Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session Y39: Focus Session: Iron Based Superconductors: Spectroscopy I |
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Sponsoring Units: DCMP DMP Chair: Dongui Lu, Stanford University Room: F150 |
Friday, March 19, 2010 8:00AM - 8:36AM |
Y39.00001: ARPES studies of LaFePO based compounds Invited Speaker: |
Friday, March 19, 2010 8:36AM - 8:48AM |
Y39.00002: Optical investigation of the parent compounds of iron-based superconductors Jason Hancock, Seyed Mirzaei, Romain Viennois, Suchitra Sebastian, Jack Gillett, Jeremie Teyssier, Enrico Giannini, Dirk van der Marel We present a comparative study of the dynamical charge response of the parent compounds of representative members of the iron pnictide and iron chalcogenide compounds. We find that strong temperature-dependent changes accompany the magnetostructural phase transition of the pnictide system, but that these are strongly suppressed and qualitatively different in the chalcogenide. This contrasting behavior is discussed in connection to the different magnetic ground state in each of these two systems. [Preview Abstract] |
Friday, March 19, 2010 8:48AM - 9:00AM |
Y39.00003: Infrared spectroscopy of a novel iron-based superconductor Fe$_{1.06}$Te$_{0.88}$S$_{0.14}$ S.V. Dordevic, N. Stojilovic, Adam Koncz, Rongwei Hu, C. Petrovic We will present the results of our infrared and optical studies of a novel iron-based superconductor Fe$_{1.06}$Te$_{0.88}$S$_{0.14}$ with T$_c$ = 8 K. Measurements have been performed over a broad range of frequencies (50 - 50,000 cm$^{-1}$) and temperatures (10 - 300 K). Our results reveal that the superconducting state develops from an unconventional normal state, without well defined quasiparticles. The structural and magnetic phase transitions at T$\simeq$~23~K do not seem to have significant effect on optical properties of Fe$_{1.06}$Te$_{0.88}$S$_{0.14}$. We introduce ``generalized spectral weight'' analysis, and use it to track the redistribution of spectral weigh with temperature. [Preview Abstract] |
Friday, March 19, 2010 9:00AM - 9:12AM |
Y39.00004: Optical Spectroscopy of BaFe$_{1.8}$Co$_{0.2}$As$_{2}$ in the normal state Sarah Purdy, Jing Yang, Greg Egan, Jules Carbotte, Thomas Timusk, Athena Sefat, Brian Sales, David Mandrus, Ewald Schachinger Sefat et al.[1] have found that electron doping of the iron arsenide BaFe$_{2}$As$_{2}$ by cobalt doping of the iron site induces superconductivity, as high as 38K. We will report a comprehensive optical spectroscopy study of normal state BaFe$_{1.8}$Co$_{0.2}$As$_{2}$ (Tc = 22K) over a wide range of frequencies from 50 to 40,000 cm$^{-1}$ and temperatures. Measuring the reflectance of single crystal samples we acquire a spectra from which we will show the optical conductivity and the dielectric function. Using Eliashberg formalism we determine the electron boson spectral function. [1] A.S. Sefat et al., Phys. Rev.Lett. 101, 117004 (2008) [Preview Abstract] |
Friday, March 19, 2010 9:12AM - 9:24AM |
Y39.00005: Infrared Spectroscopy on superconducting and non-superconducting iron pnictide compounds C. Martin, Xiaoxiang Xi, D. Koukis, K. Miller, J.S. Kim, G.R. Stewart, D.B. Tanner, L.L. Zhao, V. Leyva, E. Morosan We performed infrared reflectivity measurements on single crystals of the hole-doped BaK-122 iron pnictide superconductor ($T_{c}\approx 33$~K) and on the non-superconducting compound CaFe$_{4}$As$_{3}$. In BaK-122, similar to previous reports, we observed an increases in optical reflectance in the infrared below T$_{c}$, associated with the superconducting condensate. The non-superconducting CaFe$_{4}$As$_{3}$ shows two magnetic transitions: one associated with a spin density wave (SDW) transition at $T_{N}\approx88$~K and another one at lower temperature ($T_{2}\approx 26$~K), with unclear origin up to date. The change in measured reflectivity and in optical conductivity, obtained from Kramers-Kronig analysis, at the two transitions will be discussed in the framework of Drude and Lorentz models. [Preview Abstract] |
