ANDRZEJ ŚLEBARSKI

University of Silesia,
Faculty of Mathematics, Physics and Chemistry,
Institute of Physics, Division of Physics of Solid Body;
Uniwersytecka 4, 40-007 Katowice
e-mail: andrzej.slebarski@us.edu.pl

Description popularizing the research project

There is nothing better than cloud gazing. It is something you can do in a meadow and on a beach, even indoor when your body is chained to the desk with a computer and only the imagination wanders freely. Clouds are very important in our life. They are used in forecasting weather, expressing moods and building metaphors. From thin white tatters in the blue sky to the heavy and moody storm clouds, each of them may serve some purpose.
So what can we compare a quasicrystalline system if not to clouds? But of course not any ordinary cloud. Not every cloud can be used in the comparison. It cannot be a common fleecy cloud, a cloudlet or even a solid layered cloud known as stratus. The horribly complicated, strongly-correlated system which characterizes distribution of forces and energy among electrons, can be shown as a beautiful cumulonimbus mammatus hanging low over a city. It is hard to expect a cloud to be internally organized. Once it is a dragon, then a castle or a butterfly. Their organization resembles the one of cells in tissue or a honeycomb, but it is superficial. Within it is hard to find any kind of order. The whole cloud we can describe with equation used in describing quasicrystals. So could we use the same variables to describe mess in a typical teenager's room? Surely not, although they would like to have some scientific argument from the branch of crystallography in a confrontation with the parents. Yet the universal rules of nature make clouds behave like electrons, and electrons like clouds.

Abstract

In case of metals, the conduction electrons are treated as almost free quasiparticles which, however, are interacting with the periodic atomic lattice of the crystal, and among themselves. Therefore, these electrons are not quite free, but they are correlated due to the electrostatic interaction U.
Such interacting (correlated) electrons form rather quantum liquid which consists of the fermions (electrons), than the quantum gas state. In rare cases of cerium or uranium compounds the interaction energy U between the conduction electrons (quantum states) is very strong at very low temperatures, i.e., comparable with their kinetic energy EK (EK is of the order of bonding states); the systems are known as strongly correlated f-electron systems. However, due to delicate interplay between the competing mechanisms: the local on-site Kondo screening of the f-electron moments by the conduction states and the long range magnetic interaction f-f-type, there have been known for about 10 years new Ce, or U materials with non-Fermi liquid behaviors at very low temperatures. Very recently we have discovered new scaling: susceptibility • resistivity = const for Kondo insulators, i.e., for the strongly correlated Ce or U systems, having a small energetic gap of about 1 meV at the Fermi level (see, A. Ślebarski and J. Spałek, Phys. Rev. Lett, 2005). The physics of these phenomena seems to be difficult, therefore the picture with clouds well translates understanding of the low-temperature phenomena of strongly correlated metals. The picture shows very rare clouds cumulonimbus mammatus observed in 1997. The clouds were very fast, which means that their kinetic energy EK was large, however, due to strong interactions (correlations) U between the ice particles, they formed in short time shapes similar to that of quasicrystals. There is in my understanding very loose reference of the macroscopic behavior to the microscopic effect which can be observed in strongly correlated electron systems. I treat it more philosophically, one can suggest that the nature has very universal character.