Scientists make detailed map of current between insulators

September 13, 2013 by Glennda Chui, SLAC National Accelerator Laboratory
Scientists make detailed map of current between insulators
SLAC and Stanford researchers used an ultrasensitive device to map currents flowing between two insulating materials. This image shows higher current (brighter red) flowing in narrow paths from left to right and, more faintly, down to up. These paths are related to the crystal structure of one of the materials, known as STO. Credit: Eric Spanton/SLAC

When scientists found electrical current flowing where it shouldn't be – at the place where two insulating materials meet– it set off a frenzy of research that turned up more weird properties and the hope of creating a new class of electronics.

Now SLAC and Stanford scientists have mapped those currents in microscopic detail and found another surprise: Rather than flowing uniformly, the currents are stronger in some places than others, like river currents shaped by underlying rock.

Their finding, published Sunday in Nature Materials, is an important step in understanding why these two materials – and (LAO and STO for short) – dramatically change behavior when sandwiched together.

It looks like the currents at the interface are shaped by subtle distortions in the of the STO layer, said Kathryn A. "Kam" Moler of the Stanford Institute for Materials and Energy Sciences (SIMES), who led the research team. The cube-shaped arrangement of the atoms in the structure stretches when chilled, and variations in the direction of that stretching from one tiny region to another seem to channel the current flow.

But it will take more digging to pin down what is happening at the , she said. The team hopes to do those studies at SLAC's Stanford Synchrotron Radiation Lightsource, which generates brilliant X-ray beams for probing atomic structure.

"I think it's interesting that LAO/STO is a very well-studied system, but this effect was never expected," said Eric M. Spanton, a SLAC graduate student on the research team. "By understanding what is happening in the regions where the current flows stronger, maybe we can figure out how to make the whole sample conduct better."

The more scientists understand how materials operate at the scale of individual atoms, the better they can design them with exactly the properties they want, said Harold Hwang, a professor at SLAC and Stanford who was part of the research team.

It was Hwang and a colleague who in 2004 discovered currents flowing at the interface between LAO and STO. Three years later, another group showed that when the layers are chilled, the interface becomes superconducting; that is, it conducts electricity with zero resistance and 100 percent efficiency. Yet another group found that magnetism could exist in the gap, even though neither of the layers is inherently magnetic. And in 2011, Moler, Hwang and colleagues, along with a separate group at MIT, announced an even more improbable finding: The chilled interface can contain superconducting regions and magnetic regions at the same time. The two aren't supposed to coexist.

Today's devices use several types of switches to control the flow of information. Changes in voltage are used to switch transistors that guide the flow of current on computer chips. Changes in magnetism operate switches that store information on hard drives, and electric switches store data on flash drives. The promise of the LAO/STO interface – one of a class of materials known as engineered heterostructures – is that it might allow two or three types of switches to operate in a single device.

"The hope is you would be able to get more stuff into a smaller space and switch it faster using less energy," Moler said, "but that's all very speculative."

For the latest study, much of the work was carried out by Spanton and Beena Kalisky, a SIMES postdoctoral researcher who left midway through to take a faculty position at Bar-Ilan University in Israel.

They would put a sample of the LAO/STO sandwich into a vacuum chamber, run an electrical current through it and use an exquisitely sensitive device called a SQUID to measure the magnetic field the current induced. Those measurements showed the current was stronger in some places and weaker in others, creating striped patterns. Kalisky then used a polarized light microscope to map out the underlying patterns of structural distortion in the STO layer.

Explore further: Man-made material shows surprisingly magnetic personality

More information: Nature Materials (2013) DOI: 10.1038/nmat3753

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5 / 5 (2) Sep 13, 2013
Now wouldn't it be the biggest slap in the face if High temp superconductors were easily created on the borders of insulators...

5 / 5 (1) Sep 13, 2013
@El_Nose - before copper-oxide superconductors that suggestion would have been laughed at, but after superconducting temperature records were shattered by superconducting layers within what is normally an insulator, such 'heresy' started looking plausible.
1 / 5 (2) Sep 13, 2013
Would this explain the leakage current that we see in capacitors, flowing through the dielectric?
5 / 5 (1) Sep 13, 2013

Capacitors generally just have one layer of dielectric, and any boundaries would be perpendicular to the direction of leakage. Capacitor leakage is through the insulator rather than along its surface.

Also this appears to be a rare phenomenon - most insulators can be put together quite happily without forming a conductive plane at their interface. My guess is that in this case each insulator has a surface studded with electrons, and when the two layers are put together the electrons interfere enough that a few become free and allow the current to flow, and that this conduction will only apply to pairs of such insulators.
1 / 5 (3) Sep 14, 2013
Now wouldn't it be the biggest slap in the face if High temp superconductors were easily created on the borders of insulators...

This is exactly where they are created: Superconduction occurs by tunnelling of charge-carriers through thin insulating layers. This is the ONLY way that a charge-carrier can move from one position (at the edge of the insulating laye) to another position (at the other side of the insulating layer) without having to have a change in its potential energy: All other types of motion require a change in the charge-carrier's potential energy even when the charge-carrier does not scatter. If you have a change in potential energy of the charge-carriers, then you must measure a voltage acros a conductor;

Superconduction occurs by means of a relay race of charge carriers each of which tunnels through an insulating layer. The tunneling-formula adds a factor 2, which has been incorrectly interpreted as being a charge -2e of the charge-carriers.

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