Past events

Physiology or Medicine Victor Ambros and Gary Ruvkun discovered microRNA, a new class of tiny RNA molecules that play a crucial role in gene regulation. Their groundbreaking discovery in the small worm C. elegans revealed a completely new principle of gene regulation. This turned out to be essential for multicellular organisms, including humans. MicroRNAs are proving to be fundamentally important for how organisms develop and function. Physics John Hopfield introduced a spin model that can store and reconstruct information. Geoffrey Hinton built on Hopfield’s idea to invent the Boltzmann Machine, that is able to learn from examples to reconstruct a set of desired patterns. He also popularized and improved Backpropagation of Errors, a method actually used in today’s advanced AI technology (e.g. Deep Learning). Chemistry The Nobel Prize in Chemistry 2024 is about proteins, life’s ingenious chemical tools. David Baker has succeeded with the almost impossible feat of building entirely new kinds of proteins. Demis Hassabis and John Jumper have developed an AI model to solve a 50-year-old problem: predicting proteins’ complex structures. These discoveries hold enormous potential.

Protein function is the combined product of chemical and mechanical interactions encoded in the gene. Thus, the function of enzymes relies on finetuning the chemical groups at the active site, but also on large-scale mechanical motions, allowing enzymes to bind to substrates selectively, reach the transition state, and release products. We will discuss recent work aiming to probe directly the linkage between these collective internal motions and the functionality of enzymes, using nano-rheological measurements, AI-prediction of point mutation effects, and physical theory. This work proposes a physical view of enzymes as viscoelastic catalytic machines with sequence-encoded mechanical specifications, which are modulated via long-ranged force transduction.

Biological systems regulate their action at multiple levels of organization, from molecular circuits to physiological function. This “homeostasis” maintains stability of the system in the face of external and internal perturbation. How exactly this is achieved remains a topic of ongoing investigation; challenges are high dimensionality, many coupled positive and negative feedback loops, conflicting regulation demands and interaction with the environment. Here I will introduce an empirical approach to the fundamental question – how do we know what it is that the system really “cares about”? What variable, or combination of variables, is under regulation? Two data-driven methods will be presented. one based on statistical analysis and applied to bacterial growth and division, revealing a hierarchy of regulation – from tightly regulated to sloppy variables. The second is based on a machine-learning algorithm we developed to identify regulation with minimal assumptions. This provides a different angle on the problem and highlights directions for future research. FOR THE LATEST UPDATES AND CONTENT ON SOFT MATTER AND BIOLOGICAL PHYSICS AT THE WEIZMANN, VISIT OUR WEBSITE: https://www.biosoftweizmann.com/

A cooperative resonance effect in a stochastic nonlinear dynamical system subjected to external weak periodic forcing, called stochastic resonance (SR), has been extensively studied for the past forty years. Here I discuss the experimentally unexpected observation of SR above an elastic non-modal instability of an inertia-less channel flow of polymer solution (much more complicated than stochastic dynamical flow) due to finite-size white noise perturbations. This flow is shown to be linearly stable similar to Newtonian parallel shear flow. First, I briefly describe viscoelastic flow with curved streamlines, where linear elastic normal mode instability at the critical Weissenberg number, Wic, has been observed and characterized, and the elastic instability mechanism has been explained and experimentally validated. Furthermore, at Wi>>Wic, “elastic turbulence” (ET), a chaotic flow arising via secondary instability, is experimentally discovered, characterized and theoretically explained, while elastic instability in straight channel flow is found from the direct transition from laminar to chaotic flow in the transition flow regime is found. At the secondary instability, ET is observed, and further on the next transition to the unexpected drag reduction flow regime takes place, accompanied by elastic waves previously discovered and characterized earlier. Moreover, we propose and experimentally validate a mechanism of amplification of the wall normal fluctuating vortices by the elastic waves. The elastic waves play the key role in the energy transfer from the main flow to the wall-normal fluctuating vortices. Finally, we report on recently discovered SRs only in a limited subrange of weak elastic waves just above Wic, their characterization, and their role in the transition to a chaotic flow.

