Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:

 

Quantum magnets moving along

13.03.2013
LMU/MPQ-team of scientists observes coherent propagation of a single spin impurity in a chain of ultracold atoms.

Many discoveries in physics came as a big surprise – for example the phenomenon, that some materials loose almost all their electrical resistance at low temperatures, or that others become superconductors at unexpectedly high temperatures.


Figure 1: Illustration of the propagation of the spin impurity (red) through a chain of atoms with initially opposite spin.

Graphics: LMU, Quantum Many Body Systems Division

In the past it was mainly due to theoreticians to develop models explaining these unusual properties. Unfortunately it is not possible to have a direct look into a solid state crystal and follow up the motion of charge carriers as this process happens at extremely short time and length scales.

A team around Professor Immanuel Bloch (Chair for Experimental Physics at the Ludwig-Maximilians-Universität Munich and Director at MPQ) has now observed the coherent propagation of single spin excitations in an ultracold quantum gas of strongly correlated atoms (Nature Physics, Advance Online Publication, 24 February 2013). This is one of the most fundamental processes in the magnetism of quantum systems.

In close collaboration with theoretical physicists from the Ludwig-Maximilians-Universität Munich and the University of Geneva the scientists were able to demonstrate that the propagation of the spin wave in less strongly correlated systems is being slowed down by the emergence of quasi-particles, so-called polarons.

Properties of condensed matter such as magnetism, electrical conductivity, or superconductivity are the result of the behaviour of electrons in the periodic crystal of the solid. In this respect, the intrinsic angular momentum, i.e. the spin of the electrons, is playing a key role. For example, the high-temperature conductivity exhibited by a class of cuprates is thought to go back to the spin coupling of strongly correlated electrons. Ultracold atoms in an optical lattice are ideally suited to investigate such quantum magnetic phenomena under controlled experimental conditions.

The experiment starts with cooling rubidium atoms down to temperatures near absolute zero. The ensemble is then kept in a light field which divides it into several parallel one-dimensional tubes along which the atoms are allowed to move. Now the tubes are superimposed with yet another light field, a standing laser light wave. By the periodic sequence of dark and bright areas an optical lattice builds up in which each site is occupied with exactly one atom fixed to its position. This highly ordered state is called a Mott insulator (named after the British physicist Sir Neville Mott). After all, an array of several chains of atoms each containing around 15 atoms is formed.

The atoms in the optical lattice take the role of the electrons in a solid state crystal. They are in a similar way characterized by an intrinsic angular momentum (a spin). However, in this case the scientists have control over the spins which can – as if they were little magnetic needles – align in two opposite directions. In the beginning, all spins are pointing into the same direction. Then, one single atom in the centre of each chain is picked out by a laser beam, and its spin is flipped by irradiating microwave pulses. Afterwards the motion of this deterministically generated spin impurity through the chain is followed up (see figure 1).

An imaging technique developed in the group makes it possible to visualize each atom on its particular lattice site with very high resolution. Using this method the position of the spin impurity can be precisely determined for various evolution times. This measurement is performed on all atomic chains at the same time. The emerging distributions exhibit a structure that is characteristic of an interference pattern, as it is expected from the interference of coherent waves. “Our model describes the process of spin propagation by a mechanism called ‘correlated super exchange’,” Dr. Christian Groß explains, scientist at the experiment. “The same instance the spin impurity moves one site to the right the neighbouring atom takes its place. As this exchange takes place in the opposite direction at the same time and with the same probability the observed interference pattern results. If the system was a classic one only a broadening of the distribution would have been observed over time. Thereby we have proved that the spin wave propagates coherently.”

In the insulating Mott phase the barriers between the lattice sites are very high, and the atoms are tightly bound to their position, except for the case of the correlated super exchange mentioned above. When the height of the barrier, i.e. the intensity of the laser beams, is lowered below a certain threshold, the atoms are allowed by the rules of quantum mechanics to “tunnel” through the barrier and reach a neighbouring site. In this ‘superfluid phase’ the mobility of the atoms is enhanced, however, the motion of the impurity gets slowed down, as was demonstrated in the measurement. “The tunnelling happening everywhere in the lattice increases the complexity of the interaction of the spin impurity with the background atoms,” Dr. Takeshi Fukuhara points out, who works on the experiment as a postdoctoral researcher. “In the end, the interaction is repulsive, creating a hole in the distribution of the background atoms.” On its way through the chain the spin impurity has to drag this hole all along, that way getting kind of heavy. “This is quite similar to passing a crowd on a subway station: it will take a long time since one has to create the necessary space on each step,” Fukuhara says. “The motion of the impurity observed in our experiment is in good agreement with the forming of quasi particles in the lattice, so-called polarons, as they are known from solid state physics.”
The results obtained in this series of measurements are of high interest: on the one hand, the experiments demonstrate the outstanding control of ultracold quantum systems that can be achieved at present. This is a precondition for the simulation of collective solid state excitations, which give, for example, rise to quantum magnetic phenomena. On the other hand the measurements give a direct insight into the propagation of charge carriers and impurities in solid state crystals, which in the end determine the macroscopic properties of materials.
Olivia Meyer-Streng

