Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session D35: Focus Session: Iron Pnictides and Other Novel Superconductors IV: General Experiment |
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Sponsoring Units: DMP Chair: Rick Greene, University of Maryland Room: 405 |
Monday, March 16, 2009 2:30PM - 3:06PM |
D35.00001: Iron Pnictide Superconductors: discovery and advances Invited Speaker: Superconducting transition in a layered ZrCuSiAs-type crystal was first reported for LaFePO in 2006 [1] and subsequently, a similar Tc was found for LaNiPO with the same crystal structure in 2007. However, Tc of these compounds reminded low ($\sim $4K). On February 23, 2008, our paper reporting a layered compound in LaFeAsO$_{1-x}$F$_{x}$(x=0.1) exhibiting a superconducting critical temperature Tc (mid-point) = 26K was published [3]. In this presentation I talk the background of this discovery and the subsequent advance in materials. The following points have been clarified to date; (1) Iron-based superconductors reported are 4-types crystal structures, the 1111[3], 122[4], 111[5], and 11 [6] type. All the high Tc iron-based superconductors contain a Fe square lattice and the Fe 3d orbitals dominate the Fermi-level. (2) The occurrence of a crystallographic transition accompanying anti-ferromagnetic to paramagnetic state in the parent compound is a requisite for a high Tc. (3) There exist a vast number of materials containing the Fe square lattice. (4). A partial substitution of Fe with other transition metal is possible without serious reduction of Tc. (4) A new insulating layer AEF (AE=Ca, Sr)was found to be effective in the 1111 phase [7]. (5) High pressure synthesis was effective to obtain the 1111 phases with higher Tc, (6) Epitaxial thin films exhibiting a Tc almost the same as that in the bulk were fabricated for CaFeAsO:Co[8]. Epitaxial thin films of LaFeAsO was recently reported as well [9]. \\[4pt] [1] Y.Kamihara et al. JACS,\textbf{ 28} (2006)10012, [2] T.Watanabe et al.Inorg.Chem,\textbf{46}(2007) 7719, [3 ]Y.Kamihara et al. J.Am.Chem.Soc.\textbf{130}(2008)3296., [4]M.Rotter et al. PRL, \textbf{101}(2008) 107006, [5] J.H.Tapp et al. PRB,\textbf{78}(2008)060505 [6] F.C.Hsu et al. PNAS,\textbf{105}(2008)14262., [7] S.Matsuishi et al. JACS \textbf{130}(2008)14428 [8] H.Hiramatsu et al. Appl.Phys.Express \textbf{1}(2008)101702, [9] H.Hiramatsu et al. APL.\textbf{ 93}(2008) 162504. [Preview Abstract] |
Monday, March 16, 2009 3:06PM - 3:42PM |
D35.00002: Materials and Physics in Pnictide Superconductors Invited Speaker: Superconductivity in the pnictides has shown itself to be very interesting and attractive. Some experimental results have revealed that the superconducting mechanism could be unconventional. In this talk I will survey our recent progress of both material synthesizing and physical properties of this rich family. We have made several major contributions to the synthesizing of new pnictide superconductors. (1) Fabrication of the hole doped RE$_{1-x}$Sr$_{x}$FeAsO samples (RE=La and Pr); (2) Fabrication of a series of new parent compounds DvFeAsF (Dv=divalent metals: Sr, Ca, Eu etc.) and many new superconductors with T$_{c}$ beyond 50 K by doping electrons into the system; (3) Invention of the new material (Sr$_{3}$Sc$_{2}$O$_{5})$Fe$_{2}$As$_{2}$ with rather large spacing distance between the FeAs planes. We have successfully grown the NdFeAsO$_{1-x}$F$_{x}$ and Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$ single crystals. It is found that the anomalous electron scattering in the normal state cannot be simply attributed to the multiband effect. The influence given by the magnetic correlation may play an important role. Specific heat, lower critical field and point contact tunneling all indicate the unconventional superconductivity and multigap features, while the paring symmetry of the superconducting gap may be a non-trivial issue. In the 1111 phase, the superfluid density is rather low and contains probably a nodal feature. While in the 122 phase, both the superfluid density and the quasiparticle density of states is about 5-10 times higher than that in the 1111 phase. An s-wave component was found in the 122 phase. I will also report the measurements on anisotropy, critical current density, critical fields and vortex phase diagram. Small anisotropy, high upper critical field and fish-tail effect (in 122) were observed. All these suggest very good potential applications. In collaboration with Gang Mu, Zhaosheng Wang, Huiqian Luo, Huan Yang, Xiyu Zhu, Ying Jia, Yonglei Wang, Fei Han, Bing Zeng, Bing Shen, Cong Ren, Lei Shan. [Preview Abstract] |