The interplay between spontaneous symmetry breaking and topology can result in exotic quantum states of matter. A celebrated example is the quantum anomalous Hall (QAH) effect, which exhibits an integer quantum Hall effect at zero magnetic field due to topologically nontrivial bands and intrinsic magnetism. In the presence of strong electron-electron interactions, fractional-QAH (FQAH) effect at zero magnetic field can emerge, which is a lattice analog of fractional quantum Hall effect without Landau level formation. In this talk, I will present experimental observation of FQAH effect in twisted MoTe2 bilayer, using combined magneto-optical and -transport measurements. In addition, we find an anomalous Hall state near the filling factor -1/2, whose behavior resembles that of the composite Fermi liquid phase in the half-filled lowest Landau level of a two-dimensional electron gas at high magnetic field. Direct observation of the FQAH and associated effects paves the way for researching charge fractionalization and anyonic statistics at zero magnetic field. Reference 1. Observation of Fractionally Quantized Anomalous Hall Effect, Heonjoon Park et al., Nature, https://www.nature.com/articles/s41586-023-06536-0 (2023); 2. Signatures of Fractional Quantum Anomalous Hall States in Twisted MoTe2 Bilayer, Jiaqi Cai et al., Nature, https://www.nature.com/articles/s41586-023-06289-w (2023); 3. Programming Correlated Magnetic States via Gate Controlled Moiré Geometry, Eric Anderson et al., Science, https://www.science.org/doi/full/10.1126/science.adg4268 (2023).

In the most common phase of liquid crystals, called the nematic phase, molecules are aligned up or down along some axis, so that the net electrostatic polarization is zero. Recent experiments have found a new class of liquid crystals, called ferroelectric nematic, in which molecules align predominantly in one direction along the axis, leading to a nonzero polarization. From the perspective of statistical mechanics, the ferroelectric nematic phase has two special features. First, it has flexoelectricity, meaning that the polarization induces a splay of the molecular orientation. Second, the energy includes an electrostatic interaction, which favors a domain structure. In this talk, we discuss the competition between those two effects to control the phase behavior. FOR THE LATEST UPDATES AND CONTENT ON SOFT MATTER AND BIOLOGICAL PHYSICS AT THE WEIZMANN, VISIT OUR WEBSITE: https://www.biosoftweizmann.com/

Quantum dots (QDs) are conducting regions which can localize few charge carriers, and where the energy spectrum is dominated by Coulomb repulsion. QDs can be as large as few hundreds of nanometers, or as small as a single molecule, their sizes depending on their physical realization – whether in two-dimensional materials, nanowires, molecular systems. In my talk I will describe our work on a new type of an atomically-sized QD, realized in defects residing in ultrathin two-dimensional insulators. These defect-dots are found in layered materials such as hexagonal Boron Nitride (hBN), which we study by their assembly into stacked devices. By using graphene electrodes, we are able to electronically couple to the QD, while allowing the QD energy to be externally tuned exploiting the penetration of electric field through graphene. A consequence of the structure of our devices is that the defect QDs are placed at atomic distance to the conductors on both sides. I will show how the presence of such energy-tunable, atomically sized QDs at nanometer proximity to other conducting systems opens new opportunities for sensitive measurements, including use of QDs as highly sensitive spectrometers [1], or as single electron transistors, unique in their sensitivity to local electric fields at the nanometer scale [2]. I will discuss our future prospects of using defect QDs as quantum sensors. References 1. Devidas, T.R., I. Keren, and H. Steinberg, Spectroscopy of NbSe2 Using Energy-Tunable Defect-Embedded Quantum Dots. Nano Letters, 2021. 21(16): p. 6931-6937. 2. Keren, I., et al., Quantum-dot assisted spectroscopy of degeneracy-lifted Landau levels in graphene. Nature Communications, 2020. 11(1): p. 3408.

