Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session T39: Focus Session: Iron Based Superconductors: Scanning Probe |
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Sponsoring Units: DCMP DMP Chair: Ruslan Prozorov, Ames Laboratory Room: F150 |
Wednesday, March 17, 2010 2:30PM - 3:06PM |
T39.00001: Comparison of Nematic Electronic Structure in the ``Parent States'' of Ca(Fe$_{1-x}$Co$_{x})_{2}$As$_{2}$ and of Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta }$ Invited Speaker: The mechanism of high-temperature superconductivity in both the cuprate and iron-based superconductors is unresolved. We use spectroscopic imaging STM to compare the electronic structure of representative compounds Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta }$ and CaFe$_{1.94}$Co$_{0.06}$As$_{2}$-- both in the `parent' state from which this superconductivity emerges. Evidence for fundamental electronic nematicity in both these systems will be presented and discussed.\footnote{In collaboration with: T.-M. Chuang, Cornell University, Florida State University; M.P. Allan, Cornell University, University of St. Andrews; Jinho Lee, Cornell University, Brookhaven National Laboratory; Yang Xie, Cornell University; Ni Ni, S.L. Bud'ko, Ames Laboratory U.S. DOE and Iowa State University; G.S. Boebinger, Florida State University; P.C. Canfield, Ames Laboratory U.S. DOE and Iowa State University; M. J. Lawler, Cornell University, Binghamton University; K. Fujita, Cornell University, Brookhaven National Laboratory, University of Tokyo; Jhinhwan Lee, A.R. Schmidt, Cornell University, Brookhaven National Laboratory; Y. Kohsaka Cornell University, Brookhaven National Laboratory, RIKEN; Chung Koo Kim, Cornell University, Brookhaven National Laboratory; H. Eisaki, Institute of Advanced Industrial Science and Technology; S. Uchida, University of Tokyo; J.P. Sethna, Eun-Ah Kim, Cornell University.} [Preview Abstract] |
Wednesday, March 17, 2010 3:06PM - 3:18PM |
T39.00002: Domain walls and boundaries on the Surface of BaFe2As2 (001) Xiaobo He, Guorong Li, V.B. Nascimento, Jiandi Zhang, Rongying Jin, E. Ward Plummer, A.S. Sefat, M.A. McGuire, B.C. Sales, David G. Mandrus Phase separation and domains of the same phase are integral building blocks of functionality in complex materials. We have used scanning tunneling microscopy/spectroscopy (STM/STS) to investigate the surface geometric and electronic structure of parent compound BaFe2As2 (001) of the iron-based superconductor. Here, we describe our observations of domains of the antiferromagnetic structure with zigzag boundaries, which are found in the p(1x1) As terminated metallic phase. Bias dependent STM measurements on these boundaries at 80K clearly show that the boundaries are mainly due to the electronic contribution in the local density of states. [Preview Abstract] |
Wednesday, March 17, 2010 3:18PM - 3:30PM |
T39.00003: Local measurement of superfluid density in iron-pnictide superconductors $Ba(Fe_{1-x} Co_x )_2 As_2 $ by magnetic scanning probes Lan Luan, Ophir M. Auslaender, Thomas M. Lippman, Clifford W. Hicks, Beena Kalisky, Jiun-Haw Chu, James G. Analytis, Ian R. Fisher, John R. Kirtley, Kathryn A. Moler We use magnetic force microscopy (MFM) to measure the local penetration depth $\lambda $ and its temperature dependence $\Delta \lambda $ in $Ba(Fe_{1-x} Co_x )_2 As_2 $ ($x\approx 0.05$,$T_c =18.5K)$ single crystals, complemented by the use of scanning SQUID susceptometry to measure $\Delta \lambda $ down to 0.4 K. We observe that the superfuid density $\rho _s $ over the full temperature range is well described by a clean two-band fully gapped model. We demonstrate that MFM can measure the important and hard-to-determine absolute value of $\lambda $, as well as obtain its temperature dependence and spatial homogeneity in the same measurement. We find $\lambda _{ab} =325\pm 50$nm, hinting that the superconductivity is not limited by phase fluctuation in this underdoped sample. We observe uniform $\rho _s $ on the submicron scale despite the highly disordered vortex pinning. [Preview Abstract] |
Wednesday, March 17, 2010 3:30PM - 3:42PM |