Monday, March 16, 2009 3:42PM - 3:54PM |
D35.00003: Determination of the phase diagram of the electron doped superconductor Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ Jiun-Haw Chu, James Analytis, Chris Kucharczyk, Ian Fisher Systematic measurements of the resistivity, heat capacity, susceptibility and Hall coefficient are presented for single crystal samples of the electron-doped superconductor Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$. These data delineate an $x-T$ phase diagram in which the single magnetic/structural phase transition that is observed for undoped BaFe$_2$As$_2$ at 134 K appears to split into two distinct phase transitions, both of which are rapidly suppressed with increasing Co concentration. Superconductivity emerges for Co concentrations above $x \sim 0.025$, and appears to coexist with the broken symmetry state up to $x \sim 0.06$. The optimal T$_c$ appears to coincide with the Co concentration at which the magnetic/structural phase transitions are totally suppressed. Superconductivity is observed for a further range of Co concentrations, before being completely suppressed for $x \sim 0.18$ and above. The form of this $x-T$ phase diagram is suggestive of an association between superconductivity and a quantum critical point arising from suppression of the magnetic and/or structural phase transitions. [Preview Abstract] |
Monday, March 16, 2009 3:54PM - 4:06PM |
D35.00004: The magnetic and superconducting phase diagram of PrFeAsF$_{x}$O$_{1-x}$ Costel R. Rotundu, Stephen D. Wilson, Byron K. Freelon, Edith Bourret-Courchesne, Robert J. Birgeneau The electronic phase diagram of the newly discovered iron pnictide superconductors RFeAsO$_{1-x}$F$_{x}$ (R=rare-earth) is of great interest and with implications in the understanding of the nature of superconductivity (SC) itself. Predicted by \textit{ab initio} calculations [1] and pointed by resistivity measurements [2], the relevance of a quantum critical point remains controversial in the light of the structural phase transition between the magnetic SDW and SC [3]. We present a detailed magnetic and superconducting phase diagram of PrFeAsO$_{1-x}$F$_{x}$ as inferred from magnetic susceptibility and resistivity measurements. References: [1] G. Giovannetti \textit{et al.}, Physica B \textbf{403}, 3653 (2008) [2] R. H. Liu \textit{et al.}, Phys Rev Lett \textbf{101}, 087001 (2008) [3] H. Luetkens et al., cond.mat:0806.3533 [Preview Abstract] |
Monday, March 16, 2009 4:06PM - 4:18PM |
D35.00005: Doping-Driven Collapse of the SDW Correlation Gap in SmFeAsO$_{1-x}$F$_{x}$ J.B. Kemper, Scott C. Riggs, Z. Stegen, G.S. Boebinger, R.D. Macdonald, F.F. Balakirev, Y. Kohama, A. Migliori, H. Chen, R.H. Liu, X.H. Chen We have investigated the Hall resistivity, $\rho_{xy}$ of polycrystalline SmFeAsO$_{1-x}$F$_{x}$ for four different fluorine concentrations from the onset of superconductivity through the collapse of the structural phase transition. For the two more highly-doped samples, $\rho_{xy}$ is linear in magnetic field up to 50 T with only weak temperature dependence, reminiscent of a simple Fermi liquid. For the lightly-doped samples with $x<0.15$, we find a low temperature regime characterized $\rho_{xy}(H)$ being both non-linear in magnetic field and strongly temperature dependent even though the Hall angle is small. The onset temperature for this non- linear regime is in the vicinity of the structural phase (SPT)/spin density wave (SDW) transitions. The temperature dependence of the Hall resistivity is consistent with a thermal activation of carriers across an energy gap. The evolution of the energy gap with doping is reported. [Preview Abstract] |
Monday, March 16, 2009 4:18PM - 4:30PM |
D35.00006: STM Investigation of the (001) surfaces of the Parent and Co-doped BaFe$_{2}$As$_{2}$ S. H. Pan, A. Li, D. R. Jayasundara, Y. Xuan, J. P. O'Neal, R. Jin, E. W. Plummer, R. Jin, A. S. Sefat, M. A. McGuire, B. C. Sales, D. Mandrus We have used a UHV Low Temperature STM to study the surface structure of the parent and the Co-doped BaFe$_{2}$As$_{2}$ single crystals. Various STM images with different structural symmetry were observed. The dominant apparent surface structure is ($\surd $2 x$\surd $2)R45$^{O}$ for the parent compound and 1 x 2 stripe-like for the Co-doped ones. We will compare the different surface structures and discuss the identification of the atomic plane exposure and the possible origins for such variety in surface structure. [Preview Abstract] |
