Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:

 

Elementary magnets coming in double-packs

26.09.2013
MPQ/LMU scientists directly observe bound states of elementary magnets in ferromagnetic quantum crystals predicted more than 80 years ago

Simulating solid state properties with precisely controlled quantum systems is an important goal of the Quantum Many-Body Systems Division at MPQ.


Fig. 1: Illustration of how the magnons (red) are allowed to propagate in a chain of regularly aligned atoms (blue). Upper left: free magnons move in an anti-correlated fashion. Lower right: the two spins of the compound state stay together throughout their walk.

Graphic: MPQ, Quantum Many-Body Systems Division

Now the team around Professor Immanuel Bloch (Chair for Experimental Physics at the Ludwig-Maximilians-Universität Munich and Director at MPQ) has come again a step closer to it – to be precise, to the understanding of processes in ferromagnetic solid state crystals in which elementary excitations, so-called magnons, can emerge.

About 80 years ago the German physicist Hans Bethe deduced from a theoretical model that in one-dimensional ferromagnets two of those elementary magnetic excitations can form a bound state. Like two tiny bar magnets, two atoms can stick together and form a new particle that propagates in the crystal. The MPQ team has now succeeded to observe these most elementary mobile magnetic domains, the two-magnon states, directly and to resolve their dynamics with time-resolved measurements (Nature, AOP 25 September 2013). This study complements conventional spectroscopy in solid state crystals which yields information on momentum and frequency of the magnetic excitations. Bound states of excitations can influence the thermal conductance properties of low-dimensional ferromagnets or the propagation speed of quantum information in magnetic wires.

The magnetic properties of certain materials have been known for centuries, and they have been used for a variety of applications almost quite as long. Today, this behaviour is very well understood at the microscopic level: magnetism goes back to the angular momentum – the spin – of the electrons in the outer shell of the atoms that build up a solid state crystal. In ferromagnetic matter these spins are aligned within the so-called “Weiss” domains.

Similar conditions can be prepared in quantum crystals if the atoms have a defined spin. To this end a cloud of extremely cold (i.e. extremely slow) rubidium atoms is loaded into an optical lattice, generated by crosswise superposition of standing laser waves. The periodically arranged bright patches of the lattice are the sites on which the atoms are allowed to settle down. The dark areas act as barriers, the height of which is determined by the intensity of the laser light. As long as the lattice is comparably shallow, the atoms are able to “tunnel” through the barrier to their neighbouring site. In the experiment described here, the scientists restrict their motion to one direction. As a result the atoms are arranged in ten decoupled one-dimensional “tubes” of more or less the same length.

In the beginning the laser intensity is rather high, and the quantum crystal forms a so-called “Mott-insulator”: each lattice site is occupied with exactly one atom, fixed in space. The atoms are thereby lined up like pearls on a chain with all spins pointing in the same direction – in other words the chain represents a perfect ferromagnetic quantum magnet. A couple of months ago, in a similar experimental set-up, the Bloch group were able to change the spin-direction of one individual atom and to track the propagation of the resulting spin wave throughout the quantum magnet.

Now they went one step further: by precise addressing and application of microwave pulses they flipped the spins of two neighbouring atoms in the centre of each fully magnetized atomic chain. The two spin excitations can propagate as individual, free magnons along the quantum magnet. But due to the ferromagnetic interaction between them, they can also stick together forming an elementary mobile magnetic domain with about the same probability.

Two technical achievements make it possible to both observe the bound state directly and to track its dynamical evolution. On the one hand the group has developed a method of imaging individual atoms on their individual lattice sites. With a special kind of a microscope the scientists can distinguish atoms with a flipped spin from the unflipped ones. On the other hand, they can trigger precisely at which moment the particles start to move. This is done by lowering the lattice height, which enhances the magnetic interactions between the atoms. When the lattice height is raised again after a certain time interval, the magnetic interaction is turned off again and the spin orientation of the atoms is “frozen out”. By taking snapshots of the actual spin patterns for a series of different evolution times the propagation of the ferromagnetic excitations can be measured with high resolution in space and time.

“With this method we can unambiguously separate states with bound magnons from those with free excitations,” Dr. Christian Groß, project leader at the experiment, explains. “In the first case, the two adjacent flipped spins stay together on their walk along the quantum magnet. In the second, the free magnons show anti-bunching in their motion. This means, if one spin excitation has travelled three lattice sites to the right the other one has moved the same distance in the opposite direction.

