Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session B26: Focus Session: Iron Based Superconductors -- Growth |
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Sponsoring Units: DMP DCOMP Chair: Leonardo Degiorgi, ETH Zurich Room: D162/164 |
Monday, March 21, 2011 11:15AM - 11:27AM |
B26.00001: Influences of material processing on the microstructure and intergranular current of polycrystalline Sm1111 iron-pnictides A. Yamamoto, J. Jiang, F. Kametani, A. Polyanskii, J. Weiss, E. Hellstrom, D. Larbalestier, A. Martinelli, A. Palenzona, M. Tropeano, M. Putti We have prepared polycrystalline Sm1111 bulk samples by sintering and hot isostatic pressing (HIP) and studied the influence of processing on the microstructure and intergranular current. After sintering and HIPping, samples are denser with much less impurity SmOF and grain boundary wetting FeAs. They then show significantly larger hysteresis loops than as- prepared samples. But Tc of HIPed samples is lower than samples with optimal doping, indicating loss of fluorine during later processing. Nevertheless, even after the fluorine loss and its associated carrier density reduction produced by the HIP treatment, both Magneto-Optical and remanent magnetization analyses showed that the intergranular current was enhanced. We conclude that the denser microstructure and smaller impurity phase content of HIPped samples enhances their connected fraction, even as the doping state of the sample is degraded by F loss. Our study emphasizes that current transport in polycrystalline pnictides is a complex balance involving the need to control carrier density, especially at grain boundaries, while densifying the microstructure and avoiding normal-state grain boundary wetting phases like FeAs. [Preview Abstract] |
Monday, March 21, 2011 11:27AM - 11:39AM |
B26.00002: Synthesis and superconducting properties of FeTe1-xSe Single Crystals under high magnetic fields Tesfaye Gebre, G. Li, J. Whalen, B. Conner, M. Kostov, T. Siegrist, L. Balicas Single crystals of superconductor FeTe$_{1-x}$Se$_{x}$ (0.1 $\le $ x $\le $ 0.5) were synthesized using optical floating zone, Bridgeman technique, and solid stat reaction. The samples were synthesized under various temperature gradients and cooling rates. Crystals were characterized via EDX, X-ray scattering, magnetization and transport measurements. Upper critical fields Hc$_{2 }$as estimated through the Werthamer-Hohenberg-Helfand (WHH) formalism indicate that these materials strongly surpass the weak coupling Pauli limiting field indicating that the shape of their phase diagram under field is essentially controlled by the Pauli effect. Annealing, leads to a metallic temperature dependence of the resistivity, and to sharper superconducting transitions. Despite the relatively small increase in single crystallinity, as quantified by single crystal x-ray diffraction measurements, we observe a different phase diagram under high magnetic fields when compared to non-annealed samples. [Preview Abstract] |
Monday, March 21, 2011 11:39AM - 11:51AM |
B26.00003: Single crystal growth and superconducting properties of LiFeAs Bumsung Lee, Seunghyun Khim, Jae Wook Kim, Eun Sang Choi, Jung Soo Kim, G. R. Stewart, Kee Hoon Kim LiFeAs, a representative compound for `111' system of Fe-based superconductors, has a tetragonal Cu$_{2}$Sb-type structure and possesses a single Fe-As tetrahedral layer sandwiched by the double Li layers. This structural characteristic provides a unique opportunity to realize homogeneous Li terminating surface upon cleaving, and this possibility makes LiFeAs attractive for investigating the intrinsic properties of Fe-based superconductors with various spectroscopic tools. In this sense, growing a large LiFeAs single crystal is a necessary step for studying intrinsic properties of the Fe-based superconductors. We report the successful growth of high quality LiFeAs single crystal by Sn-flux. Electrical resistivity studies reveal the superconducting onset temperature is 18.2 K with a transition width less than 1.1 K and RRR is about 24. Bulk superconductivity is supported by perfect shielding in the magnetic susceptibility and a clear jump in the specific heat. Upper critical field slopes of d$H_{c2}^{c}$/d$T\approx $-1.39 and d$H_{c2}^{ab}$/d$T\approx $-2.99 T/K near $T_{c}$ predict $H_{c2}^{c}$(0)$\approx $17.2 and $H_{c2}^{ab}$(0)$\approx $36.9 T, and it points to a modest superconducting anisotropy about 2.3 near $T_{c}$. This observed modest anisotropy is a bit smaller than the band calculation results based on the GGA approximation but larger than the prediction considered effects of electron correlation through the dynamic mean field theory. [Preview Abstract] |
