Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session V26: Focus Session: Iron Based Superconductors -- Doping Studies |
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Sponsoring Units: DMP DCOMP Chair: Ian Fisher, Stanford University Room: D162/164 |
Thursday, March 24, 2011 8:00AM - 8:12AM |
V26.00001: Isotope substitution induced lattice expansion in the iron based superconductors Mats Granath, Oleg Kim, Itai Panas In the iron based superconductors there are indications of a wide range of exponents for the iron isotope effect on Tc in different families of these materials. In this work we explore to what extent this spread of exponents may be a result of different isotope induced lattice expansions in combination with an extreme sensitivity of Tc to the interatomic distances. We estimate the magnitude of anharmonicity in the dominant Fe-As(Se) bond based on a model fit of the temperature dependence of Raman phonon frequencies. Using a variational approach to treat the effect of the anharmonic potential we calculate the magnitude and sign of the expansion of lattice parameters and interatomic distances due to Fe isotope substitution in several different iron-based superconductors. [Preview Abstract] |
Thursday, March 24, 2011 8:12AM - 8:24AM |
V26.00002: Doping evolution of the electronic structure in Ba(Fe$_{1-x}$Co$_{x})_{2}$As$_{2}$ as revealed by polarization dependent ARPES and soft X-ray absorption Paolo Vilmercati, Christine Cheney, Sung-Kwan Mo, Federica Bondino, Elena Magnano, Marco Malvestuto, Athena Sefat, Michael Mcguire, Brian Sales, David Singh, David Mandrus, Norman Mannella Here we present a study based on ARPES and X-ray absorption spectroscopies in order to unveil the electronic structure evolution upon Co-doped in BaFe2As2 high Tc superconductors, for the doping levels x=(0,6,8,12,22){\%}. This study focuses on two points: i) the effective role of Co at different doping levels; ii) the shift upon doping of the band structure. X-ray absorption experiments carried out at the Co L23 edge highlight the chemical state of cobalt at the various doping levels, thus unveiling its role as charge donor. The orbital selectivity of polarization dependent ARPES is used to show the filling evolution of each band. The spectra have been collected in different geometries along the $\Gamma $X and $\Gamma $M high symmetry crystallographic directions and at two different photon energies. The experimental results show a general rearrangement of the charge within the various orbitals upon doping, with a non rigid band shift. [Preview Abstract] |
Thursday, March 24, 2011 8:24AM - 8:36AM |
V26.00003: Doping dependence of the optical spectra in Fe$_{1.02}$Te$_{1-x}$Se$_{x}$ Catalin Martin, X. Xi, K. Miller, D. Koukis, J. Hu, Z.Q. Mao, G.L. Carr, D.B. Tanner We measured the optical reflectivity and extracted the optical conductivity over a broad spectral range of the Fe-chalcogenide Fe$_{1.02}$Te$_{1-x}$Se$_{x}$, at various doping levels, from $x$=0.12 to $x$=0.45. Except for the highest concentration of Se ($x$=0.45), optical properties show unusual frequency and temperature dependence. The zero frequency Drude peak is absent in optical conductivity, a plasma edge cannot be unambiguously defined and both reflectivity and optical conductivity have non-monothonic temperature dependence. For the superconducting samples, we found a weak enhancement of the reflectivity in the far-infrared region upon cooling below Tc and its possible association with the superconducting gap will be discussed. [Preview Abstract] |
Thursday, March 24, 2011 8:36AM - 8:48AM |
