Counting the 'Holes' in High-Temperature Superconductors

Mar 25, 2010 by Kendra Snyder
Layer-resolved hole density in a superconducting 2 x LCO - 4 x LSCO superlattice (dots) compared to results of calculations for different screening lengths\lambda (lines).

( -- As part of the effort to better understand how superconductors transport electricity with zero resistance, a team of researchers has demonstrated a new way to count the number of a material's "holes" - locations where electrons are absent. Knowing more about holes, which, like electrons, are thought to interact with each other to produce superconducting current within a material, could help scientists develop superconductors for applications like more-efficient power transmission or magnetic-levitation high-speed trains.

The researchers, from the University of Illinois, Brookhaven National Laboratory, and the University of Paris, focused on copper-oxide compounds, called cuprates, which operate at temperatures warmer than traditional but still far below freezing. The ultimate goal for researchers is to design superconducting materials that function closer to room temperature, and therefore, are practical for everyday use.

Cuprates normally act as insulators but become superconductors when electrons are removed in a process known as "doping" holes into the material. A material is considered optimally doped when it is superconducting at the highest temperature possible. When the material is doped past this point, the superconductivity often vanishes.

In this study, the researchers built a sandwich of alternating layers of two cuprates: LSCO, which contains lanthanum, strontium, copper, and oxygen; and LCO, which lacks the strontium. They grew this complex structure using a unique molecular beam epitaxy system in Brookhaven's and Materials Science Department. This machine allows researchers to deposit materials atom by atom, giving precise control of each layer's thickness.

"Using molecular beam epitaxy, our collaborators at Brookhaven were able to achieve excellent control of the amount of material that goes in the sample," said University of Illinois researcher Serban Smadici. "That's very difficult to do, and it's one of the driving forces of this project."

This particular structure was made of 15 segments of precisely measured two unit cells of LCO and four unit cells of LSCO. Anything less than the highest precision, Smadici said, would alter the layer's properties.

Neither of the two materials used in the study is superconducting by itself - LCO was used in its naturally insulating form and LSCO was overdoped to the point where its superconductivity disappeared. Surprisingly, though, the overall structure is superconducting at 38 Kelvin, about -235 degrees Celsius.

"When you put these layers together, the overall structure actually becomes superconducting," Smadici said. "The question is why? Figuring out the answer to this question requires determining where the holes are in this structure."

They did that at NSLS beamline X1B using resonant soft x-ray scattering, a technique that allows researchers to selectively probe the holes. By measuring the reflectivity of the structure at different angles, they pinpointed the number of holes in each individual layer.

Their results, which were published in the March 13, 2009, edition of Physical Review Letters, show that many of the holes actually moved from the overdoped LSCO to the LCO. This creates optimally doped LCO - the driver of the surprise in the multilayered structure.

"Our results demonstrate the utility of resonant soft x-ray scattering for probing the holes near interfaces separately of the atomic structure," Smadici said. "They also provide evidence on the importance of interfaces to novel superconducting materials."

Now, the researchers are using this analytic synchrotron technique on other multilayer superconducting structures.

"This work is far from over," Smadici said.

Other authors include James Lee, Shuai Wang, and Peter Abbamonte, from the University of Illinois; Gennady Logvenov, Adrian Gozar, and Ivan Bozovic, from Brookhaven National Laboratory; and Catherine Deville Cavellin, from Brookhaven and the University of Paris.

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More information: S. Smadici, J.C.T. Lee, S. Wang, P. Abbamonte, G. Logvenov, A. Gozar, C. Deville Cavellin, I. Bozovic, "Superconducting Transition at 38 K in Insulating-Overdoped La2CuO4-La1.64Sr0.36CuO4 Superlattices: Evidence for Interface Electronic Redistribution from Resonant Soft X-Ray Scattering," PRL, 102, 107004 (2009).

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not rated yet Mar 25, 2010
Presence of hole stripes is crucial for high Tc superconductivity. Electrons are attracted to them, thus forming dense elongated clouds of high electron density around it. Because free electrons are heavily compressed there, they're moving without friction: each electron near hole is surrounded by so many electrons from all sides, their mutual repulsive forces are nearly balanced there and such electron is moving smoothly without shaking, i.e. without ohmic loss.

This mechanism is sensitive to hole density. Low concentration of holes leads to high electron density around holes, but their islands are isolated and they manifest by so called "pseudogap" state, where bulk characteristics of material remind true superconductor - but material is still insulator. A high concentration of holes is harmless too, because repulsive forces between electrons increase distance between atoms and the pressure inside of stripes decreases. Therefore the proper hole density is crucial there.
3.7 / 5 (3) Mar 25, 2010
IMO, if we would collect electrons in vacuum tube across thin layer of insulator with high dielectric strength (for example in thin bornitride pipe), the electrons would cumulate along inner surface of pipe, thus creating a thin superconductive layer in it.

In 2001 prof. J.F.Prins proved presence of superconductive layer of electrons at the surface of diamond stripes, into which oxygen holes were injected. The electrons were strongly attracted to holes, thus forming a continuous superconductive layer at the surface of diamond above oxygen ions even at room temperature.


Unfortunately, this great discovery was widely ignored by mainstream physics. If confirmed, it would become the 2nd most significant finding of the physics of the last century after cold fusion.
4.5 / 5 (2) Mar 26, 2010
What is happening to Johan F. Prins is truly disheartening and that sad state of affairs is not exclusive to Physics.

Dogma, financial and political interests, ignorance and bad science galore (bad usage of basic statistics, twisted to get the results/significance you want).

--I apologize for being offtopic and will just bite my tongue now. Sorry.
1 / 5 (1) Mar 26, 2010
What is happening to Johan F. Prins is truly disheartening
The problem rather is, whole civilization is wasting time and money in superconductor research - while completely useless & potentially dangerous collider research is favored, because of many influential theorists is involved in it. Theoretical physics is worth of replacement due the lack of effectivity.
1 / 5 (1) Mar 29, 2010
to bad there hasn't been anyone who could repeat the cold fusion experiment
2.7 / 5 (3) Mar 30, 2010
Cold fusion is replicated routinely. By September 1990, 92 groups from 10 countries reported successful replications of Fleischmann and Pons experiments.