Bound states usually don’t hold forever – this is as valid in the quantum world as it is in real life. “According to our observation the number of magnon pairs decreases for long observation times,” Dr. Takeshi Fukuhara points out. “One possible reason for this is that the fragile spin waves couple to some extent to thermal excitations. At present we are working on a model which will describe the decay of bound states both quantitatively and qualitatively.”

The observation of elementary quantum magnetic phenomena helps to understand fundamental magnetic processes in solid state crystals. But the investigation of the dynamics of spin waves in one-dimensional quantum magnets is also relevant for potential applications. For example, the transport of quantum information along a chain of qubits can be strongly influenced by the formation of bound two-magnon states. Olivia Meyer-Streng

Original publication:
Takeshi Fukuhara, Peter Schauß, Manuel Endres, Sebastian Hild, Marc Cheneau, Immanuel Bloch, and Christian Groß

Microscopic observation of magnon bound states and their dynamics Nature, AOP 25 September 2013, DOI: 10.1038/nature12541

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

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

More articles from Physics and Astronomy:

nachricht Applicability of dynamic facilitation theory to binary hard disk systems
08.12.2016 | Nagoya Institute of Technology

nachricht Will Earth still exist 5 billion years from now?
08.12.2016 | KU Leuven

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: Significantly more productivity in USP lasers

In recent years, lasers with ultrashort pulses (USP) down to the femtosecond range have become established on an industrial scale. They could advance some applications with the much-lauded “cold ablation” – if that meant they would then achieve more throughput. A new generation of process engineering that will address this issue in particular will be discussed at the “4th UKP Workshop – Ultrafast Laser Technology” in April 2017.

Even back in the 1990s, scientists were comparing materials processing with nanosecond, picosecond and femtosesecond pulses. The result was surprising:...

Im Focus: Shape matters when light meets atom

Mapping the interaction of a single atom with a single photon may inform design of quantum devices

Have you ever wondered how you see the world? Vision is about photons of light, which are packets of energy, interacting with the atoms or molecules in what...

Im Focus: Novel silicon etching technique crafts 3-D gradient refractive index micro-optics

A multi-institutional research collaboration has created a novel approach for fabricating three-dimensional micro-optics through the shape-defined formation of porous silicon (PSi), with broad impacts in integrated optoelectronics, imaging, and photovoltaics.

Working with colleagues at Stanford and The Dow Chemical Company, researchers at the University of Illinois at Urbana-Champaign fabricated 3-D birefringent...

Im Focus: Quantum Particles Form Droplets

In experiments with magnetic atoms conducted at extremely low temperatures, scientists have demonstrated a unique phase of matter: The atoms form a new type of quantum liquid or quantum droplet state. These so called quantum droplets may preserve their form in absence of external confinement because of quantum effects. The joint team of experimental physicists from Innsbruck and theoretical physicists from Hannover report on their findings in the journal Physical Review X.

“Our Quantum droplets are in the gas phase but they still drop like a rock,” explains experimental physicist Francesca Ferlaino when talking about the...

Im Focus: MADMAX: Max Planck Institute for Physics takes up axion research

The Max Planck Institute for Physics (MPP) is opening up a new research field. A workshop from November 21 - 22, 2016 will mark the start of activities for an innovative axion experiment. Axions are still only purely hypothetical particles. Their detection could solve two fundamental problems in particle physics: What dark matter consists of and why it has not yet been possible to directly observe a CP violation for the strong interaction.

The “MADMAX” project is the MPP’s commitment to axion research. Axions are so far only a theoretical prediction and are difficult to detect: on the one hand,...

All Focus news of the innovation-report >>>

Anzeige

Anzeige

Event News

ICTM Conference 2017: Production technology for turbomachine manufacturing of the future

16.11.2016 | Event News

Innovation Day Laser Technology – Laser Additive Manufacturing

01.11.2016 | Event News

#IC2S2: When Social Science meets Computer Science - GESIS will host the IC2S2 conference 2017

14.10.2016 | Event News

 
Latest News

Closing the carbon loop

08.12.2016 | Life Sciences

Applicability of dynamic facilitation theory to binary hard disk systems

08.12.2016 | Physics and Astronomy

Scientists track chemical and structural evolution of catalytic nanoparticles in 3-D

08.12.2016 | Materials Sciences

VideoLinks
B2B-VideoLinks
More VideoLinks >>>