T39.00004: Doping Dependence of Scanning Tunneling Spectroscopy on Single Crystal Ba(Fe$_{1-x}$Co$_{x})_{2}$As$_{2}$ Shuheng Pan, Ang Li, Jihua Ma, Jared O'Neal, A. Sefat, M. McGuire, B. Sales, D. Mandrus, R. Jin, E. Plummer We have used a low temperature STM to investigate the density of states (DOS) on atomically clean (001) surface of single crystals Ba(Fe$_{1-x}$Co$_{x})_{2}$As$_{2 }$with Co doping level from x = 0 to 0.20 at 4.2 K. The data set includes the parent compound, the entire superconducting dome, and into the normal metal. With increasing doping concentration the tunneling spectrum shows a systematic evolution. We will present the details of such a spectral evolution and compare them with the band structure derived from ARPES experiments and theoretical calculations. [Preview Abstract] |
Wednesday, March 17, 2010 3:42PM - 3:54PM |
T39.00005: Scanning Tunneling Microscopy/Spectroscopy on Optimally Ni doped Single Crystal BaFe$_{2}$As$_{2}$ Jihua Ma, Ang Li, Jared O'Neal, Fang Zhou, Zhongxian Zhao, Shuheng Pan Doping the iron pnictide parent compound brings the material into superconducting state. It is intriguing that different dopants can result in different maximum Tc's. In this talk we will present the scanning tunneling microscopy and spectroscopy data on (001) surface of the optimally doped single crystal BaFe$_{1.9}$Ni$_{0.1}$As$_{2}$ (Tc $\sim $ 20 K) and compare them to the results from the optimally Co doped BaFe$_{2}$As$_{2}$. [Preview Abstract] |
Wednesday, March 17, 2010 3:54PM - 4:06PM |
T39.00006: Cryomagnetic Scanning Tunneling Spectroscopy of Superconducting FeSe$_{1-x}$Te$_{x}$ Single Crystals John Y.T. Wei, Igor Fridman, Kuo-Wei Yeh, Maw-Kuen Wu We report cryomagnetic scanning tunneling spectroscopy measurements on single crystals of the iron-based superconducting compound FeSe$_{1-x}$Te$_{x}$. Atomically-resolved conductance spectra are observed down to 300 mK and up to 5 Tesla. A gap structure is seen against a linear spectral background, showing non-trivial spatial variation, temperature dependence and field evolution. We discuss our data in relation to possible doping inhomogeneities and other recent spectroscopic measurements on iron-based superconductors. [Preview Abstract] |
Wednesday, March 17, 2010 4:06PM - 4:18PM |
T39.00007: Spatially resolved superconductivity on the surface of Ba(Fe$_{1-x}$Co$_{x})_{2}$As$_{2}$ single crystals Tae-Hwan Kim, R. Jin, L. R. Walker, J. Y. Howe, J. R. Thompson, A. S. Sefat, M. A. McGuire, B. C. Sales, D. Mandrus, M. H. Pan, J. F. Wendelken, A. P. Li We have used a cryogenic four-probe scanning tunneling microscope and scanning electron microscope (SEM) to investigate local electrical transport properties on the surface of optimally-doped superconducting Ba($_{Fe1-x}$Co$_{x})_{2}$As$_{2}$ single crystal. For the regions that showed uniform SEM contrast, a superconducting transition has been observed with a very sharp transition width of 0.2 K. On the other hand, superconducting transition temperature was found to vary with transition widths up to 3 K in the non-uniform SEM contrast region. The wavelength dispersive x-ray spectroscopy reveals that dopant (Co) concentration remains a fixed value in the uniform region, but changes gradually in the non-uniform region. We will show that local superconductivity is connected with local chemical-compositional change. [Preview Abstract] |
Wednesday, March 17, 2010 4:18PM - 4:30PM |
T39.00008: Low Temperature STM of the Iron Chalcogenide Fe$_{1+y}$Se$_{1-x}$Te$_{x}$ Francis Niestemski, Yoshinori Okada, Xianglin Ke, Vidya Madhavan The recent years have shown great diversity in high temperature superconductors. Whereas copper was once thought to be essential to the physics it is now seen that high temperature superconductivity can exist with iron based compounds. To gain further insight into this phenomena it is important to study these iron based materials of both high and low superconducting transition temperature. We examine the chalcogenide material Fe$_{1+y}$Se$_{0.64}$Te$_{0.36}$ with a low-temperature ultra-high vacuum scanning tunneling microscope (STM) at 4 K. We discuss our results on the topography, spectra and energy mapping of this novel superconductor. [Preview Abstract] |
Wednesday, March 17, 2010 4:30PM - 4:42PM |
T39.00009: Direct observation of two-gap superconductivity in SrFe$_{1.85}$Co$_{0.15}$As$_{2}$ single crystals by scanning tunneling microscopy and spectroscopy Jewook Park, Seunghyun Khim, Gun Sang Jeon, Jun Sung Kim, Kee Hoon Kim, Kookrin Char Scanning tunneling microscopy and spectroscopy (STM/S) studies on the recently discovered superconductor, SrFe$_{1.85}$Co$_{0.15}$As$_{2 }$single crystal, and its parent material SrFe$_{2}$As$_{2}$ reveal a 0.8 nm stripe pattern in the atomic resolution topographic images of both samples. In the conductance spectra taken along a line on the SrFe$_{1.85}$Co$_{0.15}$As$_{2}$\underline { }surface robust superconducting gaps (2$\Delta _{large}$ = 17.3 meV) as well as additional small gap (2$\Delta _{small}$ = 2.9 meV) structures were simultaneously observed. To the best of our knowledge, these are the first observations of such two-gap structures in pnictides in STS/M studies. [Preview Abstract] |