Original publication:
Takeshi Fukuhara, Adrian Kantian, Manuel Endres, Marc Cheneau, Peter Schauß,
Sebastian Hild, David Bellem, Ulrich Schollwöck, Thierry Giamarchi, Christian Groß, Immanuel Bloch, and Stefan Kuhr
Quantum dynamics of a mobile spin impurity
Nature Physics, Advance Online Publication, 24 February 2013

Contact:
Prof. Dr. Immanuel Bloch
Chair of Quantum Optics, LMU Munich
Schellingstr. 4, 80799 München, and
Max-Planck-Institute of Quantum Optics
Hans-Kopfermann-Straße 1
85748 Garching b. München
Phone: +49 (0) 89 / 32 905 -138
E-mail: immanuel.bloch@mpq.mpg.de

Dr. Christian Groß
Max-Planck-Institute of Quantum Optics
Phone: +49 (0) 89 / 32 905 -713
E-mail: christian.gross@mpq.mpg.de

Dr. Takeshi Fukuhara
Max-Planck-Institute of Quantum Optics
Phone: +49 (0) 89 / 32 905 -677
E-mail: takeshi.fukuhara@mpq.mpg.de

Prof. Dr. Stefan Kuhr
University of Strathclyde
Department of Physics
107 Rottenrow East
Glasgow, U.K.
G4 0NG
Phone: +44 141 548 3364
E-mail: stefan.kuhr@strath.ac.uk

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

More articles from Physics and Astronomy:

nachricht Molecule flash mob
19.01.2017 | Technische Universität Wien

nachricht Magnetic moment of a single antiproton determined with greatest precision ever
19.01.2017 | Johannes Gutenberg-Universität Mainz

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: Traffic jam in empty space

New success for Konstanz physicists in studying the quantum vacuum

An important step towards a completely new experimental access to quantum physics has been made at University of Konstanz. The team of scientists headed by...

Im Focus: How gut bacteria can make us ill

HZI researchers decipher infection mechanisms of Yersinia and immune responses of the host

Yersiniae cause severe intestinal infections. Studies using Yersinia pseudotuberculosis as a model organism aim to elucidate the infection mechanisms of these...

Im Focus: Interfacial Superconductivity: Magnetic and superconducting order revealed simultaneously

Researchers from the University of Hamburg in Germany, in collaboration with colleagues from the University of Aarhus in Denmark, have synthesized a new superconducting material by growing a few layers of an antiferromagnetic transition-metal chalcogenide on a bismuth-based topological insulator, both being non-superconducting materials.

While superconductivity and magnetism are generally believed to be mutually exclusive, surprisingly, in this new material, superconducting correlations...

Im Focus: Studying fundamental particles in materials

Laser-driving of semimetals allows creating novel quasiparticle states within condensed matter systems and switching between different states on ultrafast time scales

Studying properties of fundamental particles in condensed matter systems is a promising approach to quantum field theory. Quasiparticles offer the opportunity...

Im Focus: Designing Architecture with Solar Building Envelopes

Among the general public, solar thermal energy is currently associated with dark blue, rectangular collectors on building roofs. Technologies are needed for aesthetically high quality architecture which offer the architect more room for manoeuvre when it comes to low- and plus-energy buildings. With the “ArKol” project, researchers at Fraunhofer ISE together with partners are currently developing two façade collectors for solar thermal energy generation, which permit a high degree of design flexibility: a strip collector for opaque façade sections and a solar thermal blind for transparent sections. The current state of the two developments will be presented at the BAU 2017 trade fair.

As part of the “ArKol – development of architecturally highly integrated façade collectors with heat pipes” project, Fraunhofer ISE together with its partners...

All Focus news of the innovation-report >>>

Anzeige

Anzeige

Event News

Sustainable Water use in Agriculture in Eastern Europe and Central Asia

19.01.2017 | Event News

12V, 48V, high-voltage – trends in E/E automotive architecture

10.01.2017 | Event News

2nd Conference on Non-Textual Information on 10 and 11 May 2017 in Hannover

09.01.2017 | Event News

 
Latest News

New Study Will Help Find the Best Locations for Thermal Power Stations in Iceland

19.01.2017 | Earth Sciences

Not of Divided Mind

19.01.2017 | Life Sciences

Molecule flash mob

19.01.2017 | Physics and Astronomy

VideoLinks
B2B-VideoLinks
More VideoLinks >>>