New neutron studies support magnetism's role in superconductors

Feb 02, 2010
A simulation of the nature of the spin excitations in a superconducting material's structure. Studies performed at DOE's Oak Ridge National Laboratory support theories that magnetic properties play an important role in high-temperature superconductivity.

(PhysOrg.com) -- Neutron scattering experiments performed at the Department of Energy's Oak Ridge National Laboratory give strong evidence that, if superconductivity is related to a material's magnetic properties, the same mechanisms are behind both copper-based high-temperature superconductors and the newly discovered iron-based superconductors.

The work, published in a recent , was performed at ORNL's (SNS) and High Flux Isotope Reactor (HFIR) along with the ISIS Facility at the United Kingdom's Rutherford Appleton Laboratory.

High-temperature superconducting materials, in which a material conducts electricity without resistance at a relatively high temperature, have potential for application to energy efficient technologies where little electricity is lost in transmission.

The research community was stirred in 2008 when a Japanese team reported in an iron-based material. Previously, only copper-based, or cuprate, materials were known to have those properties. The discovery elicited widespread and intense analysis of the material's structure and properties.

"The pairing up of electrons is essential for the formation of the macroscopic quantum state giving rise to superconductivity," said lead researcher Mark Lumsden of ORNL. "One of the leading proposals for the pairing mechanism in the iron-based superconductors is that magnetic interactions, provide the glue that binds the electrons together."

Detailed studies of the magnetic excitations of materials are essential for understanding high-temperature superconductivity. Although superconductivity near absolute zero is common, only certain materials exhibit the property at "high" (i.e., the temperature of liquid nitrogen) temperatures.

The ORNL researchers subjected single crystals of an iron, tellurium and selenium material to analysis at the SNS, HFIR and ISIS.

"Even in comparison to cuprates, this is experimentally the best indication of what the spin excitations are doing. One of the prominent views is that spin excitations are a key ingredient. The first step in evaluating this proposal is understanding what the spin excitations are and what they are doing," Lumsden said.

scattering analysis is considered to be among the most powerful techniques to understand the molecular structure and interactions in advanced materials. The newly available pulsed-neutron source intensity at the accelerator-based SNS, combined with steady-state neutron beams from the recently upgraded HFIR, give researchers at Oak Ridge valuable new tools for understanding these properties.

"Neutron scattering is the only way to study the full wave vector and energy dependence of the spin excitations that are believed to be behind these superconducting properties," said co-author Andrew Christianson, who with Lumsden is a member of ORNL's Neutron Scattering Science Division.

The researchers performed time-of-flight neutron scattering measurements with the Wide Angular Range Chopper Spectrometer at the SNS, which opened in 2006 and is currently the world's most powerful pulsed-beam neutron source. Time-of-flight experiments were also conducted in England with the Merlin chopper spectrometer at ISIS, another leading neutron source. Triple-axis neutron scattering experiments were performed at HFIR in Oak Ridge. The single crystals were synthesized and characterized by ORNL's Correlated Electron Materials group.

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Jarek
4 / 5 (2) Feb 02, 2010
So what glues electrons together?
In Cooper pairs or atomic orbitals (of opposite spins!)
Another(?) question: why observed in sun's corona magnetic reconnections "is not well understood: once started, it proceeds many orders of magnitude faster than predicted by standard models"(from wikipedia)?

Maybe the answer to both question is to understand what spin is - something around which quantum phase makes some number of rotations - this property isn't 'pointwise' but rather 'curvelike'! - on each plane orthogonal to such curve quantum phase makes something like here:
http://demonstrat...arities/
And generally looking at Aharonov-Bohm effect in vacuum (e.g. electron+proton), we see that quantum flux doesn't have to be quantified only inside superconducting rings, but these fluxons/spin curves could 'glue' fermion pairs, could have energy density required for reconnections, explain selection rules..(4th section of http://arxiv.org/abs/0910.2724 )
broglia
5 / 5 (1) Feb 03, 2010
So what glues electrons together?
At low temperature (Type I) superconductors it's a spin of electrons. But such force isn't sufficient for high temperature superconductors. The layered structure of cuprates and iron-based superconductors indicates, this force is exerted by surrounding atoms instead. Electrons are attracted to hole stripes (islands of positively charged atoms), which increases their repulsive forces up to level, they become geometrically degenerated. Illustratively speaking, each electron there is surrounded by so many electrons from all possible directions, it literally "doesn't know", where to move first. The forces exerted by surrounding electrons are compensating mutually and as the result, electrons near hole stripes are moving freely through atom lattice.

This mechanism is conceptually quite simple, but the large number of electrons involved makes trouble in its quantitative description. But we aren't required to compute thing for being able to use it.
gwrede
1 / 5 (1) Feb 04, 2010
Broglia: Thanks for an exceptionally clear explanation!
broglia
not rated yet Feb 09, 2010
Of course, this explanation basically collides with the meaning of the above article, in which superconductivity is related to a material's magnetic properties. But by Röser's semi-empirical equation, the temperature of supercritical transition depends basically on the geometry only (so called doping distance) with high precision for most superconductors known so far.

http://jpsj.ipap..../142/pdf

It basically means, supercritical temperature doesn't care about magnetic properties of neighboring atoms - it just depends on the compression of electron fluid inside of atom lattice. The more sparse the hole stripes are, the better compression level of electrons could be achieved, because electrons are repulsing mutually across layers at distance.

On the other hand, the other characteristics of superconductor - like the critical current or intensity of magnetic field, which destroys superconductivity - could still depend on the nature of neighboring atoms.
CoralCastleCode
not rated yet Mar 25, 2010
I'm not sure why they are acting as though they just discovered the atoms are held together by magnetic bonding ,It has been shown at this website for almost 6 years.also be sure not to overlook some fundamental unrecognized principles of magnetism & magnetic harmony in regards to what is now known as the "Equilibrius Grid"
http://www.coralcastlecode.com