June 22, 2012 report
Researchers find evidence of 'quantum critical point' in high temperature superconductivity material
(Phys.org) -- In the seemingly never ending search for a superconductor that can operate at or near room temperature, researchers have discovered a lot of things about superconductors in general. One of these is that there appears to be two classes of them - those materials that only take on their superconductive properties when exposed to an environment just a few degrees above absolute zero, and those that do the same at somewhat higher temperatures. Those in the latter group are said to have high critical-temperature superconductivity (Tc). Unfortunately, researchers have made little progress in understanding how such materials are able to do what they do. Now, new research by a team of Japanese, British and American researchers has found that at least one of these Tc materials likely has a quantum critical point (QCP), which if true, would go a long way towards explaining why some materials are able to become superconductors at higher temperatures than others. They have published a paper detailing their findings in the journal Science.
A materials QCP is analogous to the phase changes in ordinary materials, such as ice melting to water. In the quantum world, rather than occurring due to the ordering of atoms, they are based on electronic carriers, and come about as a result of interactions rather than organization. At the QCP, the properties of the material change suddenly, due to quantum fluctuations. Researchers have suspected that the reason materials with Tc are able to become super conductive (have no resistance to an electrical charge) is because they have a QCP, but thus far have not been able to prove it.
In this new research, the team has come close. They worked with a Tc known as an iron pnictide, because of its location on the periodic table; in this case it was arsenic. To test the material for a QCP, the team first added a doping factor to provide an extra charge. When put in an extremely cold environment and adding just the right amount of the doping factor, the material becomes a superconductor.
To test the material to see if it had a QCP, the team turned to a measurement method known as the London penetration depth, which measures the depth to which a magnetic field penetrates a given material. In so doing, the team was able to see a very noticeable jump in the penetration depth with their sample as the doping factor was tweaked to its optimal value.
Unfortunately, while the jump is a good sign that the Tc sample does likely have a QCP, its not definitive proof, though it does come close. Close enough that it gives researchers a renewed belief that their theoretical models have been correct all along, likely leading to new research that might just uncover some other mysteries still surrounding super conducting materials.
In a superconductor, the ratio of the carrier density, n, to its effective mass, m*, is a fundamental property directly reflecting the length scale of the superfluid flow, the London penetration depth, λL. In two-dimensional systems, this ratio n/m* (~1/λL2) determines the effective Fermi temperature, TF. We report a sharp peak in the x-dependence of λL at zero temperature in clean samples of BaFe2(As1xPx)2 at the optimum composition x = 0.30, where the superconducting transition temperature Tc reaches a maximum of 30 kelvin. This structure may arise from quantum fluctuations associated with a quantum critical point. The ratio of Tc/TF at x = 0.30 is enhanced, implying a possible crossover toward the Bose-Einstein condensate limit driven by quantum criticality.
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