Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session T35: Focus Session: Iron Pnictides and Other Novel Superconductors XII: Phonons and Miscellaneous Experiment |
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Sponsoring Units: DMP Chair: Pengcheng Dai, Oak Ridge National Laboratory Room: 405 |
Wednesday, March 18, 2009 2:30PM - 2:42PM |
T35.00001: Role of fluorine in the iron pnictides: phonon softening and effective hole-doping Jesse Noffsinger, Feliciano Giustino, Steven G. Louie, Marvin L. Cohen Using first principles techniques, we investigate the influence of fluorine doping on the electronic structure, lattice dynamics and electron-phonon coupling in LaFeAsO. We explicitly simulate the F-doping using a supercell model in order to explore properties not described by virtual crystal models. Our analysis reveals that local lattice relaxation accompanies the fluorine doping and modifies the lattice dynamics, in accord with recent experimental data. In addition, it is found that the charge density of the doped electrons cannot be described by simplified models of electron-doping in the two-dimensional Fe-plane. [Preview Abstract] |
Wednesday, March 18, 2009 2:42PM - 2:54PM |
T35.00002: Effects of Magnetic Ordering on Phonon Spectra in Iron-based Superconductors: First Principle Calculation and Theoretical Analysis Hiroki Nakamura, Masahiko Machida, Alfred Baron, Tatsuo Fukuda, Shinich Shamoto Recently, inelastic x-ray scattering measurements on single crystals of PrFeAsO$_y$ (y$\sim $0.2) have reported that phonons related with Fe-Fe and Fe-As bondings are significantly more softened than those obtained by the first principle calculations [1]. However, it is noted that any previous calculations do not include the magnetic degree of freedom. Therefore, we performed the phonon structure calculations by taking into account the magnetic structure in mother compounds. The magnetic calculations are in better agreement with the observed softening. We show the results and clarify the reason. \\[3pt] [1] T. Fukuda et al, J. Phys. Soc. Jpn. 77, 103715~ (2008). [Preview Abstract] |
Wednesday, March 18, 2009 2:54PM - 3:06PM |
T35.00003: Cubic anharmonicity of the Fe-As bond of the iron-pnictides estimated from Raman spectroscopy. Mats Granath, Johan Bielecki, Joakim Holmlund, Chris Knee, Nan Lin Wang, Lars B\"orjesson We study the phonon spectrum of the iron-pnictide superconductors, (Ce,Nd)O$_{1-x}$F$_{x}$FeAs, using Raman spectroscopy. Based on the temperature dependent softening of the out of plane optical Fe-mode (B$_{1g}$ at $\Gamma$) we estimate the magnitude of the cubic anharmonicity of the Fe-As bond by calculating the self-energy due to phonon-phonon interactions. This also gives an estimate of the lattice expansion or contraction due to Fe isotope substitution which may in turn influence electronic properties. [Preview Abstract] |
Wednesday, March 18, 2009 3:06PM - 3:18PM |
T35.00004: Phonon Softening in PrFeAsO$_{1-y}$ (y$\sim $0.2) by Inelastic X-Ray Scattering Alfred Q.R. Baron, Tatsuo Fukuda, Shin-ichi Shamoto, Hiroshi Uchiyama, Jun-ichiro Mizuki, Hiroki Nakamura, Masahiko Machida, Motoyuki Ishikado, Masatoshi Arai, Hijiri Kito, Hiroshi Eisaki We present phonon dispersion measurements from single crystals of~PrFeAsO$_{1-y}$ with T$_{c}$ (onset) of 42 to 45 K made using inelastic x-ray scattering with 1.5 meV resolution at BL35XU of SPring-8. In agreement with our previous results on powders and crystals [1] we see pronounced softening of the in-plane Fe-As modes compared to phonon calculations using pseudopotential methods in the tetragonal (non-magnetic) structure. C-axis modes are somewhat harder. No strong changes in phonon spectra across T$_{c}$ were observed at the momentum transfers investigated. We also compare our results against calculations of phonons in the magnetic parent material. [1] Fukuda\textit{, et al}, J. Phys. Soc. Japan, \textbf{77} (2008), 103715. [Preview Abstract] |
Wednesday, March 18, 2009 3:18PM - 3:30PM |
T35.00005: CaFe$_{2}$As$_{2}$ Phonons via Inelastic X-ray Scattering and First Principles Calculations Steven Hahn, Ahmet Alatas, Bogdan Leu, Yongbin Lee, Ni Ni, Duck Young Chung, Iliya Todorov, Mercouri Kanatzidis, Ercan Alp, Paul Canfield, Alan Goldman, Robert McQueeney, Bruce Harmon In the iron pnictides, the sensitivity of the iron magnetic moment to the arsenic position suggests a strong relationship between phonons and magnetism. We measured the phonon dispersion of several branches in the high temperature tetragonal phase of CaFe$_{2}$As$_{2}$ using inelastic x-ray scattering on single-crystal samples. These measurements were compared to ab-initio calculations of the phonons. Spin polarized calculations imposing the antiferromagnetic order present in the low temperature orthorhombic phase dramatically improves agreement between theory and experiment. This is discussed in terms of the strong antiferromagnetic correlations that are known to persist in the tetragonal phase. [Preview Abstract] |
