In a study exploring the coupling between heat and particle currents in a gas of strongly interacting atoms, physicists at ETH Zurich find puzzling behaviours
From everyday experience we know that metals are good conductors for both electricity and heat -- think inductive cooking or electronic devices warming up upon intense use. That intimate link of heat and electrical transport is no coincidence. In typical metals both sorts of conductivity arise from the flow of 'free' electrons, which move like a gas of independent particles through the material.
But when fermionic carriers such as electrons interact with one another, then unexpected phenomena can arise, as Dominik Husmann, Laura Corman and colleagues in the group of Tilman Esslinger in the Department of Physics at ETH Zurich -- in collaboration with Jean-Philippe Brantut at the École Polytechnique Fédérale de Lausanne (EPFL) -- report in a paper published this week in the journal Proceedings of the National Academy of Sciences.
Studying heat and particle conduction in a systems of strongly interacting fermionic atoms they found a range of puzzling behaviours that set this system apart from known systems in which the two forms of transport are coupled.
In metals, the connection of thermal and electrical conductivity is described by the Wiedemann-Franz law, which has first been formulated in 1853. In its modern form the law states that at a fixed temperature, the ratio between the two types of conductivity is constant. The value of that ratio is quite universal, being the same for a remarkably wide range of metals and conditions.
That universality breaks down, however, when the carriers interact with one another. This has been observed in a handful of exotic metals hosting strongly correlated electrons. But Husmann, Corman and their co-workers have now explored the phenomenon in a system in which they had exquisite control over all relevant parameters, enabling them to monitor particle and heat transport in unprecedented detail.
The carriers in their experiments are fermionic lithium atoms, which they cooled to sub-microkelvin temperatures and trapped using laser beams. Initially, they confined a few hundred thousand of these atoms to two independent reservoirs that can be heated individually. Once a temperature difference between the two reservoirs had been established, they opened a tiny restriction between them -- a so-called quantum point contact -- thus initiating transport of particles and heat (see the figure).
The transport channel is defined and controlled using laser light as well. The experiment therefore provides an extraordinarily clean platform for studying fermionic transport. For example, in real materials, the lattice through which the electrons flow starts to melt at high temperatures. In contrast, in the cold-atom setup, with the structures defined by light, no such 'lattice heating' occurs, making it possible to focus on the carriers themselves.
When Husmann et al. determined the ratio between thermal and particle conductivity in their system, they found it to be an order of magnitude below the predictions of the Wiedemann-Franz law. This deviation indicates a separation of the mechanisms responsible for particle and heat currents, in contrast to the situation so universally observed for free carriers. As a result, their system evolved into a state in which heat and particle currents vanished long before an equilibrium between the two reservoirs in terms of temperature and particle number has been reached.
Moreover, another measure for thermoelectric behaviour, the Seebeck coefficient, was found to have a value close to that expected for a non-interacting Fermi gas. This is puzzling, because in some regions of the channel the strongly interacting atoms were in the superfluid regime (in which a gas or liquid flows without viscosity) and in the prototypical superfluid, helium-4, the Seebeck coefficient is zero. This discrepancy signals a different thermoelectric character for the fermionic gas studied by the ETH team.
These findings therefore pose new challenges for microscopic modelling of strongly interacting fermion systems. At the same time, the platform established with these experiments could help to explore novel concepts for thermoelectric devices, such as coolers and engines that are based on interconverting temperature differences into particle flow, and vice versa.
Andreas Trabesinger | EurekAlert!
Exotic spiraling electrons discovered by physicists
19.02.2019 | Rutgers University
Astronomers publish new sky map detecting hundreds of thousands of previously unknown galaxies
19.02.2019 | Universität Bielefeld
Up to now, OLEDs have been used exclusively as a novel lighting technology for use in luminaires and lamps. However, flexible organic technology can offer much more: as an active lighting surface, it can be combined with a wide variety of materials, not just to modify but to revolutionize the functionality and design of countless existing products. To exemplify this, the Fraunhofer FEP together with the company EMDE development of light GmbH will be presenting hybrid flexible OLEDs integrated into textile designs within the EU-funded project PI-SCALE for the first time at LOPEC (March 19-21, 2019 in Munich, Germany) as examples of some of the many possible applications.
The Fraunhofer FEP, a provider of research and development services in the field of organic electronics, has long been involved in the development of...
For the first time, an international team of scientists based in Regensburg, Germany, has recorded the orbitals of single molecules in different charge states in a novel type of microscopy. The research findings are published under the title “Mapping orbital changes upon electron transfer with tunneling microscopy on insulators” in the prestigious journal “Nature”.
The building blocks of matter surrounding us are atoms and molecules. The properties of that matter, however, are often not set by these building blocks...
Scientists at the University of Konstanz identify fierce competition between the human immune system and bacterial pathogens
Cell biologists from the University of Konstanz shed light on a recent evolutionary process in the human immune system and publish their findings in the...
Laser physicists have taken snapshots of carbon molecules C₆₀ showing how they transform in intense infrared light
When carbon molecules C₆₀ are exposed to an intense infrared light, they change their ball-like structure to a more elongated version. This has now been...
The so-called Abelian sandpile model has been studied by scientists for more than 30 years to better understand a physical phenomenon called self-organized...
11.02.2019 | Event News
30.01.2019 | Event News
16.01.2019 | Event News
21.02.2019 | Earth Sciences
21.02.2019 | Trade Fair News
21.02.2019 | Life Sciences