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The magnetic fluctuations associated with a quantum critical point (QCP) arewidely believed to cause the non-Fermi liquid behaviors and unconventionalsuperconductivities, for example, in heavy fermion systems and high temperaturecuprate superconductors. Recently, superconductivity has been discovered iniron-based layered compound $LaO_{1-x}F_xFeAs$ with $T_c$=26 K\cite{yoichi},and it competes with spin-density-wave (SDW) order\cite{dong}. Neutrondiffraction shows a long-rang SDW-type antiferromagnetic (AF) order at $\sim134$ K in LaOFeAs\cite{cruz,mcguire}. Therefore, a possible QCP and its role inthis system are of great interests. Here we report the detailed phase diagramand anomalous transport properties of the new high-Tc superconductors$SmO_{1-x}F_xFeAs$ discovered by us\cite{chenxh}. It is found thatsuperconductivity emerges at $x\sim$0.07, and optimal doping takes place in the$x\sim$0.20 sample with highest $T_c \sim $54 K. While $T_c$ increasesmonotonically with doping, the SDW order is rapidly suppressed, suggesting aQCP around $x \sim$0.14. As manifestations, a linear temperature dependence ofthe resistivity shows up at high temperatures in the $x<0.14$ regime, but atlow temperatures just above $T_c$ in the $x>0.14$ regime; a drop in carrierdensity evidenced by a pronounced rise in Hall coefficient are observed, whichmimic the high-$T_c$ cuprates. The simultaneous occurrence of order, carrierdensity change and criticality makes a compelling case for a quantum criticalpoint in this system.

Phase Diagram and Quantum Critical Point in Newly Discovered Superconductors: SmO_{1-x}F_xFeAs