# Invited speakers

## Invited Talk Abstracts

 Institute for Theoretical Physics, University of Amsterdam, Science Park 904, 1098 XH, Amsterdam The Netherlands  Dynamics and relaxation in integrable quantum systems Recent years have witnessed rapid progress in the use of integrability in characterizing the out-of-equilibrium dynamics of low-dimensional systems such as interacting atomic gases and quantum spin chains. This talk will provide an introduction to these developments, with a particular focus on the Quench Action method. Exact solutions to the interaction turn-on quench in the Lieb-Liniger model and to the Néel-to-XXZ quench in spin chains will be presented. Particular emphasis will be given to interesting open issues and challenges, including the failure of the (local) Generalized Gibbs Ensemble to properly describe post-quench steady-state properties and the necessity to include quasilocal conserved charges to obtain correct answers. Theoretical Physics, Oxford University , 1, Keble Road , Oxford OX1 3NP, United Kingdom Classical loop models and quantum magnets  I will give an overview of recent work on the statistical physics of lattice models of close-packed loops and their relation to quantum magnets. These models describe a number of problems in classical statistical physics -- where, for example, the loops may be vortex lines in a three-dimensional random field -- and also in antiferromagnets -- where the loops are world-lines of quantum particles. The loop models have two phases -- one in which all loops are finite, and another in which some loops are extended -- corresponding ground states of an antiferromagnet with respectively valence bond and N\'eel order. I will discuss the continuum description of these systems and present results from Monte Carlo simulations. In particular, these loop models are a convenient way to access the transition from the valence bond solid to a Neel states, which is a candidate deconfined critical point. Sergio Ciliberto Laboratoire de Physique de l'ENS de Lyon , 46 Allée d'Italie, 69364 Lyon, France A protocol for reaching equilibrium arbitrary fast When  a control parameter of a system  is suddenly changed, the accessible phase space changes too and  the system needs its characteristic relaxation time to reach the final equilibrium distribution.  An important and relevant question is whether it is possible to travel from an equilibrium state to another in an arbitrary  time, much shorter  than the natural relaxation time. Such strategies are reminiscent of those worked out in the recent field of Shortcut to Adiabaticity, that aim at developing protocols, both in quantum and in classical regimes, allowing the system to move as fast as possible from one equilibrium position to a new one, provided that there exist an adiabatic transformation relating the two. Proof of principle experiments have been carried out for isolated systems.   Instead  in  open system the reduction of the relaxation time, which is frequently desired and necessary, is often obtained by complex feedback processes. In this talk, we present  a protocol,named Engineered Swift Equilibration (ESE), that shortcuts time-consuming relaxations. We tested experimentally this protocol  on  a Brownian particle trapped in an optical potential first and then on an AFM cantilever. We show that applying a specific driving, one can reach equilibrium in an arbitrary short time.  We also estimate  the energetic cost to get such a time reduction.  Beyond its fundamental interest, the ESE method paves the way for applications in micro and nano devices, in high speed AFM, or in monitoring mesoscopic chemical or biological process. [1] Engineered Swift Equilibration,  Ignacio A Martinez; Artyom Petrosyan; David Guéry-Odelin; Emmanuel Trizac; Sergio Ciliberto, Nature Physics, Vol 12, 843 (2016). [2] Arbitrary fast modulation of an atomic force microscope, Anne Le Cunuder; Ignacio A Martinez; Artyom Petrosyan; David Guéry-Odelin; Emmanuel Trizac; Sergio Ciliberto. Applied Physics Letters, 109, 113502 (2016) Irene Giardina Dipartimento di Fisica, Universita' di Roma La Sapienza, P. A. Moro 2, 00185 Rome, Italy Information propagation and collective swings in biological groups Collective changes in biological groups require all individuals in the group to go through a behavioral change of state. Sometimes these changes are triggered by external perturbations, as in evasive maneuvers of animal groups under predatory attacks. Often, however, they occur spontaneously and are only due to internal behavioral fluctuations. In all cases, the efficiency of information transport is a key factor to prevent cohesion loss and preserve collective robustness.In this talk, I will present an experimental and theoretical study of collective movements in animal groups. Starting from experimental data on collective turns in starling flocks, I will discuss what is the mechanism that triggers a collective