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The physics of quantum critical phase transitions connects to some of
the most difficult problems in condensed matter physics, including
metal-insulator transitions, frustrated magnetism and high-temperature
superconductivity. Near a quantum critical point, a new kind of metal
emerges, the thermodynamic and transport properties of which do not fit
into the unified phenomenology for conventional metalsÃ¢e Landau
Fermi-liquid theory. Studying the evolution of the temperature
dependence of these observables as a function of a control parameter
leads to the identification of both the presence and the nature of the
quantum phase transition in candidate systems. In this study we measure
the transport properties of BaFe2(As1-xPx)2 below the critical
temperature Tc by suppressing superconductivity with high magnetic
fields. At sufficiently low temperatures, the resistivity of all
compositions (x>0.31) crosses over from a linear to a quadratic
temperature dependence, consistent with a low-temperature Fermi-liquid
ground state. As compositions with optimal Tc are approached from the
overdoped side, this crossover becomes steeper, consistent with models
of quantum criticality where the effective Fermi temperature TF goes to
zero. These measurements also point to a unversality class of
quantum-criticality which extends to the physics of heavy fermions and
the cuprates.
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