Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session B23: Focus Session: Iron Based Superconductors -- Electronic Structure, Theory and Spectroscopy |
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Sponsoring Units: DCOMP DMP Chair: Bruce Harmon, Iowa State University Room: D165 |
Monday, March 21, 2011 11:15AM - 11:27AM |
B23.00001: Point contact spectroscopy (PCS) on the Fe122 pnictides and Fe11 chalcogenides H.Z. Arham, C.R. Hunt, W.K. Park, L.H. Greene, J. Gillett, S. Sebastian, Z.J. Xu, J.S. Wen, Z.W. Lin, Q. Li, G. Gu, A. Thaler, S.L. Budko, P.C. Canfield We present PCS results on Ba(Fe$_{1-x}$Co$_{x})_{2}$As$_{2}$ and Fe$_{1+y}$Te. The superconducting (S) crystals (x=0.08) show multigap like Andreev peaks. The non-S crystals (x=0.015, y=0.03) also show a conductance enhancement with split peaks at low temperatures (T). This conductance enhancement does not match with the bulk antiferromagnetic (AFM) transition T and survives up to 90 K for y=0.03 (T$_{N}\sim $69 K) and 130 K for x=0.015 (T$_{N}\sim $115 K). For the S samples in the coexisting regime (x=0.05 {\&} 0.055), in addition to the Andreev peaks below T$_{C}$, a zero bias conductance enhancement develops and survives for $\sim $5K above T$_{C}$. PCS detects conductance changes due to quasiparticles scattering off charge or spin ordering. These conductance enhancements may arise from orbital ordering as detected by photoemission spectroscopy$^{1}$ and AFM ordering (Q-scattering), respectively.$^{2}$ $^{1}$Yi et.al, arXiv:1011.0050. $^{2}$Bobkova et.al, PRL 94, 037005 (2005). UIUC work supported by NSF-DMR-0706013, U.S. DOE Award No.DE-AC02-98CH10886, BNL work by DOE Award No.DE-AC0298CH10886, Cambridge work by EPSRC, Trinity College, the Royal Society, the Commonwealth Trust. Ames Lab operated by ISU under DOE Contract No.DE-AC02-07CH11358. [Preview Abstract] |
Monday, March 21, 2011 11:27AM - 11:39AM |
B23.00002: Point Contact Andreev Reflection Studies on Iron Pnictide Superconductors Xiaohang Zhang, S.R. Saha, N.P. Butch, K. Kirshenbaum, J. Paglione, R.L. Greene, I. Takeuchi, Y.-S. Oh, Y. Liu, L.Q. Yan, K.-H. Kim We have systematically investigated the temperature, doping and the directional dependence of the gap structure for various types of single crystal iron pnictide superconductors by point contact Andreev reflection spectroscopy. Our studies were performed on highly transparent junctions evidenced by sharp and dramatic conductance enhancements at low temperatures. For the 122 family, despite some small features occasionally observed on the spectroscopy curves which may originate from the multiband superconductivity, a more conclusive characteristic of our obtained spectra is the presence of one predominant superconducting gap. By applying the BTK model, we find that the determined gap size scales well with the transition temperature, resulting in the 2$\Delta $/$k_{B}T_{C}$ value of $\sim $ 3.1 for both potassium doped and cobalt doped single crystals. Directional studies suggest that this gap is highly isotropic. Results on chalcogenide and nickel doped 122 superconductors will also be discussed. [X. H. Zhang et al., Phys. Rev. B 81, 024518 (2010)]. [Preview Abstract] |
Monday, March 21, 2011 11:39AM - 11:51AM |
B23.00003: Point contact spectroscopy of Co-doped pnictide superconductors John Timmerwilke, Alessandra Gallastegui, J.S. Kim, G.R. Stewart, Amlan Biswas, N.H. Sung, M.S. Kim, B.K. Cho Iron pnictides are an interesting material for superconductivity research, since they provide a contrasting system for high temperature superconductivity to the cuprates. Point-contact spectroscopy is a unique method which has been used for investigating the density of states of cuprate superconductors and can help shed light on the pnictide superconductors as well. Point-contact measurements have an advantage over tunneling spectroscopy since they are less sensitive to sample surface quality and the dimensionless barrier parameter Z, can be varied with contact pressure. We have performed point contact measurements using an apparatus with a capacitive displacement sensor which enables us to systematically vary Z. We will present point contact spectra obtained on single crystal Ba(Fe$_{0.926}$Co$_{0.074)2}$As$_{2}$ samples and other cobalt doped iron pnictide samples at various temperatures, magnetic fields, and Z. [Preview Abstract] |