Wednesday, March 18, 2009 3:30PM - 3:42PM |
T35.00006: Magnetic and lattice coupling in the AFe$_2$As$_2$ (A=Ca, Ba, and Sr) compounds Haifeng Li, David Vaknin, Jerel Zarestky, Wei Tian, Andreas Kreyssig, Ni Ni, Sergey Bu'dko, Paul Canfield, Robert McQueeney, Alan Goldman Systematic elastic and inelastic neutron scattering studies of the AFe$_2$As$_2$ (A=Ca, Ba, and Sr) compounds reveal some common and distinguished properties that may shed light on the nature of the coupling between the magnetic and lattice degrees of freedom. We find, that for all three samples, the structural and antiferromagnetic (AFM) transition temperatures coincide within the experimental uncertainty of measurements. We also find that the AFM propagation vector is unequivocally along the long a- orthorhombic axis for all three compounds. The coupling between the magnetic and chemical structure is in play below the transition down to the lowest temperatures. More results and discussion on the nature of the transitions will be presented. [Preview Abstract] |
Wednesday, March 18, 2009 3:42PM - 3:54PM |
T35.00007: Large electron-A$_{1g}$ phonon interaction in doped LaOFeAs: coupling with antiferromagnetism Felix Yndurain, Jose Soler We present first principles calculations of the atomic and the electronic structure of electron-doped LaOFeAs. We find that whereas the undoped compound has an antiferromagnetic arrangement of magnetic moments at the Fe atoms, the doped system becomes non magnetic at a critical electron concentration. We have studied the electron-phonon interaction in the doped paramagnetic phase. For the A$_{1g}$ phonon, the separation between the As and Fe planes induces a non-collinear arrangement of the Fe magnetic moments. This arrangement is anti parallel for interactions mediated by As, and perpendicular for Fe-Fe direct interactions, thus avoiding frustration. This coupling of magnetism with vibrations induces anharmonicities and an electron-phonon interaction much larger than in the pure paramagnetic case. We propose that such enhanced interactions play an essential role in superconducting compounds close to an atiferromagnetic phase transition. [Preview Abstract] |
Wednesday, March 18, 2009 3:54PM - 4:06PM |
T35.00008: Phonon and magnetic excitations in the novel BaFe1.8Co0.2As2 superconductor Dan Parshall, Konstantin Lokshin, Matthew Stone, Douglas Abernathy, Mark Lumsden, Andrew Christianson, David Mandrus, Athena Safa-Sefat, Takeshi Egami Phonon and magnetic excitations in the BaFe1.8Co0.2As2 superconductor single crystal were studied by inelastic neutron scattering using the ARCS time-of-flight spectrometer at the Spallation Neutron Source. Most of the phonon branches show a good agreement with the density functional theory calculations. However, the As-Raman vibrations along the c-axis demonstrate strong softening contrary to the flat behavior expected from the LDA calculations. The softening is strongest along the (0.5, 0.5, L) direction, by up to 4 meV. At the same time a sharp magnetic response was found along the same (0.5, 0.5, L) direction over a wide range of L-values at energy transfer of 10-25 meV. This dynamic magnetic responds indicates on a 2-D character of antiferromagnetic spin fluctuations in the superconducting phase, in strong contrast to the 3-D static antiferromagnetism in the undoped non-superconducting BaFe2As2. Thus, in Fe-As based superconductors magnetism shows strong sensitivity to the lattice, suggesting a possibility of spin-phonon coupling playing a role in superconductivity. [Preview Abstract] |
Wednesday, March 18, 2009 4:06PM - 4:18PM |
T35.00009: 3d crystal field excitations in iron pnictides studied by Raman spectroscopy Tao Zhou, Zhen Qin, Sean O'Malley, Kwok Lo, Chenglin Zhang, Sang-Wook Cheong We have measured the Raman spectra of LaFeAsO$_{1-x}$F$_{x}$ (x = 0, 0.1 and 0.33) as well as AFe$_{2}$As$_{2}$ (A = Ca, Sr, Ba) polycrystalline samples at different temperatures. We found that in addition to the phonon excitations at low frequency below 250 cm$^{-1}$, there are many strong excitations in the range between 250 cm$^{-1}$ and 1000 cm$^{-1}$. We attribute them to the crystal field excitations of Fe 3d electrons. The temperature and doping dependence of these excitations will be presented, and possible implications for the physics model of this new high temperature superconductor family will be discussed. [Preview Abstract] |
