Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session L3: Fe-based Superconductors: Competing Orders |
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Sponsoring Units: DCMP Chair: Hong Ding, Institute of Physics, CAS Room: 301/302 |
Tuesday, March 17, 2009 2:30PM - 3:06PM |
L3.00001: Competing orders and spin density wave instabilities in FeAs-based systems Invited Speaker: The discovery of superconductivity with T$_c$ up to 55 K in layered FeAs-based compounds has generated tremendous interest in the scientific community. Except for relatively high T$_c$, the Fe pnictides display many interesting properties. Among others, the presence of competing orders is one of the most intriguing phenomena. In the early stage of our study on the compounds, we identified a spin-density-wave (SDW) ordered state for the parent compound with a stripe (or collinear) type spin structure based on the transport, specific heat, optical spectroscopy measurements and the first- principle calculations. The proposed spin structure from a nesting of the Fermi surfaces is confirmed by subsequent neutron experiments. However, it could also be explained by a local superexchange picture. In this talk I shall focus on our recent optical data on single crystal samples, trying to address the debating issue about itinerant or localized approaches to the SDW order. We found that the undoped compounds are quite metallic with relatively high plasma frequencies above T$_{SDW}$. Upon entering the SDW ordered state, a large part of the Drude component is removed by the gapping of Fermi surfaces. Meanwhile, the carrier scattering rate is even more dramatically reduced. Those observations favor an itinerant description for the driving mechanism of SDW instability. Nevertheless, our experiments also indicate that Fe pnictides are not simple metals. A high energy gap-like feature is present even above T$_{SDW}$, which seems to be linked with the antiferromagnetic spin fluctuations. For the superconducting samples, a superconducting pairing energy gap is clearly observed in the far-infrared reflectance measurement. The Ferrell-Glover- Tinkham sum rule is satisfied at a low energy scale. Work done in collaboration with: G. F. Chen, J. L. Luo, Z. Fang, X. Dai, W. Z. Hu, J. Dong, G. Li, Z. Li, P. Dai, J. Lynn, H. Q. Yuang, J. Singleton. [Preview Abstract] |
Tuesday, March 17, 2009 3:06PM - 3:42PM |
L3.00002: Magnetic order close to superconductivity in the iron-based layered RFeAsO1?xFx (R = La, Ce) systems Invited Speaker: |
Tuesday, March 17, 2009 3:42PM - 4:18PM |
L3.00003: Competing Magnetic Interactions, Structural Phase Transition, and the Unprecedented Giant Coupling of Fe-spin State and the As-As Interactions in Iron-Pnictide Invited Speaker: From all-electron fixed-spin-moment calculations [1], we showed that the ferromagnetic and checkerboard antiferromagnetic ordering in LaOFeAs were not stable and the stripe Fe-spin configuration (i.e. SDW) was the only stable ground state. The main exchange interaction between Fe ions are large, antiferromagnetic, and frustrated. The magnetic stripe SDW phase breaks the tetragonal symmetry, removes the frustration, and causes a structural distortion. We unravel surprisingly strong interactions between arsenic ions, the strength of which is controlled by the Fe-spin state in an unprecedented way [2]. Reducing the Fe-magnetic moment, weakens the Fe-As bonding, and in turn, increases As-As interactions, causing giant reduction in the c-axis. For CaFe$_{2}$As$_{2}$ system, this reduction of c-axis with the loss of the Fe-moment is as large as 1.4 {\AA}, an unheard of giant coupling of local spin-state of an ion to its lattice. Since the calculated large c-reduction has been recently observed only under high-pressure, our results suggest that the iron magnetic moment should be present in Fe-pnictides at all times at ambient pressure. Implications of these findings on the mechanism of superconductivity in iron-pnictides will be discussed. \\[4pt] [1] T. Yildirim, Phys. Rev. Lett. 101, 057010 (2008) (arXiv:0804.2252). \\[0pt] [2] T. Yildirim, arXiv:0807.3936 (2008) [Preview Abstract] |
Tuesday, March 17, 2009 4:18PM - 4:54PM |
L3.00004: Superconductivity and Magnetism in LaO$_{1-x}$F$_{x}$FeAs Invited Speaker: Measuring $^{75}$As, $^{139}$La, and $^{57}$Fe Nuclear Magnetic Resonance (NMR) as well as $\mu $SR, transport and thermodynamic properties we have determined the phase diagram of LaO$_{1-x}$F$_{x}$AsFe superconductors [1-6]. In my talk, I will show experimental studies of the magnetic ordering [2, 5], properties of the superconducting state [1, 3, 5] and the normal state properties [1, 4, 6] in the superconducting regions of the phase diagram. While the temperature dependence of the London penetration as determined from $\mu $SR points to an isotropic s wave state [3], our early NMR data suggest singlet pairing and nodes of the order parameter [1]. Extending the NMR work to lower temperatures we find evidence for a deviation of the T$^{3}$ behaviour of the spin lattice relaxation, which would agree with the extended s-wave symmetry suggested in recent theoretical work. In the paramagnetic normal state, NMR on all three nuclei shows that the local electronic susceptibility rises with increasing temperature. This had led to suggest the presence of a pseudogap, which I will discuss in detail. The scaling of all NMR shifts with respect to the macroscopic susceptibility indicates that there is no apparent multiband effect through preferential hyperfine couplings. Relaxation measurements indicate a similar temperature-dependence for (T$_{1}$T)$^{-1}$, and suggest that the dynamical susceptibility changes uniformly in q space with varying temperature. The transport properties show some striking similarities to the findings in cuprates [6] and, finally, susceptibility [4] as well as NMR studies point to the antiferromagnetic fluctuations, whose relevance is also discussed in many theoretical models of the superconducting pairing mechanism. In collaboration with Hans-Joachim Grafe, Christian Hess, R\"udiger Klingeler, G\"unter Behr, Agnieszka Kondrat, Norman Leps, and Guillaume Lang, IFW Dresden; Hans-Henning Klauss, TU Dresden; and Hubertus Luetkens, PSI Villigen. \\[4pt] References: \\[0pt] [1] H.-J. Grafe et al., Phys. Rev. Lett. \textbf{101}, 047003 (2008) \\[0pt] [2] H.-H. Klauss et al., Phys. Rev. Lett. \textbf{101}, 077005 (2008) \\[0pt] [3] H. Luetkens et al., Phys- Rev. Lett. \textbf{101}, 097009 (2008) \\[0pt] [4] R. Klingeler et al., arXiv: 0808.0708 (2008) \\[0pt] [5] H. Luetkens et al., arXiv: 0806.3533 (2008) \\[0pt] [6] C. Hess et al., arXiv: 0811.1601 (2008) [Preview Abstract] |
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