Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session C39: Fe-based Superconductors: Tunneling and SpectroscopyFocus
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Sponsoring Units: DMP Chair: Tatiana Webb, Harvard University Room: 386 |
Monday, March 13, 2017 2:30PM - 2:42PM |
C39.00001: Low temperature scanning tunneling microscopy and spectroscopy investigation of FeSe$_{\mathrm{1-x}}$S$_{\mathrm{x}}$ single crystals C. Di Giorgio, A. Putilov, E. Lechner, D. Trainer, O.S. Volkova, A.N. Vasiliev, D. Chareev, G. Karapetrov, J.F. ZASADZINSKI, M. Iavarone FeSe has the simplest crystal structure among the Fe-based superconductors, and this very simplicity could provide the most appropriate venue of understanding the superconducting mechanism of Fe-based superconductors. Low temperature scanning tunneling microscopy and spectroscopy measurements on high quality FeSe and S substituted FeSe single crystals will be presented. Multiband superconductivity, symmetry of the order parameter, role of disorder and vortex matter in this system will be discussed. [Preview Abstract] |
Monday, March 13, 2017 2:42PM - 2:54PM |
C39.00002: Towards a quantitative description of tunneling conductance of superconductors Andreas Kreisel, R. Nelson, T. Berlijn, W. Ku, R. Aluru, P. Wahl, H. Zhou, U.R. Singh, S. Chi, R. Liang, W.N. Hardy, D.A. Bonn, P.J. Hirschfeld, B.M. Andersen Since the discovery of iron-based superconductors, a number of theories have been put forward to explain the qualitative origin of pairing, but there have been few attempts to make quantitative, material-specific comparisons to experimental results. We use first principles electronic structure calculations to make predictions for the superconducting gap and calculate the surface wavefunctions within the same framework. We present a detailed comparison between theory and scanning tunneling experiments for the material LiFeAs, where nonpolar surfaces have allowed collection of very high resolution data[1]. For the homogeneous system we find that the calculated topographic images show a transition between registered As and Li lattices as a function of setpoint bias as it is also seen experimentally[2]. We show further that generic weak to intermediate strength impurity potentials lead to resonances tied to the lower gap edge, and discuss conclusions that can be drawn about the superconducting order parameter from these observations. [1] S. Chi, et al., Phys. Rev. B 94, 134515 (2016) [2] A. Kreisel, et al., arXiv:1610.00619 [Preview Abstract] |
Monday, March 13, 2017 2:54PM - 3:06PM |
C39.00003: Spectroscopic-imaging STM studies of superconductivity and nematicity in FeSe$_{1-x}$S$_x$ T. Hanaguri, Y. Kohsaka, K. Iwaya, T. Machida, T. Shibauchi, T. Watashige, S. Kasahara, Y. Matsuda FeSe$_{1-x}$S$_x$ exhibits electronic nematic order that is suppressed with increasing sulfur content $x$. The nematic order disappears in the bulk above $x\sim$0.17, whereas superconducting transition temperature $T_c = 9\sim10$~K remains almost unchanged~[1]. We performed spectroscopic-imaging STM experiments on FeSe$_{1-x}$S$_x$ to investigate the change in the band structure and the superconducting gap across the nematic quantum critical point at $x\sim$0.17. We have found that anisotropy of the in-plane band structure diminishes with increasing $x$ but survives at least locally even at $x>$0.17. Superconducting gap is hardly affected by sulfur doping in the nematic phase but becomes blunt at $x>$0.17. This result may suggest that superconductivity and nematicity are interrelated. [1] S. Hosoi {\it et al}., PNAS {\bf 113}, 8139 (2016). [Preview Abstract] |
Monday, March 13, 2017 3:06PM - 3:18PM |