Over the last decade, several exciting directions have been initiated by laser-driven plasmas, e.g., compact particle accelerators, inertial fusion and laboratory astrophysics. The first has known rapid progress, in terms of current, energy, stability; fusion has gone through a historic step, with the news of ignition being achieved at NIF in 2022; and laboratory astrophysics has known also spectacular developments, demonstrating the possibility to perform fully scalable experiments relevant to various objects such as forming stars and supernovae. A particularly interesting aspect is that all these fields are strongly synergistic, i.e., that advances in one can push the others as well. I will present examples of such synergies, through recent results we have obtained in all these domains, and in particular how ultra-bright neutron beams can be developed using latest generation multi-PW lasers [1,2]. These could open interesting perspectives in terms of cargo inspection, but also for fusion plasma measurements. I will also show how fusion can benefit from external magnetization [3]. Finally, I will discuss advances in laboratory astrophysics, particularly the first-stage acceleration of ions leading to cosmic rays [4,5], understanding the universal nature of collimated outflows in the Universe [6], and probing the intricacy of 3D magnetic reconnection [7] [1] High-flux neutron generation by laser-accelerated ions from single-and double-layer targets, V Horný et al., Scientific Reports 12 (1), 19767, 2022 [2] Numerical investigation of spallation neutrons generated from petawatt-scale laser-driven proton beams, B Martinez et al., Matter and Radiation at Extremes 7 (2), 024401, 2022 [3] Dynamics of nanosecond laser pulse propagation and of associated instabilities in a magnetized underdense plasma, W. Yao et al., https://doi.org/10.48550/arXiv.2211.06036 [4] Laboratory evidence for proton energization by collisionless shock surfing, W Yao et al., Nature Physics 17 (10), 1177-1182, 2021 [5] Enhancement of the Nonresonant Streaming Instability by Particle Collisions, A Marret et al., Physical Review Letters 128 (11), 115101, 2022 [6] Laboratory disruption of scaled astrophysical outflows by a misaligned magnetic field, G Revet et al., Nature communications 12 (1), 762, 2021 [7] Laboratory evidence of magnetic reconnection hampered in obliquely interacting flux tubes, S Bolaños et al., Nature Communications 13 (1), 6426, 2022

Holes in elastic metamaterials, defects in 2D curved crystals, localized plastic deformations in amorphous solids and T1 transitions in epithelial tissue, are typical realizations of stress-relaxation mechanisms in different solid-like structures, interpreted as mechanical screening. In this talk I will present a mechanical screening theory that generalizes classical theories of solids, and introduces new moduli that are missing from the classical theories. Contrary to its electrostatic analog, the screening theory in solids is richer even in the linear case, with multiple screening regimes, predicting qualitatively new mechanical responses. The theory is tested in different physical systems, including disordered granular solids that do not have a continuous mechanical description. These materials are shown to violate energy conservation and are best described by Odd-Screening: a screening model that does not derive from an energy function. Experiments reveal a mechanical response that is strictly different from classical solid theory and is completely consistent with our mechanical-screening theory. Finally, I will discuss the relevance of this theory to 3D solids and a new Hexatic-like state in 3D matter.

In three-dimensional space, elementary particles are divided between fermions and bosons according to the properties of symmetry of the wave function describing the state of the system when two particles are exchanged. When exchanging two fermions, the wave function acquires a phase, φ=π. On the other hand, in the case of bosons, this phase is zero, φ=0. This difference leads to deeply distinct collective behaviors between fermions, which tend to exclude themselves, and bosons which tend to bunch together. The situation is different in two-dimensional systems which can host exotic quasiparticles, called anyons, which obey intermediate quantum statistics characterized by a phase φ varying between 0 and π [1,2]. For example in the fractional quantum Hall regime, obtained by applying a strong magnetic field perpendicular to a two-dimensional electron gas, elementary excitations carry a fractional charge [3,4] and have been predicted to obey fractional statistics [1,2] with an exchange phase φ=π/m (where m is an odd integer). Using metallic gates deposited on top of the electron gas, beam-splitters of anyon beams can be implemented. I will present how the fractional statistics of anyons can be revealed in collider geometries, where anyon sources are placed at the input of a beam-splitter [5,6]. The partitioning of anyon beams is characterized by the formation of packets of anyons at the splitter output. This results in the observation of strong negative correlations of the electrical current, which value is governed by the anyon fractional exchange phase φ [5,7]. [1] B. I. Halperin, Phys. Rev. Lett. 52, 1583–1586 (1984). [2] D. Arovas, J. R. Schrieffer, F. Wilczek, Phys. Rev. Lett. 53, 722–723 (1984). [3] R. de Picciotto et al., Nature 389, 162–164 (1997). [4] L. Saminadayar, D. C. Glattli, Y. Jin, B. Etienne, Phys. Rev. Lett. 79, 2526–2529 (1997) [5] B. Rosenow, I. P. Levkivskyi, B. I. Halperin, Phys. Rev. Lett. 116, 156802 (2016). [6] H. Bartolomei et al. Science 368, 173-177 (2020). [7] Lee, JY.M., Sim, HS, Nature Communications 13, 6660 (2022).

