Dynamics of Light Harvesting Complexes and Chromophores Through Ultrafast Spectroscopy

Jessica M. Anna
Department of Chemistry, University of Pennsylvania

Understanding electronic energy transfer and electron transfer reactions in multichromophoric assemblies is an active area of research spanning the fields of natural and artificial photosynthesis and photovoltaics. In this talk I will discuss our recent investigations in this area focusing on two different systems: Photosystem I and BODIPY chromophores. Photosystem I is a natural light-harvesting complex that catalyzes oxygenic photosynthesis through a trans-membrane electron transfer reaction. In its trimeric form PSI consists of ~300 chlorophyll molecules that are tightly packed in a protein scaffold leading to the formation of excitonic states upon the absorption of a photon. Applying two-dimensional electronic spectroscopy (2DES) to PSI we gain insight into the photophysics of this system on the femtosecond to picosecond timescales. In the second part of the talk, I will discuss our recent studies on BODIPY chromophores, molecules used as donor/acceptor materials in organic photovoltaics and artificial photosynthetic systems. Applying 2DES and pump-probe spectroscopies to BODIPY derivatives we monitor the solvent dependent dynamics of the system.

High-mobility electron liquid in graphene: insights by infrared nano-imaging of plasmonic waves

Dimitri N. Basov
Department of Physics, Columbia University, New York

Optical spectroscopies are an invaluable resource for exploring new physic of new quantum materials. Surface plasmon polaritons and other forms of hybrid light-matter polaritons provide new opportunities for advancing this line of inquiry. In particular, polaritonic images obtained with modern nano-infrared tools grant us access into regions of the dispersion relations of various excitations beyond what is attainable with conventional optics. I will discuss this emerging direction of research with two examples from graphene physics: i) ultrafast dynamics of hot photo-excited electrons [2]; and ii) ballistic electronic transport at low temperatures [3].

[1] D.N. Basov, M.M. Fogler and F. J. Garcia de Abajo “Polaritons in van der Waals materials”, Science 354, 195 (2016).

[2] G. X. Ni, L. Wang, M. D. Goldflam, M. Wagner, Z. Fei, A. S. McLeod, M. K. Liu, F.Keilmann, B. Özyilmaz, A. H. Castro Neto, J. Hone, M. M. Fogler and D. N. Basov Nature Photonics 10, 244 (2016)

[3] G. X. Ni, A. S. McLeod, L. Xiong et al. [in preparation].

Semiconductor microcavities: a non-linear photonic emulator

Jacqueline Bloch
Centre de Nanosciences et de Nanotechnologies (C2N), CNRS / Paris Sud University

Semiconductor microcavities appear today as a new platform for the study of quantum fluids of light. They enable confining both light and electronic excitations (excitons) in very small volumes. The resulting strong light-matter coupling gives rise to hybrid light-matter quasi-particles named cavity polaritons. Polaritons propagate like photons but strongly interact with their environment via their matter part: they are fluids of light and show fascinating properties such as superfluidity or nucleation of quantized vortices. Sculpting microcavities at the micron scale, we fabricate at C2N lattices of various geometries and use this photonic platform for the emulation of different Hamiltonians.
After a general introduction, I will illustrate with several examples the diversity of physical problems we have implemented with polariton lattices: Dirac physics and edge states in honeycomb lattices, fractal spectrum in quasi-periodic 1D lattice, or topological edge state in a 1D SHH chain.
I will conclude with the discussion of polariton-polariton interactions, which gives the system giant non-linearities, thus opening the way to the exploration of complex non-linear dynamics and in a near future quantum many body physics with light.

Electron Spin Resonance of single atoms on a surface

Andreas Heinrich
IBS Research, Center for Quantum Nanoscience, Seoul + Ewha Womans University, Physics Department, Seoul

The scanning tunneling microscope is an amazing experimental 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 I will apply these techniques to the investigation of the quantum spin properties of transition metal atoms on surfaces. We will conclude with our recent measurements of electron spin resonance in an STM on individual Fe atoms supported on an insulating thin film, offering unprecedented energy resolution on the atomic scale. This tool can be used as an ESR sensor to measure the magnetic field (dipolar interaction) from neighboring atoms, enabling the high-precision measurement of the magnetic moment of individual atoms on surfaces.