C39.00004: Spin Polarised STM study of Magnetism in the non-superconducting parent compound of Iron Chalcogenide Superconductors Christopher Trainer, Chi Ming Yim, Peter Wahl, Christopher Stock, Vladimir Tsurkan, Alois Loidl Magnetism is thought to play an important role in the development of the superconductivity in the Iron chalcogenide superconductors. In the parent material, FeTe, the magnetic phase diagram is very complex with unusual magnetic states developing as a function of excess Iron doping.[1] We have conducted a spin-polarized scanning tunnelling microscopy study on samples of FeTe with different levels of excess Iron concentrations by in-situ prepared magnetic tips [2,3]. Using a ferromagnetic tip in an STM in a vector magnetic field, we have been able to map out the magnetic order at the surface on an atomic scale in all three spatial directions. I will show how the magnetic structure changes locally as well as a function of excess iron doping, and discuss the results in comparison with neutron scattering. [1] – E.Rodriguez et al. Phys. Rev. B 84 064403 [2] – M.Enayat et al. Science. 345, 6197, p. 653-656 4 p. [3] – U.R.Singh et al. Physical Review. B 91, 16, 161111 [Preview Abstract] |
Monday, March 13, 2017 3:18PM - 3:30PM |
C39.00005: Impact of iron-site defects on superconductivity in LiFeAs R. Aluru, P. Wahl, S. Chi, R. Liang, W. N. Hardy, D. A. Bonn, A. Kreisel, B. M. Andersen, U. R. Singh, R. Nelson, T. Berlijn, W. Ku, P. J. Hirschfeld In iron-based high temperature superconductors the symmetry of the order parameter still remains a controversial topic where for the same compound sign changing and non sign-changing order parameters have been proposed theoretically. Among the iron-based superconductors, LiFeAs takes a special role (together with FeSe) by being a stoichiometric superconductor, minimizing intrinsic scattering. Here, we study engineered iron-site defects in LiFeAs by low temperature scanning tunneling microscopy and spectroscopy (STM/STS). The tunneling spectra obtained on individual defects show signatures of impurity bound states[1]. A detailed comparison of the tunneling spectra measured on impurities with theoretical simulations [2] enables us to draw conclusions about the superconducting order parameter in LiFeAs. Studying Ni, Co, Mn impurities and native defects, we find a continuous evolution from negligible impurity bound states at the smaller gap edge to detectable states as the scattering potential increases. [1] R. Aluru, et al., PRB 94, 134515 (2016) [2] A. Kreisel, et al., arXiv:1610.00619 [Preview Abstract] |
Monday, March 13, 2017 3:30PM - 3:42PM |
C39.00006: Point Contact Spectroscopy of Iron Pnictide: Probing Multigap Superconductors to Observe Energy Gaps of Ba$_{\mathrm{(1-x)}}$K$_{\mathrm{x}}$Fe$_{\mathrm{2}}$As$_{\mathrm{2}}$. Luke Conover, Samuel Bische, Oberon Wackwitz, Joseph Lambert, Pencheng Dai, Chenglin Zhang, Yu Song, Guotai Tan, Roberto Ramos Multi-band superconductors, such as the iron pnictides, can exhibit multiple energy gaps depending on the crystal growth conditions and on which tunneling directions are made accessible by the sample fabrication process. The gaps are often anisotropic with respect to the crystal lattice, with some gaps predominantly conducting parallel or perpendicular to the c-axis of the lattice.~ Using point contact spectroscopy (PCS), it is possible to measure the energy gaps of various K-doped pnictides (x$=$0.33 and x$=$0.60) along the axes simultaneously at low temperatures. We describe our progress in measuring the energy gaps of iron pnictide single crystals (K-doped Ba-based 122 family) using PCS. Furthermore, we also discuss the effects of fritting, or electrically manipulating, the point contact for better resolution of the measured conductance of iron pnictide. [Preview Abstract] |
Monday, March 13, 2017 3:42PM - 4:18PM |
C39.00007: Real space imaging of magnetic order and excitations in iron-based superconductors Invited Speaker: Peter Wahl The proximity of magnetic order to superconductivity in the phase diagrams of many of the iron-based superconductors indicates an intimate relationship between the two. In my talk, I will discuss local measurements by low temperature scanning tunnelling microscopy and spectroscopy of the magnetic order and magnetic excitations in iron-based superconductors. In the first part of my talk I will discuss detection of magnetic excitations in the non-superconducting parent compound, FeTe, of the iron-chalcogenides by inelastic tunnelling spectroscopy. I will then show the influence of inelastic tunnelling on spectra obtained in the superconducting state and show how the inelastic signal can be used to image the spin resonance mode of the iron-based superconductors in real space, information not available from other methods. [Preview Abstract] |
