Plastic deformation of metals usually occurs as a series of rapid and localized deformation events of various sizes. During these events, the line-like defects of the crystal lattice, called dislocations, move collectively in a local part of the crystal. The experimental investigation of this avalanche-like phenomenon was initially carried out on bulk samples using acoustic emission (AE) measurements. It was found that the distribution of both the energy and amplitude of individual events are scale-free, which indicates the critical nature of plastic deformation. This phenomenon was later also observed during the compression of micropillars, typically a few um in size, where it manifests in random jumps in the stress-strain curve.

 

In the first half of the lecture, the outcome of the combination of these two experimental techniques will be presented, the aim of which was to obtain a more detailed picture of the dynamics of dislocation avalanches. To this end, micropillars were milled from Zn single crystals using the focused ion beam technique. The samples were attached to an AE sensor and compressed in situ in a scanning electron microscope using a special device designed for this purpose. We found that the acoustic events measured during compression are perfectly correlated with the stress drops experienced during compression. The statistical analysis of the data obtained by the two methods revealed the complex spatio-temporal dynamics of a single stress drop: a dislocation avalanche consists of many smaller events that show similarities to earthquakes through different phenomenological laws. These results, which are also confirmed by discrete dislocation dynamics simulations, provide the missing link between the quantities measured by AE and the mechanical characteristics of individual events [1]. As a continuation of the research, we present the stability analysis of the discrete dislocation systems, which reveals the development of dynamic modes that can span the entire volume. The emergence of these modes provides an explanation for the previously experienced anomalous system size dependence of avalanche sizes and the unusual behavior of local flow stresses [2].

 

[1] PD Ispánovity, D Ugi, G Péterffy, M Knapek, Sz Kalácska, D Tüzes, Z Dankházi, K Máthis, F Chmelík, I Groma, Nature Communications 13, 10 (2022).

[2] D Berta, G Péterffy, PD Ispánovity, arXiv:2202.08224 (2022).

The Nobel Prize in Physics in 2022 was awarded to Alain Aspect, John F. Clauser, and Anton Zeilinger (shared equally) “for experiments with entangled photons, establishing the violation of Bell inequalities and pioneering quantum information science”. In this talk I will review how entanglement and quantum nonlocality was first introduced, what the Bell inequalities are, how this year's Nobel laureates and others measured their violation, and how this work "pioneered quantum information science". 

 

The Majorana fermion was originally proposed by Ettore Majorana in 1937 as an elementary particle which is its own antiparticle. Although this elementary particle has not yet been detected, its analogons, Majorana states (and Majorana Zero Modes, often called Majoranas) can appear as quasiparticle excitations in condensed matter physics. These latter have promising applications as building blocks of topological quantum bits, based on their non-Abelian anyonic exchange statistics. Thanks to the rapid development in scanning-tunneling microscopy techniques, today it is possible to build the systems used in model calculations that should host Majoranas, and measure their spectral properties. It remains challenging to prove the presence of Majoranas from the data, and ab initio calculations can help arrive at definite conclusions.

 

I will present an overview of our ab initio based results on the formation of Majorana zero modes. I will start from a single magnetic impurity on the surface of an s-wave superconductor,  introduce the Yu-Shiba-Rusinov states, and then move on to magnetic chains. For some spin-spiral configurations we found an induced minigap, which has zero energy states localized to the ends of the chain in a spin triplet (superconducting) state. By varying the spiraling angle we observed a phase transition to another gapped state without zero energy states, which we expect to be a topological phase transition.