Wednesday, March 18, 2009 4:18PM - 4:30PM |
T35.00010: Evidence for two gaps from specific heat in LiFeAs single crystals Fengyan Wei, Bing Lv, Feng Chen, Yuyi Xue, Arnold M. Guloy, Chingwu Chu The mechanism and the paring state of the newly discovered FeAs based superconductors are still open questions. We report here the specific heat of several single crystals of LiFeAs from 1.8K to 160K. The Cp anomaly around the transition temperature Tc = 17 K is clear, and a change of the slope d(Cp/T)/dT$^{2}$ is observed around 4 K. This down turn in the Cp/T vs. T$^{2}$ plot, however, is suppressed in both poly-crystal samples and under high fields. We attribute this feature to the second gap, and will compare the data with calculations. [Preview Abstract] |
Wednesday, March 18, 2009 4:30PM - 4:42PM |
T35.00011: Coexistence of superconductivity and ferromagnetism in BaFe$_{1.8}$Co$_{0.2}$As$_2$ Mario S. da Luz, R. K. Bollinger, J. J. Neumeier, A. Sefat, M. A. McGuire, R. Jin, B. C. Sales, D. Mandrus Thermal expansion and heat capacity measurements were performed on three single crystals of BaFe$_{1.8}$Co$_{0.2}$As$_2$ with superconducting transition temperatures $T_c$ = 16.5, 19, and 22 K . The thermal expansion coefficients $\mu_i$ (i = a and c axis) are highly anisotropic. Magnetization measurements reveal the presence of ferromagnetism at the same transition temperature as superconductivity in some of the samples. The ferromagnetism has a small moment on the order of 0.5x10$^{-3}$ $\mu_B$/Fe ($\mu_B$ is the Bohr magneton). Thus, two phases: superconductivity and magnetism coexist in some BaFe$_{1.8}$Co$_{0.2}$As$_2$ samples. This ferromagnetism could be associated with a canted (non- collinear) antiferromagnetic order. This material is based upon work supported by the U. S. Department of Energy (DE-FG02-07ER46269) and the National Science Foundation (DMR-050476 and DMR-0552458). [Preview Abstract] |
Wednesday, March 18, 2009 4:42PM - 4:54PM |
T35.00012: Comprehensive Characterization of Superconductivity in Co-doped BaFe$_{2}$As$_{2}$ Tsuyoshi Tamegai, Yasuyuki Nakajima, Toshihiro Taen We have grown high-quality single crystals of Ba(Fe$_{1-x}$Co$_{x})_{2}$As$_{2}$ by self-flux method. Superconducting properties of these single crystals are characterized by measuring magnetization, resistivity, upper critical field, Hall coefficient, and magneto-optical images. A sharp drop of susceptibility is observed around 24 K for $x$=0.1. Irreversible magnetization shows fish-tail feature in a wide temperature range, indicating the presence of inhomogeneities in the crystal. The critical current density $J_{c}$ for $x$=0.1 is over 10$^{5}$ A/cm$^{2}$ at low temperatures up to 50 kOe. Upper critical field determined by resistive transition is anisotropic with anisotropic parameter $\sim $ 3.5. Hall effect measurements indicate that Ba(Fe$_{1-x}$Co$_{x})_{2}$As$_{2}$ is a multiband system with a dominant conduction by electron. Magneto-optical image for $x$=0.1 at 25 K reveals the presence of trapped vortices in a part of the crystal, which leads us to expect that bulk superconductivity above 25 K can be realized by fine tuning the Co-doping level. [Preview Abstract] |
Wednesday, March 18, 2009 4:54PM - 5:06PM |
T35.00013: Superconductivity in Sr(Fe,Ni)$_2$As$_2$ single crystals Nicholas Butch, Shanta Saha, Kevin Kirshenbaum, Johnpierre Paglione Iron pnictide compounds are the subject of intense research efforts because of their relatively high superconducting critical temperatures and the interplay of magnetic, structural, and superconducting phases found in these materials. Of the known superconducting iron pnictide compounds, those with the ThCr$_2$Si$_2$ structure appear to have the best chemical homogeneity. We present transport, magnetic, and specific heat measurements of Ni-substituted SrFe$_2$As$_2$ flux-grown single crystals. [Preview Abstract] |
Wednesday, March 18, 2009 5:06PM - 5:18PM |
T35.00014: Physical Properties in a 5-Band Spin Flucutuation Theory of Ferropnictides Greg Boyd, Siegfried Graser, Vivek Mishra, Peter Hirschfeld Within a 5 band spin fluctuation model for the ferropnictides, we give predictions for experimentally measurable quantities in the superconducting state. A BCS-RPA approach is used to examine the leading superconducting instabilities and determine the thermodynamically stable ground state. We then present results for superfluid density, nuclear magnetic relaxation, and Raman scattering. [Preview Abstract] |
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