The magnetic fluctuations associated with a quantum critical point (QCP) are widely believed to cause the non-Fermi liquid behaviors and unconventional superconductivities, for example, in heavy fermion systems and high temperature cuprate superconductors. Recently, superconductivity has been discovered in iron-based layered compound $LaO_{1-x}F_xFeAs$ with $T_c$=26 Kcite{yoichi}, and it competes with spin-density-wave (SDW) ordercite{dong}. Neutron diffraction shows a long-rang SDW-type antiferromagnetic (AF) order at $sim 134$ K in LaOFeAscite{cruz,mcguire}. Therefore, a possible QCP and its role in this system are of great interests. Here we report the detailed phase diagram and anomalous transport properties of the new high-Tc superconductors $SmO_{1-x}F_xFeAs$ discovered by uscite{chenxh}. It is found that superconductivity emerges at $xsim$0.07, and optimal doping takes place in the $xsim$0.20 sample with highest $T_c sim $54 K. While $T_c$ increases monotonically with doping, the SDW order is rapidly suppressed, suggesting a QCP around $x sim$0.14. As manifestations, a linear temperature dependence of the resistivity shows up at high temperatures in the $x<0.14$ regime, but at low temperatures just above $T_c$ in the $x>0.14$ regime; a drop in carrier density evidenced by a pronounced rise in Hall coefficient are observed, which mimic the high-$T_c$ cuprates. The simultaneous occurrence of order, carrier density change and criticality makes a compelling case for a quantum critical point in this system.
Download