Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session Y40: Focus Session: Iron Based Superconductors: Magnetism and Transport |
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Sponsoring Units: DCMP DMP Chair: Athena Safa-Sefat, Oak Ridge National Laboratory Room: F151 |
Friday, March 19, 2010 8:00AM - 8:12AM |
Y40.00001: Investigation of SmFe$_{1-x}$Co$_x$AsO using $^{57}$Fe M\"{o}ssbauer spectroscopy as a function of temperature and applied magnetic field G. Long, J. Steiner, M. DeMarco, D. Coffey, H. Zeng, Y. Li, G. Cao, Z. Xu The M\"{o}ssbauer spectra(MS) of SmFe$_{1-x}$Co$_x$AsO, where $x$=0.0, 0.05, and 0.1, were measured in applied fields up to 9 T and at temperatures between 4.2 K and 298 K. SmFeAsO is magnetically ordered with T$_N$=138 K and has a hyperfine magnetic field of 5.05 T at 4.2 K. In a 9 T magnetic field the MS is consistent with a distribution of hyperfine magnetic fields and has a width of $H_{apllied}+H_{hyperfine} \simeq 13 T$, suggesting that the sample is not polarized and evidence of strong magnetic anisotropy. The MS of SmFe$_{0.95}$Co$_{.05}$AsO is well-described by a Lorentzian peak from room temperature to 4.2 K. From room temperature to $\sim$25 K the linewidth is 0.30 mm/sec but then increases to 0.72 mm/sec when cooled from 25 K to 10 K. Below 10 K it remains constant. The MS of SmFe$_{0.9}$Co$_{.1}$AsO shows no evidence of internal field at any temperature. [Preview Abstract] |
Friday, March 19, 2010 8:12AM - 8:24AM |
Y40.00002: Revealing electronic, structural and magnetic phases in NdFeAsO with temperature-dependent electron energy-loss spectroscopy Juan C. Idrobo, Matthew F. Chisholm, Micah Prange, Stephen J. Pennycook, Sokrates T. Pantelides, Jing Tao, Yimei Zhu, Zhi-An Ren, Zhong-Xian Zhao Iron pnictides, quaternary compounds of the form (ReFeAsO, Re=rare-earth elements), present structural and magnetic phase transitions in the temperature range between 120 K to 150 K. In this talk, we report momentum transfer- and temperature-dependent electron energy-loss spectroscopy (EELS) studies of the parent compound NdFeAsO using (scanning) transmission electron microscopy. In particular, we find that both the Fe L$_{23}$-ratio and the Nd M$_{45}$-ratio increase, the intensities of the Fe L$_{1}$-edge and Nd M$_{3}$-edge reduce, and the O K- edge fine structure remains unchanged as the temperature decreases. Using a combination of experiments and total-energy first-principles calculations within density functional theory, we show that the changes of the EELS Fe and Nd fine structure can be directly correlated with changes on the electronic structure of NdFeAsO and Fe and Nd magnetic moments. [Preview Abstract] |
Friday, March 19, 2010 8:24AM - 8:36AM |
Y40.00003: Magneto-volume effects in the iron-pnictide superconductors D. Parshall, Jennifer Niedziela, A. Subedi, D.J. Singh, T. Egami The iron-pnictide superconductors display extraordinary sensitivity to the lattice, and in particular to the height (angle) of the pnictogen atom relative to the iron plane. We have developed a Landau theory based upon the magneto-volume effect to explain this phenomenon, and provide experimental evidence to support this interpretation. In addition, we demonstrate a correlation between the local moment predicted by our theory and the superconducting critical temperature. We note that the strong dependence of the iron spin moment on the pnictogen atom height provides one possible mechanism for unconventional electron-phonon coupling. Finally, we provide evidence showing the relationship between negative thermal expansion (which is associated with the magneto-volume effect) and superconducting behavior in these compounds. [Preview Abstract] |
Friday, March 19, 2010 8:36AM - 9:12AM |
