Researchers find evidence of 'quantum critical point' in high temperature superconductivity material

June 22, 2012 by Bob Yirka report

( -- 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 material’s 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, it’s 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.

Explore further: Putting the Pressure on Iron-Based Superconductors

More information: A Sharp Peak of the Zero-Temperature Penetration Depth at Optimal Composition in BaFe2(As1–xPx)2, Science 22 June 2012: Vol. 336 no. 6088 pp. 1554-1557 DOI: 10.1126/science.1219821

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(As1–xPx)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.

Related Stories

Putting the Pressure on Iron-Based Superconductors

March 5, 2009

( -- Traditionally, magnetism and superconductivity don't mix. For more than 20 years, the only known superconductors that worked at so-called "high" temperatures (above 30 K, or about -406 degrees Fahrenheit) ...

New property in warm superconductors discovered

November 17, 2010

( -- Led by Simon Fraser University physicist Jeff Sonier, scientists at TRIUMF have discovered something that they think may severely hinder the creation of room-temperature (37 degrees Celsius) superconductors.

A chemical detour to quantum criticality

July 6, 2011

( -- Physicists and chemists use different techniques to study essentially the same thing — the nature and behavior of matter. Usually the particular path is of little consequence, because they all lead ultimately ...

Recommended for you

'Material universe' yields surprising new particle

November 25, 2015

An international team of researchers has predicted the existence of a new type of particle called the type-II Weyl fermion in metallic materials. When subjected to a magnetic field, the materials containing the particle act ...

CERN collides heavy nuclei at new record high energy

November 25, 2015

The world's most powerful accelerator, the 27 km long Large Hadron Collider (LHC) operating at CERN in Geneva established collisions between lead nuclei, this morning, at the highest energies ever. The LHC has been colliding ...

Exploring the physics of a chocolate fountain

November 24, 2015

A mathematics student has worked out the secrets of how chocolate behaves in a chocolate fountain, answering the age-old question of why the falling 'curtain' of chocolate surprisingly pulls inwards rather than going straight ...

1 comment

Adjust slider to filter visible comments by rank

Display comments: newest first

5 / 5 (1) Jun 23, 2012
Interesting in connection to article Physicists show standard 'quasiparticle' theory breaks down at 'quantum critical point', which essentially means, above the quantum critical point the classical BCS theory of low temperature superconductivity is not valid anymore (compare my explanations of it here)

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.