Featured Research

from universities, journals, and other organizations

Controversy clarified: Why two insulators together can transport electricity

Date:
July 3, 2012
Source:
Paul Scherrer Institut (PSI)
Summary:
In 2004, researchers discovered something amazing: If the two materials, SrTiO3 and LaAlO3, both of which are electrical insulators, are joined together, a thin, electrically conducting region is formed at their interface. Over the subsequent three years, a number of hypotheses were proposed to explain this effect, which has led to controversy ever since.

Structure of the material investigated; Lower: Pure SrTiO3 (dark blue: strontium Sr; turquoise: titanium Ti; grey: oxygen O) with alternating planes of SrO and TiO2. Upper: Mixed layer of SrTiO3 and LaAlO3 (red: lanthanum La; orange: aluminium Al).
Credit: Image courtesy of Paul Scherrer Institut (PSI)

In 2004, researchers discovered something amazing: If the two materials, SrTiO3 and LaAlO3, both of which are electrical insulators, are joined together, a thin, electrically conducting region is formed at their interface. Over the subsequent three years, a number of hypotheses were proposed to explain this effect, which has led to controversy ever since. "There are conference sessions at which the majority of contributions are about this effect," says Mathilde Reinle-Schmitt, a researcher at PSI and the first author of the work presented here.

Of the various explanations forwarded, effectively only two competing hypotheses have survived. In order to compare their relative merits, an appropriate experiment was performed by scientists at PSI, together with colleagues from the University of Geneva. In addition, scientists from the University of Liège have contributed important theoretical insights.

Intermixing or polar catastrophe?

Both substances are complex metal-oxide materials – so-called perovskites – with a typical layered structure. In SrTiO3, the structure consists of alternating planes of strontium oxide (SrO) and titanium dioxide (TiO2), while in LaAlO3, they are lanthanum oxide (LaO) and aluminium dioxide (AlO2). The two materials, however, differ in one crucial aspect: In SrTiO3, both planes are electrically neutral, while in LaAlO3 they are alternately positively and negatively charged. The combination of two such materials leads, in the opinion of one school of thought, to the transfer of mobile electrons to the interface between the two materials, thereby making this ultrathin interfacial region conducting. This happens, however, only when the LaAlO3 film is thick enough. Otherwise, the combination of materials remains an insulator. This hypothesis is known as the «polar catastrophe». Other researchers are nevertheless convinced that conductivity is created because both compounds are known to combine at their interface and this intermixed material is conducting.

The appropriate experiment

In order to clarify this controversy, the researchers wanted to answer the question, "Is a mixture of the two substances conducting?" Mathilde Reinle-Schmitt says, "We used again SrTiO3 as the substrate, but now, instead of depositing pure LaAlO3 on top of this, we used a mixture of SrTiO3 und LaAlO3 in various proportions and thicknesses and then measured the conductivity. The results were surprising. For thin layers of deposited mixtures, the system was insulating, while for thicker layers the interface was conducting, although the layers themselves were not, just as for pure LaAlO3. What changed was that the mixed layer had to be correspondingly thicker the less LaAlO3 there was in the mixture. This result agrees perfectly with the polar catastrophe hypothesis," says Claudia Cancellieri, PSI scientist and second author of the article. "It would be very difficult to explain these results in terms of intermixing."

Numerous applications in view

Perovskites, the category to which the materials being investigated belong, often have interesting electrical and magnetic properties, some of which have never been observed in any other materials. An additional important property is highlighted by Philip Willmott, leader of the investigating group at PSI, "In contrast to traditional semiconductors used today, different perovskites have a similar crystalline and chemical structure and can thus be easily incorporated together, so that materials with different characteristics can be combined into a single device, such as a superconductor with a material which reacts sensitively to magnet fields, or one which can detect substances in the air." Materials in which the current does not flow in all three dimensions, but in only one or, as here, in two, are also current topics of research that could lead to numerous applications.