V26.00004: Doping-Induced Evolution of Superconducting Order Parameter in Ba(Fe1-xNix)2As2 Single Crystals Cong Ren, Zhaosheng Wang, Huiqian Luo, Huan Yang, Lei Shan, Haihu Wen We report a systematic investigation on the $c$-axis point- contact Andreev reflection (PCAR) in BaFe$_{2-x}$Ni$_x$As$_2$ superconducting single crystals with the Ni concentrations from underdoped to overdoped regions (0.075 $\leq x\leq 0.15$). At low temperatures, an in-gap peak at low-bias voltage is observed in PCAR for overdoped samples, in contrast to the case of underdoped junctions, in which an in-gap plateau is observed. The spectra are fitted using a generalized Blonder- Tinkham-Klapwijk (BTK) formalism with two gaps: one isotropic and another angle dependent. The second gap, resulted from the fitting, shows a clear crossover from a nodeless in the underdoped side to a nodal feature in the overdoped region. This intriguing observation provides strong evidence of the doping induced evolution of the superconducting order parameter when the inter-pocket and intra-pocket scattering are tuned through doping, as expected in the $s_{\pm}$ scenario. [Preview Abstract] |
Thursday, March 24, 2011 8:48AM - 9:00AM |
V26.00005: First-principles study of light-element doping effects on iron-based superconductors Hiroki Nakamura, Masahiko Machida Since the discovery of the iron-based superconductor, LaFeAsO$_{1-x}$F$_x$ whose Tc reached 26K, various types of iron-based superconductors have been fabricated to attain higher $T_c$. Recently, it is reported that Tc of an iron-based superconductor LaFeAsO$_{1-y}$ is enhanced to 35K by doping hydrogen. This result implies that atoms of light elements penetrate into the crystal of iron-based superconductors and transform their structures into more useful ones for superconductivity. In this talk, we investigate how the light elements are doped in the iron-based superconductors by using the first-principles density functional theory. Furthermore, we evaluate the effects of doping on the crystal structures and electronic states and explore the origin of the $T_c$ enhancements. [Preview Abstract] |
Thursday, March 24, 2011 9:00AM - 9:12AM |
V26.00006: Electron spin resonance in iron pnictides H.-A. Krug von Nidda, J. Deisenhofer, N. Pascher, S. Schaile, E. Dengler, A. Loidl, H.S. Jeevan, P. Gegenwart We report on electron spin resonance studies in Eu based 122-superconductors where the Eu$^{2+}$ ions serve as a probe of the normal and superconducting state. In polycrystalline Eu$_{0.5}$K$_{0.5}$Fe$_2$As$_2$ the spin-lattice relaxation rate $1/T_1^{\rm ESR}$ obtained from the ESR linewidth exhibits a Korringa-like linear increase with increasing temperature above $T_{\rm c}$ evidencing a normal Fermi-liquid behavior. Below $T_{\rm c}$ the spin lattice relaxation rate $1/T_1^{\rm ESR}$ follows a $T^{1.5}$-behavior without any appearance of a coherence peak. In superconducting EuFe$_2$As$_{1.8}$P$_{0.2}$ single crystals we find a similar Korringa slope in the normal state and observe anisotropic spectra for measuring with the external field parallel and perpendicular to the $c$-axis. In addition, we will discuss the ESR properties of selected systems from the 1111 and 11 families. [Preview Abstract] |
Thursday, March 24, 2011 9:12AM - 9:48AM |
V26.00007: Effect of Isoelectronic Doping at the As Site in Iron-based Superconducting Systems Invited Speaker: FeAs-based superconductivity can be induced with various doping strategies---either electron doping, hole doping or isoelectronic doping. In this talk, we will focus on the unique isoelectronic doping at the As site. By using different dopants such as P [1,2] or Sb [3], positive or negative chemical pressure can be generated onto the FeAs layers. The positive chemical pressure suppresses/destroys the spin-density-wave (SDW) ordering, and then superconductivity emerges around a quantum critical point. In contrast, the negative pressure tends to recover the suppressed/hidden SDW ordering. The isoelectronic doping also influences the electronic and magnetic state of 