Friday, March 19, 2010 9:24AM - 9:36AM |
Y39.00006: A doping dependent study of nonequilibrium quasiparticle dynamics in electron and hole doped BaFe$_2$As$_2$ Darius Torchinsky, James McIver, David Hsieh, G.F. Chen, J.L. Luo, N.L. Wang, Nuh Gedik We report on measurements of the doping dependence of quasiparticle dynamics in single-crystals of Ba$_{1-x}$K$_{x}$Fe$_2$As$_2$ and Ba(Fe$_{1-x}$Co$_{x}$)$_2$As$_2$ in the superconducting state via ultrafast pump-probe spectroscopy. Quasiparticles are injected into the samples by ultrashort laser pulses. Once injected, they cause a reflectivity change of the sample at the laser frequency, allowing time-resolved optical measurements of their density. Our measurements reveal band-dependent quasiparticle recombination which provide clues on gap symmetry and electron-boson coupling. [Preview Abstract] |
Friday, March 19, 2010 9:36AM - 9:48AM |
Y39.00007: Band and momentum dependent electron dynamics in Co doped BaFe$_2$As$_2$ Rudi Hackl, Bernhard Muschler, Wolfgang Prestel, Hans-Martin Eiter, Thomas P. Devereaux, James G. Analytis, Jiun-Haw Chu, Ian R. Fisher We present results from electronic Raman scattering experiments on high quality ${\rm Ba(Fe_{1-x}Co_{x})_2As_2}$ single crystals revealing details of carrier properties in the normal and in the superconducting states. Due to a light penetration depth on the order of 30~nm the experiment is bulk sensitive. We show that Raman scattering is capable of essentially projecting out the hole and the electron bands separately by using different polarization combinations of the incident and scattered photons. The experiments indicate a strong band and momentum anisotropy of the electron dynamics above and below the superconducting transition highlighting the importance of complex band-dependent interactions. The presence of low energy spectral weight deep in the superconducting state suggests a gap with accidental nodes which may be lifted by doping and/or impurity scattering. Both, the normal state relaxation and the superconducting gap vary substantially with doping $x$ but we do not find evidence of true gap nodes at any $x$. Along with other experiments we conclude that, in contrast to the cuprates, there is no generic behavior below $T_c$. [Preview Abstract] |
Friday, March 19, 2010 9:48AM - 10:00AM |
Y39.00008: Ultrafast Optical Spectroscopy of Ba(Co$_{x}$Fe$_{1-x}$)$_{2}$As$_{2}$ Superconductors Tianqi Li, Liang Luo, Ni Ni, Sergey Bud'ko, Paul Canfield, Jigang Wang High-temperature superconductivity (HTS) represents one of the most visited macroscopic quantum phenomena. A particularly interesting example is FeAs-based systems showing superconductivity from 20-50K. The development of ultrafast spectroscopy techniques has provide unique access to transiently photoexcited quasiparticles and their dynamics. Although the understandings of static thermodynamics, electronic transport, and magnetic properties are progressing, ultrafast optical studies on these systems are scarce. Here, we investigated the femtosecond optical responses of optimally-doped superconducting Ba(Co$_{x}$Fe$_{1-x})_{2}$As$_{2 }$(x=0.7) and antiferromagnetic parent compound BaFe$_{2}$As$_{2}$. Time-resolved pump-probe differential reflectivity revealed several distinct dynamic decay processes -- initial drop and slow recovery. The timescales and amplitudes of these components exhibited strong temperature dependence. Our results showed that the ultrafast optical response represents a new tool to study FeAs-based HTS. [Preview Abstract] |
Friday, March 19, 2010 10:00AM - 10:12AM |
