Talks

Collective Dynamics: Colloids and Active Spinners

The anomalous behavior of particulate systems has long been studied in soft matter physics. Of particular interest is the connection between micro-scale properties of particles and the emergent bulk behavior such as nonlinear mechanical response and out of equilibrium collective dynamics. Here, I will talk about my recent experimental investigations on two such systems.First, I will describe the reversibility dynamics of a colloidal suspension adsorbed at an oil-water interface. The interplay between Brownian motion and applied shear force causes the particle to rearrange. Particles under cyclic shear undergo either reversible or irreversible rearrangements after completing each cycle. We compared two system with different effective temperature, and found that even minimal degrees of thermal noise significantly affect the reversibility dynamics. In the next part, I will describe the collective dynamics of an active system in which the energy is surged to the rotational degrees of freedom. The interparticle hydrodynamic interaction is negligible, and the dynamics is dominated by hyperelastic collisions and dissipative forces. I will sow that despite the out of equilibrium nature of the particle dynamics, the system of active spinners effectively behave as a hard sphere system.

Time Crystals

Time crystals are quantum systems which are able to reveal condensed matter behaviour but in the time domain. Research on time crystals can be divided into a few stages: (i) The initial Wilczek idea about time crystal formation. (ii) Proposals of crystallization of motion of periodically driven many-body systems and experimental demonstration of such discrete time crystals. (iii) Research of condensed matter phenomena in the time domain. Time is becoming our new ally and time engineering allows for realization of various solid state phenomena in the time domain like: Anderson localization and many-body localization in time, time quasi-crystals, topological time crystals or exotic condensed matter-like systems in the time domain. During my talk I will concentrate on the items (ii) and (iii) mentioned above.

Journal Club

Motional and spin dynamics of individual impurities coupled to an ultracold bath

Impurities can significantly modify the properties of materials; prominent examples are colors of gemstones or the conductance of a semiconductor. In the quantum realm, the coupling of individual atomic impurities to a many-body system is a paradigm forming the foundation of many highly-successful models, such as the spin-boson or the central-spin model, revealing detailed, local information about the environment. I will present our experimental approach to immerse individual Cesium atoms as tightly-controlled atomic impurities into an ultracold Rubidium cloud, being either a thermal gas or a Bose-Einstein condensate. We study the interaction between impurity and gas, where all relevant parameters, including inter-species interaction strength, can be experimentally controlled. We have implemented a range of methods to retrieve precise information about motional/thermal as well as internal (spin) states of the impurity, reflecting a snapshot of the local state of the many-body system. I will present the current state of the project, aiming on local, non-destructive probing of the relaxation dynamics of a quantum gas out of equilibrium.

Colloquium: On Growth and Form of Microorganisms on Liquid Substrates

The interplay between fluid flows and living organisms plays a major role in the competition and organization of microbial populations in liquid environments. Hydrodynamic transport leads to the dispersion, segregation or clustering of biological organisms in a wide variety of settings. To explore such questions, we have created microbial range expansions in a laboratory setting by inoculating two identical strains of S. cerevisiae (Baker’s yeast) with different fluorescent labels on a nutrient-rich fluid 10^4 to 10^5 times more viscous than water. The yeast metabolism generates intense flow in the underlying fluid substrate several times larger than the unperturbed colony expansion speed. These flows dramatically impact colony morphology and genetic demixing, triggering in some circumstances a fingering instability that allows these organism to spread across an entire Petri dish in roughly 24 hours. We argue that yeast colonies create fluid flow by consuming nutrients from the surrounding fluid, decreasing the fluid’s density, and ultimately triggering a baroclinic instability when the fluid’s pressure and density contours are no longer parallel. Our results suggest that microbial range expansions on viscous fluids will provide rich opportunities to study the interplay between advection and spatial population genetics.

