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Významné výsledky dosiahnuté na FÚ SAV

Global Λ hyperon polarization in nuclear collisions

The extreme energy densities generated by ultra-relativistic collisions between heavy atomic nuclei produce a state of matter that behaves surprisingly like a fluid, with exceptionally high temperature and low viscosity. Non-central collisions have angular momenta of the order of 1,000ћ, and the resulting fluid may have a strong vortical structure that must be understood to describe the fluid properly. The vortical structure is also of particular interest because the restoration of fundamental symmetries of quantum chromodynamics is expected to produce novel physical effects in the presence of strong vorticity. However, no experimental indications of fluid vorticity in heavy ion collisions have yet been found. Since vorticity represents a local rotational structure of the fluid, spin–orbit coupling can lead to preferential orientation of particle spins along the direction of rotation. Here we present measurements of an alignment between the global angular momentum of a non-central collision and the spin of emitted particles (in this case the collision occurs between gold nuclei and produces Λ baryons), revealing that the fluid produced in heavy ion collisions is the most vortical system so far observed. (At high energies, this fluid is a quark–gluon plasma.) We find that Λ and anti-Λ hyperons show a positive polarization of the order of a few per cent, consistent with some hydrodynamic predictions. (A hyperon is a particle composed of three quarks, at least one of which is a strange quark; the remainder are up and down quarks, found in protons and neutrons.) A previous measurement that reported a null result, that is, zero polarization, at higher collision energies is seen to be consistent with the trend of our observations, though with larger statistical uncertainties. These data provide experimental access to the vortical structure of the nearly ideal liquid created in a heavy ion collision and should prove valuable in the development of hydrodynamic models that quantitatively connect observations to the theory of the strong force.

Spoluator článku: Mgr. Peter Filip, PhD.
Zdroj: Nature Letters

Direct measurement of individual phonon lifetimes in the clathrate compound Ba7.81Ge40.67Au5.33

Engineering lattice thermal conductivity requires to control the heat carried by atomic vibration waves, the phonons. The key parameter for quantifying it is the phonon lifetime, limiting the travelling distance, whose determination is however at the limits of instrumental capabilities. Here, we show the achievement of a direct quantitative measurement of phonon lifetimes in a single crystal of the clathrate Ba7.81Ge40.67Au5.33, renowned for its puzzling ‘glass-like’ thermal conductivity. Surprisingly, thermal transport is dominated by acoustic phonons with long lifetimes, travelling over distances of 10 to 100 nm as their wave-vector goes from 0.3 to 0.1 Å−1. Considering only low-energy acoustic phonons, and their observed lifetime, leads to a calculated thermal conductivity very close to the experimental one. Our results challenge the current picture of thermal transport in clathrates, underlining the inability of state-of-the-art simulations to reproduce the experimental data, thus representing a crucial experimental input for theoretical developments.

Spoluator článku: RNDr. Marek Mihalkovič, CSc.
Zdroj: Nature Communications

Nová metóda zmerania vektorových veličín v nanosvete

Medzinárodný tím, v ktorom pôsobili aj výskumníci z Fyzikálneho ústavu SAV, prof. Ivan Štich, Robert Turanský, Ján Brndiar, zverejnili svoj nový výskum v magazíne Nature Physics. Ako vôbec prví dokázali odmerať vektorové veličiny v nanosvete pomocou upravenej metódy bezkontaktnej silovej mikroskópie. Výsledkom je nová metóda, ktorá umožňuje zmerať presnejšie vektorové veličiny na subatomárnej úrovni.

Schopnosť merať  vektorové veličiny na subatomárnej škále je pritom dôležitá, ak chceme nanomateriálom lepšie porozumieť a aplikovať ich. Predpokladá sa, že metóda otvorí nové okno pre materiály na subatomárnej škále, umožní lepšie pochopenie morfológie povrchov, ich chemického zloženia, chemických reakcií na ich povrchoch, prispeje k zlepšeniu techník atomárnej a molekulárnej manipulácie a k lepšiemu pochopeniu správania sa nanostrojov.

Vo fyzike sú tri rôzne typy veličín: takzvané skalárne, ktoré sú jednoduchými číslami, vektorové veličiny, ktoré majú smer – ako napríklad sila či magnetický moment a tenzorové veličiny, ktoré sú zovšeobecnením vektorov. Práve fyzikom sa teraz podarilo ukázať, ako ich na subatomárnej úrovni nielen odhadovať. Pritom práve sledovanie týchto vektorových veličín je dôležité, ak chceme nanomateriálom lepšie porozumieť a potom ich používať.

Spoluautori článku: prof. Ing. Ivan Štich, DrSc., RNDr. Robert Turanský, PhD., Mgr. Ján Brndiar PhD.
Zdroj: tech.sme.skNature Physics

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