Monday, March 21, 2011 11:51AM - 12:27PM |
B23.00004: First-principles studies for understanding diverse high-T$_{c}$ Invited Speaker: In this talk, I survey results and insights gained from first-principles calculations on materials that exhibit superconducting behavior at temperatures higher than those characteristic of conventional BCS superconductors. These range from highly correlated cuprate Mott insulators as represented by the bismuth-strontium-calcium-copper-oxides (BSCCOs) to border-line itinerant-Mott systems such as the recently discovered 1111 and 122 pnictides. ultimate goal of our studies is to correlate T$_{c}$ with specific material composition using detailed first-principles calculations in conjunction with many-body physics techniques via the critical step of constructing real-materials model Hamiltonians. By manipulating impurity doping, which plays a crucial role in the phase diagrams of high T$_{c}$ materials, we hope to find guidance for designing candidate systems with T$_{c}$ higher than ones currently known. BSCCO material, density functional calculations using a good generalized-gradient approximation (GGA) yield structural information that is correlated to the experimentally observed (STM) super-modulation and impurity peak in the high energy regime ($\sim $1 eV), even though the Kohn-Sham bands from such functionals fail to have a band gap. For FeAs-based high-T$_{c}$ systems, DFT band-structure calculations provide a very good starting point for constructing model Hamiltonians for studies of spin fluctuation and electron pairing mechanisms. Fermi sheets that have been constructed using Wannier transformed Kohn-Sham states have provided critical information for understanding this family of superconducting materials. Analysis of the details of magnetic ordering, density of states, and 2D vs. 3D features in both the 1111 and 122 materials have been valuable in understanding sometimes perplexing experimental findings. Effects of Co impurities have been studied and fully analyzed as well., I will discuss persistent challenges related to calculations on the structure of the non-magnetic state Ba$_{1}$Fe$_{2}$As$_{2}$ system. Both further examination of the underlying physics and development of new approximate functionals are needed. [Preview Abstract] |
Monday, March 21, 2011 12:27PM - 12:39PM |
B23.00005: Point-Contact Andreev Reflection Spectroscopy of Iron-Based Superconductors Yi-Tang Yen, J.Y.T. Wei, S.R. Saha, T. Drye, K. Kirshenbaum, J. Paglione, Rongwei Hu*, C. Petrovic, K.W. Yeh, M.K. Wu We carry out point-contact Andreev reflection spectroscopic studies at cryogenic temperatures on single crystals of Fe-chalcogenide and Fe-pnictide superconductors, including FeS$_{1-x}$Te$_{x}$, FeSe$_{1-x}$Te$_{x}$, SrFe$_{1.84}$Pt$_{0.16}$As$_{2}$ and BaFe$_{1.9}$Pt$_{0.1}$As$_{2}$. Our data are analyzed using current theoretical models involving multiband superconductivity. Further interpretations will be made in comparison with scanning tunneling spectroscopy measurements on Fe-chalcogenides and NbSe$_{2}$. *Present address: Ames Laboratory, Iowa State University [Preview Abstract] |
Monday, March 21, 2011 12:39PM - 12:51PM |
