LABORATOIRE DE PHYSIQUE THEORIQUE DE LA MATIERE CONDENSEE

 

 

Attention : désormais les séminaires ont lieu tous les lundis à 10h45 en salle  523 du LPTMC - Tour 12-13 


 

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.

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.

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

Synchronization in coupled oscillator systems

Abstract: 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.

Andrej Mesaros
Cornell university

Short-ranged charge modulations in cuprates: r-space or k-space?

Electronic spatial charge modulations (CM) have been recently established as integral to the phase diagram of high-transition-temperature (high-Tc) cuprate superconductors. It is not yet understood if CM are involved in establishing high-Tc superconductivity, or are detrimental to it. The answer hinges on determining, in face of disorder, the relationship between periodicities of electronic CM and underlying lattice. Here we determine that CM are commensurate in the entire pseudogap region of Bi2Sr2CaCu2O8 phase diagram. The found commensurability points towards a strong coupling perspective on electrons, likely implying cooperation with superconductivity. We introduce a general analysis that exploits phase-sensitive measurements to find the periodicity of inhomogeneous modulations, and apply it to scanning tunneling microscopy images.
 


François Crépin
LPTMC

Appariement impair en fréquence d'électrons dans les canaux de bord d'isolants topologiques.

Les liquides hélicaux sont des gaz uni-dimensionnels d'électrons ayant la particularité d'une forte corrélation entre la valeur du spin et le sens de propagation. Ils émergent naturellement aux bords de certains semi-conducteurs communément appelés isolants topologiques [1]. En plus d'une certaine robustesse face aux impuretés non-magnétiques, ces nouveaux états électroniques présentent de nombreuses propriétés remarquables, notamment lorsqu'on les place à proximité d'un supraconducteur [2]. Dans ce séminaire, je propose tout d'abord de passer en revue plusieurs phénomènes directement causés par l'hélicité et caractéristiques d'une supraconductivité non-conventionnelle, comme des effets Josephson fractionnaires ou l'apparition d'états-liés exotiques. Dans une deuxième partie, je présenterai une analyse détaillée des symétries d'appariement dans des jonctions hybrides supraconducteur/isolant topologique [3]. En particulier, nous discuterons d'un appariement exotique, dit impair en fréquence, et de ses signatures possibles dans des mesures non-locales de la conductance de ces jonctions.

[1] M. Z. Hasan and C. L. Kane, Rev. Mod. Phys. 82, 3045 (2010)
[2] Liang Fu and C. L. Kane, Phys. Rev. Lett. 100, 096407 (2008)
[3] François Crépin, Pablo Burset, and Björn Trauzettel, Phys. Rev. B 92, 100507(R) (2015)