change (a turn) and grants efficient and fast information propagation through the system. Finally, I will discuss the role of heterogeneities, network unbalance, and boundary effects in initiating a collective change of state. Physics Department, Cavendish Laboratory, Cambridge, CB3 0HE, United Kingdom Quantum gas in a box For the past two decades harmonically trapped ultracold atomic gases have been used with great success to study fundamental many-body physics in a flexible experimental setting. In 2013 we created the first atomic Bose-Einstein condensate (BEC) in an essentially uniform potential of an optical box trap [1]. Compared to the traditional setting of a harmonic trap, this has opened new possibilities for closer connections with other many-body systems and the theories that rely on the translational symmetry of the system. I will give an overview of our recent experiments on this system, including studies of the (Kibble-Zurek) dynamics of spontaneous symmetry breaking [2] and the emergence of turbulence in a periodically driven gas [3].   [1] A. L. Gaunt et al., PRL 110, 200406 (2013) [2] N. Navon et al., Science 347, 167 (2015) [3] N. Navon et al., Nature 539, 72 (2016) Jozef Stefan Institute and Faculty of Mathematics and Physics, University of Ljubljana., SI-1000 Ljubljana, Slovenia Many-body localization in disordered spin and Hubbard chains The many-body localization (MBL) is the quantum phenomenon involving the interplay of disorder and  particle interaction, characterized mainly by the nonergodic behaviour. Recently it is intensively investigated theoretically within one-dimensional many-body models, and experimentally in optical lattices of cold atoms, but might be relevant also for materials with spin chains. In the talk the evidence for the transition to the MBL will be presented as it emerges from numerical investigations on the one-dimensional disordered spin and Hubbard models. It will be shown that within a random- eld spin chain dynamical staggered correlations, which are frequently used as the order parameter for the MBL phase, are closely related to the uniform dynamical spin conductivity and d.c. transport, whereby the transition is best characterized by the universal critical dynamics. On the other hand, an analogous numerical investigation of the disordered Hubbard chain indicates that disordered potential does not induce full MBL, but only charge localization while spin correlations vanish for large times. Virginie Simonet Institut NEEL CNRS/UGA UPR2940, 25 rue des Martyrs BP 166, 38042 Grenoble, France Magnetic charge injection in spin ice   The spin ice state emerges in pyrochlore lattices of vertex sharing tetrahedra when the magnetic moments are subjected to an effective ferromagnetic interaction and are constrained along the local directions joining the corners to the center of each tetrahedron. It is a macroscopically degenerate ground state called a Coulomb phase, in which the spins obey locally the ice-rule, which means that two spins point in and two spins point out of each tetrahedron. The spin ice elementary excitations, the magnetic monopoles, are obtained by reversing one spin at the center of a pair of tetrahedra.We propose a new mechanism to inject monopoles in a spin ice through a staggered magnetic field. We show experimentally that this is realized in a rare-earth pyrochlore iridate where the iridium sublattice produces, at the rare-earth site, a staggered magnetic field, pointing inwards/outwards adjacent tetrahedra. A new peculiar ground state is stabilized originating from the competition between the antiferromagnetic-like molecular field and the ferromagnetic spin-ice correlation. Compared to conventional spin ices, the different nature of the excitations in this new state opens the way to novel field-induced and dynamical behaviors. Department of Mathematics, King's College London, Strand London WC2R 2LS, United Kingdom Non-affine fluctuations and pleating transitions in crystalline solids I will review our recent attempts to understand the equilibrium and non-equilibrium behaviour of non-affine fluctuations in solids. We measure local non-affinity $\chi$ by a systematic coarse graining of microscopic atomic displacements. This generates a local elastic deformation tensor $D$ and the non-affinity as the extent to which the displacements are not representable as affine deformations of a reference crystal. We calculate the statistics of $\chi$ and $D$ and their spatio-temporal correlations for solids at low temperatures, within a harmonic approximation. The calculation allows us to identify the dominant non-affine fluctuation modes, which have an interpretation as precursors for the nucleation of lattice defects.In a second part I describe a phase transition that results when total non-affinity is biased by an appropriate conjugate field, in a two-dimensional network solid. Monte Carlo simulations reveal that the network