Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:

 

Topological insulators: Magnetism is not causing loss of conductivity

22.02.2016

Topological insulators appeared to be rather well-understood from theory until now. The electrons that can only occupy "allowed" quantum states in the crystal lattice are free to move in only two dimensions, namely along the surface, behaving like massless particles.

Topological insulators are therefore highly conductive at their surfaces and electrically insulating within. Only magnetic fields should destroy this mobility, according to theory.


In pure bismuth-selenide (left) no bandgap is found. With the addition of magnetic manganese (4 percent; 8 percent), a band gap (dashed line) arises, and electrical conductivity disappears. This effect shows even at room temperature.

Credit: HZB

Now physicists headed by Oliver Rader and Jaime Sánchez-Barriga from HZB along with teams from other HZB departments, groups from Austria, the Czech Republic, Russia, and theoreticians in Munich have disproved this hypothesis.

They investigated samples for this purpose made of bismuth-selenide - a classic topological insulator - built up from enormous numbers of extremely thin layers, like puff pastry. These samples were doped with the magnetic element manganese (Mn), forming (Bi1_xMnx)2Se3 with various concentrations of Mn.

Theoretically, what is known as a band gap should have opened between the allowed electron states as a result of doping with magnetic impurities so that the previously conductive surface becomes insulating. As a result of the appearance of the band gap, the electrons also regain part of their mass. The magnetism of the impurities should be the critical influence in this process.

Theory disproved: Magnetism is not influencing the mobility of electrons

The physicists were able to actually detect the formation of a band gap in the doped samples. The mass of the electrons climbed from zero to one-sixth the mass of free electrons. They showed, however, that this band gap is not the result of ferromagnetic ordering in the interior or at the surface of the material, nor of the local magnetic moments of the manganese. The band gap formed independent of the strength of the magnetisation and even when the sample was doped with nonmagnetic impurities.

"We even measured surface band gaps that are ten times larger than the theoretically predicted magnetic band gaps, and actually independent of whether we had incorporated magnetic or nonmagnetic impurities", says Jaime Sánchez-Barriga.

Instead, they suggest an entirely different process in these samples that causes the band gap at the Dirac point: with the help of what is known as resonant photoemission spectroscopy, they were able to observe scattering processes that might be responsible for opening a band gap. The fundamental properties of topological insulators do not offer many possibilities for these kinds of scattering processes. The researchers think it is conceivable that the presence of the impurities enables the electrons to leave the surface and disappear into the bulk.

"It is always more interesting for experimentalists like us, of course, when the experiment does not confirm the theoretical expectation. This band gap is considerably larger than predicted by theory and additionally involves a different causal mechanism. In order be sure that we are not mistaken, we used the entire arsenal at BESSY II, such as photoelectron microscopy and magnetic fields up to seven tesla. This enabled us to really preclude magnetism occurring as a possible cause down to roughly the nanometre scale", explains Oliver Rader.

Two conclusions can already be drawn from this work: on one hand, that topologically shielded states are still far from being completely understood. On the other, it means that problems previously overlooked are now in the spotlight. How can scattering processes be minimised by the choice of magnetic impurities? And what is the role of lattice location of the impurities in the host? Since Topological insulators are promising candidates for new information technologies, those questions should be explored in depth.

Media Contact

Antonia Roetger
antonia.roetger@helmholtz-berlin.de
49-308-062-43733

 @HZBde

http://www.helmholtz-berlin.de 

Antonia Roetger | Helmholtz-Zentrum Berlin für Materialien und Energie

More articles from Physics and Astronomy:

nachricht Study offers new theoretical approach to describing non-equilibrium phase transitions
27.04.2017 | DOE/Argonne National Laboratory

nachricht SwRI-led team discovers lull in Mars' giant impact history
26.04.2017 | Southwest Research Institute

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: Making lightweight construction suitable for series production

More and more automobile companies are focusing on body parts made of carbon fiber reinforced plastics (CFRP). However, manufacturing and repair costs must be further reduced in order to make CFRP more economical in use. Together with the Volkswagen AG and five other partners in the project HolQueSt 3D, the Laser Zentrum Hannover e.V. (LZH) has developed laser processes for the automatic trimming, drilling and repair of three-dimensional components.

Automated manufacturing processes are the basis for ultimately establishing the series production of CFRP components. In the project HolQueSt 3D, the LZH has...

Im Focus: Wonder material? Novel nanotube structure strengthens thin films for flexible electronics

Reflecting the structure of composites found in nature and the ancient world, researchers at the University of Illinois at Urbana-Champaign have synthesized thin carbon nanotube (CNT) textiles that exhibit both high electrical conductivity and a level of toughness that is about fifty times higher than copper films, currently used in electronics.

"The structural robustness of thin metal films has significant importance for the reliable operation of smart skin and flexible electronics including...

Im Focus: Deep inside Galaxy M87

The nearby, giant radio galaxy M87 hosts a supermassive black hole (BH) and is well-known for its bright jet dominating the spectrum over ten orders of magnitude in frequency. Due to its proximity, jet prominence, and the large black hole mass, M87 is the best laboratory for investigating the formation, acceleration, and collimation of relativistic jets. A research team led by Silke Britzen from the Max Planck Institute for Radio Astronomy in Bonn, Germany, has found strong indication for turbulent processes connecting the accretion disk and the jet of that galaxy providing insights into the longstanding problem of the origin of astrophysical jets.

Supermassive black holes form some of the most enigmatic phenomena in astrophysics. Their enormous energy output is supposed to be generated by the...

Im Focus: A Quantum Low Pass for Photons

Physicists in Garching observe novel quantum effect that limits the number of emitted photons.

The probability to find a certain number of photons inside a laser pulse usually corresponds to a classical distribution of independent events, the so-called...

Im Focus: Microprocessors based on a layer of just three atoms

Microprocessors based on atomically thin materials hold the promise of the evolution of traditional processors as well as new applications in the field of flexible electronics. Now, a TU Wien research team led by Thomas Müller has made a breakthrough in this field as part of an ongoing research project.

Two-dimensional materials, or 2D materials for short, are extremely versatile, although – or often more precisely because – they are made up of just one or a...

All Focus news of the innovation-report >>>

Anzeige

Anzeige

Event News

Fighting drug resistant tuberculosis – InfectoGnostics meets MYCO-NET² partners in Peru

28.04.2017 | Event News

Expert meeting “Health Business Connect” will connect international medical technology companies

20.04.2017 | Event News

Wenn der Computer das Gehirn austrickst

18.04.2017 | Event News

 
Latest News

Wireless power can drive tiny electronic devices in the GI tract

28.04.2017 | Medical Engineering

Ice cave in Transylvania yields window into region's past

28.04.2017 | Earth Sciences

Nose2Brain – Better Therapy for Multiple Sclerosis

28.04.2017 | Life Sciences

VideoLinks
B2B-VideoLinks
More VideoLinks >>>