Y39.00009: Observation of phonon splitting in BaFe$_{2}$As$_{2}$ single crystals using infrared spectroscopy Alexander Schafgans, Andrew LaForge, Mumtaz Qazilbash, Athena Sefat, David Mandrus, Dimitri Basov We present infrared spectroscopic results on the pnictide parent compound BaFe$_{2}$As$_{2}$ (Ba122) detailing direct observation of phonon splitting in the pnictide materials following the tetragonal to orthorhombic structural phase transition. We then compare these results with the optimally Co-doped superconductor Co-Ba122 in which only one, weak infrared-active phonon is observed. Finally, we explore these results to glean insights into the interplay between structural and spin density wave order. [Preview Abstract] |
Friday, March 19, 2010 10:12AM - 10:24AM |
Y39.00010: Raman spectra of Fe-pnictide superconductors including the effect of disorder G.R. Boyd, P.J. Hirschfeld, T.P. Devereaux The nature of the superconducting state in the Fe-pnictide materials is still an open question. Data from NMR, penetration depth, ARPES, and other techniques support nodal or fully gapped states depending on the probe, the doping, and which member of the iron pnictide family is being measured. Recent Raman scattering measurements on Cobalt doped barium-122 for different dopings provide evidence of gap nodes which are removed with increasing Co content. Here we report calculations of the Raman intensity in extended s-symmetry superconducting states and an electronic structure appropriate for the Fe-pnictide materials. Including disorder treated in the T-matrix approximation, we show how intraband scattering can lift the nodes and provide an explanation for the data in various polarizations. We give predictions for the evolution of these spectra with further disorder. [Preview Abstract] |
Friday, March 19, 2010 10:24AM - 10:36AM |
Y39.00011: Infrared phonon anomaly in BaFe$_2$As$_2$ C. C. Homes, A. Akrap, J. J. Tu, L. J. Li, G. H. Cao, Z. A. Xu The detailed in-plane optical properties of single-crystal BaFe$_2$As$_2$ have been determined over a wide frequency range above and below the structural and magnetic transition at $T_N \simeq 138$~K. Both infrared-active $E_u$ modes are observed at 94 and 253~cm$^{-1}$ (11.6 and 31.4~meV) at 295~K. Below $T_N$ the modes are expected to split, $E_u \rightarrow B_{2u} + B_{3u}$. The 94~cm$^{-1}$ mode displays little temperature dependence, but may split at low temperature. In contrast, the 253~cm$^{-1}$ vibration softens discontinuously at $T_N$ but does not split; for $T < T_N$ the frequency of this mode displays almost no temperature dependence, yet it nearly doubles in intensity.\footnote{A. Akrap {\em et al.}, Phys. Rev. B {\bf 80}, 180502(R), (2009).} This anomalous behavior appears to be a consequence of orbital ordering in the Fe-As layers.\footnote{C.-C. Lee, W.-G. Yin, and W. Ku, arXiv:0905.2957.} [Preview Abstract] |
Friday, March 19, 2010 10:36AM - 10:48AM |
Y39.00012: Coherent phonon in superconductor iron pnictide Davide Boschetto, Barbara Mansart, Annunziata Savoia, Marino Marsi The role of phonons in high critical temperature superconductivity has been subjected to a large debate and the description of the superconductivity mechanism is still controversial. Recently, the attention of the scientific community has been focused on the new family of iron pnictide superconductors, in which all the degrees of freedom of the crystal, such as spin, lattice and charges, compete giving rise to a fascinating phase diagram. However, such strong correlations make harder to assert on the mechanism of the electrons pair formation. In order to shine light on this key point linked to the occurrence of superconductivity, we study a coherent A$_{1g}$ optical phonon mode in time domain by measuring the transient reflectivity in iron pnictide Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ (x=0.06 and 0.08) across the superconducting phase transition [1]. The phonon parameters such as frequency, amplitude and damping time are discussed as a function of both sample temperature and doping. [1] Mansart et al., Pys. Rev. B 80, 172504 (2009) [Preview Abstract] |
Friday, March 19, 2010 10:48AM - 11:00AM |
Y39.00013: ABSTRACT WITHDRAWN |
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