Y40.00004: Electron itinerancy, orbital symmetry and itinerant spin fluctuations in Fe-based superconductors as revealed by soft x-ray spectroscopies Invited Speaker: The discovery of superconductivity with critical temperature exceeding 55 K in the iron-pnictides (FeSC) compounds has offered the community a new set of materials hosting high temperature superconductivity [1]. In this talk, I will discuss some of our most important results obtained with complementary soft x-ray spectroscopies such as core level and Angle Resolved Photoemission (ARPES) and x-ray absorption (XAS) on different families of FeSC compounds. Results concerning two main aspects particularly important for the physics of the FeSC materials will be discussed, namely 1) the bonding topology/orbital symmetry, and 2) the presence of spin fluctuations. The symmetry of the orbitals and their hybridization play a central role in the interplay between magnetism and superconductivity, and in the nature of the superconducting pairing. I will discuss the symmetry and topology of the occupied and unoccupied orbitals by presenting results of polarization-dependent ARPES and element-specific XAS measurements of the electronic structure in the normal state of BaFe$_{2}$As$_{2}$ and BaFe$_{1.8}$Co$_{0.2}$As$_{2}$ single crystals. I will then focus on discussing how the presence of exchange multiplets in the Fe 3$s$ photoemission spectra in different FeSC materials are indicative of the presence of fluctuating spin moments on the Fe sites. Due to extremely fast time scales involved, the detection of magnetic fluctuations by means of magnetic probes has so far remained elusive. Our experiment provides a strong test case for the occurring of itinerant magnetic fluctuations, whose detection has been made possible by the extremely fast time scales proper of the photoemission process. The Fermi surface topology revealed by ARPES experiments in different FeSC compounds and its possible relation to the presence of magnetic fluctuation will also be discussed. \\[4pt] [1] Y. Kamihara, et al., \textit{J. Am. Chem. Soc}. \textbf{130}, 3296 (2008). [Preview Abstract] |
Friday, March 19, 2010 9:12AM - 9:24AM |
Y40.00005: Temperature dependence of the penetration depth in pnictides measured locally with scanning SQUIDs Thomas Lippman, Clifford Hicks, Lan Luan, Martin Huber, James Analytis, Jiun-Haw Chu, Ann Erickson, Ian Fisher, Kathryn Moler Using a Scanning SQUID Microscope, we locally measure changes in the ab-plane penetration depth $\Delta\lambda_{ab}(T)$ of superconducting samples. Local measurements allow us to check for electronic homogeneity, including $T_c$ homogeneity, as well as to minimize the possibility of sample-geometry-dependent errors, including contributions from the c-axis penetration depth. We report results on lead as well as on pnictide superconductors, including $Ba(Fe_{0.95}Co_{0.05})_{2}As_{2}$ and LaFePO. In LaFePO, we find a linear-T dependence with a slope of 143$\pm$15 {\AA}/K, suggesting well-developed line nodes in the superconducting order parameter. [Preview Abstract] |
Friday, March 19, 2010 9:24AM - 9:36AM |
Y40.00006: Resistivity, Hall coefficient, and magnetoreistance of superconducting FeSe$_{1-x}$Te$_{x}$ thin films Hsien-Hung Chang, Jiu-Yong Luo, Chun-Te Wu, Fong-Chi Hsu, Ta-Kun Chen, Mau-Kuen Wu, Ming-Jye Wang We report the normal state transport properties of superconducting FeSe$_{1-x}$Te$_{x}$ films, for 0$<$x$<$0.9. The thin films were grown on MgO substrate by pulsed laser deposition (PLD) technique. All of films exhibit tetragonal structure with c-axis preferred orientation. The superconducting transition temperature of FeSe$_{1-x}$Te$_{x}$ films depends on Te substitution level and the normal state resistivity changes from metal-like to semiconductor-like behavior gradually as Te substitution level is increased. The strong temperature and Te substitution level dependences of Hall coefficients