Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:

 

A Traffic Jam of Quantum Particles

05.03.2012
LMU/MPQ-scientists discover surprising transport phenomena in ultracold quantum many body systems.

Transport properties such as thermal or electrical conductivity are of great importance for technical applications of materials. In particular the electrical conductivity stems from the behaviour of the electrons in the solid and is very difficult to predict. This is true especially in the case of strongly correlated electrons, when the position and the dynamics of each single electron is strongly influenced by the behaviour of all other electrons.


Figure 1: A system of fermionic atoms in an optical lattice (top) is brought out of equilibrium and exhibits different dynamics for non-interacting (left) and interacting atoms (right). Grafik: MPQ

Ultracold atoms in optical lattices can be used as model systems that allow the study of analogues processes in a clean and well controlled environment where all relevant parameters can be manipulated by external lasers and magnetic fields. Scientists in the group of Professor Immanuel Bloch (Ludwig-Maximilians-Universität Munich and Max Planck Institute of Quantum Optics, Garching) in collaboration with the theory group of Prof. Achim Rosch (University of Cologne) have now demonstrated that the dynamics of a system of ultracold potassium atoms, trapped in an optical lattice, depend surprisingly strongly on the particle interaction strength (Nature Physics 8, 213-218 (2012), DOI: 10.1038/NPHYS2205). Investigations of this kind give new insights into properties like electrical conductivity, superconductivity or magnetism, and may help to develop materials with ‘tailored’ properties.

So-called optical lattices are generated by superimposing several laser beams. The resulting periodic structure of light resembles the geometry of simple solid state crystals. In fact, atoms trapped in such an artificial lattice, at a temperature of a few nano-Kelvin above absolute zero, experience forces similar to the ones that act on electrons in solid state systems. However, concerning their dynamics, only fermionic atoms behave exactly the same way as electrons, which are fermions as well. These particles have to differ in at least one quantum property if they happen to be at the same place at the same time. Bosonic particles, on the other hand, prefer to gather in exactly the same quantum state.

In the experiment, atoms of the fermionic isotope potassium-40 are cooled down to an extremely low temperature with the help of laser beams and magnetic fields. Then they are loaded into an optical lattice as described above. Initially, the edges of the egg carton-like lattice structure are bent upwards (see figure 1, the colours red and green represent different spin states of the atoms) and the particles sit in the centre with a constant density distribution. Subsequently, the external confining field – responsible for the upwards bending of the lattice – is suddenly eliminated. The egg carton becomes flat and the particle cloud starts to expand. Now the physicists monitor exactly how the density distribution changes during the expansion.
An important feature of this experimental setup is the use of a so-called Feshbach resonance, which makes it possible to change the interaction between the atoms by magnetic fields almost at will. This holds for the sign – attractive or repulsive – as well as for the strength of the interaction. In fact, the interaction can be switched off completely. In this case the atoms don’t ‘see’ each other. They move through the lattice unhindered, and their velocity depends on the lattice depth only. During this free expansion, the symmetry of the cloud changes from the spherical initial density distribution to a square symmetry that is governed by the symmetry of the lattice (figure 1, left).

As soon as there are small interactions present the atoms collide and ‘hinder’ each other, such that the expansion velocity of the cloud decreases. For larger interactions, more and more atoms ‘remain stuck’ in the core of the cloud, which remains spherical. For very strong interactions the dynamics of the high density core change qualitatively: the essentially frozen core dissolves by emitting particles and therefore shrinks in size, similarly to a melting ice cube (figure 1, right).
Surprisingly, only the magnitude, but not the sign of the interaction matters. The observed dynamics of the expansion is identical for repulsive and attractive interactions, as long as they are of the same strength. “This symmetry between attractive and repulsive interaction is an interesting feature of these lattice systems,” Dr. Ulrich Schneider, project leader at this experiment, explains. “In free space, interactions with opposite signs would give rise to opposite effects. Here they can lead to a quantum mechanical entanglement of distant atoms and allow the generation of either ‘normally’ or ‘repulsively’ bound particle pairs.”

