Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:

 

Spin Currents in Topological Insulators

12.06.2012
Once again, Würzburg physicists provide new insights into spintronics: In ultra-thin topological insulators, they have identified spin-polarized currents, which were first theoretically predicted six years ago. They also present a method of application for the development of new computers.

Electrons have an intrinsic angular momentum, called spin. As a consequence, not only do they carry charge, but they also behave like tiny magnets, which can be aligned. In our everyday use of computers, however, so many electron magnets point randomly in all directions as to cancel out as a whole.


The edge currents of a topological insulator serve as a source of spin-polarized electrons. Graphics: Luis Maier


Electron microscopic image of the circuit: The semiconductor H is shown in red, the gate contacts in yellow. The picture shows a section of about three by three micrometers. Photo: Luis Maier

But if the spin were to be controlled, conventional computers might suddenly become a lot faster: In the field of so-called spintronics, the magnetic orientation of the electrons is used for information transfer, which generates much less heat than is produced by continually switching the current on and off as is required in conventional electronics.

Metal and insulator at the same time: Topological insulators

Topological insulators represent a very promising class of materials for the implementation of spintronic devices. They conduct electricity only on their surface, but not in their interior. In the thin layers of some of these materials, the edge current consists of exactly two channels in which the individual electrons flow. The flow direction in the two channels is opposite to each other as is the spin orientation. This behavior is called the quantum spin Hall effect due to its analogy to the quantum Hall effect. The QSH effect was discovered in 2007 by the research group of Professor Laurens Molenkamp at the University of Würzburg.

Physicists at the department of Laurens Molenkamp and the research group of Professor Ewelina Hankiewicz now demonstrate – together with researchers of Stanford University in California – how the spin polarization of the channels can be experimentally verified. They also present an electronic device that can generate and measure spin-polarized currents and thus possesses some basic qualities required for spintronics. The results are published in the prestigious journal "Nature Physics".

From theory to experiment: Successful with an H-shaped nanostructure

Until recently, the spin-polarization of the channels was just mathematically described; experimentally, it could only be indirectly inferred. "However, the quantum spin Hall effect requires an actual spin-polarized transport as a condition for its existence," says research group leader Hartmut Buhmann of Molenkamp's department.

Würzburg physicist Christoph Brüne managed to furnish the desired experimental proof with an ingenious experimental set-up. Critical to the success was an H-shaped nanostructure, consisting of mercury telluride and fitted with an additional gold electrode at each leg.

With this configuration, it is possible to induce a quantum spin Hall state in one leg of the H-structure by means of an applied gate voltage. The other leg causes an imbalance between the two spin currents at the connection point, the cross bar of the H. As a consequence, only electrons with magnetic alignment can be extracted and measured. This also works in the reverse direction so that you can inject a spin-polarized current and measure the induced voltage in the QSH material.

Editors of "Nature Physics" turn the spotlight on the research

The theory required for the clear identification of the measured values as spin-currents, including some sophisticated simulations, comes from the group of Ewelina Hankiewicz and her colleagues in the research group of Professor Shou-Cheng Zhang in Stanford: "It wasn't easy to calculate how the spin edge currents get into the metal of the second leg," Professor Hankiewicz says.

However, all the hard work paid off in the end. The editors of "Nature Physics" even dedicated a "News & Views" review article to the Würzburg research. "This is equivalent to a high distinction, classifying our results as particularly important," explains Laurens Molenkamp.

Next research steps: Development of the concept

So far, the configuration presented by the Würzburg physicists only works at extremely low temperatures of typically minus 271 degrees Celsius. To make it work at room temperature, the scientists still need to find suitable materials. In the future, the Würzburg researchers intend as a first step to develop the concept into a spin transistor, thus providing all the basic elements required for application in spintronics.

In addition, topological insulators have even more potential: They are a safe bet for further exotic discoveries, such as Majorana fermions, i.e. particles that are their own anti-particles. So it doesn't come as a surprise that the German Research Foundation (DFG) intends to establish a new priority program for "topological insulators" this year.

