Place and time: Friday November 26th at 10:00, VRII-152

Speaker: Cristobal Arratia, NORDITA

Title: A story on hydrodynamics, instabilities, and transition to turbulence

Fluid mechanics is an old subject. It is present in our everyday experience and its governing equations were written nearly two centuries ago, yet we have long struggled to explain very common phenomena. In this talk we will see a part of its history, starting with the experiments of Reynolds (1883). These contain examples of the two ways in which complex behaviour arises in fluids and other systems: through an unstable eigenmode of the linearized dynamics around a simple state, and in the absence of such an instability. After describing some general insights for the study of instabilities that were learnt during the last few decades, we will return to the main subject of Reynolds paper, the transition to turbulence in a pipe. We will describe how this problem has finally been understood during the last decade thanks to a model by Barkley. We will end on some current perspectives on how this model can be extended to other transitional flows with much richer dynamics.

Seminar by Habib Rostami (Nov. 5th 10:00)

Place and time: Friday November 5th at 10:00, Oddi 206

Speaker: Habib Rostami

Title: Strain physics in 2D materials: pseudo gauge field, piezoelectricity and acoustoelectric

Abstract: In the context of applied physics, we are always interested in controlling and modifying the intrinsic electronic properties of electronic systems. For instance, we can use electric and magnetic fields for this purpose. However, mechanical deformation is another external field that is not as common as electric and magnetic fields. Two-dimensional van der Walls layered materials such as graphene and transition metal dichalcogenides (TMDs) show remarkable elastic properties with strong in-plane Yang modulus and weak bending rigidity. Accordingly, they provide a promising platform for flexible electronics, optoelectronic and acousto-optic devices. I will review the manifestation of a fictitious (pseudo) gauge field in 2D materials owing to the lattice deformation. Pseudo gauge field results in an ultra-strong pseudo magnetic field preserving the time-reversal symmetry and leading to the flat-band (Landau level) formation. I will discuss strain-induced electric polarization due to the piezoelectric effect related to a topological quantity known as the valley Chern number in inversion broken TMDs. I will talk about highly efficient exciton drift mechanism in single-layer TMD owing to the strain-induced deformation potential, the built-in piezoelectric field and the pseudo magnetic field. I will present acoustogalvanic effect as a rectification process of propagating pseudo electromagnetic field (sound wave) in 2D and 3D Dirac materials. I will discuss a transverse acoustoelectric current due to a pseudo gauge field that can propagate transverse to sound propagation.