Penetrating the quantum nature of magnetism

Jun 17, 2013
Schematic representation of the magnetic excitations in a spin-1/2 (Heisenberg) antiferromagnetic chain and overview of the neutron scattering results for CuSO4- 5D2O.

Antiferromagnets are materials that lose their apparent magnetic properties when cooled down close to absolute zero temperature. Different to conventional magnets, which can be described with classical physics even at the atomic level, antiferromagnets such as copper sulfate are quantum systems where electrons behave in a complex cooperative manner. They belong to the field of quantum many-body physics, a branch of science that studies the collective behavior of vast assemblies of interacting particles. Thus far, only very few exact solutions to quantum many-body problems are known and even fewer have been realized experimentally.

In a recent publication in Nature Physics, scientists from EPFL's Laboratory for Quantum Magnetism (LQM) have together with collaborators from Institut Laue Langevin and the University of Amsterdam have measured the collective quantum-mechanical magnetism in crystals of copper sulfate and show how it relates to properties of sub-electron quasiparticles known as spinons.

Copper sulfate pentahydrate is a commonplace material that is often used to keep swimming pools clear of algae. Because it easily grows as large crystals, it is also very popular in schools' chemistry classes where many students across the world have been inspired by their beautiful and intense blue color. But copper sulfate has some other fascinating properties that most of us might not realize.

When cooled down close to absolute zero temperature, this material hosts a fascinating state of matter called a "quantum spin-liquid". Magnetism essentially originates from a material's electrons spins, sometimes referred to as moments. When conventional magnets (such as ferromagnets) are cooled down to low temperatures, their electrons' spins align together in a simple static pattern. In contrast, in a quantum spin-liquid state like discovered in copper sulfate, the electrons' spins are interrupted from lining up by quantum fluctuations. This causes the spins to constantly change direction as the molecules floating in a liquid. Despite of this constant change in direction, they remain correlated over long distances, a property that is known as quantum entanglement and which is the key property scientist's hope will lead to future quantum computers.

The LQM scientists, led by Henrik M. Rønnow, cooled down a copper sulfate crystal close to absolute zero (about 0.01 K) to turn it into a quantum spin liquid and then used inelastic neutron scattering to investigate the motion of electrons' spins. The experiments reveal that the magnetic properties of can no longer be described by the individual behavior of the magnetic moments carried by each individual electron in the sample. Instead, flipping the magnetic moment of one single electron creates two spatially separated quantum objects called spinons.

The accuracy of the experiments made it possible to detect not only such spinon-pairs but even splitting (also called fractionalization) into four-spinon states. By carefully accounting for the intensity of their experimental signal, the EPFL team thereby proved and quantified the actual existence of states composed of more than two spinons. Their paradigm-shifting discovery is not only expected to affect future physics textbooks, but, more importantly, allows researchers to develop a simple picture for understanding multi-particle excitations in quantum systems.

Explore further: Finding faster-than-light particles by weighing them

More information: Mourigal M, Enderle M,Klöpperpieper A, Caux J-B, Stunault A, Rønnow HM. Fractional spinon excitations in the quantum Heisenberg antiferromagnetic chain. Nature Physics (2013) doi:10.1038/nphys2652 Published online 16 June 2013… /full/nphys2652.html

Related Stories

Quantum bar magnets in a transparent salt

Jun 15, 2012

Scientists have managed to switch on and off the magnetism of a new material using quantum mechanics, making the material a test bed for future quantum devices.

Researchers forward quest for quantum computing

May 23, 2013

Research teams from UW-Milwaukee and the University of York investigating the properties of ultra-thin films of new materials are helping bring quantum computing one step closer to reality.

A quantum simulator for magnetic materials

May 23, 2013

Physicists understand perfectly well why a fridge magnet sticks to certain metallic surfaces. But there are more exotic forms of magnetism whose properties remain unclear, despite decades of intense research. ...

Recommended for you

Finding faster-than-light particles by weighing them

Dec 26, 2014

In a new paper accepted by the journal Astroparticle Physics, Robert Ehrlich, a recently retired physicist from George Mason University, claims that the neutrino is very likely a tachyon or faster-than-light par ...

Controlling core switching in Pac-man disks

Dec 24, 2014

Magnetic vortices in thin films can encode information in the perpendicular magnetization pointing up or down relative to the vortex core. These binary states could be useful for non-volatile data storage ...

Atoms queue up for quantum computer networks

Dec 24, 2014

In order to develop future quantum computer networks, it is necessary to hold a known number of atoms and read them without them disappearing. To do this, researchers from the Niels Bohr Institute have developed ...

New video supports radiation dosimetry audits

Dec 23, 2014

The National Physical Laboratory (NPL), working with the National Radiotherapy Trials Quality Assurance Group, has produced a video guide to support physicists participating in radiation dosimetry audits.

Acoustic tweezers manipulate cell-to-cell contact

Dec 22, 2014

Sound waves can precisely position groups of cells for study without the danger of changing or damaging the cells, according to a team of Penn State researchers who are using surface acoustic waves to manipulate ...

User comments : 1

Adjust slider to filter visible comments by rank

Display comments: newest first

no fate
not rated yet Jun 18, 2013
"flipping the magnetic moment of one single electron creates two spatially separated quantum objects called spinons."

So instead of a holon, orbiton and spinon, they have created 2 spinons, each of which conserves the other 2 properties. Spatial separation then allows each spinon to be influenced differently by the surrounding medium, hence different spin.

"the electrons' spins are interrupted from lining up by quantum fluctuations."

No,you can be more specific than "quantum fluctuations", the spins don't line up because you are cooling a COMPOUND, each element of the compound will reach ground state at a different temperature which would "confuse" the valence electrons.

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.