Monday, March 13, 2017 4:18PM - 4:30PM |
C39.00008: Identify the chemical and electronic inhomogeneity in optimally Co doped BaFe$_{\mathrm{\mathbf{2}}}${As}$_{\mathrm{\mathbf{2}}}$ Qiang Zou, Zhiming Wu, Mingming Fu, Chunmiao Zhang, Shivani Rajput, Yaping Wu, Li Li, D. Parker, Junyong Kang, A.S. Sefat, Zheng Gai Combined scanning tunneling microscopy, spectroscopy and local barrier height (LBH) image studies show that low temperature cleaved optimal doped Ba(Fe$_{\mathrm{1-x}}$Co$_{\mathrm{x}})_{\mathrm{2}}$As$_{\mathrm{2\thinspace }}$single crystals have complicate morphologies, by means of the mixture of the Ba dominated bright patches and As dominated stripes. Although the surface morphologies are different, the superconducting gap maps show same gap width and similar nanometer size inhomogeneity. Based on the spectroscopy and LBH maps, the bright patches and dark stripes variations in the morphologies were identified as Ba or As dominated surface terminations. Magnetic impurities are believed to create local in-gap state and suppressed coherent peaks of the SC. This study will clarify the debate on the terminations of the cleavage surface of the Fe based 122 superconductors. This research was conducted at the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility. [Preview Abstract] |
Monday, March 13, 2017 4:30PM - 4:42PM |
C39.00009: Spatial competition of the ground states in 1111 iron pnictides Guillaume Lang, Louis Veyrat, Uwe Gr\"{a}fe, Franziska Hammerath, Dalibor Paar, G\"{u}nther Behr, Sabine Wurmehl, Hans-Joachim Grafe Using nuclear quadrupole resonance, the phase diagram of 1111 pnictides is constructed as a function of the local charge distribution in the paramagnetic state, which features low-doping-like (LD-like) and high-doping-like (HD-like) regions. Comparison of compounds based on magnetic (Ce, Sm) and nonmagnetic (La) rare earths reveals the detrimental role of static iron $3d$ magnetism on superconductivity (SC). It is found that the LD-like regions fully account for the orthorhombicity of the system, and are thus the origin of any static iron magnetism. Orthorhombicity and static magnetism are not hindered by SC but limited by dilution effects, in agreement with 2D (respectively 3D) nearest-neighbor square lattice site percolation when the rare earth is nonmagnetic (respectively magnetic). The LD-like regions are not intrinsically supportive of SC, contrary to the HD-like regions, as evidenced by the Uemura relation between $T_c$ and the superfluid density when accounting for the proximity effect. We propose\footnote{G. Lang et al., PRB 94, 014514 (2016)} a complete description of the interplay of ground states in 1111 pnictides, where nanoscopic regions compete to establish the ground state through suppression of SC by static magnetism, and extension of SC by proximity effect. [Preview Abstract] |
Monday, March 13, 2017 4:42PM - 4:54PM |
C39.00010: Laser-Based ARPES Study on Superconducting Gap of Optimally-Doped (Ba0.6K0.4)Fe2As2 Superconductor Jianwei Huang The determination of the superconducting gap and its symmetry are important in understanding the underlying superconductivity mechanism in the iron-based superconductors. However, the results on the superconducting gap in a prototypical iron-based superconductor (Ba$_{\mathrm{0.6}}$K$_{\mathrm{0.4}})$Fe$_{\mathrm{2}}$As$_{\mathrm{2}}$ remains controversial after eight years of extensive investigations. Here we report our high resolution laser-based angle-resolved photoemission (ARPES) measurements on the superconducting gap of an optimally-doped (Ba$_{\mathrm{0.6}}$K$_{\mathrm{0.4}})$Fe$_{\mathrm{2}}$As$_{\mathrm{2}}$ superconductor. With ultra-high energy and momentum resolutions from our new generation laser-based ARPES system, we have carried out precise measurements of the superconducting gap and its temperature and momentum dependences in (Ba$_{\mathrm{0.6}}$K$_{\mathrm{0.4}})$Fe$_{\mathrm{2}}$As$_{\mathrm{2}}$. Our results provide a unified picture for the superconducting gap in (Ba$_{\mathrm{0.6}}$K$_{\mathrm{0.4}})$Fe$_{\mathrm{2}}$As$_{\mathrm{2}}$ superconductor and solve a long-standing controversy. They also put an upper limit on the anisotropy of the superconducting gap in the iron-based superconductors. [Preview Abstract] |