Efficient approximations of (spatio-temporal) stochastic biological processes using machine learning

Many systems in nature consist of stochastically interacting agents or particles. Stochastic processes have been widely used to model such systems, yet they are notoriously difficult to analyse. In this talk I will show how ideas from statistics and machine learning can be used to tackle some challenging problems in the field of stochastic processes. In the first part, I will consider the problem of inference from experimental data for stochastic reaction-diffusion processes. I will show that multi-time distributions of such processes can be approximated by spatio-temporal Cox processes, a well-studied class of models from computational statistics. The resulting approximation allows us to naturally define an approximate likelihood, which can be efficiently optimised with respect to the kinetic parameters of the model. In the second part, we consider more general path properties of a certain class of stochastic processes. Specifically, we consider the problem of computing first-passage times for Markov jump processes, which are used to describe systems where the spatial locations of particles can be ignored. I will show that this important class of generally intractable problems can be exactly recast in terms of a Bayesian inference problem by introducing auxiliary observations. This leads us to derive an efficient approximation scheme to compute first-passage time distributions by solving a small, closed set of ordinary differential equations.

Colloquium: Active field theories: Statistical physics and the arrow of life

Active matter comprises particles whose microscopic dynamics breaks time-reversal symmetry (TRS) via the continuous conversion of fuel into motion, resulting in entropy production at the microscopic scale. Examples include bacteria and synthetic self-propelled colloids. Equilibrium statistical physics concepts, for instance the Boltzmann distribution, are not applicable to active matter because these concepts assume the TRS of the underlying microscopic laws. When viewed at a coarse grained level, the microscopic absence of TRS may either remain obvious at large scales or become almost undetectable. In the latter case, we retain the hope that equilibrium concepts, at least in some modified form, may be applicable. This talk will discuss the conceptual toolbox needed to characterize the presence or absence of TRS at coarse-grained level, focusing on global and local measures of entropy production. Put differently: when, and how, can you tell whether life's movie is running backwards?

A unifying perspective on rigidity in under-constrained materials

I present a novel approach to understanding rigidity in under-constrained materials, including sub-isostatic 2D spring networks as well as 2D and 3D vertex models. I show that in all these models the onset of rigidity is determined by a purely geometric criterion, represented by a minimal length function with generic scaling properties. This allows to analytically predict the elastic material properties close to the transition, which depend only on two or three geometric coefficients. For instance, I obtain exact expressions for the magnitudes of bulk modulus and shear modulus discontinuities at the rigidity transition, several scaling relations of the shear modulus, and the magnitude of the anomalous Poynting effect. Moreover, the ratio of the excess shear modulus to the shear stress is inversely proportional to the critical shear strain with a prefactor of three, which I expect to be a general hallmark of rigidity in under-constrained materials induced by geometric incompatibility. This could be used in experiments to distinguish whether strain-stiffening as observed for instance in biopolymer networks arises from nonlinear mechanics of the microscopic material components or from geometric incompatibility effects discussed here.

Colloquium: An effective molecular orbital approach to electron phonon and pairing interactions in skipped valence and negative charge transfer gap Oxides

In high oxidation state oxides like the trivalent Nickel oxides, tetravalent Co and Fe oxides as well as the parent superconductors BaBiO3 and SrBiO3 and High Tc hole doped cuprates, the cation electron affinity in the formal valence could end up larger than the O 2- ionization potential leading to a so called negative charge transfer gap. If the charge transfer energy is strongly negative, then we should really adopt starting electronic configurations such as Ni2+ rather than 3 + or Bi 3+ rather than 4+ with compensation holes in the O 2p valence band for charge neutrality. If in addition the lowest energy cation ionization states are strongly hybridized with the valence O 2p states the low energy scale electronic structure and be well described by a molecular orbital type of approach (1,2). This is a new approach to the Wannier function description (3) but with explicit inclusion of the O states which provides a natural path to inclusion of the electron phonon coupling, charge density wave formation, potential bipolaron formation and paring interactions in superconductors. We discuss recent developments in this approach and show that the effective electron phonon coupling involving these molecular like orbitals is much stronger than that estimated from density function approaches. We also show that this leads to Peierls like charge density wave like ground states and we describe how the electron phonon coupling involving the hopping integrals rather than the on-site energies evolves into a large effective attractive interaction between low energy scale electrons. I will also briefly describe how these effects lead to our coelution that the ion battery material LiNiO2 should be viewed as an “entropy-stabilized charge- and bond-disproportionated glass”.