Monday, March 21, 2011 11:51AM - 12:03PM |
B26.00004: ABSTRACT WITHDRAWN |
Monday, March 21, 2011 12:03PM - 12:15PM |
B26.00005: Superconductivity and wire fabrication of FeSe family Toshinori Ozaki, Keita Deguchi, Yoshikazu Mizuguchi, Hiroaki Kumakura, Yoshihiko Takano The 11 family is an fascinating iron-based superconducting system for not only elucidation of superconducting mechanism but also technological applications because of the simplest crystal structures, the less toxic and high critical field. Recently, we found that the superconductivity appears in the specimen immersed in alcoholic beverages. Focused on the pressure dependence of Se height from Fe layer in FeSe, we found that the Tc is correlated to Se height. Moreover, the anion height dependence of $T_{c}$ for all FeAs-based superconductor obeyed a universal curve with a peak around 1.38 {\AA}. We succeeded in observing the transport $J_{c}$ in the single- and 7-core wires of FeTe$_{x}$Se$_{1-x}$ superconductor using an in-situ powder-in-tube (PIT) method. The $J_{c}$ values in single- and 7-core wire are as high as 159 A/cm$^{2}$ and 100 A/cm$^{2}$ at 4.2 K, respectively. It is considered that the optimization of the composition, together with the improvement of the grain boundary in FeTe$_{x}$Se$_{1-x}$ superconducting wires, will lead to higher $J_{c}$. [Preview Abstract] |
Monday, March 21, 2011 12:15PM - 12:27PM |
B26.00006: Synthesis of Large Single Crystals of LaMnPO Greg Smith, Jack Simonson, Carlos Marques, Victor Leyva, Meigan Aronson The compound LaMnPO is isostructural with LaFeAsO, a recently discovered high-temperature superconductor, but optical spectroscopy and transport measurements of this compound have been heretofore limited by small crystal size. Accordingly, crystal syntheses from Sn, Pb and molten salt fluxes (including NaCl/KCl, LiCl/NaCl, KCl, CaCl$_2$, BaCl$_2$/CaCl$_2$ and KCl/CaCl$_2$) were investigated. Fluorine doping was explored; concentrations less than 30 at.\% (nominal) had no effect on crystal size; concentrations greater than 40 at.\% (nominal) did not yield crystals. Once growth parameters were optimized, the crystals grew in a flat rectangular shape with black luster; their composition was verified with powder and single crystal x-ray diffraction. Successful growths yielded crystals with dimensions up to 3.2 mm by 1 mm by 10 $\mu$m, a significant improvement upon previously reported growths in the literature. These large crystals enabled our group to perform a wide range of experiments that were previously restricted to polycrystalline materials. It may be feasible to extend these methods to the synthesis of similar compounds. [Preview Abstract] |
Monday, March 21, 2011 12:27PM - 12:39PM |
B26.00007: ABSTRACT WITHDRAWN |
Monday, March 21, 2011 12:39PM - 12:51PM |
B26.00008: Faceted nanocrystalline growth of FeTe on SrTiO$_{3}$(001) Yi Li, Zhaoliang Liao, Lina Chen, Jianneng Li, Amar Karki, Rongying Jin, Ward Plummer, Jiandi Zhang The new class of iron based superconductors has ignited the materials community. Of all of these new superconductors, the Fe chalcogenide system is probably the simplest at least in structure. One of most interesting questions is how the properties of these systems change as the structure is manipulated. We have studied ultra thin films of FeTe grown on a SrTiO$_{3}$(001) surface by using Pulsed Laser Deposition (PLD) and characterized with surface techniques. We observe the formation of faceted nanocrystalline islands at the surface of thin films. The structure of the islanded surface, including the faceting angle of nanostructures, has been studied by Low Energy Electron Diffraction (LEED) as well as scanning tunneling microscope (STM). The correlation of such a 3D-type growth with growth conditions and substrate-induced strain in the ultra-thin films are discussed. *Supported by NSF-DMR1005562 [Preview Abstract] |
