Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session Q35: Focus Session: Iron Pnictides and Other Novel Superconductors X: Spectroscopy |
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Sponsoring Units: DCMP Chair: Michelle Johannes, Naval Research Laboratory Room: 405 |
Wednesday, March 18, 2009 11:15AM - 11:51AM |
Q35.00001: NMR Studies of Iron-Oxypnictide Superconductor LaFeAs(O$_{1-x}$F$_x$) Invited Speaker: We present experimental results of $^{75}$As and $^{139}$La nuclear magnetic resonance (NMR) in the layered oxypnictide system LaFeAs(O$_{1-x}$F$_x$) ($x$ = 0.0, 0.04, 0.07, 0.11 and 0.14) where superconductivity occurs in $x$ greater than 0.04 [1,2]. In the undoped LaFeAsO, $1/T_1$ of $^{139}$La exhibits a distinct peak at $T_N \sim$ 142 K below which the La-NMR spectra become broadened due to the internal magnetic field attributed to an antiferromagnetic (AFM) ordering[1]. In the $x$=0.04 sample, $1/T_1T$ of $^{75}$As exhibits a Curie-Weiss temperature dependence down to 30 K, suggesting the development of AFM spin fluctuations with decreasing temperature. The AFM fluctuations are significantly suppressed with F-doping, and pseudogap behavior is observed in $1/T_1T$ in the $x$=0.11 sample with a gap value of $\Delta_{PG} \sim$ 175 K[1]. The spin dynamics vary markedly with F-doping, which is ascribed to the change of the Fermi-surface structure by the electron doping. As for the superconducting properties for $x$=0.04, 0.07 and 0.11, $1/T_1$ of $^{75}$As in all compounds does not exhibit a coherence peak just below $T_c$ and follows a $T^3$ dependence at low temperatures. These results seemingly suggest that unconventional superconductivity with zero gap along lines, whereas the lack of the residual density of states at the low temperatures is incompatible with the presence of the line-nodes. We discuss similarity and difference between LaFeAs (O$_{1-x}$F$_x$) and cuprates, and also discuss the relationship between spin dynamics and superconductivity on the basis of F-doping dependence of $T_c$ and $1/T_1$[2]. \\[4pt] [1] Y. Nakai, K. Ishida, Y. Kamihara, M. Hirano, and H. Hosono; J. Phys. Soc. Jpn. 77, 073701 (2008). \\[0pt] [2] Y. Nakai, S. Kitagawa, K. Ishida, Y. Kamihara, M. Hirano, and H. Hosono; cond/mat 0810.3569. [Preview Abstract] |
Wednesday, March 18, 2009 11:51AM - 12:03PM |
Q35.00002: Electron itinerancy and Strong Itinerant Spin Fluctuations in the Normal State of CeFeAsO$_{0.89}$F$_{0.11}$ Iron-Oxypnictides F. Bondino, E. Magnano, M. Malvestuto, F. Parmigiani, M.A. McGuire, A.S. Sefat, B.C. Sales, R. Jin, D. Mandrus, E.W. Plummer, D.J. Singh, N. Mannella The recent discovery of high-temperature superconductivity in iron-oxypnictides and related materials has generated enormous excitement in the community. The electronic structure of the normal state of CeFeAsO$_{0.89}$F$_{0.11}$ has been measured with photoemission spectroscopy (PES) and x-ray absorption spectroscopy (XAS). The Fe XAS and PES spectra do not display satellite features commonly found in the Cu spectra of cuprates HTCS, indicative of the absence of strong electron correlation and localization effects in the electronic structure. In sharp contrast to the cuprates HTSC, the Fe XAS and PES spectra exhibit spectral signatures which are typical of delocalized, itinerant electrons. The Fe 3$s$ spectra show exchange multiplets due to the coupling of the final Fe 3$s$ core hole state with the conduction band states, indicative of the presence of fluctuating spin moments on the Fe sites. These findings indicate that the FeSC must be considered a new class of materials, quite unlike the cuprate HTSC or conventional BCS superconductors [F. Bondino et al. http://arxiv.org/abs/0807.3781 (arXiv:0807.3781)]. [Preview Abstract] |
Wednesday, March 18, 2009 12:03PM - 12:15PM |
Q35.00003: Core Level and Valence Band Studies of the Novel Iron Pnictide Superconductors Daniel Garcia, Chris Jozwiak, Choonkyu Hwang, Alexei Fedorov, Stephen Hanrahan, Steven Wilson, Costel Rotundu, Byron Freelon, Robert Birgeneau, Edith Bourret-Courchesne, Alessandra Lanzara Towards understanding the physics of the superconducting iron pnictides, critical information can be gained through exploring the electronic structure of these novel materials. We have used photoemission spectroscopy to study the LaFeAsO$_{1-x}$F$_{x}$ and the PrFeAsO$_{1-x}$F$_{x}$ superconductor. The evolution of valence band density of states, hybridization energy, Fermi surface topology and many body interaction are presented as a function of doping, photon energy and temperature. We explore the significance of these results to the question of electron correlation and spin physics in these Fe-based superconductors. [Preview Abstract] |
