High-temperature superconductivity starts at nanoscale

May 31, 2012 By Bill Steele
Scanning tunneling microscope image of a partially doped cuprate superconductor shows regions with an electronic "pseudogap" (rounded rectangle) others with no progress from the original insulator (dashed circles). As doping increases, pseudogap regions spread and connect, making the whole sample a superconductor.

(Phys.org) -- High-temperature superconductivity doesn't happen all it once. It starts in isolated nanoscale patches that gradually expand until they take over.

That discovery, from observations at Cornell and the University of Tokyo, offers a new insight into the puzzling "pseudogap" state observed in high-temperature superconductors; it may be another step toward creating new materials that superconduct at temperatures high enough to revolutionize .

Using extremely precise scanning tunneling microscopes (STM) that can observe the states of around , an international research team led by J.C. Séamus Davis, the J.G. White Distinguished Professor in the Physical Sciences, and by Hidenori Takagi, professor of physics at the University of Tokyo, has for the first time observed how a high-temperature superconductor evolves as its chemical composition is modified. They found that as more "dopant" atoms are added, small, scattered superconducting areas, some just a few atoms across, appear. These grow until they touch and eventually fill the entire space, whereupon the entire material becomes a superconductor.

"Some theorists have imagined that this is what happens," Davis said, "but there has been no evidence until now." The research was reported May 20 in the online edition of the journal Nature Physics.

Superconductivity, in which an electric current flows with zero resistance, was first discovered in metals cooled very close to absolute zero (-273 degrees Celsius). called cuprates -- copper oxides "doped" with other atoms -- superconduct as "high" as -123 Celsius.

Observations of with the STM and other instruments show an "energy gap" where electronic states are missing. Theory says that electrons have left to join into "Cooper pairs" that can carry an electric current without interference. A puzzler for physicists is that sometimes this energy gap appears but the material still does not superconduct -- a so-called "pseudogap" phase. The pseudogap appears at higher temperatures than any superconductivity, offering the promise of someday developing materials that would superconduct at or near room .

The researchers use STMs to scan a surface in steps smaller than an atom, measuring what electron energy levels are occupied and what electrons are conspicuous by their absence. They examined a series of samples of a material known as sodium-doped calcium cuprate, prepared with gradually increasing sodium content. As more sodium is added to the mix it displaces calcium atoms, changing the crystal structure and the arrangement of electrons in ways not completely understood. This particular cuprate was chosen because its simple chemistry allows fine tuning, Davis said. The phenomena observed had not been seen before because most cuprates make abrupt transitions from insulator to pseudogap to superconductor, he explained.

At a moderate level of doping, the finds small, scattered areas with the pseudogap signature. These areas also show a "broken symmetry" where the arrangement of electrons between copper and oxygen atoms differs between "north and south" and "east and west" in the square crystal lattice. Davis and colleagues had found this broken symmetry in earlier observations of the same superconductors.

As doping increases, these areas become larger until finally they touch, and the entire sample becomes a superconductor. It's presumed that the scattered pseudogap regions occur in the vicinity of dopant atoms, but those atoms were not observed in the current study, Davis said.

Previously, the researchers noted, it was thought that the pseudogap phase in cuprates might be in competition with superconductivity, something that had to be gotten out of the way before superconductivity could happen. This work, they said, suggests that it is beneficial -- a necessary step in the evolution of a superconductor.

The research was supported by the U.S. Department of Energy and the Japan Society for the Promotion of Science.

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gmurphy
5 / 5 (3) May 31, 2012
This sort of of really precise observational science will provide the insight needed to crack this difficult problem, like all other scientific fields, it's the slow steady grind that wins the day.
Origin
1 / 5 (2) May 31, 2012
It's well known and imaged already. The situation, when these isolated nanoscale patches aren't still connected is called a pseudogap state. At the certain level of doping it's the only state, which the material can achieve during its cooling.
Macksb
1.9 / 5 (7) May 31, 2012
I have suggested previously that the "Cooper pairs" for the cuprates are actually a four way dynamic pairing--two by two, dynamically interconnected, the outlines of which create a slightly misshapen 4 leaf clover. This new report is mildly supportive of my "4 way pairing" theory, since it describes a broken symmetry: one north-south, the other east-west.

I have also said in past comments that cuprate superconductivity may be due to mass synchrony, with these four way pairings as the unit, replicated throughout the superconducting material. Mass synchrony implies a larger stability that might persist at higher temps. Again, this new report is supportive...more than mildly supportive on this "mass synchrony" theory, in my opinion, because the pseudogap islands must connect to produce the super result.

This is good work by Cornell and U. Tokyo, wherever it might lead us.


Minich
1 / 5 (2) Jun 03, 2012
Pseudogap state is the insulating state in some momentum direction of electron states.
Electron Cooper paring is nonsence, bull...
Superconducting electron is electron in unique state without P,-P degeneracy. Only spin degeneracy up/down is allowed.
Infinum
1 / 5 (1) Jun 03, 2012
It seems analogous to nucleation sites during crystallization process.

When energy is drained from the system it cools off and electrons condense (in opposition to boiling off in plasma) to form Cooper pairs.

Here it seems the temperature is being hold constant but the ratio of chemical elements is changed so that at some point the superconductive "nucleation" starts and eventually all the electrons are "frozen" into Cooper pairs.
Terriva
1 / 5 (1) Jun 03, 2012
The nature of superconductivity (at the high temperature one in particular) is not in based on Cooper pairing. Such mechanism doesn't explain for example, why some elements are superconductive, whereas others not. Its primary mechanism is, the movable electrons are attracted to hole stripes (line of positively charged atoms within the lattice) and they're compressed mutually there into dense chaotic fluid. When electrons are getting too close mutually, their repulsive forces do overlap and such an electrons are moving freely along the hole stripes. You can model this situation with market hall with long tables, around whose the customers are condensing being attracted with their curiosity. But the customers are repulsing mutually at distance and from this equilibrium of attractive and repulsive forces the superconductivity arises.
Minich
1 / 5 (1) Jun 05, 2012
The nature of superconductivity (at the high temperature one in particular) is not in based on Cooper pairing. Such mechanism doesn't explain for example, why some elements are superconductive, whereas others not.
The ONLY reason to be superconductor is the SPECIAL form of electron energy spectrum. For ordinary (phonon initiated) superconductors energy bands as a rule half filled with an approximated 1/2 probability to have positive Hall constant. So we have 1/2 elements are superconductors and 1/2 of all elements are nonsuperconductors.
For HTSC we as a rule have pozitiv Hall constant, so all of such material are superconductors.
For T
Minich
1 / 5 (1) Jun 05, 2012
For T less than Tc the spectrum of electronic orbital changes, Felix Bloch and Wilson band theory becomes invalid. There appear SUPERCODUCTING electron orbitals.
For conventional superconductors order parameter is phonon superradiance at Kohn anomaly, for HTSC the order parameter is more effective SOLID :) parameter :)