2021

Electron and Positron Scattering by Cyclic Molecules.

Alessandra Souza Barbosa (UFPR)

Streamed live in 12 . 04 . 2021

Electron and positron physics attracts great interest due to its fundamental, technological and biological applications. Since many of its applications are based on basic interactions of electrons or positrons with molecules, scattering studies become a cornerstone towards understanding the underlying physics of these interactions.

Since a pioneer work of the early 2000s indicating that single and double strand breaks in DNA can be induced by low-energy electrons [1], and later works showing that the dissociation process can be mediated by the formation of a resonance in specific sites of the DNA chains, many theoretical and experimental groups have been working on electron interactions with molecules of biological relevance and their precursors. In particular, one of the goals is to characterize the temporary electron attachment to the molecules (resonance) and/or the subsequent dissociation pathways.

Regarding positron scattering, its calculations can be a difficult task due some intrinsic problems. For example, besides the inelastic process, which are also observed in electron-scattering, the incoming positron can capture one of the electrons of the target to form positronium (Ps). Moreover, Ps formations can be responsible for a large amount of the total cross section for positron impact energies over the positronium formation threshold. Even more troublesome, at energies below the Ps formation and electronic excitation thresholds, the agreement between the theoretical calculations and experimental data is far from good. This is mainly due to a poor description of the polarization potential for the positron scattering. In recent years, we have put a lot of effort in order to provide some reliable results for positron scattering by molecules and systems of technological and biological relevance. For example, employing the Schwinger multichannel method [2], we have performed extensive studies in order to improve the description of the polarization potential for two small non-polar molecules, allene and silane [3] and we were able to predict a positron bound state employing an ab initio scattering calculation.

In this talk we will discuss some of our recent theoretical results on electron and positron scattering by cyclic molecules of technological and/or biological relevance. Some of the target molecules include unsaturated and saturated molecules, such as benzene and some of its derivatives, proline, cyclohexane and other close-related molecules.

[1] B. Boudaïffa et al., 2000 Science 287, 1658 (2000).

[2] J. S. E. Germano and M. A. P. Lima, Phys. Rev. A 47, 3976 (1993).

[3] A. S. Barbosa, S. Sanchez, and M. H. F. Bettega, Phys. Rev. A 96, 062706 (2017); A. S. Barbosa and M. H. F. Bettega, Phys. Rev. A 96, 042715 (2017).

Quantum Fractals

Cristiane de Marais Smith (Institute for Theoretical Physics, Ultrecht University, The Netherlands)

Streamed live in 05 . 07 . 2021

The human fascination for fractals dates back to the time of Christ, when structures known nowadays as a Sierpinski gasket were used in decorative art in churches. Nonetheless, it was only in the last century that mathematicians faced the difficult task of classifying these structures. In the 80’s and 90’s, the foundational work of Mandelbrot triggered enormous activity in the field. The focus was on understanding how a particle diffuses in a fractal structure. However, those were classical fractals.

This century, the task is to understand quantum fractals. Last year, in collaboration with experimental colleagues from the Debye Institute, we realized a Sierpinski gasket using a scanning tunneling microscope to pattern adsorbates on top of Cu(111) and showed that the wavefunction describing electrons in a Sierpinski gasket fractal has the Hausdorff dimension d = 1.58 [1,2]. However, STM techniques can only describe equilibrium properties.

Now, we went a step beyond and using state-of-the-art photonics experiments in collaboration with colleagues at Jiao-Tong University in Shanghai, we unveiled the quantum dynamics in fractals. By injecting photons in waveguide arrays arranged in a fractal shape, we were able to follow their motion and understand their quantum dynamics with unprecedented detail. We built and investigated 3 types of fractal structures to reveal not only the influence of different Hausdorff dimension, but also of geometry [3].

[1] S.N. Kempkes, M.R. Slot, S.E. Freeney, S.J.M. Zevenhuizen, D. Vanmaekelbergh, I. Swart, and C. Morais Smith, “Design and characterization of electronic fractals”, Nature Physics 15, 127(2019).

[2] Physics Today 72, 1, 14 (2019) https://physicstoday.scitation.org/doi/full/10.1063/PT.3.4105

[3] X.-Y. Xu, X.-W. Wang, D.-Y. Chen, C. Morais Smith, and X.-M. Jin, “Shining light on quantum transport in fractal networks,” ArXiv: 2005.13385, in print Nature Photonics (2021).

Global Quantum Systems: exploring the coherent nature of spontaneous emission

Daniel Felinto (UFPE)

Streamed live in 16 . 08 . 2021

The formation of collective entangled states on atomic ensembles is heralded by detection of photons spontaneously emitted by the sample. Its origin is the coherent interaction of atoms with the quantum reservoir of vacuum modes. In the last two decades, many groups have explored this process to devise applications in quantum information. Here we present our recent work on the problem, focusing on two fronts. We first develop a bottom-up approach to the study of superradiance from particular quantum states. Later, we present our results on ensembles of pure two-level atoms, aiming to explore quantum correlations in the simplest systems. We particularly discuss the peculiar position of this problem on the backdrop of the broad discussion about the role of quantum entanglement in our macroscopic world.

Formation of supersolids and complex patterns in quantum fluids

Thomas Pohl (Aarhus University - Denmark)

Streamed live in 25 . 10 . 2021

Ensembles of particles with finite-range interactions can exhibit a rich spectrum of self-organization phenomena that have long been of fascination in many areas of physics. Recent developments in cold-atom research are now opening up exciting prospects for laboratory explorations of such effects in the quantum domain. In this talk, we will consider the behaviour of ultracold quantum gases in which finite-range interactions between atoms arise from static dipoles or are induced by external optical driving. We will discuss how such kind of interactions can cause the emergence of distinct regular structures and break the symmetry of the system in sometimes unexpected ways. The role of quantum and thermal fluctuations as well as experimental progress and prospects will also be discussed.