Quantum criticality –The physics of quantum critical phase transitions connects to some of the most challenging and technologically relevant problems in condensed matter physics, including metal-insulator transitions, frustrated magnetism and high temperature superconductivity. Near a quantum critical point (QCP) a new kind of metal emerges, one whose thermodynamic and transport properties differ from the unified phenomenology with which we understand conventional metals – the Landau-Fermi liquid theory – which are characterized by a low temperature T 2 resistivity. Studying the evolution of this temperature dependence identifies a quantum phase transition at the heart of pnictide superconductivity. We probe the transport properties of BaFe 2 (As 1−x P x ) 2 at low temperatures by suppressing superconductivity with high magnetic fields. At sufficiently low temperatures, all compositions cross-over from a linear to quadratic temperature dependence, consistent with a low-temperature Landau-Fermi liquid groundstate. At optimal doping, we find a divergence of the electronic effective mass, derived from the T 2 resistivity coefficient via the Kadowaki-Woods ratio, indicating increased electron-electron correlations near the quantum critical point. This same doping gives the most robust superconductivity, indicating that these mass-enhancing electronic correlations are intimately tied to superconducting pairing in the pnictides. Transport in the quantum critical regime of the iron arsenide superconductor BaFe 2 (As 1-x P x ) 2 J.G. Analytis 1,2, H-H. Kuo 2, R.D. McDonald 3, M. Wartenbe 3, P.M.C. Rourke 4, N. E. Hussey 4 and I. R. Fisher 1 1. Stanford University; 2. UC Berkeley; 3. National High Magnetic Field Laboratory; 4. University of Bristol Funding Grants: G.S. Boebinger (NSF DMR ); I.R. Fisher (DOE BES DE-AC02-76SF00515); N. Harrison (DOE BES ‘Science of 100 tesla’) Facilities: Pulsed and DC Field facilities. Instrument/Magnet: 65 T short pulse, 36 T resistive, 45 T Hybrid. Citation: Transport near a quantum critical point in BaFe 2 (As 1-x P x ) 2, J.G. Analytis, H-H. Kuo, R.D. McDonald, M. Wartenbe, P.M.C. Rourke, N. E. Hussey and I. R. Fisher, Nature Physics, 10, 194 (2014) - The resistance of conventional metals arises from electron scattering, which increases as the square of temperature (the blue region of the phase diagram). - By contrast, where superconductivity is most robust the resistance remains close to linear in temperature down to the onset of superconductivity (the red region of the phase diagram). - When high magnetic fields suppress superconductivity, the resistance returns to quadratic at low temperature, revealing that the T 2 coefficient diverges – the signature of a quantum critical point at the same doping that gives rise to the most robust superconductivity ρ = ρ 0 + AT n a) Color plot of the power-law dependence n as a function of temperature and composition. b) divergence of the T 2 coefficient and effective mass approaching the quantum critical point