# Séminaires du Wednesday 15 November

## Physics-Biology interface seminar: Felix Rico

#### 11:00:00

## Molecular to cellular mechanics probed by high-speed atomic force microscopy

### Felix Rico (Aix-Marseille Université)

The mechanical properties of individual proteins, filaments, and supramolecular assemblies provide structural stability and mechanical flexibility to the living cell. Thus, molecular understanding of the mechanics from the single molecule to the whole cell is relevant to understand biological function. High-speed atomic force microscopy (HS-AFM) is a unique technology that combines nanometric-imaging capabilities at video rate. In this talk, I will present our recent applications of HS-AFM to probe protein and cellular mechanics. In the first part, I will introduce the development of high-speed force spectroscopy (HS-FS) to probe protein unfolding at the timescales of molecular dynamics simulations (1). This provides a unique approach to acquire atomistic understanding of biomolecular processes based on experimental results. In the second part, I will present our recent work on the adaptation of HS-AFM to probe the microrheology of living cells at high frequencies (up to 100 kHz), revealing cytoskeletal dynamics (2). We show that the mechanical response at high frequencies depends on the actin filament tension and pathological state of the cell. Microrheology over a wide dynamic range—up to the frequency characterizing the molecular components—provides a mechanistic understanding of cell mechanics.

1. F. Rico, L. Gonzalez, I. Casuso, M. Puig-Vidal, S. Scheuring, High-Speed Force Spectroscopy Unfolds Titin at the Velocity of Molecular Dynamics Simulations. Science 342, 741 (2013).

2. A. Rigato, A. Miyagi, S. Scheuring, F. Rico, High-frequency microrheology reveals cytoskeleton dynamics in living cells. Nat Phys 13, 771 (2017).

## Séminaire exceptionnel du LPTMS: Natalia Menezes Silva Da Costa

#### 14:00:00

**Quantum field theory in low-dimensional condensed-matter systems**

### Natália Menezes Silva Da Costa (Utrecht University)

In this talk I will present two examples of how quantum field theories may be applied to describe the long wavelength regime of condensed-matter systems.
The first example [1] concerns the study of electronic interactions on the boundary of a two-dimensional time-reversal-invariant topological insulator. While the bulk of this two-dimensional material is insulating, the boundary exhibits propagating modes that may be described in terms of a one-dimensional Dirac theory. By assuming that there is an underlying electromagnetic theory mediating the e-e interaction on the edges, and by employing a dimensional reduction procedure, I will show that the effective one-dimensional theory is a non-Fermi liquid, known as the helical Luttinger liquid (HLL). This HLL resembles a theory of free bosons, however, with a parameter in its kinematics that indicates the strength of the e-e interactions. Within the quantum-field theoretical formalism, I will show that such parameter can be written in terms of the fine structure constant, which allows one not only to predict its value but also to manipulate the nature attractive/repulsive of the interaction.
The second example [2] concerns the topological response of a fermionic model defined on the Lieb lattice in presence of an electromagnetic field. The tight-binding model is built in terms of three species of spinless fermions and supports a topological Varma phase due to the spontaneous breaking of time-reversal symmetry. In the low-energy regime, the emergent effective Hamiltonian coincides with the so-called Duffin-Kemmer-Petiau (DKP) Hamiltonian, which describes relativistic pseudospin-0 quasiparticles and goes beyond the commonly studied spin-1/2 Dirac/Weyl paradigm. By considering a minimal coupling between the DKP quasiparticles and an external Abelian gauge field, I will present both the Landau-level spectrum and the emergent Chern-Simons theory. The corresponding Hall conductivity reveals an atypical quantum Hall effect, which can be simulated in an artificial Lieb lattice.
[1] N. Menezes, G. Palumbo and C. Morais Smith, Sci. Rep. 7, 14175 (2017).
[2] N. Menezes, C. Morais Smith and G. Palumbo, arXiv:1710.07916 (2017).