Wednesday, March 18, 2009 12:15PM - 12:27PM |
Q35.00004: Core-level study of high-temperature superconductivity iron arsenide (Ba$_{1-x}$K$_{x})$Fe$_{2}$As$_{2}$. Yi Li, Haizhong Guo, Jiandi Zhang, Darwin Urbina, H. Ding, Genfu Chen, N.L. Wang We have used high-resolution x-ray photoemission spectroscopy (XPS) techniques to investigate the core-level x-ray photoemission spectra for high-temperature superconductivitor iron arsenide (Ba$_{1-x}$K$_{x})$Fe$_{2}$As$_{2}$ ($T_{C}$ = 32 K) and its parent compound BaFe$_{2}$As$_{2}$. One important issue for understanding the nature of the superconductivity in the compound is the determination of the electron-electron correlation in the system which in principle should be reflected by the appearance of core-level satellites. We have measured the temperature-dependence of Fe-$2p$ and $3s$ core-level spectrum in both parent and doped superconductor compounds and found that the core-level electronic structure is quite different from that observed in cuprates. The origin and nature of the core spectra in these iron-based materials will be discussed. [Preview Abstract] |
Wednesday, March 18, 2009 12:27PM - 12:39PM |
Q35.00005: $^{75}$As NMR study of spin-spin relaxation and impurity effects in (Ba,K)Fe$_2$As$_2$ and CaFe$_2$As$_2$ S. Mukhopadhyay, S. Oh, A.M. Mounce, M. Lee, W.P. Halperin, A.P. Reyes, P. Kuhns, P.C. Canfield, N. Ni, S. Bud'ko We report here NMR measurements on Ba$_{0.55}$K$_{0.45}$Fe$_2$As$_2$ single crystals ($T_c \sim 30 $ K) grown from Sn flux. The variations of the NMR line widths and shifts with temperature (40 -- 150 K) and with magnetic field (6.4 -- 14 T) are indicative of the presence of local magnetic impurities in these crystals, in contrast with crystals of CaFe$_2$As$_2$. Both the shift and line width are linearly dependent on the bulk magnetization and are sufficiently large that the impurity magnetic moments must be coupled through the hyperfine interaction to the As nuclei. However, this coupling is somewhat weaker than for impurities in YBCO and $^{17}$O NMR~[1]. Measurements of the spin-spin relaxation from the Hahn echo decay envelope and CPMG sequences indicate slow magnetic fluctuations, $\approx 350$ Hz, whose origin will be discussed.\\[3pt] [1] J.~Bobroff. Ph. D. Thesis, Universite De Paris-Sud (1997) [Preview Abstract] |
Wednesday, March 18, 2009 12:39PM - 12:51PM |
Q35.00006: NMR study of the FeAs parent compounds, AFe$_2$As$_2$ (A=Ba,Ca) Eric Bauer, Seung-Ho Baek, Nicholas Curro, Filip Ronning, Joe Thompson We present $^{75}$As NMR results of the FeAs 122 parent compounds, AFe$_2$As$_2$ (A=Ba,Ca) single crystals. For BaFe$_2$As$_2$, we find that Sn impurities in the single crystal dramatically alter the low energy spin fluctuations and suppress the ordering temperature from 138 K to 85 K, and that the temperature dependence of the $^{75}$As NMR spectra and spin lattice relaxation rates reveal a second order phase transition to a state of incommensurate magnetic order. On the other hand, CaFe$_2$As$_2$ shows a commensurate first order magnetic transition which is coupled to the structural transition. By comparing the two compounds, we show that the static and dynamic properties of the FeAs systems is extremely sensitive to the microscopic out-of-plane structure in microscopic level. Our results may shed light on the superconductivity observed under pressure. [Preview Abstract] |
Wednesday, March 18, 2009 12:51PM - 1:03PM |
Q35.00007: NMR relaxation rate in the FeAs superconductors Meera Parish, Jiangping Hu, B. Andrei Bernevig We consider how different symmetries of the superconducting order parameter will affect the NMR spin relaxation rate in the newly discovered iron-based superconductors. We particularly focus on a nodeless order parameter of unconventional extended s-wave symmetry, which changes sign between the electron and hole Fermi surfaces. Using a two-band model, we show that the extended s-wave order parameter is consistent with the results of recent NMR measurements, which exhibit a characteristic $T^3$ dependence of the NMR spin relaxation rate, only if the inter-band contribution dominates the response. [Preview Abstract] |
