Researchers may have discovered the key to high temperature superconductivity — quantum criticality. A quantum critical point occurs where a material undergoes a continuous transformation at absolute zero. For superconducting cuprates and iron-arsenides, the curve of the superconducting transition temperature, Tc, versus doping (or pressure) is dome shaped. It wasn’t clear until now if superconductivity prevents a quantum critical point or if quantum critical behavior is hidden beneath the dome. An international team studied a barium-iron arsenic superconductor where arsenic is partially substituted with phosphorous, BaFe2(As1-xPx)2. Phosphorous substitution suppresses magnetism and induces superconductivity leading to a maximum Tc when magnetism is fully suppressed. The team measured the characteristic decay of the magnetic field at the surface, the so-called London penetration depth, and found quantum critical behavior coexists with and may actually be protected by superconductivity. Better understanding what drives high temperature superconductivity will accelerate the search for new, higher temperature superconductors.
A Sharp Peak of the Zero-Temperature Penetration Depth at Optimal Composition in BaFe2(As1-xPx)2