supports, apart from the homogeneous phase, a "pleated" phase that has stress localised in rows of pleats and eliminated from the rest of the lattice. The kinetics of the phase transition is extremely slow in molecular dynamics simulation near coexistence, due to very large free energy barriers. When the external field is increased beyond coexistence to lower these barriers, the network exhibits rich dynamic behaviour: it transforms into a metastable phase with the stress now localised in a disordered arrangement of pleats. The pattern of pleats shows ageing dynamics and slow relaxation to equilibrium. Our predictions should be amenable to experimental testing using tethered colloidal solids in dynamic laser traps.References: Phys Rev E 87:042801, 2013; Soft Matter 11:4517, 2015; JSTAT P06025, 2015; arXiv:1612.00574 LPTENS, 24 rue Lhomond, 75005 Paris, France The Field theory of avalanches When elastic systems like contact lines on a rough substrate, domain walls in disordered magnets, or tectonic plates are driven slowly, they remain immobile most of the time, before responding with strong intermittent motion, termed avalanche. I will describe the field theory behind these phenomena, explain why its effective action has a cusp, and how such intricate objects as the temporal shape of an avalanche can be obtained. Finally, an exact mapping to the Manna sandpile model is discussed. CNR-ISC: Institute of Complex Systems and Dipartimento di Fisica Universita’ “La Sapienza”  Rome,  Italy Anomalous slow dynamics in soft matter  Dynamic arrest in soft matter systems manifests in a variety of arrested states, including gels, attractive and repulsive glasses. Usually, the approach to an arrested state happens through a two-step relaxation of the dynamic auto-correlation functions, where the microscopic, fast relaxation is separated from the structural, slow relaxation by an intermediate plateau often interpreted in terms of the so-called cage effect. However, anomalous dynamics where this picture in not valid is observed in many situations for soft matter systems. In this case, the two-step behavior is replaced by a logarithmic relaxation of the density auto-correlation functions, which is accompanied by a sub-diffusive behavior of the mean-squared displacement. In this talk I will present some examples of anomalous slow dynamics in simulations and experiments, and I will discuss the current theoretical understanding of this behavior. Institut de Physique Theorique, Orme des Merisiers,  CEA/SACLAY, F-91191 Gif-sur-Yvette Cedex, France Spectral analysis of sparse data: Translating physics into algorithmsA number of problems in data science can be solved using algorithms based on the spectrum of a matrix associated with the data. Principal component analysis is a well-known example of such a spectral method. There are many examples of applications that require the matrix associated with the data to be sparse. However, the traditionally considered sparse matrices associated to the data develop spurious large eigenvalues associated with localised eigenvectors that harm the algorithmic performance. These so-called Lifshitz tails are analogous to impurity states in disordered systems. Inspired by the theory of spin glasses, we introduce the non-backtracking operator that is able to mitigate this problem and has no spurious eigenvalues. We discuss properties of this operator, as well as its applications to several algorithmic problems such as clustering of networks, percolation, matrix completion or inference from pairwise comparisons. Institute for Theoretical Physics, Friedrich-Hund-Platz 1, D-37077 Göttingen Germany Dense Granular Flow A jamming scenario of frictional particles is discussed and interpreted in terms of a nonequilibrium first order phase transition [1]. Results of numerical simulations will be presented and analyzed in the framework of a simple model which can account for both, the continuous frictionless case and the discontinuous frictional case. The most important features of the frictional phase diagram are reentrant behavior and a critical jamming point at finite stress. In the simulations, we observe that small systems settle into a stationary state, whereas large systems do not relax to a stationary state on the timescale of observation, but rather display chaotic time dependence [2]. We propose a hydrodynamic model which couples stress relaxation to a scalar variable accounting for the microstructure of the packing. Linear stability analysis reveals an extended phase diagram which in addition to regions of stationary flow and jammed states displays chaos. We also develop a microscopic picture of reentrant flow and shear thickening, which emphasizes the role of friction. [1] M. Grob, C. Heussinger, and A. Zippelius, Phys. Rev. E89, 050201 (R) (2014).[2] M. Grob, A. Zippelius and C. Heussinger, Phys. Rev. E93, 030901 (R) (2015).
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