MaNEP e-Newsletter for members - Published monthly             Issue Nr. 11 - September 2006 / page 3

Free Column
How to find a pin in a haystack
by Prof. Jürg Hülliger (chemist/UniBE - head of Project 4)
Separating superconductive matter from normal state diamagnetic grains : new method
Doing chemistry in the solid state, say, by reacting granular starting materials exposed to high temperature offers opportunities and causes difficulties quite different from those we are used to when performing reactions in liquids and gases. Performing chemistry in liquids and gases can take a fortune of highly advanced separation techniques to purify and finally identify compounds.

Contrary, solid state production of materials often has to enface samples that are not phase-pure, which can not be processed to separate them into individual phases, including minority contributions.

Here, I think we have made a real step forward by developing a new method for separating superconductive matter from normal state diamagnetic grains : if a ceramic grain or single crystal in the Meissner or vortex state gets located into the near field distribution of a magnetized wire (perpendicular to its length), the particle may be pushed into the region of the lowest total field, depending on the ratio of magnetic and gravitational forces.

Phenomenologically, the particle can get attracted side wise to a wire, i.e. in the direction perpendicular to the external field. Magnetic forces at considerably low external fields of less than 50 G can be sufficient to hold

 superconducting particles on a thin wire, allowing to separate such particles from an assembly of grains.

Basics of the theoretical aspects and a first experimental demonstration can be found in Supercond. Sci. Technol. 19, 748-755, 2006. [ see publications ]







 Up to now we have been able to demonstrate a sensitivity of 1 ppm and we are quite confident to rise sensitivity up to about 10 ppb.

Given now a rather simple method to find “a pin in a haystack” we are well equipped for a quest to search for new phases by combinatorial syntheses, performed within project 4 of MaNEP II.

First trials have already retrieved superconducting particles at 77 K from highly multiphase reaction products. Important to notice, that by changing the external field, a superconductor, magnetically described by its hysteresis loop, will behave totally different than normal state matter.

This means, a particle large enough to be observed visually would be sufficient to demonstrate the existence of superconductivity at any temperature at which such an experiment is performed.

Having such a tool in hand, we might once more speculate about the existence of superconductivity above 135 K. Thus, we will revisite published work (Phys. Rev. B and many others!) reporting on higher Tc’s, which finally failed in reproducing bulk samples.

However, it might well be that such samples nevertheless contained traces of interesting superconductors, difficult to synthesize in bulk quality.
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Figures description

The capturing of superconducting particles by a magnetized wire

From top to bottom: YBCO at zero field; grains captured sidewise at maximum field; paramagnetic behaviour when decreasing the field; loss of particles at low field. Note that particles moved sidewise when lowering the field below capturing.
The investigators


left : Prof. Jürg Hulliger
right : Dr. Hans Willems

[ click here to see big picture ]
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