4f electrons in the rare-earth atomic layers and 3d electrons of the Fe planes, especially in the case of proximity between 4f and 3d energy levels. This was manifested by the observation of local-moment ferromagnetism of 4f electrons in EuFe$_{2}$(As$_{1-x}$P$_{x})_{2}$, CeFeAs$_{1-x}$P$_{x}$O and CeFeAs$_{1-x}$P$_{x}$O$_{0.95}$F$_{0.05}$ [4] systems. Our results demonstrate the intriguing interplay/competition of intersite RKKY coupling among 4f-moments, intrasite Kondo interaction between 4f- and 3d- electrons, and \textbf{k}-space Cooper pairing of 3d electrons. This work was done in collaboration with Zhu-An Xu and Jian-Hui Dai, and was supported by National Basic Research Program of China (Grant Nos. 2007CB925001 and 2010CB923003). \\[4pt] [1] Zhi Ren et al., \textit{Phys. Rev. Lett}. 102, 137002 (2009). \\[0pt] [2] C. Wang et al., \textit{EPL }86, 47002 (2009)~; S. Jiang et al., \textit{JOP-CM }21, 382203 (2009)~. \\[0pt] [3] C. Wang et al., \textit{Science China G }53, 1225 (2010). \\[0pt] [4] Y. K. Luo et al., \textit{Phys. Rev. B} 81, 134422 (2010)~; Y. K. Luo et al., to be published. [Preview Abstract] |
Thursday, March 24, 2011 9:48AM - 10:00AM |
V26.00008: Suppression of critical temperature in proton irradiated Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$ Yasuyuki Nakajima, Toshihiro Taen, Yuji Tsuchiya, Tsuyoshi Tamegai, Hisashi Kitamura, Takeshi Murakami The study on the superconducting gap structure of iorn-pnictide superconductor is one of the most important issues to uncover the pairing mechanism of high temperature superconductivity. To elucidate the superconducting gap structure, a detailed study on the effect of defects is very crucial because the pair-breaking effects due to scattering centers are phase-sensitive. We report the suppression of $T_{c}$ due to the pair- breaking effect introduced by 3 MeV proton irradiation in Ba(Fe$_{1- x}$Co$_{x}$)$_{2}$As$_{2}$ single crystals at under-, optimal-, and over-doping levels. We find that $T_{c}$ decreases and residual resistivity increases monotonically with increasing the proton dose. We also find no resistive upturn at low temperatures, which suggests that the proton irradiation provides nonmagnetic scattering centers. The critical scattering rate for all samples estimated by three different ways is much higher than that expected in $s_{\pm}$-pairing scenario based on inter-band scattering due to antiferro-magnetic spin fluctuations. [Preview Abstract] |
Thursday, March 24, 2011 10:00AM - 10:12AM |
V26.00009: Nanometer Scale Phase Separation and Chemical Inhomogeneity in the Iron Chalcogenide Superconductor Fe$_{1+y}$Te$_{x}$Se$_{1-x}$ Hefei Hu, Jian-Min Zuo, Jinsheng Wen, Zhijun Xu, Zhiwei Lin, Qiang Li, Genda Gu, Wan Kyu Park, Laura Greene We report direct evidences of phase separation and chemical inhomogeneity in Fe$_{1+y}$Te$_{x}$Se$_{1-x}$ single crystals from scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS). In STEM, images recorded using an annular dark field (ADF) detector show characteristic nanometer scale patterns of phase separation from the Z dependent contrast. The separation was observed in both non-superconducting samples with excess iron as well as superconducting samples. Using the line scan EELS technique, we determined $\sim $20{\%}, or less, fluctuation in Te concentration from the local average compositions by integrating the intensity of the Te-M$_{4,5}$ edge. The energy-loss near-edge structure (ELNES) of the Fe-L$_{2,3}$ edge changes as the composition varies, especially the L$_{3}$ and L$_{2}$ ratio, which is sensitive to the d-state occupancy of the Fe atom. The results suggest a miscibility gap in the Fe$_{1+y}$Te$_{x}$Se$_{1-x}$ system and changes in the d-electron states at the nanometer scale from the separated phases. [Preview Abstract] |
Thursday, March 24, 2011 10:12AM - 10:24AM |