Monday, March 21, 2011 12:51PM - 1:03PM |
B26.00009: Superconducting Iron-Chalcogenide Thin Films and Coated Conductors Qiang Li, Weidong Si Superconducting iron-chalcogenide superconducting films have been grown on a variety of substrates, including single crystalline and metallic substrates. In this presentation, we will report transport and structural properties of these films. The superconducting transition temperature of these films is significantly higher than that of the corresponding bulk materials. Analytical electron microcopy analysis indicates substantial structural difference between thee films and single crystals at atomistic level. Detailed measurement of the angular dependence of upper critical field and critical current density demonstrated that iron chalcogenide superconductors have lower anisotropy factor in comparison to the high Tc cuprates, that presents the unique opportunity for this class of materials in high field application. [Preview Abstract] |
Monday, March 21, 2011 1:03PM - 1:15PM |
B26.00010: Superconducting properties of FeSe$_{0.5}$Te$_{0.5}$ and FeTe:O$_x$ thin films Weidong Si, Ayan Bhattacharya, Su Jung Han, Ivo Dimitrov, Lijun Wu, Qiang Li High quality superconducting thin films of FeSe$_{0.5}$Te$_{0.5} $ and FeTe:O$_x$ have been grown by pulsed laser deposition on various substrates including coated conductor. Thin films of FeSe$_{0.5}$Te$_{0.5}$ have a higher superonducting transition temperature Tc (onset start around 20K and zero resistance about 16~17K) than that of bulk. High resolution transmission electron microscopy has identified a microstructure only in thin films, which may be associated with the higher Tc. Magneto-transport measurement has been carried out to check the angular dependence of upper critical field Hc2 and the critical current density of the films with the direction of magnetic field. Both have shown a weak anisotropy. These films have a special high dHc2/dT at Tc, especially for FeTe:O$_x$ film. [Preview Abstract] |
Monday, March 21, 2011 1:15PM - 1:27PM |
B26.00011: Maze-like surface reconstruction on pure SrFe$_2$As$_2$ observed by STM/STS Michael Dreyer, Mark Gubrud, Hui Wang, Shanta Saha, Nick Butch, Kevin Kirshenbaum, Johnpierre Paglione We measured undoped SrFe$_2$As$_2$ samples using a low temperature scanning tunneling microscope. Similar samples showed superconductivity in up to 15\% of its volume in measurements of the diamagnetic screening although being undoped as confirmed by energy dispersive x-ray spectroscopy. The samples where cleaved at room temperature at a pressure $P<10^{-8}$ mbar before introducing them into the low temperature scanning tunneling microscope (LTSTM) operating at a temperature of $T=4.2$ K. Beside the usual striped surface reconstruction and disordered regions we observed areas showing a maze like reconstruction. Atomically resolved images strongly suggest that the maze structure is formed by adatoms and thus, by inference, is the striped phase. Scanning tunneling spectroscopy (STS) measurements showed a superconducting gap on both the striped and maze phase of the sample, similar to the spectra on a cobalt doped sample. The results will be discussed in detail. [Preview Abstract] |
Monday, March 21, 2011 1:27PM - 1:39PM |
B26.00012: Cleavage behavior and surface states in iron pnictides Klaus Koepernik, Alexander Lankau, Helmut Eschrig, Jeroen van den Brink, Sergey Borisenko, Erik van Heumen, Mark S. Golden We present a density functional study of the surface electronic structure and the cleavage behavior of LiFeAs and Co-doped BaFe$_{2}$As$_{2}$. The results are discussed together with angle resolved photo emission (ARPES) and low energy electron diffraction (LEED) data. The two systems behave rather differently and we conclude that LiFeAs will be the ideal system for surface sensitive probes among the iron pnictide familly. [Preview Abstract] |
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