Wednesday, March 18, 2009 1:03PM - 1:15PM |
Q35.00008: Infrared properties of Sr(Fe,Ni)2As2 superconducting single crystals. Kevin Kirshenbaum , A.B. Sushkov, S.R. Saha, N.P. Butch, J. Paglione, H.D. Drew We report on temperature dependence of the bulk single crystal reflectivity and transmission of thin crystals of iron pnictides Sr(Fe,Ni)$_2$As$_2$ in the broad frequency range from far infrared to UV. We will discuss our data in comparison to results of other experiments and theory. [Preview Abstract] |
Wednesday, March 18, 2009 1:15PM - 1:27PM |
Q35.00009: Nonequilibrium quasiparticle dynamics in single crystals of Ba$_{1-x}$K$_{x}$Fe$_2$As$_2$ Darius Torchinsky, G.F. Chen, J.L. Luo, N.L. Wang, Nuh Gedik We report on measurements of the quasiparticle dynamics in single-crystals of Ba$_{1-x}$K$_{x}$Fe$_2$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. We describe the temperature and excitation density dependence of the quasiparticle recombination rate and discuss the implications of these measurements on the nature of the superconducting gap and the electron-phonon coupling in these materials. [Preview Abstract] |
Wednesday, March 18, 2009 1:27PM - 1:39PM |
Q35.00010: Charge dynamics of the spin-density-wave state in BaFe$_2$As$_2$ F. Pfuner, L. Degiorgi, J.G. Analytis, J.-H. Chu, I.R. Fisher We report on a thorough optical investigation of BaFe$_2$As$_2$ over a broad spectral range and as a function of temperature, focusing our attention on its spin- density-wave (SDW) phase transition at $T_{SDW}=135$ K. While BaFe$_2$As$_2$ remains metallic at all temperatures, we observe a depletion in the far infrared energy interval of the optical conductivity below $T_{SDW}$, ascribed to the formation of a pseudogap-like feature in the excitation spectrum. This is accompanied by the narrowing of the Drude term consistent with the $dc$ transport results and suggestive of suppression of scattering channels in the SDW state. About 30\% of the spectral weight in the far-infrared energy interval of the optical conductivity is affected by the SDW phase transition. [Preview Abstract] |
Wednesday, March 18, 2009 1:39PM - 1:51PM |
Q35.00011: Temperature Dependent Local Structure of LaO$_{1-x}$F$_{x}$FeAs T.A. Tyson, T. Wu, J. Woicik, B. Ravel, A. Ignatov, C. Zhang, Z. Qin, T. Zhou, S.-W. Cheong The local structure of the parent and doped LaO$_{1-x}$F$_{x}$FeAs compounds were studied by x-ray absorption spectroscopy. The Fe-As correlations are well modeled by an Einstein model with no low temperature anomalies. While the Einstein temperatures are identical for the doped (11{\%}) and undoped samples, the doped sample is found to have a lower level of static disorder in the Fe-As distribution. For the Fe-Fe correlation, doping enhances the effective Einstein temperature. Comparisons with the temperature dependent structure of the simpler FeSe$_{0.88}$ systems will be made. This work is supported by DOE Grant DE-FG02-07ER46402. [Preview Abstract] |
Wednesday, March 18, 2009 1:51PM - 2:03PM |
Q35.00012: Exploring Local Density of States in FeSe-based Superconductors D. Zhang, C.S. Ting Starting from two Fe ions per unit cell and two degenerate orbitals per Fe ion, we construct an effective four-band model for the FeSe-based superconductors, which Fermi surfaces are consistent with those from the ARPES experiments and LDA calculations. The hole pockets at $(0,0)$ and the electron pockets at $(\pi,\pi)$ are controlled by hopping between the same orbitals on the nearest and second neighboring sites while the intervals between the inner and outer Fermi surfaces around $(0,0)$ and $(\pi,\pi)$ are determined by hopping between different orbitals on the same sublattice. On the basis of the mean field theory for superconductivity, we also investigate the local density of states for different pairing symmetries and compare them with the recent STM experiments. [Preview Abstract] |
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