Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:

 

Studying fundamental particles in materials

17.01.2017

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 to observe particle properties that have no realization in elementary particles.


Stimulated by special laser pulses Weyl-cones dance in a Dirac-fermion material on a laser-controlled path (loop). One cone includes right-handed, the other left-handed Weyl-fermions.

Jörg M. Harms/MPSD

In the present study, an international research team led by Angel Rubio from the Max Planck Institute for the Structure and Dynamics of Matter at CFEL in Hamburg and the University of the Basque Country in Donostia-San Sebastián predicted how laser light can be used to create Weyl fermion states in 3D Dirac materials and to switch between Weyl semimetal, Dirac semimetal and topological insulator states on ultrafast timescales.

Besides its relevance for fundamental quantum physics, the results might lead to applications in ultrafast switching of material properties. The findings are published online in the journal Nature Communications today.

In the standard model of particle physics, the fundamental particles that make up all matter around us – electrons and quarks – are so-called fermions, named after the famous Italian physicist Enrico Fermi. Quantum theory predicts that elementary fermions could exist as three different kinds:

Dirac, Weyl, and Majorana fermions, named after Paul Dirac, Hermann Weyl, and Ettore Majorana. However, despite being predicted almost a hundred years ago, of these three kinds of particles only Dirac fermions have been observed as elementary particles in nature so far.

With the discovery of graphene in 2004, however, it was realized that the behaviour of relativistic free particles could be observed in the electronic properties of materials. This sparked the search for materials where these fundamental particles could be observed and only last year the first materials hosting Weyl fermions were discovered.

While any known material only hosts one kind of these fermions in its equilibrium state, in the present work it is demonstrated how one can transform the fermion nature within specific materials by using tailored light pulses.

First observation of Dirac fermions in graphene

The observation of Dirac fermions in the properties of graphene originates from a complex interaction of the large number of electrons and ions that make up the material. Although each individual electron interacts with its surrounding ions and electrons via electrostatic forces, the particular pattern of carbon ions in graphene’s honeycomb layer structure causes the electrons to behave collectively like massless, free fermions – Dirac fermions. These particles cooperatively forming new particles with different properties are called quasiparticles. The hunt for other materials hosting quasiparticles behaving like fundamental particles has thus focused on the crystal structure of materials so far.

Creating laser-driven topological states

It has now been found, however, that by irradiating a material with a laser, it is also possible to combine a quasiparticle with the photons of the laser field to form a new quasiparticle that, again, can behave fundamentally differently. In particular, the coupling to photons can affect the topology of quasiparticles. Topology is a property of the particles that leads to peculiar properties, for example metallic chiral edge states that form a collisionless one-way quantum highway along the edge of a topological insulator. This chirality, or handedness, is topological in the sense that right-handed and left-handed chiralities are discrete states that cannot be continuously deformed into each other. The 2016 Nobel Prize in Physics was just awarded to Michael Kosterlitz, Duncan Haldane, and David Thouless for the discovery of such topological phases of matter.

Dirac and Weyl fermions differ by their chirality. Just like our left and right hands, Weyl fermions occur in pairs, where one particle is a mirrored version of the other. The two partners are almost identical, yet they cannot be superimposed. Dirac fermions, by contrast, do not have this property.

One approach to create chirality in a material is to drive it with a laser beam. “It was realized about ten years ago that the so-called Floquet theory − a theory for laser-driven systems that oscillate periodically in time − allows us to engineer parameters and symmetries in materials that can change their topology,” explains Michael Sentef, Emmy Noether group leader at the MPSD in Hamburg. Inducing chirality into a Dirac fermion material by combining those fermions with photons from the laser beam to form new quasiparticles can thus transform it into a Weyl fermion material.

In the present work, the team around Angel Rubio used high-level computational simulations of material properties to show how this optical transformation from Dirac fermions to Weyl fermions can be achieved in a real material – Na3Bi. This material is a so-called three-dimensional Dirac semimetal. It consists of layers of sodium and bismuth atoms that arrange to form a three-dimensional equivalent of graphene. This three-dimensionality is required for the transformation of Dirac into Weyl fermions to take place. It cannot happen in a two-dimensional sheet of graphene.

“The crucial challenge in this work was to take the ideas of Floquet theory and topology from the conceptual level of model systems to the world of real materials and to demonstrate that such non-equilibrium topological phase transitions can be realized in a materials science context,” says Hannes Hübener, Marie Curie fellow at the University of the Basque Country in San Sebastián and lead author of the work.

From topological stability to ultrafast electronics

In particular, the authors could show how topological protection of the handedness of Weyl fermions arises and can be made more robust the stronger the laser field is. “We realized in our simulations that when we cranked up the field, the two different right- and left-handed Weyl fermions moved further apart from each other in the so-called momentum space, in which quasiparticles live,” says Sentef. “Since right- and left-handed particles are antiparticles of each other, they have to come together to destroy each other. The separation thus protects them from getting destroyed, which means that we achieve topological stability of these quasiparticles.”