Imaging the Quantum Wigner Crystal of Electrons in One-Dimension

Shahal Ilani
Weizmann Institute of Science

The quantum crystal of electrons, predicted more than eighty years ago by Eugene Wigner, is still one of the most elusive states of matter. Experiments have searched for its existence primarily via measurements of macroscopic properties, but since these resemble those of non-interacting electrons, a clear-cut observation of this crystal is still lacking. In this talk, I will present our recent experiments that observe the one-dimensional Wigner crystal directly, by imaging its charge density in real space. To measure this fragile state without perturbing it, we developed a new scanning probe platform that utilizes a pristine carbon nanotube as a scanning charge detector to image, with minimal invasiveness, the many-body electronic density within another nanotube. The imaged density looks utterly different than that predicted by single-particle physics, but matches nicely that of a strongly interacting crystal, in which the electrons are ordered like pearls on a neckless. The quantum nature of the crystal emerges when we explore its tunneling through a potential barrier. Whereas for non-interacting electrons only a single electron should tunnel across the barrier, images of the density change upon tunneling show that in our system the crystal tunnels collectively, involving the motion of multiple electrons. These experiments provide the long-sought proof for the existence of the electronic Wigner crystal, and open the way for studying even more fragile interacting states of matter by imaging their many-body density in real space.

Imaging ultrafast spin and charge dynamics at the atomic scale

Sebastian Loth
Institute for Functional Matter and Quantum Technologies, University of Stuttgart + MPI for Solid State Research, Stuttgart + MPI for the Structure and Dynamics of Matter, Hamburg

Spin and charge correlations are particularly pronounced in nanoscale materials where they give rise to exciting many-body effects such as colossal responses to minimal electronic fluctuations or quantum critical behavior. Accessing these correlations on their intrinsic length and time scales is an important step towards a microscopic understanding of correlated-electron physics.
We combine scanning tunneling microscopy with electronic [1] and optical pump probe schemes to achieve ultrafast spectroscopy of spin and charge dynamics with atomic spatial resolution and pico- to femtosecond temporal resolution. In this talk I will focus on two recent experiments: one in which we utilize dynamic read-out of a few-atom sensor spin [2] to quantify minute magnetic interactions and correlations between quantum magnets [3]; and one in which we image the ultrafast motion of charge density waves around atomic defects. These experiments shed light onto the impact of correlations and coherences in quantum materials and highlight pathways to design and control matter at the single atom level.

[1] S. Loth, M. Etzkorn, C. P. Lutz, D. M. Eigler, A. J. Heinrich, Science 329, 1628 (2010).
[2] S. Yan, D.J. Choi, J. Burgess, S. Rolf-Pissarczyk, S. Loth, Nat. Nanotech. 10, 40 (2015).
[3] S. Yan, L. Malavolti, J. Burgess, A. Droghetti, A. Rubio, S. Loth, Science Advances, in press (2017).

Quantum plasmonics, polaritons and strong light-matter interactions with 2d material heterostructures

Frank Koppens
ICFO-Institut de Ciencies Fotoniques, The Barcelona Institute of Science and Technology
ICREA – Institució Catalana de Recerça i Estudis Avancats, Barcelona

The control of polaritons are at the heart of nano-photonics and opto-electronics. Two-dimensional materials have emerged as a toolbox for in-situ control of a wide range of polaritons: plasmons, excitons and phonons. By stacking these materials on top of each other, heterostructures of these materials can be controlled at atomic scale, with extremely high quality and clean interfaces.

In this talk, we will show several examples of 2d material heterostructure devices with novel ways of exciting, controlling and detecting polaritons [1,2,3]. We challenge the limits of quantum light-matter interactions [5,6] as well as extremes in propagating plasmon confinement, down to the scale of a few nanometers.

The advances on ultra-high quality materials allow for plasmon propagation at extremely small electron densities, with de Broglie wavelength above 50 nm. This is an excellent platform for testing quantum theories of the dynamic response of the electron system, including spatial dispersion and electron-electron correlation effects.

Finally, we present novel results on Super-Planckian energy transfer between hot electrons and hyperbolic phonon polaritons [7]. Future directions on new directions in quantum materials will be addressed.


[1] Near-field photocurrent nanoscopy on bare and encapsulated graphene. A. Woessner et al., Nature Communications (2016)
[2] Thermoelectric detection and imaging of propagating graphene plasmons. Lundeberg et al., Nature Materials (2016)
[3] Ultra-confined acoustic THz graphene plasmons revealed by photocurrent nanoscopy. Alonso-Gonzalez et al., Nature Nanotechnology (2016)
[4] Real-space mapping of tailored sheet and edge plasmons in graphene nanoresonators. Nikitin et al., Nature Photonics (2016)
[5] Electro-mechanical control of optical emitters using graphene. Reserbat-Plantey et al.,Nature Communications (2016)
[6] Electrical Control of Optical Emitter Relaxation Pathways enabled by Graphene. K.J. Tielrooij et al., Nature Physics (2015)
[7] Super-Planckian electron cooling in a van der Waals stack. Principi et al., Arxiv 1608.01516 (2016)