Former experiments with fermionic atoms in optical lattices focused on the properties of systems in equilibrium. Here, on the contrary, the scientists observe the dynamics of the atoms in an out-of equilibrium system. These measurements are an important step towards a better understanding of the electronic motion in condensed matter. The physicists hope that this knowledge will lead to an explanation of complex phenomena in solid state physics and material science, and consequently to new tailored materials. [Olivia Meyer-Steng]

Original publication:
Ulrich Schneider, Lucia Hackermüller, Jens Philipp Ronzheimer, Sebastian Will, Simon Braun, Thorsten Best, Immanuel Bloch, Eugene Demler, Stephan Mandt, David Rasch and Achim Rosch

Fermionic transport and out-of-equilibrium dynamics in a homogeneous Hubbard model with ultracold atoms
Nature Physics 8,213-218 (2012), DOI: 10.1038/NPHYS2205 (AOP, 15 January 2012)

Contact:
Prof. Dr. Immanuel Bloch
Chair of Quantum Optics
LMU Munich, Schellingstr. 4
80799 München, Germany, and
Max Planck Institute of Quantum Optics
Hans-Kopfermann-Straße 1
85748 Garching b. München
Phone: +49 89 / 32905 -138
E-mail: immanuel.bloch@mpq.mpg.de
Dr. Ulrich Schneider
Fakultät für Physik
LMU Munich, Schellingstr. 4
80799 München, Germany,
Phone: +49 89 / 2180 -6129
E-mail: ulrich.schneider@lmu.de
Dr. Olivia Meyer-Streng
Press & Public Relations
Max Planck Institute of Quantum Optics
Press & Public Relations
Phone: +49 89 / 32905 -213
E-mail: olivia.meyer-streng@mpq.mpg.de

Dr. Olivia Meyer-Streng | Max-Planck-Institut
Further information:
http://www.quantum-munich.de

More articles from Physics and Astronomy:

nachricht New manifestation of magnetic monopoles discovered
08.12.2017 | Institute of Science and Technology Austria

nachricht NASA's SuperTIGER balloon flies again to study heavy cosmic particles
07.12.2017 | NASA/Goddard Space Flight Center

All articles from Physics and Astronomy >>>

The most recent press releases about innovation >>>

Die letzten 5 Focus-News des innovations-reports im Überblick:

Im Focus: Towards data storage at the single molecule level

The miniaturization of the current technology of storage media is hindered by fundamental limits of quantum mechanics. A new approach consists in using so-called spin-crossover molecules as the smallest possible storage unit. Similar to normal hard drives, these special molecules can save information via their magnetic state. A research team from Kiel University has now managed to successfully place a new class of spin-crossover molecules onto a surface and to improve the molecule’s storage capacity. The storage density of conventional hard drives could therefore theoretically be increased by more than one hundred fold. The study has been published in the scientific journal Nano Letters.

Over the past few years, the building blocks of storage media have gotten ever smaller. But further miniaturization of the current technology is hindered by...

Im Focus: Successful Mechanical Testing of Nanowires

With innovative experiments, researchers at the Helmholtz-Zentrums Geesthacht and the Technical University Hamburg unravel why tiny metallic structures are extremely strong

Light-weight and simultaneously strong – porous metallic nanomaterials promise interesting applications as, for instance, for future aeroplanes with enhanced...

Im Focus: Virtual Reality for Bacteria

An interdisciplinary group of researchers interfaced individual bacteria with a computer to build a hybrid bio-digital circuit - Study published in Nature Communications

Scientists at the Institute of Science and Technology Austria (IST Austria) have managed to control the behavior of individual bacteria by connecting them to a...

Im Focus: A space-time sensor for light-matter interactions

Physicists in the Laboratory for Attosecond Physics (run jointly by LMU Munich and the Max Planck Institute for Quantum Optics) have developed an attosecond electron microscope that allows them to visualize the dispersion of light in time and space, and observe the motions of electrons in atoms.

The most basic of all physical interactions in nature is that between light and matter. This interaction takes place in attosecond times (i.e. billionths of a...

Im Focus: A transistor of graphene nanoribbons

Transistors based on carbon nanostructures: what sounds like a futuristic dream could be reality in just a few years' time. An international research team working with Empa has now succeeded in producing nanotransistors from graphene ribbons that are only a few atoms wide, as reported in the current issue of the trade journal "Nature Communications."

Graphene ribbons that are only a few atoms wide, so-called graphene nanoribbons, have special electrical properties that make them promising candidates for the...

All Focus news of the innovation-report >>>

Anzeige

Anzeige

Event News

Innovative strategies to tackle parasitic worms

08.12.2017 | Event News

AKL’18: The opportunities and challenges of digitalization in the laser industry

07.12.2017 | Event News

Blockchain is becoming more important in the energy market

05.12.2017 | Event News

 
Latest News

Making fuel out of thick air

08.12.2017 | Life Sciences

Rules for superconductivity mirrored in 'excitonic insulator'

08.12.2017 | Information Technology

Smartphone case offers blood glucose monitoring on the go

08.12.2017 | Information Technology

VideoLinks
B2B-VideoLinks
More VideoLinks >>>