Just recently, Japanese scientists have discovered that the Tc of iron-based layer compound LaO1-xFxFeAs would be increased till ~43 K for x=0.11 and at ~4 GPa mechanical pressure, which is the highest Tc has been reported for this family of superconductors. The result indicates that this system would have potential to reach higher Tc in comparable with the Tc of copper-oxide HTSCs. Figure 1 shows the crystal structure of this system, where the substitution of F1- at the O2- site, creates the induced charge in the FeAs layer and increase the Tc till the optimal doping x=0.11 with maximum Tc of ~26 K. In this study the effect of mechanical pressure up to 31 GPa has been investigated on the Tc of polycrystalline LaO1-xFxFeAs with x=0.05 (underdoped) and x=0.11 (optimal doped). In below the most important results of this study would be discussed briefly.
Results:
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The resistivity measurements show that the Onset Tc and Midpoint Tc of x=0.11 sample increase by pressure until 3 GPa respectively to ~43 and ~36 K with a step like shape near 1 GPa. The Zero-point Tc increases much slower and thus results in a broadening of superconducting transition by applying pressure (Fig. 2a and inset).
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Magnetic measurements confirm the increasing of Tc by mechanical pressure in this system.
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The Onset Tc of x=0.11 sample increases slowly after 3 GPa to reach highest Tc~43 at ~4 GPa and then decreases sharply with a further increase in pressure above 4 GPa (Fig. 2c &d).
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For x=0.05 with Onset Tc~25 K (which could be considered as an underdoped system in this family), the pressure increases the Onset Tc by the rate of 1.0-2.0 K/GPa which is considerably smaller than the value for 0.11 optimal doped sample. The pressure also changes the semiconducting behavior of resistivity just above Tc to the metallic behavior (Fig. 3).
The authors suggest that the increasing of Tc by mechanical pressure in this system could be resulted from the pressure induced anisotropic shrinking of crystal structure, as comparable to the pressure induced charge transfer effect in copper oxide HTSCs.
Ref.: H. Takahashi, K. Igawa, K. Arii, Y. Kamihara, M. Hirano & H. Hosono, Nature 453 (2008) 376.