Structure formation of soft matter near adsorbing interfaces

Kurt Kremer
MPI for Polymer Research

Functional soft matter systems often are created by adsorption processes or by growth from a surface. Two different approaches will be discussed, which relate to organic electronics on the one side and to biominerals on the other. For the latter we study the crystal growth in binary Lennard-Jones mixtures by molecular dynamics simulations. We show how the growth dynamics, the structure of the liquid-solid interface as well as droplet incorporation into the crystal vary with solution properties. For demixed systems we observe a strongly enhanced crystal growth at the cost of enclosed impurities. Furthermore, we find different interface morphologies depending on solubility. We can relate our observations to growth mechanisms based on the Gibbs-Thomson effect as well as to predictions of the Kardar-Parisi-Zhang theory in 2 + 1 dimensions. In the case of organic electronics graphene nanoribbons play an important role. Despite their importance it is very difficult to prepare them in a controlled way, i.e. making materials with a well defined band gap. One way is to synthesize phenylene based polymers, which are then adsorbed at a surface, where they form nanoribbons. We try to relate the properties of the synthesized polymers to the resulting nanoribbons.

M. Radu, K. Kremer, PRL 118, 055702 (2017) Enhanced Crystal Growth in Binary Lennard-Jones Mixtures N. C. Forero-Martinez, B. Baumeier, K. Kremer, preprint (2017) Backbone contribution to the Kuhn lengths of polyphenylene precursors

Materials under Strain: an Atomic-Scale Perspective

Abhay Pasupathy
Columbia University

What is the effect of stretching a crystal along a given direction? In general, one might not expect much: a change in lattice constant, accompanied by corresponding changes in the electronic and vibrational properties of a crystal. I will describe a few cases of materials where the effect of stretching (ie, uniaxial strain) lead to large and unexpected effects. These effects are probed with new experimental techniques where we can apply calibrated uniaxial strain to single crystals or thin films and measure their response with atomic-resolution scanning tunneling microscopy techniques. I will discuss results on the iron-based superconductors (where large electronic nematic effects are seen), layered transition-metal dichalcogenides (where we can observe the formation of strain solitons) and two-dimensional semiconductors (where we can cause large changes in electronic and vibrational properties and even cause structural phase transitions).

Photons on a lattice: from quantum simulations to brain-inspired computing

Sebastian Schmidt
ETH Zürich

Strongly interacting light-matter systems can be routinely engineered in various quantum-electrodynamic architectures (QED), reaching from real atoms in optical cavities to integrated systems such as excitons in microcavities or superconducting qubits embedded in microwave circuitry. A key challenge for practical applications is to explore the physics of coupled QED systems, i.e., arrays of cavities, qubits and waveguides. Such interacting photonic lattices constitute a novel type of quantum metamaterial, which may be utilized for metrology, computation and the simulation of fundamental many-body phenomena out of equilibrium such as driven, dissipative phase transitions. This talk will give an overview on recent theoretical and experimental advances in this emerging field of science.

Strong coupling in organic cavities: Quantum optics for molecular chemistry

Tal Schwartz
Tel Aviv University

Strong coupling of light and matter is a regime in which quantum-coherent interactions prevail over dissipation. In particular, organic dye molecules are very attractive for studying strong coupling effects, as they typically display coupling strengths approaching 1eV at room temperature. On the other hand, organic molecules display a very rich behavior, (e.g. molecular functionality or chemical reactivity) which can lead to new effects when brought into the realm of Cavity Quantum-Electrodynamics. In my talk I will present how strong light-matter coupling in such hybrid organic/photonic structures can be used to tailor the energetic landscape in molecular systems, and how this mechanism provides new possibilities for controlling photochemistry and molecular properties.

Probing Valley Dynamics in WS2/WSe2 heterostructures

Feng Wang
Physics Department, University of California Berkeley

Van der Waals heterostructures composed of stacked atomically thin layers can exhibit novel phenomena due to the unique layer-layer interactions. In this talk, I will describe our studies of ultrafast charge transfer between different transition metal dichalcoginide layers. This not only leads to ultrafast charge separation, but also enables the generation of high-purity and long-lived valley polarization with microsecond lifetime in WS2/WSe2 heterostructures. I will also discuss spatially and temporally resolved imaging of pure valley and spin current in the WS2/WSe2 heterostructure.