Monday, March 13, 2017 4:54PM - 5:06PM |
C39.00011: Coherent phonon excitation in K-doped BaFe$_2$As$_2$ studied by trARPES Heike Pfau, Hadas Soifer, Jonathan Sobota, Alexandre Gauthier, Harlyn Silverstein, Johanna Palmstrom, Costel Rotundu, Ian Fisher, Patrick Kirchmann, Zhi-Xun Shen The phase diagrams of Fe-based superconductors are complex and contain magnetism, nematicity and superconductivity. To understand the emergence of superconductivity in these materials, it is crucial to study the interplay of charge, spin, orbital and lattice degrees of freedom. Here, we focus on electron-phonon coupling, which is intimately connected to the properties of the nematic phase and has been suggested to be enhanced in the presence of electronic correlations. We use time- and angle-resolved photoemission spectroscopy to study the electron-phonon coupling in K-doped BaFe$_2$As$_2$. The coherent response after optical excitation reveals at least four bosonic modes. One of them corresponds to the E$_g$-phonon mode, which is directly connected to the symmetry of the structural transition. This observation provides the opportunity to further study the interplay of electronic and lattice degrees of freedom in a nematically ordered system. [Preview Abstract] |
Monday, March 13, 2017 5:06PM - 5:18PM |
C39.00012: Scanning tunneling spectroscopy as a probe of multi-Q magnetic states of itinerant magnets Maria Navarro Gastiasoro, Ilya Eremin, Rafael M Fernandes, Brian M Andersen The combination of electronic correlations and Fermi surfaces with multiple nesting vectors can lead to the appearance of complex multi-\textbf{Q} magnetic ground states, hosting unusual states such as chiral density-waves and quantum Hall insulators. Distinguishing single-\textbf{Q} and multi-\textbf{Q} magnetic phases is however a notoriously difficult experimental problem. Here we propose theoretically that the local density of states (LDOS) near a magnetic impurity, whose orientation may be controlled by an external magnetic field, can be used to map out the detailed magnetic configuration of an itinerant system and distinguish unambiguously between single-\textbf{Q} and multi-\textbf{Q} phases. We demonstrate this concept by computing and contrasting the LDOS near a magnetic impurity embedded in three different magnetic ground states relevant to iron-based superconductors -- one single-\textbf{Q} and two double-\textbf{Q} phases. Our results open a promising avenue to investigate complex magnetic configurations in itinerant systems via standard scanning tunneling spectroscopy, without requiring spin-resolved capability. [Preview Abstract] |
Monday, March 13, 2017 5:18PM - 5:30PM |
C39.00013: Observation of particle-hole asymmetry and other abnormalities in the normal state of a prototypical electron-doped pnictide high-temperature superconductor. Norman Mannella, Paolo Vilmercati, Sung-Kwan Mo, Alexei Fedorov, Michael McGuire, Athena Sefat, Brain Sales, DAvid Mandrus, David Singh, Wei Ku, Steve Johnston Understanding high-T$_{\mathrm{c}}$ superconductivity requires a correct description of the normal state, a notoriously challenging task due to varying degrees of electronic correlations and competing orders. The nature of the normal state is still debated, even in the case of the iron-pnictides, some of which are the least correlated of the high-T$_{\mathrm{c}}$ superconductors. Using Angle Resolved Photoemission spectroscopy (ARPES), we show that the hole and electron bands in the prototypical electron-doped pnictide Ba(Fe$_{\mathrm{1-x}}$Co$_{\mathrm{x}})_{\mathrm{2}}$As$_{\mathrm{2}}$ exhibit different types of excitations characterized by fundamentally different lineshapes in the normal state. This occurs even when these bands are formed from the same Fe 3d orbitals, indicating that it is the nature of the carriers rather than their orbital character that characterizes the elementary excitations in momentum space. Other anomalies consisting in a highly non-monotonic dependence of electronic properties on Co concentration, including band filling and quasiparticle coherence, will be discussed. [Preview Abstract] |
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