Publications on the topic

Christoph Brüne, Andreas Roth, Hartmut Buhmann, Ewelina M. Hankiewicz, Laurens W. Molenkamp, Joseph Maciejko, Xiao-Liang Qi & Shou-Cheng Zhang: Spin polarization of the quantum spin Hall edge states; Nature Physics 8, 486–491 (2012), doi:10.1038/nphys2322

Yi Zhou & Fu-Chun Zhang: Quantum spin Hall effect: Left up right down; Nature Physics 8, 448–449 (2012), doi: 10.1038/nphys2335

Markus König, Steffen Wiedmann, Christoph Brüne, Andreas Roth, Hartmut Buhmann, Laurens W. Molenkamp, Xiao-Liang Qi and Shou-Cheng Zhang: Quantum Spin Hall Insulator State in HgTe Quantum Wells; Science 318, 766-770 (2007), doi: 10.1126/science.1148047

Contact person

Prof. Dr. Laurens Molenkamp, Institute of Physics of the University of Würzburg, T (0931) 31-84925, molenkamp@physik.uni-wuerzburg.de

Robert Emmerich | idw
Further information:
http://www.uni-wuerzburg.de

More articles from Physics and Astronomy:

nachricht First Juno science results supported by University of Leicester's Jupiter 'forecast'
26.05.2017 | University of Leicester

nachricht Measured for the first time: Direction of light waves changed by quantum effect
24.05.2017 | Vienna University of Technology

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: Can the immune system be boosted against Staphylococcus aureus by delivery of messenger RNA?

Staphylococcus aureus is a feared pathogen (MRSA, multi-resistant S. aureus) due to frequent resistances against many antibiotics, especially in hospital infections. Researchers at the Paul-Ehrlich-Institut have identified immunological processes that prevent a successful immune response directed against the pathogenic agent. The delivery of bacterial proteins with RNA adjuvant or messenger RNA (mRNA) into immune cells allows the re-direction of the immune response towards an active defense against S. aureus. This could be of significant importance for the development of an effective vaccine. PLOS Pathogens has published these research results online on 25 May 2017.

Staphylococcus aureus (S. aureus) is a bacterium that colonizes by far more than half of the skin and the mucosa of adults, usually without causing infections....

Im Focus: A quantum walk of photons

Physicists from the University of Würzburg are capable of generating identical looking single light particles at the push of a button. Two new studies now demonstrate the potential this method holds.

The quantum computer has fuelled the imagination of scientists for decades: It is based on fundamentally different phenomena than a conventional computer....

Im Focus: Turmoil in sluggish electrons’ existence

An international team of physicists has monitored the scattering behaviour of electrons in a non-conducting material in real-time. Their insights could be beneficial for radiotherapy.

We can refer to electrons in non-conducting materials as ‘sluggish’. Typically, they remain fixed in a location, deep inside an atomic composite. It is hence...

Im Focus: Wafer-thin Magnetic Materials Developed for Future Quantum Technologies

Two-dimensional magnetic structures are regarded as a promising material for new types of data storage, since the magnetic properties of individual molecular building blocks can be investigated and modified. For the first time, researchers have now produced a wafer-thin ferrimagnet, in which molecules with different magnetic centers arrange themselves on a gold surface to form a checkerboard pattern. Scientists at the Swiss Nanoscience Institute at the University of Basel and the Paul Scherrer Institute published their findings in the journal Nature Communications.

Ferrimagnets are composed of two centers which are magnetized at different strengths and point in opposing directions. Two-dimensional, quasi-flat ferrimagnets...

Im Focus: World's thinnest hologram paves path to new 3-D world

Nano-hologram paves way for integration of 3-D holography into everyday electronics

An Australian-Chinese research team has created the world's thinnest hologram, paving the way towards the integration of 3D holography into everyday...

All Focus news of the innovation-report >>>

Anzeige

Anzeige

Event News

Marine Conservation: IASS Contributes to UN Ocean Conference in New York on 5-9 June

24.05.2017 | Event News

AWK Aachen Machine Tool Colloquium 2017: Internet of Production for Agile Enterprises

23.05.2017 | Event News

Dortmund MST Conference presents Individualized Healthcare Solutions with micro and nanotechnology

22.05.2017 | Event News

 
Latest News

How herpesviruses win the footrace against the immune system

26.05.2017 | Life Sciences

Water forms 'spine of hydration' around DNA, group finds

26.05.2017 | Life Sciences

First Juno science results supported by University of Leicester's Jupiter 'forecast'

26.05.2017 | Physics and Astronomy

VideoLinks
B2B-VideoLinks
More VideoLinks >>>