MaNEP e-Newsletter MaNEP Nr. 28
Scientific Highlight – p2

Spectroscopic evidence for Fermi liquid like energy and temperature dependence of the relaxation rate in the pseudogap of the cuprates
by Dirk van der Marel

After an article published in Proceedings of the National Academy of Sciences

The electrical current is carried by electrons, each of which has an elementary charge. The number of electrons in a solid is typically of the order of Avogadro’s number, which is roughly one million times one million times one million times one million - a huge number.
Given the number of charges and the fact that they move at an astronomical speed, it is not surprising that many collisions happen. These collisions (physicists call them “correlations”) determine the properties of any material. If a substance is insulating, magnetic or conducting, this is the result of correlations among the electrons. Although theoretical physics has explained the Big Bang and predicted many elementary particles such as the Higgs boson, the theory of correlations for a set of many elementary particles (not to mention an Avogadro number of particles) is a major headache, and is in fact one of the "big  open problems" of theoretical physics.

Thanks to the recent discovery of the "quantum materials" group, the fog finally clears around the metallic nature of high-temperature superconducting materials, that is to say below 200 degrees Celsius. Experiments showed that these materials, albeit considered to be highly correlated, can be described simply and completely as Fermi liquids. This means that from this complicated soup emerge new "quasi-particles" that behave much like electrons. But the difference is that, unlike electrons, quasi-particles do not undergo collisions. They are a little heavier than an electron, and that is all. This description has very important implications for superconductivity itself and should, eventually, provide new ways towards superconductivity at room temperature.

Illustration of the pseudogap phase of the high Tc cuprates. The green hilly landscape represents the energy (vertical direction) as a function of momentum in the CuO2 planes (planar dimensions). The Fermi-liquid condenses in the lowest energy states of this landscape, and forms lakes. The finite size and filling of these lakes is a manifestation of the Fermi-liquid character of the condensate. The periodicity of the energy-momentum landscape is indicated by the yellow markers on the hilltops. Each of those is surrounded by 4 lakes, which have become disconnected from each other by an energy-barrier representative of the pseudogap. Illustration courtesy of Damien Stricker.

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