demonstrate the multi-band characteristic and the modification of Fermi surface by Te substituted. The magnetoresistance of films with low Te substitution is positive and shows a regular quadratic dependence on magnetic field. For higher Te substitution, the magnetoresistance changes from positive to negative and that can be attributed to the weak localization effect. The magnitude of Hall coefficient and magnetoresistance changes quickly below the temperature of structural distortion. Our results strongly demonstrate the importance of low temperature structural distortion on the scattering rate of carriers. [Preview Abstract] |
Friday, March 19, 2010 9:36AM - 9:48AM |
Y40.00007: Superconducing SrFe$_{1.75}$Co$_{.25}$As$_{2}$ thin films grown by pulsed laser deposition Richard Suchoski, Richard Greene, Johnpierre Paglione, Nicholas Butch, Shanta Saha, Kevin Kirshenbaum, Paul Bach, Kui Jin, Xiao Hang Zhang, Ichiro Takeuchi We are growing SrFe$_{1.75}$Co$_{.25}$As$_{2}$ thin films by pulsed laser deposition. The target was prepared by grinding and sintering a superconducting single crystal of SrFe$_{1.75}$Co$_{.25}$As$_{2}$. Typical deposition conditions are 700C and base pressure of 7x10$^{-8}$ torr. The films are grown on substrates LSAT (100) and STO (100), and their T$_{c}$(onset) range is 21-15K. We will report on structural analysis and transport measurements on the films. [Preview Abstract] |
Friday, March 19, 2010 9:48AM - 10:00AM |
Y40.00008: Transport characteristics of Co-doped BaFe$_{2}$As$_{2}$ epitaxial thin film 24deg grain-boundaries P. Panyajirawut, N. Smith, C. Tarantini, S. Lee, J. Jiang, C.W. Bark, J.D. Weiss, C.M. Folkman, S.H. Baek, E.E. Hellstrom, C.B. Eom, D.C. Larbalestier, M.S. Rzchowski We investigate [001] 24deg tilt GBs of epitaxial thin film of the pnictide superconductor Co-doped Ba(Fe$_{0.92}$Co$_{0.08})_{2}$As$_{2}$ on (001) SrTiO$_{3}$ bicrystal substrates. The films were grown by pulsed laser deposition (PLD) with KrF (248 nm) UW excimer laser in vacuum at a base pressure of 2x10$^{-7}$ Torr. The chemical composition of the films determined by wavelength dispersive x-ray spectroscopy (WDS) is Ba : Fe : Co : As : O = 1 : 1.7 : 0.13 : 1.7 : 0.3. Four-circle x-ray diffraction exhibits excellent expitaxy with cube-on cube in-plane epitaxial relationship. The films have an onset T$_{c}$ of 20.5~K, and a T$_{co}$ of 19.8K. The intragrain $J_{c}$ is over 1 MA/cm$^{2}$ (4.2K, self field) which is significantly higher than in previously reported thin films. The grain boundary couples at a slightly lower temperature of 19.7~K. We discuss magnetic field and temperature dependence of the IV characteristics, and microwave response. [Preview Abstract] |
Friday, March 19, 2010 10:00AM - 10:12AM |
Y40.00009: Nano-structured SmFeAs(O,F) single crystals: Nearly isotropic transport up to 65 T Philip Moll, Roman Puzniak, Fedor Balakirev, Janusz Karpinski, Nikolai Zhigadlo, Bertram Batlogg Electric 4-probe transport measurements were performed on Focused Ion Beam (FIB) cut single crystals with sub-$\mu$m$^2$ cross-section, with current along and perpendicular to the crystallographic c-axis. The 4-probe geometry was defined accurately, resulting in large signals and excellent signal-to- noise ratio. Our study of the transport properties of SmFeAs (O,F) (T$_c$ $\approx$ 52 K) single crystals reveals a promising combination of high ($>$ 2 10$^6$ A/cm$^2$) and nearly isotropic critical current densities, which agree well with magnetization measurements. We find the onset of resistivity in pulsed fields up to 65T for currents along the c-axis (jIIc) very close to those for currents in the ab-plane. This favorable intragrain current transport in SmFeAs(O,F) is a crucial requirement for possible applications. [Preview Abstract] |
Friday, March 19, 2010 10:12AM - 10:24AM |
