3d and 4f correlations in quaternary iron pnictides: the special case of CeFe(As_(1-x)P_x)O

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Date/Time:Thursday, 29 Sep 2011 - Thursday, 29 Sep 2011
Location:ROOM 5 PHYSICS Hall
Channel:Condensed Matter Physics
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J. Anton Ames Lab and Iowa State University

The alloy CeFe(As1-xPx)O presents a unique situation among Fe-pnictides because substituting P for As not only suppresses antiferromagnetic (AFM) order of Fe as in many homologous systems, but it also suppresses AFM order of Ce since CeFePO was shown to be a paramagnetic heavy fermion system close to a ferromagnetic (FM) instability. Thus, one expects at least two critical points in this alloy, with both strong 3d and 4f correlations.
Our results show a continuous suppression of the AFM order of Fe down to TN-Fe = 40 K until the first critical point at xc1 ≈ 0.3. A further increase of x does not reduce TN-Fe anymore, instead the signature of this transition fade away. This suggests that the Fe-AFM state disappears at a classical critical endpoint and not at a quantum critical point. Interestingly, near xc1 the magnetic order of Ce changes from AFM to FM, which likely suppresses the superconductivity typically observed at the disappearance of Fe-magnetism in these compounds. Nevertheless, our single crystals show evidence for a narrow superconducting phase in the critical region. Simultaneous emergence of FM ordering and zero-resistivity at Tc = 4 K has been observed for several samples with x = 0.30, whereas no indication for superconductivity was found for x < 0.27 and x > 0.35. Upon further increasing the P-content, the FM order of Ce is first stabilized, with TC increasing to 9 K, but then weakens again.
The second critical point, where the FM-Ce ordering is suppressed, is very close to pure CeFePO. No superconductivity was observed near this critical point.