Monday, March 16, 2009 4:30PM - 4:42PM |
D35.00007: Electronic Structure on (001) Surface of BaFe$_{2}$As$_{2}$ Parent Compound Studied with Scanning Tunneling Spectroscopy D. R. Jayasundara, A. Li, Y. Xuan, J. P. O'Neal, S. H. Pan, R. Jin, E. W. Plummer, R. Jin, A. S. Sefat, M. A. McGuire, B. C. Sales, D. Mandrus Doping can drive some metallic pnictide compounds to superconducting phase. The microscopic mechanism of this phase transition has still not been understood. Starting with the parent compound, we have used a UHV Low Temperature STM to study the density of states on the (001) surface of single crystal BaFe$_{2}$As$_{2}$. The tunneling spectrum varies depending on the local environment. All the spectrums have the same background with density of states depression near the Fermi energy, but some of them show different anomalies. We attribute some of these anomalies to surface states. These results may provide useful information to those surface techniques other than STM. [Preview Abstract] |
Monday, March 16, 2009 4:42PM - 4:54PM |
D35.00008: Structural Investigation of the BaFe$_{2}$As$_{2}$(001) Surface Using LEED and STM Von Braun Nascimento, X.B. He, R. Jin, E.W. Plummer, T.Y. Chien, Biao Hu, Guorong Li, M.H. Pan, J.F. Wendelken, A.S. Sefat, M.A. McGuire, B.C. Sales, D. Mandrus, Ang Li, Dilushan R. Jayasundara, Yi Xuan, Jared O'Neal, Shuheng Pan BaFe$_{2}$As$_{2}$, a parent compound to one of the newly discovered high-$T_{c}$ superconductors, presents very interesting physical properties such as a structural transition occurring prior to the formation of a spin density wave. It is surely of interest to investigate the effect of breaking the symmetry by creating a surface. Single crystals of BaFe$_{2}$As$_{2 }$were cleaved in vacuum exposing a (001) surface. Quantitative LEED I-V measurements and low-temperature STM topography revealed an ordered As surface with disorded Ba atoms present. LEED shows a clear (1 $\times $ 1) periodicity with a surface structure slightly different than the bulk. STM reveals a weak C(2 $\times $ 2) periodicity. We will explore the possibility that the C(2x2) STM image is electronic in origin. [Preview Abstract] |
Monday, March 16, 2009 4:54PM - 5:06PM |
D35.00009: Temperature and Spatial Dependence of the Superconducting and Pseudogap of NdFeAsO$_{0.86}$F$_{0.14}$. X.B. He, M.H. Pan, G.R. Li, J.F. Wendelken, R.Y. Jin, A.S. Sefat, M.A. McGuire, B.C. Sales, D. Mandrus, E.W. Plummer Scanning tunneling microscopy/spectroscopy are used to investigate the superconducting gap and pseudogap of Fe based high-Tc superconducting material NdFeAsO0.86F0.14 at various temperatures from 17 K to 150 K. The superconducting gap (SG) in the tunneling spectra follows the BCS prediction and closes at Tc of the bulk material. Surprisingly, a pseudogap (PG) opens abruptly just above Tc and closes at 120 K, strongly suggesting that the SG and PG states have competing order parameters in contrast to the cuprates. The PG state may be related to spin fluctuations in the doped materials. Research was supported in part at ORNL by Laboratory Directed Research and Development funds and by the Division of Materials Sciences and Engineering, Office of Basic Energy Sciences, US DOE. [Preview Abstract] |
Monday, March 16, 2009 5:06PM - 5:18PM |
D35.00010: Scanning Tunneling Spectroscopy and Vortex Imaging in the Iron-Pnictide Superconductor BaFe$_{1.8}$Co$_{0.2}$As$_2$ Yi Yin, M. Zech, T.L. Williams, X.F. Wang, G. Wu, X.H. Chen, J.E. Hoffman We present an atomic resolution scanning tunneling spectroscopy study of superconducting BaFe$_{1.8}$Co$_{0.2}$As$_2$ single crystals in magnetic fields up to 9\, Tesla. At zero field, a single gap with coherence peaks at $\overline{\Delta}=6.25$\, meV is observed in the density of states. At 9\, T and 6\, T, we image a disordered vortex lattice, consistent with isotropic, single flux quantum vortices. Vortex locations are uncorrelated with strong scattering surface impurities, demonstrating bulk pinning. The vortex-induced sub-gap density of states fits an exponential decay from the vortex center, from which we extract a coherence length $\xi=27.6\pm 2.9$\, {\AA}, corresponding to an upper critical field $H_{c2}=43$\, T. [Preview Abstract] |
Monday, March 16, 2009 5:18PM - 5:30PM |
D35.00011: STM measurements on iron pnictides Zhanybek Alpichshev Some results of scanning tunneling microscopy/spectroscopy of cobalt doped pnictides at differrent doping levels are presented. [Preview Abstract] |
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