Wednesday, March 17, 2010 4:42PM - 4:54PM |
T39.00010: Atomically resolved images and spectroscopy of superconducting Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$ single crystals N.-C. Yeh, M.L. Teague, P. Cheng, H.H. Wen We report atomically resolved images and spectroscopy of electron-doped superconducting Ba(Fe$_{1-x}$Co$_{x})_{2}$As$_{2}$ single crystals, with x = 0.12, 0.08 and 0.06 corresponding to T$_{c}$ = 20 K, 24 K and 14 K, respectively. Scanning tunneling microscopic images of the crystals reveal somewhat disordered atomic arrangements of the Ba, As and Fe/Co layers, with averaged lattice constant $\sim $ 0.40 nm for Ba and As layers, and $\sim $ 0.28 nm for the Fe/Co layers. In contrast, spectroscopic studies indicate relatively homogeneous tunneling conductance at constant bias voltages, which differ from the findings on hole-doped (Ba$_{1-x}$K$_{x})$Fe$_{2}$As$_{2}$. Two types of spectra are observed, corresponding to two energy gaps $\sim $ 5 meV and $\sim $ 10 meV for Ba(Fe$_{0.88}$Co$_{0.12})_{2}$As$_{2}$, and the spatial distribution of the larger gap exhibits quasi super-modulations. The smaller gap may be attributed to superconductivity and the larger gap may be related to remnant spin density waves. Finally, doping dependent spectral evolution of Ba(Fe$_{1-x}$Co$_{x})_{2}$As$_{2}$ will be discussed. Acknowledgement: NSF Grant DMR-0907251. [Preview Abstract] |
Wednesday, March 17, 2010 4:54PM - 5:06PM |
T39.00011: Quasiparticle Interference Imaging of the Nematic Electronic Structure in the Parent State of Iron-Based Superconductor Ca(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$ Tien-Ming Chuang, Milan Allan, Jinho Lee, Yang Xie, Ni Ni, Sergey Bud'ko, Gregory Boebinger, Paul Canfield, J. C. Seamus Davis Spectroscopic imaging STM is used to study the electronic structure of Ca(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$ (x=0.03) in the parent state from which the superconductivity emerges. The electronic structure in k-space determined by Fourier transform of real-space conductance images reveals the unidirectional dispersion along b-axis with additional apparent band folding along a-axis. These C2 symmetric quasiparticle interference modulations rotate with orthorhombic twin boundaries. These observations indicate a more complicated electronic nematicity than originally expected in the ferropnictide parent state. [Preview Abstract] |
Wednesday, March 17, 2010 5:06PM - 5:18PM |
T39.00012: Static Nematic Electronic Structure in the ``Parent'' State of the Iron-Based Superconductor Ca(Fe$_{1-x}$Cox)$_{2}$As$_{2}$ Milan P. Allan, Tien-Ming Chuang, Jinho Lee, Yang Xie, Ni Ni, Sergey L. Bud'ko, Gregory S. Boebinger, Paul C. Canfield, J.C. Seamus Davis We use spectroscopic imaging STM to study the electronic structure of CaFe$_{1.94}$Co$_{0.06}$As$_{2}$ in the antiferromagnetic and orthorhombic `parent' state from which the iron-based high temperature superconductivity emerges with increased carrier density. We find static unidirectional nanostructures self similar at $\sim $ 8 a$_{FeFe}$, aligned along the crystalline a (AFM) axis. This introduces an unidirectionality (nematicity) much stronger than one could expect from the musicale lattice distortion; on a lengthscale different than the AFM lengthscale. [Preview Abstract] |
Wednesday, March 17, 2010 5:18PM - 5:30PM |
T39.00013: Temperature Dependence of Scanning Tunneling Spectroscopy on Single Crystal Ba(Fe$_{1-x}$Co$_{x})_{2}$As$_{2}$ Ang Li, Jihua Ma, Jared O'Neal, A. Sefat, M. McGuire, B. Sales, D. Mandrus, R. Jin, E. Plummer, Shuheng Pan Tunneling spectroscopic measurements on single crystal Ba(Fe$_{1-x}$Co$_{x})_{2}$As$_{2}$ were performed at temperatures from 1.5K to above the superconducting transition temperature. In this talk we will present the temperature dependence of the density of states along with detailed analyses of the low energy spectrum line shape, and the finite density of states at the Fermi energy. We will then discuss the possible pairing symmetries through numerical fittings with different models. [Preview Abstract] |
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