The theoretical results suggest that experimentalists should be able to measure the transformation between Dirac and Weyl fermions in ultrafast laser experiments. One way of doing this is to use the photoelectric effect to eject electrons from the laser-driven material, a technique called pump-probe photoemission spectroscopy, which is available at the Max Planck Institute for the Structure and Dynamics of Matter under Otto-Hahn group leader Isabella Gierz and Director Andrea Cavalleri.

Angel Rubio, Director of the MPSD Theory Department, adds: “This work opens exciting new avenues to manipulate the properties of materials and molecules using fundamental light-matter interaction. It paves the way for ultimately controlling their behaviour at the nanoscale and with ultrafast switching cycles.” The scientists even hope that there might be a way to stabilize the light-induced states for longer times while retaining the ability to switch them at terahertz or even faster frequencies. This may enable new ultrafast electronics for superfast computers in the future.

Long figure caption:
Dancing Weyl cones: When excited by tailored laser pulses (white spiral), the cones in a Dirac fermion material dance on a path (8-shape) that can be controlled by the laser light. This turns a Dirac material into a Weyl material, changing the nature of the quasiparticles in it. One of the cones hosts right-handed Weyl fermions; the other cone hosts left-handed ones.

Weitere Informationen:

http://www.mpsd.mpg.de/326856/2016-11-floquet-weyl-huebener Institutes press release
http://dx.doi.org/10.1038/ncomms13940 Original publication
http://www.mpsd.mpg.de/en/research/theo Theorie Department at the Max Planck Institute for the Structure and Dynamics of Matter in Hamburg

Dr. Joerg Harms | Max-Planck-Institut für Struktur und Dynamik der Materie

Further reports about: Electrons Max-Planck-Institut fermions fundamental particles graphene ions topology

More articles from Physics and Astronomy:

nachricht Ice on the spin liquid
11.06.2018 | Universität Augsburg

nachricht How solar prominences vibrate
08.06.2018 | Instituto de Astrofísica de Canarias (IAC)

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: Sharp images with flexible fibers

An international team of scientists has discovered a new way to transfer image information through multimodal fibers with almost no distortion - even if the fiber is bent. The results of the study, to which scientist from the Leibniz-Institute of Photonic Technology Jena (Leibniz IPHT) contributed, were published on 6thJune in the highly-cited journal Physical Review Letters.

Endoscopes allow doctors to see into a patient’s body like through a keyhole. Typically, the images are transmitted via a bundle of several hundreds of optical...

Im Focus: Photoexcited graphene puzzle solved

A boost for graphene-based light detectors

Light detection and control lies at the heart of many modern device applications, such as smartphone cameras. Using graphene as a light-sensitive material for...

Im Focus: Water is not the same as water

Water molecules exist in two different forms with almost identical physical properties. For the first time, researchers have succeeded in separating the two forms to show that they can exhibit different chemical reactivities. These results were reported by researchers from the University of Basel and their colleagues in Hamburg in the scientific journal Nature Communications.

From a chemical perspective, water is a molecule in which a single oxygen atom is linked to two hydrogen atoms. It is less well known that water exists in two...

Im Focus: Powerful IT security for the car of the future – research alliance develops new approaches

The more electronics steer, accelerate and brake cars, the more important it is to protect them against cyber-attacks. That is why 15 partners from industry and academia will work together over the next three years on new approaches to IT security in self-driving cars. The joint project goes by the name Security For Connected, Autonomous Cars (SecForCARs) and has funding of €7.2 million from the German Federal Ministry of Education and Research. Infineon is leading the project.

Vehicles already offer diverse communication interfaces and more and more automated functions, such as distance and lane-keeping assist systems. At the same...

Im Focus: Molecular switch will facilitate the development of pioneering electro-optical devices

A research team led by physicists at the Technical University of Munich (TUM) has developed molecular nanoswitches that can be toggled between two structurally different states using an applied voltage. They can serve as the basis for a pioneering class of devices that could replace silicon-based components with organic molecules.

The development of new electronic technologies drives the incessant reduction of functional component sizes. In the context of an international collaborative...

All Focus news of the innovation-report >>>

Anzeige

Anzeige

VideoLinks
Industry & Economy
Event News

2nd International Baltic Earth Conference in Denmark: “The Baltic Sea region in Transition”

08.06.2018 | Event News

ISEKI_Food 2018: Conference with Holistic View of Food Production

05.06.2018 | Event News

12th COMPAMED Spring Convention: Innovative manufacturing processes of modern implants

28.05.2018 | Event News

 
Latest News

AI senses people's pose through walls

12.06.2018 | Information Technology

Making the oxygen we breathe, a photosynthesis mechanism exposed

12.06.2018 | Life Sciences

Why Zika is not an STD - Semen inhibits Zika virus infection

12.06.2018 | Life Sciences

VideoLinks
Science & Research
Overview of more VideoLinks >>>