Attention : désormais les séminaires auront lieu tous les lundis à 11h00 en salle  523 du LPTMC - Tour 12-13 

Benjamin Canals (Institut Néel, Grenoble)

Artificial magnets as model systems : from the fragmentation of magnetization to the square ice model

Complex architectures of nanostructures are routinely elaborated using bottom-up or nanofabrication processes. This technological capability allows scientists to engineer materials with properties that do not exist in nature, but also to manufacture model systems to explore fundamental issues which appeared in condensed matter physics. One- and two-dimensional frustrated arrays of magnetic nanostructures are one class of systems for which theoretical predictions can now be tested and challenged experimentally. These systems have been the subject of intense research in the last few years and have allowed the investigation of a rich physics and fascinating phenomena, such as the exploration of the extensively degenerate ground-state manifolds of spin ice systems, the evidence of new magnetic phases in purely two-dimensional lattices, and the observation of pseudo-excitations involving classical analogues of magnetic charges. This talk aims at providing two examples of two-dimensional artificial magnets which allow to probe the low energy manifolds of two exotic Ising systems.

The first one is related to the seminal 6-vertex model and shows that it is possible to perform a scan through the 6-vertex model phase diagram with an appropriately designed artificial magnet [1]. In particular, the symmetric point of the (Lieb) square ice is recovered, providing with the opportunity to study the signatures of an algebraic Coulomb spin liquid. Because of the experimental procedure used to reach the low energy manifold, quasi-particles are trapped in this disordered manifold, pointing to the need of thermal systems, but also emphasizing that these systems may be well suited to study out of equilibrium relaxation of monopole-(anti)monopole pairs in a near future.

The second one refers to a recent proposal, the fragmentation of magnetisation [2], in an Ising Kagomé model. Here, we show it is possible to observe this intriguing phenomena, which corresponds to the splitting of the local degree of freedom into two channels, one ordering at low effective temperatures, in an AF all-in all-out ordering despite the ferromagnetic nature of the system, the other, building a Coulomb-like low energy manifold, inside which the magnetic equivalent of the Kirchhoff law at each node of the Kagomé lattice is fulfilled [3].

[1] Y. Perrin, B. Canals, N. Rougemaille, Nature 540, 410–413 (2016).
[2] M. E. Brooks-Bartlett, S. T. Banks, L. D. C. Jaubert, A. Harman-Clarke, and P. C. W. Holdsworth, Phys. Rev. X, 4, 011007 (2014).
[3] B. Canals, I. A. Chioar, V.-D. Nguyen, M. Hehn, D. Lacour, F. Montaigne, A. Locatelli, T. O. Mentes, B. S. Burgos and N. Rougemaille, Nat. Comm. 7, 11446 (2016).

Thomas Franosch

Institut fÜr Theoretische Physik, Universität Innsbruck


Non-equilibrium dynamics of active agents and driven particles in microrheology

The paradigm of virtually all transport processes  in  soft matter or biophysics systems is Brownian motion. However, often the constituents are highly anisotropic resulting in a non-trivial coupling between orientational and translational degrees of freedom.

In this talk I will introduce an exact solution of the Smoluchowski-Perrin equation for anisotropic diffusion exploiting a mathematical analogy to the quantum pendulum. Then the single-particle dynamics can be obtained as a superposition of suitable eigenfunctions of the Smoluchowski operator.
We discuss  features emerging due to the interplay of particle anisotropy and translational motion and how they manifest themselves in the directly measurable intermediate scattering functions.

Next, we investigate the dynamics of a single active particle, i.e. an agent that undergoes self-propelled motion along an axis of orientation which slowly and randomly changes. Again the intermediate function can be elaborated analytically and reveals oscillatory behavior for intermediate wave numbers, in striking contrast to passive overdamped systems. We compare our results with recent dynamic differential microscopy measurements and demonstrate that our solution allows  reliably extracting motility parameters.