Careers Talk

Colloquium: tba

Extreme-ultraviolet high-harmonic generation and spectroscopy in condensed matter

Attosecond science, studies of electron dynamics in their natural time scale, stems from the development of mostly extreme ultraviolet sources through high-order harmonic generation in gas-phase systems. Recently, being ignited by the researches done in Stanford, high-order harmonic generation in solids has been discovered and been actively investigated. The coherent, extreme ultraviolet radiation from solids can be utilized not only as a new source for technical applications but it also offers a great tool to study electronic properties of solids. In this talk, we briefly review the developments in this emerging field and we report our newest results. In details, we demonstrate the first polarimetry measurement of high-order harmonic generation from solids and use it to uncover the non-vanishing Berry curvature underlying the generation of even harmonics in quartz, in orthogonal polarization with respect to the linearly incident electric field. First ab initio calculation of Berry curvature of quartz has been carried out and it shows a high degree of agreement to the experimentally retrieved Berry curvature which concludes an important spectroscopic application. Furthermore, we extend high-order harmonic generation in condensed matter by reporting on the unambiguous, systematic, experimental investigations of the high-order harmonic generation in liquids, the third phase of matter. By utilizing a liquid flat-jet as a target for light-matter interaction, coherent, intense extreme ultraviolet radiation is recorded in the form of multiple odd-order harmonics reaching up to 27 order and extending beyond 20 electron Volt. The intensity scaling and the ellipticity measurement show the non-perturbative, solid-like nature of the radiation. Highest cut-off energy photons were obtained using ethanol by comparison to water and other liquids. Our investigation serves as a promising first step in utilizing the new source of coherent extreme ultraviolet radiation as well as exploring electron dynamics in liquid-phase of matter.

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Colloquium: Quantum optics using atomic arrays

Ensembles of atoms or other quantum emitters are envisioned to be an important component of quantum applications, ranging from quantum memories for light to photon-photon gates to metrology. It has historically been an outstanding challenge to exactly solve for the quantum dynamics of an optical field as it propagates through and interacts with an ensemble. The standard axiomatic approach is to use the one-dimensional Maxwell-Bloch equations, which treats the interaction between the ensemble and a quasi-1D optical mode of interest, while the interaction with the remaining 3D continuum of modes is assumed to result in independent spontaneous emission of excited atoms. Strictly speaking, this assumption cannot be correct, as the emission of light is a wave phenomenon, and thus the emitted intensity must depend on interference and correlations between the atoms. Here, we discuss an alternative theoretical approach, which accounts for interference and the precise atomic positions. In this formalism, an interacting quantum spin model describes the dynamics of the atomic internal degrees of freedom under multiple photon scattering, while the field properties can subsequently be re-constructed from the spin correlations. Using this model, we then show how interference can be exploited as an extremely powerful resource to suppress the unwanted emission of light and the subsequent loss of information into undesirable directions. The effects of interference are particularly prominent in ordered arrays of emitters. As two specific examples, we construct a new protocol for a quantum memory for light based upon an ordered array, whose error rate as a function of system resources scales exponentially better than previously known bounds. We also show the interrogation time in an optical lattice clock can be significantly extended, through the excitation of collective subradiant atomic states whose spontaneous emission rates are strongly suppressed. These results raise the intriguing question of whether interference can be used to broadly re-define the performance limits of all applications involving atomic light-matter interfaces.

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Current and Future Urban Climate Simulations for Barcelona

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