B23.00006: Observation of multiple superconducting gaps in Fe$_{1+y}$Se$_x$Te$_{1-x}$ through Andreev reflection Debtanu De, Carlos Diaz-Pinto, Zheng Wu, Pei-Herng Hor, Haibing Peng Iron-based superconductors have been under intensive study because of the high transition temperature and the intriguing physical mechanisms involving the superconductivity and magnetic orders. Theoretical studies on the role of spin fluctuation suggest unconventional S wave pairing and multiple superconducting (SC) gaps due to the five disjoint Fermi surfaces. However, this multiple SC-gap scenario has yet to be confirmed in experiments. Here we report the experimental observation of five SC gaps in Fe$_{1+y}$Se$_x$Te$_{1-x}$ from Andreev reflection spectra, along with negative differential conductance dips due to the pair breaking related to the largest SC gap. The evolution of the multiple SC gaps is further investigated as a function of both temperature and magnetic field. For the largest SC gap, the Andreev reflection signal persists above bulk Tc, suggesting the existence of phase incoherent Cooper pairs. [Preview Abstract] |
Monday, March 21, 2011 12:51PM - 1:03PM |
B23.00007: Scanning Tunneling Microscopy/Spectroscopy study on Optimally Potassium Doped Single Crystal BaFe$_{2}$As$_{2}$ Jihua Ma, Ang Li, Chenglin Zhang, Pengcheng Dai, Shuheng Pan The iron pnictide parent compound material can be brought into superconducting state by chemical doping. It is worthwhile to study and compare the hole- and electron-doped iron pnictides. Among the well-known family of AEFe$_{2}$As$_{2}$ (AE=Ca, Sr, Ba), the scanning tunneling microscopy/spectroscopy study on hole-doped samples is insufficient. In this talk we will present high resolution STM/STS results on (001) surface of the optimally doped single crystal Ba$_{0.6}$K$_{0.4}$Fe$_{2}$As$_{2}$ (Tc$\sim $37K). With the data we will discuss the spatial variation of the superconducting energy gap. [Preview Abstract] |
Monday, March 21, 2011 1:03PM - 1:15PM |
B23.00008: STM Studies on the Surface Structure of Ba122 iron pnictides cleaved at Low Temperature Ang Li, Jihua Ma, A. Sefat, M. McGuire, B. Sales, D. Mandrus, R. Jin, Chenglin Zhang, Pengcheng Dai, Shuheng Pan We have performed scanning tunneling microscopy/spectroscopy (STM/STS) studies on electron- and hole-doped BaFe$_{2}$As$_{2}$ iron pnictides (Ba(Fe$_{1-x}$Co$_{x})_{2}$As$_{2}$ and Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2})$. Two types of surface topography are always found when the samples are cleaved at low temperature. One has a square-like structure and the other shows a dimerization into rows. Details of these two surfaces, particularly at their boundaries, will be utilized to argue about the surface termination. We also show the impacts of these two surface structures and some topographic features on the tunneling spectrum. [Preview Abstract] |
Monday, March 21, 2011 1:15PM - 1:27PM |
B23.00009: STM Studies of the Lattice Distortion at the Surface of Ba(Fe$_{1-x}$Co$_{x})_{2}$As$_{2}$ Shuheng Pan, Ang Li, Jihua Ma, A. Sefat, M. McGuire, B. Sales, D. Mandrus, R. Jin, E. Plummer At low temperatures the bulk of the iron pnictides such as Ba(Fe$_{1-x}$Co$_{x})_{2}$As$_{2}$ goes from an orthorhombic structure phase to the tetragonal phase with doping. This phase transition is also accompanied by a magnetic phase transition. These phenomena have been discussed in the context of the mechanism of superconductivity in the iron pnictides. With careful examination of our low temperature STM topographic images on the single crystals of Ba(Fe$_{1-x}$Co$_{x})_{2}$As$_{2}$, we found that$_{ }$the lattice distortions of the two observed surface structures, namely ``Root 2'' and ``1$\times $2'', evolve with doping concentration x across the superconducting dome. Starting from parent compound, the orthorhombic ``Root 2'' structure evolves towards rhombic, while the ``1$\times $2'' structure evolves from rhombic to orthorhombic. We will discuss the implications of such doping dependent lattice distortion. [Preview Abstract] |
Monday, March 21, 2011 1:27PM - 1:39PM |