Last we address the driven dynamics of tracer particle in a colloidal suspension of hard spheres upon switching on an external force. The force drives the system far from equilibrium and we monitor the time-dependent velocity response. Within a low-density expansion and computer
simulations we show that linear response as encoded in the fluctuation-dissipation theorem becomes qualitatively wrong.

Olga Petrova
Junior Research Chair
Department of Physics, Ecole Normale Superieure, Paris

Magnetic monopoles in quantum spin ice

The quest for quantum spin liquids is an important enterprise in strongly correlated physics, yet candidate materials are still few and far between. Meanwhile, the classical front has had far more success, epitomized by the exceptional agreement between theory and experiment for a class of materials called spin ices. It is therefore natural to introduce quantum fluctuations into this well-established classical spin liquid model, in the hopes of obtaining a fully quantum spin liquid state.

The spin-flip excitations in spin ice fractionalize into pairs of magnetic monopoles carrying opposite effective charge. Quantum fluctuations have a parametrically larger effect on monopole motion than on the spin ice ground states. Therefore, the leading manifestations of quantum behavior appear in the dynamics of the monopoles. I will first discuss a particularly crisp phenomenon that we found in the presence of weak dilution with nonmagnetic ions: the emergence of hydrogenic excited states in which a magnetic monopole is bound to a vacancy at various distances. I will show how this vacancy problem can be approximated by an exactly solvable model of a single particle hopping on the Bethe lattice. This allows us to obtain an approximate expression for the dynamic structure factor for the weakly diluted quantum spin ice, which can be directly measured in neutron scattering experiments.

In the more general, unperturbed case we find that the classical degeneracy of the excited states is only partially lifted by quantum fluctuations in the form of a transverse field term. I will discuss a family of extensively degenerate excited states that make up an approximately flat band at the first excited classical energy level. These states are exact up to the splitting of the spin ice ground state manifold.

Karim Essafi

Okinawa Institute of Science and Technology

A Kagome Map of Spin Liquids
Abstract: Competing interactions in frustrated magnets prevent ordering down to very low temperatures and stabilize exotic highly degenerate phases where strong correlations coexist with fluctuations. We study a very general nearest-neighbour Heisenberg spin model Hamiltonian on the kagome lattice which consist of Dzyaloshinskii-Moriya, ferro- and antiferromagnetic interactions. We present a three-fold mapping which transforms the well-known Heisenberg antiferromagnet (HAF) and XXZ model onto two lines of time-reversal Hamiltonians. The mapping is exact for both classical and quantum spins, i.e. preserves the energy spectrums of the HAF and XXZ model. As a consequence, our three-fold mapping gives rise to a connected network of quantum spin liquids centered around the Ising antiferromagnet. We show that this quantum disorder spreads over an extended region of the phase diagram at linear order in spin wave theory, which overlaps with the parameter region of Herbertsmithite ZnCu3(OH)6Cl2. At the classical level, all the phases have an extensively degenerate ground-state which present a variety of properties such as ferromagnetically induced pinch points in the structure factor and spontaneous scalar chirality which was absent in the original HAF and XXZ models.

16h - Bibliothèque du LPTHE (couloir 13-14, 4ème étage)

Shamik Gupta
Max Planck Institute for the Physics of Complex Systems,

Synchronization in coupled oscillator systems


Collective synchronization involves a large population of coupled oscillators of diverse frequencies spontaneously synchronizing to oscillate at a common frequency. Common examples are synchronized firings of heart cells, phase synchronization in electrical power networks, rhythmic applause in concert halls, etc. The Kuramoto model serves as a prototype to study collective synchronization. The model comprises oscillators with distributed natural frequencies interacting through a mean-field coupling. Interpreting the dynamics as that of a long-range interacting system driven out of equilibrium by quench disordered external torques, I will discuss the rich out-of-equilibrium phenomena the model exhibits, and in particular, its complete phase diagram for unimodal frequency distributions.