Condensed-matter physics: Superconducting electrons go missing

Condensed-matter physics: Superconducting electrons go missing

(Related to Lloyd's earlier post on SC Bismuth... I bet the more they expand these experiments with different temperatures and different "doped" elements... they will be creating Charge-Field effects that can't be explained that confidently with standard QT-BCS theory. I bet at some point someone may get lucky and do something extraordinary at near room temperature that just works yet without understanding the CF effects in play.  )

(Strontium's carousel is capped with Neutrons except for two alphas on each side. http://www.users.on.net/~Nevyn/science/physics/PeriodicTable/ ... "doping" is merely adding more bridges for the Charge Field? )

   Jan Zaanen1,
   Nature
   536, 282–283(18 August 2016)
   doi:10.1038/536282a
Published online
   17 August 2016

Editor's summary

Ivan Božović et al. present a comprehensive study of the key physical properties of the overdoped copper oxide superconductor La2−xSrxCuO4. Their results run counter to the common assumption that strongly correlated fermion physics evolves smoothly into conventional Bardeen–Cooper–Schrieffer (BCS) behaviour in overdoped copper oxide superconductors. Rather, in La2−xSrxCuO4 the scaling law for the critical superconducting temperature and zero-temperature phase stiffness does not conform to standard BCS physics. The authors speculate that the high critical temperature derives from local electron pairing and unusual kinematics.

http://www.nature.com/nature/journal/v536/n7616/full/536282a.html

Dependence of the critical temperature in overdoped copper oxides on superfluid density
   I. Božović1, 2, X. He1, 2, J. Wu1, A. T. Bollinger1,
   Nature
   536, 309–311 (18 August 2016)  doi:10.1038/nature19061

The physics of underdoped copper oxide superconductors, including the pseudogap, spin and charge ordering and their relation to superconductivity1, 2, 3, is intensely debated. The overdoped copper oxides are perceived as simpler, with strongly correlated fermion physics evolving smoothly into the conventional Bardeen–Cooper–Schrieffer behaviour. Pioneering studies on a few overdoped samples4, 5, 6, 7, 8, 9, 10, 11 indicated that the superfluid density was much lower than expected, but this was attributed to pair-breaking, disorder and phase separation. Here we report the way in which the magnetic penetration depth and the phase stiffness depend on temperature and doping by investigating the entire overdoped side of the La2−xSrxCuO4 phase diagram. We measured the absolute values of the magnetic penetration depth and the phase stiffness to an accuracy of one per cent in thousands of samples; the large statistics reveal clear trends and intrinsic properties. The films are homogeneous; variations in the critical superconducting temperature within a film are very small (less than one kelvin). At every level of doping the phase stiffness decreases linearly with temperature. The dependence of the zero-temperature phase stiffness on the critical superconducting temperature is generally linear, but with an offset; however, close to the origin this dependence becomes parabolic. This scaling law is incompatible with the standard Bardeen–Cooper–Schrieffer description.

http://www.nature.com/nature/journal/v536/n7616/full/nature19061.html

Scientists uncover origin of high-temperature superconductivity in copper-oxide compound
August 17, 2016

(more at link: http://phys.org/news/2016-08-scientists-uncover-high-temperature-superconductivity-copper-oxide.html )

Since the 1986 discovery of high-temperature superconductivity in copper-oxide compounds called cuprates, scientists have been trying to understand how these materials can conduct electricity without resistance at temperatures hundreds of degrees above the ultra-chilled temperatures required by conventional superconductors. Finding the mechanism behind this exotic behavior may pave the way for engineering materials that become superconducting at room temperature. Such a capability could enable lossless power grids, more affordable magnetically levitated transit systems, and powerful supercomputers, and change the way energy is produced, transmitted, and used globally.

Now, physicists at the U.S. Department of Energy's (DOE) Brookhaven National Laboratory have an explanation for why the temperature at which cuprates become superconducting is so high. After growing and analyzing thousands of samples of a cuprate known as LSCO for the four elements it contains (lanthanum, strontium, copper, and oxygen), they determined that this "critical" temperature is controlled by the density of electron pairs—the number of electron pairs per unit area. This finding, described in a Nature paper published August 17, challenges the standard theory of superconductivity, which proposes that the critical temperature depends instead on the strength of the electron pairing interaction.

"Solving the enigma of high-temperature superconductivity has been the focus of condensed matter physics for more than 30 years," said Ivan Bozovic, a senior physicist in Brookhaven Lab's Condensed Matter Physics and Materials Science Department who led the study. "Our experimental finding provides a basis for explaining the origin of high-temperature superconductivity in the cuprates—a basis that calls for an entirely new theoretical framework."

According to Bozovic, one of the reasons cuprates have been so difficult to study is because of the precise engineering required to generate perfect crystallographic samples that contain only the high-temperature superconducting phase.

"It is a materials science problem. Cuprates can have up to 50 atoms per unit cell and the elements can form hundreds of different compounds, likely resulting in a mixture of different phases," said Bozovic.

That's why Bozovic and his research team grew their more than 2,500 LSCO samples by using a custom-designed molecular beam epitaxy system that places single atoms onto a substrate, layer by layer. This system is equipped with advanced surface-science tools, such as those for absorption spectroscopy and electron diffraction, that provide real-time information about the surface morphology, thickness, chemical composition, and crystal structure of the resulting thin films.
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Brookhaven Lab physicist Ivan Bozovic explains why a copper-oxide compound can conduct electricity without resistance at temperatures well above those required by conventional superconductors. Credit: Brookhaven National Laboratory

"Monitoring these characteristics ensures there aren't any irregular geometries, defects, or precipitates from secondary phases in our samples," Bozovic explained.

In engineering the LSCO films, Bozovic chemically added strontium atoms, which produce mobile electrons that pair up in the copper-oxide layers where superconductivity occurs. This "doping" process allows LSCO and other cuprates—normally insulating materials—to become superconducting.

For this study, Bozovic added strontium in amounts beyond the doping level required to induce superconductivity. Earlier studies on this "overdoping" had indicated that the density of electron pairs decreases as the doping concentration is increased. Scientists had tried to explain this surprising experimental finding by attributing it to different electronic orders competing with superconductivity, or electron pair breaking caused by impurities or disorder in the lattice. For example, they had thought that geometrical defects, such as displaced or missing atoms, could be at play.

To test these explanations, Bozovic and his team measured the magnetic and electronic properties of their engineered LSCO films. They used a technique called mutual inductance to determine the magnetic penetration depth (the distance a magnetic field transmits through a superconductor), which indicates the density of electron pairs.