Y40.00010: Properties of SmFeAsO1-xFx Type Oxipnictide Bulks and Wires Made via Several Routes Mike Sumption, Maria Kanuchova, Milan Majoros, Mike Susner, E.W. Collings SmFeAsO1-xFx Type Oxipnictide bulks and wires were made via several routes. Using a single step route, pellets as well as encased ``wires'' (with low amounts of total reduction) were made. Polycrystalline bulk samples and wires with nominal composition SmO1-xFxFeAs (x=0.3, 0.35 and 0.4) were synthesized by conventional solid state reaction using high-purity As (we used allotrope-grey arsenic), SmF3, Sm, Fe and Fe2O3 powders as starting materials. The weighed powders were thoroughly grounded by hand with a mortar and pestle and pressed into pellets in an argon-filled glove box. The pressed pellets were sealed in an evacuated quartz ampoule and HT to 1100 $^{\circ}$C. Similar compositions were used in swaged wires, constructed with monel outer casings and Nb inner barriers. A two step method was also used in a second set of samples, in this case based on FeAs and SmAs precursors prepared in our laboratory. Magnetic measurements were made using a Quantum Design PPMS system equipped with 14 Tesla superconducting magnet. Onset Tc was seen to be 55 K for samples from the first set, and 53 K for samples made with the FeAs precursors. Birr and Bc2 values were measured at higher fields, and magnetic properties were investigated. Transport measurements were also made and the results are reported. [Preview Abstract] |
Friday, March 19, 2010 10:24AM - 10:36AM |
Y40.00011: Strong pinning centers in high quality Co-doped BaFe$_{2}$As$_{2}$ epitaxial thin film grown on templated substrates Chiara Tarantini, S. Lee, J. Jiang, Y. Zhang, C.W. Bark, J.D. Weiss, C.T. Nelson, H.W. Jang, C.M. Folkman, S.H. Baek, J.W. Park, X.Q. Pan, A. Gurevich, E.E. Hellstrom, C.B. Eom, D.C. Larbalestier We report measurements of the field and angular dependences of the critical current density J$_{c}$ of truly Co-doped BaFe$_{2}$As$_{2}$ epitaxial thin film on SrTiO$_{3}$/(La,Sr)(Al,Ta)O$_{3}$ for different SrTiO$_{3}$ thickness, which do not exhibit any evidence of electromagnetic weak links and have $J_{c}$(4K) values exceeding 1 MA/cm$^{2}$ . Those samples show a maximum of the pinning force close to H/H$_{Irr}\sim $ 0.5 indicative of strong vortex pinning, a conclusion strengthened by a very strong c-axis peak in the angular dependence $J_{c}(\theta )$ at fields up to 12 T. In opposition to the H$_{c2}$ anisotropy, $J_{c}$ for H along the $c$-axis exceeds $J_{c}$ for field applied along the \textit{ab} plane. The microstructural origin of the strong vortex pinning has been investigated by high resolution TEM which reveals a high density of columnar defects, whose nature is currently under study. [Preview Abstract] |
Friday, March 19, 2010 10:36AM - 10:48AM |
Y40.00012: Superconducting and structural properties of FeSe$_{x}$Te$_{1-x}$ epitaxial thin films Qiang Li, Weidong Si We report magnetoresistive, critical current density, and structural measurements of superconducting FeSe$_{1-x}$Te$_{x}$ epitaxial thin films grown by pulsed laser deposition. Enhanced onset superconducting transition temperature ($\sim $ 17 K) is observed in FeSe$_{0.5}$Te$_{0.5}$ epitaxial thin films. Structural analysis by x-ray diffraction and high resolution transmission electron microscopy reveal that these films generally have significantly shorter out-of-plane lattice constant $c $than the bulk value, suggesting that the out-of-plane changes have a dominating impact on the superconducting transition in iron-based superconductors. Our data also indicate that the upper critical field $H_{c2}$(0) of those films may reach over 50 T. Flux pinning properties of these films are examined with the help of detailed structural analysis. [Preview Abstract] |
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