B23.00010: Observation of multiple gaps and vortex bound states in Ba0.6K0.4Fe2As2 by Scanning Tunneling Microscopy/Spectroscopy Lei Shan, Yong-Lei Wang, Bing Shen, Bin Zeng, Jing Gong, Yan Huang, Huan Yang, Cong Ren, Hai-Hu Wen, Ang Li, Shuheng Pan, Da Wang, Qiang-Hua Wang We report on low-temperature scanning tunneling microscopy/spectroscopy studies of the electronic structure in single crystalline Ba0.6K0.4Fe2As2. Multiple superconducting gaps were observed in the density of states (DOS) and the sizes of the two dominant gaps are 7.6 meV and 3.3 meV, respectively. The flat bottom of the DOS spectra near zero bias indicates the nodeless feature of the gaps, while the global fitting to the spectra requires definitely the anisotropy. By applying magnetic fields, we observed ordered vortices with Andreev bound states in vortex cores. The bound states and their spatial evolution can be qualitatively explained by our numerical calculations when considering the multiband s-wave superconductivity. [Preview Abstract] |
Monday, March 21, 2011 1:39PM - 1:51PM |
B23.00011: Quasiparticle Interference in Iron-Based Superconductors Johannes Knolle, Alireza Akbari, Ilya Eremin, Roderich Moessner The phase diagram of iron-based superconductors exhibits an anti ferromagnetic phase at low doping, an unconventional superconducting phase at larger carrier concentration, and possibly a coexistence regime of both orders in between. What are the signatures of the different orders in the electronic spectrum and how can phase sensitive measurements distinguish between different order parameter symmetries? To address these questions we systematically calculate quasiparticle interference (QPI) signatures for the relevant candidate phases of iron-based superconductors. Experimentally, QPI can be probed through spectroscopic imaging-scanning tunneling microscopy (SI-STM) thanks to impurities unavoidably present in the sample. We show that in the anti ferromagnetic phase the rotational symmetry of the electronic structure is broken, signatures of which are also seen in the coexistence regime with both superconducting and magnetic order. In the superconducting regime the different scattering behavior for magnetic and non-magnetic impurities allows us to verify the $s\pm$ symmetry of the order parameter. Furthermore, we discuss the effect of possible gap minima or nodes. [Preview Abstract] |
Monday, March 21, 2011 1:51PM - 2:03PM |
B23.00012: Cryogenic Scanning Tunneling Spectroscopy of Superconducting Iron Chalcogenide Single Crystals J.Y.T. Wei, Igor Fridman, Kuo-Wei Yeh, Maw-Kuen Wu, Rongwei Hu, C. Petrovic We report scanning tunneling spectroscopy measurements on the iron-based superconductors of the ``11'' family including Fe$_{1-y}$Te$_{1-x}$Se$_{x}$ and Fe$_{1-y}$Te$_{1-x}$S$_{x}$. Conductance spectra and atomically-resolved images are obtained on single crystals down to 300 mK. A gap-like structure is observed, showing an asymmetric spectral background, non-trivial spatial variation and temperature dependence. We discuss our data in terms of possible gap anisotropy and doping inhomogeneities, and in relation to other recent spectroscopic measurements on iron-based superconductors. [Preview Abstract] |
Monday, March 21, 2011 2:03PM - 2:15PM |
B23.00013: Surface lattice dynamics of the 122-type iron pnictides Jing Teng, Chen Chen, Guorong Li, Amar Karki, Jiandi Zhang, Rongying Jin, Ward Plummer We present a systematic High Resolution Electron Energy Loss Spectra (HREELS) study of the surfaces lattice dynamics of the cleaved single crystals of the parent compounds AFe$_{2}$As$_{2}$ (A=Ba, Ca) as a function of temperature and sample treatment. The different phonon signatures on the competing surface reconstructions 1$\times $2 phase and ($\surd $2x$\surd $2)R45$^{^{\circ}}$ phase are studied. For Ba there are two optical phonon modes are observed at 18 and 29 meV, which can be identified as the $A_{1g}$ and $B_{1g}$ vibrations of the As and Fe atoms, respectively. A detailed discussion is given in terms of the interplay between the spin and lattice in this novel system [Preview Abstract] |
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