The communication of plants with their environment is crucial for their survival. Plants are known to use light, odors, and touch to communicate with other organisms, including plants and animals. Yet, acoustic communication is almost unexplored in plants, despite its potential adaptive value. This is the topic of the current talk. We have started exploring plant bioacoustics - what plants hear, and what they “say”. I will describe two major projects: in the first we study plant hearing, testing the responses of flowers to sounds of pollinators; in the second we investigate plant sound emission - we have shown that different species of plants emit brief ultrasonic signals, especially under stress. Using AI we can interpret these sounds and identify plant species and stress condition from the sounds. Potential implications of these projects for plant ecology, evolution and agriculture will be discussed.

Statistical physics successfully accounts for phenomena involving a large number of components using a probabilistic approach with predictions for collective properties of the system. While biological cells contain a very large number of interacting components, (proteins, RNA molecules, metabolites, etc.), the cellular network is understood as a particular, highly specific, choice of interactions shaped by evolution, and therefore difficultly amenable to a statistical physics description. Here we show that when a cell encounters an acute but non-lethal stress, its perturbed state can be modelled as random network dynamics. Strong perturbations may therefore reveal the dynamics of the underlying network that are amenable to a statistical physics description. We show that our experimental measurements of the recovery dynamics of bacteria from a strong perturbation can be described in the framework of physical aging in disordered systems (Kaplan Y. et al, Nature 2021). Further experiments on gene expression confirm predictions of the model. The predictive description of cells under and after strong perturbations should lead to new ways to fight bacterial infections, as well as the relapse of cancer after treatment.

In this talk, I introduce a theoretical framework to tailor three-dimensional defect line architecture in nematic liquid crystals. By drawing an analogy between nematic liquid crystals and magnetostatics, I will show quantitative predictions for the connectivity and shape of defect lines in a nematic confined between two thinly spaced glass substrates. I will demonstrate experimental and numerical verification of these predictions, and identify critical parameters that tune the disclination lines' curvature within an experimental setup, as well as non-dimensional parameters that allow matching experiments and simulations at different length scales. Our system provides both physical insight and powerful tools to induce desired shapes and shape changes of defect lines.

Microbial ecosystems, pivotal in global ecological stability, display a diverse array of species, influenced by complex interactions. When considering environments with changing nutrient levels, we have recently suggested an 'early bird' effect. This phenomenon, which results from changing nutrient levels, initial and fast uptake of resources confers an advantage, significantly altering microbial growth dynamics. In serial dilution cultures with varying nutrient levels, this effect leads to shifts in diversity, demonstrating that microbial communities do not adhere to a universal nutrient-diversity relationship. Using a consumer-resource, serial dilution modeling framework, we simulate scenarios of changing nutrient balance, such as variations in phosphorous availability in rainforest soils, to predict a possible lag in ecosystems response near a loss of diversity transition point. Lastly, we explore the notion of 'microbial debt', a form of the early bird advantage, where microbes initially grow rapidly at the cost of later growth or increased mortality. This dynamic, exemplified in both classical chemostat and serial dilution cultures, reveals that such debt can convey an advantage, with varying outcomes on community structure depending on the nature of the trade-off involved. Together, these studies illuminate some organizing principles behind microbial dynamics, balancing growth and survival in changing environments.