V26.00010: Local topography and spectroscopy of the doped chalcogenide FeTe$_{x}$Se$_{1-x}$ Xiaobo He, Guorong Li, Jiandi Zhang, Rongying Jin, A.S. Sefat, M.A. McGuire, B.C. Sales, D. Mandrus, E.W. Plummer The atomically resolved structural and electronic properties of FeTe and FeTe$_{0.55}$Se$_{0.45}$ have been investigated using scanning tunneling microscopy/spectroscopy. The STM topography of the doped sample clearly distinguishes two types of atoms. Statistically the topography shows the correct concentration of the two species and reveals that they are not randomly distributed but prefer to congregate with tens of identical atoms in nanometer scale. Consequently, in contrast to extremely flat surface of parent compound FeTe, FeTe$_{0.55}$Se$_{0.45}$ shows structural phase separation globally breaking the pure \textit{p4/nmm} symmetry. Surprisingly the local density of states on tellurium and selenium atoms in FeTe$_{0.55}$Se$_{0.45}$ are identical, but not the same as that in pure FeTe. This indicates an itinerant (rather than localized) electronic character in this doped system. This behavior is opposite to phase separation in many other doped materials. [Preview Abstract] |
Thursday, March 24, 2011 10:24AM - 10:36AM |
V26.00011: Revealing the electronic structure of the iron pnictides with electron energy-loss spectroscopy J.C. Idrobo, W. Zhou, M.F. Chisholm, M.P. Prange, A.S. Sefat, M.A. McGuire, B.C. Sales, S.J. Pennycook, S.T. Pantelides We report electron energy-loss spectroscopy (EELS) studies of the parent compounds (LnFeAsO, Ln=La, Ce, Pr, Nd, Sm, Gd) using scanning transmission electron microscopy. We find that all the studied LnFeAsO present a Fe L-edge fine structure closer to that of metallic iron than iron oxides. We observe a direct correlation between the Fe valence state (obtained from EELS) and $T_C$, i.e. the smaller the calculated Fe valence state, the larger is the $T_C$ for that compound. We also find an anomalous crystallographic orientation-dependence of the Ln $M_{45}$ edge fine structure. In particular, we find difference in the apparent crystal field splitting of Ce and Gd f- bands when the spectra are collected parallel and perpendicular to the c-axis. This research was partially supported by NSF Grant No. DMR-0938330 (JCI, WZ), by ORNL's Shared Research Equipment (SHaRE) User Facility, which is sponsored by the Office of Basic Energy Sciences, U.S. Department of Energy (JCI) and the Office of Basic Energy Sciences, Materials Sciences and Engineering Division, U.S. Department of Energy (MC, ASS, MAM, BCS \& SJP), DOE grant DE- F002-09ER46554 (MP, STP), and by the McMinn Endowment (STP) at Vanderbilt University. [Preview Abstract] |
Thursday, March 24, 2011 10:36AM - 10:48AM |
V26.00012: Doping dependent vortex pinning in single crystal BaFe$_{2}$(As$_{1-x}$P$_{x})_{2 }$ Lei Fang, Y. Jia, C. Chaparro, J. Schlueter, Z.L. Xiao, H. Helmut, A.E. Koshelev, U. Welp, G.W. Crabtree, W.K. Kwok We report on magnetization measurements on doped single crystals of superconducting BaFe$_{2}$(As$_{1-x}$P$_{x})_{2 }$. For optimum doped crystals, we observe a second magnetization peak effect (fish tail). With further doping of phosphur for arsenic, the fish tail effect evolves into a peak effect close to H$_{c2}$, similar to that found in conventional type II superconductors. In heavily overdoped crystals, the magnetization loop is mostly reversible and no peak effect is observed. The evolution of the peak effect with doping is attributed to the reduction in defects as the crystal's purity is increased, going from optimum doping to over-doping. Possible pinning mechanism for the peak effect will be discussed within the framework of recent heat capacity and resistivity measurements. [Preview Abstract] |
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