Story Source:

The above story is based on materials provided by Paul Scherrer Institut (PSI). Note: Materials may be edited for content and length.


Journal Reference:

  1. M.L. Reinle-Schmitt, C. Cancellieri, D. Li, D. Fontaine, M. Medarde, E. Pomjakushina, C.W. Schneider, S. Gariglio, Ph. Ghosez, J.-M. Triscone, P.R. Willmott. Tunable conductivity threshold at polar oxide interfaces. Nature Communications, 2012; 3: 932 DOI: 10.1038/ncomms1936

Cite This Page:

Paul Scherrer Institut (PSI). "Controversy clarified: Why two insulators together can transport electricity." ScienceDaily. ScienceDaily, 3 July 2012. <www.sciencedaily.com/releases/2012/07/120703172508.htm>.
Paul Scherrer Institut (PSI). (2012, July 3). Controversy clarified: Why two insulators together can transport electricity. ScienceDaily. Retrieved September 16, 2014 from www.sciencedaily.com/releases/2012/07/120703172508.htm
Paul Scherrer Institut (PSI). "Controversy clarified: Why two insulators together can transport electricity." ScienceDaily. www.sciencedaily.com/releases/2012/07/120703172508.htm (accessed September 16, 2014).

Share This



More Matter & Energy News

Tuesday, September 16, 2014

Featured Research

from universities, journals, and other organizations


Featured Videos

from AP, Reuters, AFP, and other news services

Space Race Pits Bezos Vs Musk

Space Race Pits Bezos Vs Musk

Reuters - Business Video Online (Sep. 16, 2014) — Amazon CEO Jeff Bezos' startup will team up with Boeing and Lockheed to develop rocket engines as Elon Musk races to have his rockets certified. Fred Katayama reports. Video provided by Reuters
Powered by NewsLook.com
MIT's Robot Cheetah Unleashed — Can Now Run, Jump Freely

MIT's Robot Cheetah Unleashed — Can Now Run, Jump Freely

Newsy (Sep. 16, 2014) — MIT developed a robot modeled after a cheetah. It can run up to speeds of 10 mph, though researchers estimate it will eventually reach 30 mph. Video provided by Newsy
Powered by NewsLook.com
Manufacturer Prints 3-D Car In Record Time

Manufacturer Prints 3-D Car In Record Time

Newsy (Sep. 15, 2014) — Automobile manufacturer Local Motors created a drivable electric car using a 3-D printer. Printing the body only took 44 hours. Video provided by Newsy
Powered by NewsLook.com
Refurbished New York Subway Tunnel Unveiled After Sandy Damage

Refurbished New York Subway Tunnel Unveiled After Sandy Damage

Reuters - US Online Video (Sep. 15, 2014) — New York officials unveil subway tunnels that were refurbished after Superstorm Sandy. Nathan Frandino reports. Video provided by Reuters
Powered by NewsLook.com

Search ScienceDaily

Number of stories in archives: 140,361

Find with keyword(s):
 
Enter a keyword or phrase to search ScienceDaily for related topics and research stories.

Save/Print:
Share:  

Breaking News:
from the past week

In Other News

... from NewsDaily.com

Science News

Health News

    Environment News

      Technology News



      Save/Print:
      Share:  

      Free Subscriptions


      Get the latest science news with ScienceDaily's free email newsletters, updated daily and weekly. Or view hourly updated newsfeeds in your RSS reader:

      Get Social & Mobile


      Keep up to date with the latest news from ScienceDaily via social networks and mobile apps:

      Have Feedback?


      Tell us what you think of ScienceDaily -- we welcome both positive and negative comments. Have any problems using the site? Questions?
      Mobile iPhone Android Web
      Follow Facebook Twitter Google+
      Subscribe RSS Feeds Email Newsletters
      Latest Headlines Health & Medicine Mind & Brain Space & Time Matter & Energy Computers & Math Plants & Animals Earth & Climate Fossils & Ruins