Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session Q40: Focus Session: Iron Based Superconductors Physical Properties III |
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Sponsoring Units: DCMP DMP Chair: T. Devereaux, University of Waterloo Room: F151 |
Wednesday, March 17, 2010 11:15AM - 11:51AM |
Q40.00001: Dramatic variation of electronic structure with doping in iron arsenic superconductors revealed by angle resolved photoemission spectroscopy Invited Speaker: The characteristic feature of the phase diagram of iron pnictides is coexistence and competition of the magnetic order and superconductivity in certain doping range. The interplay between magnetism and superconductivity appears to be central issue in understanding properties of these materials. Presence of the two orders has pronounced effects on the electronic structure, which is here used to elucidate their intriguing relation which then can be studied with angle resolved photoemission spectroscopy (ARPES). We have collected extensive sets of data on CaFe$_2$As$_2$ and BaFe$_2$As$_2$, two types of parent compounds as well as Co-doped BaFe$_2$As$_2$ for a wide range of carrier concentration. The data of parent compounds demonstrates that although inner most Fermi surface sheet is strongly three-dimensional, it does indeed have long parallel segments along the $k_z$ direction that can lead to the emergence of magnetic order via ($\pi,\pi$) nesting. More interestingly, we find very unusual incommensurate nesting of the Fermi surface in the $a-b$ plane that is present only at low temperatures. We speculate that this may be a signature of a failed Charge Density Wave (CDW) state that was predicted by renormalization group studies. Data from Co-doped BaFe$_2$As$_2$ samples is particularly intriguing. We found that the signature of Fermi surface reconstraction due to the antiferromagnetic ordering vanishes at the doping corresponding to the onset of the superconductivity, rather than point where the sample becomes paramagnetic. This is consistent with an abrupt change with doping of a value of hole coefficient, which is sensitive to a topology of Fermi surface. Our finding clearly demonstrates that avoiding magnetically driven, significant Fermi surface reconstruction is a key to establishing the superconductivity in iorn arsenic superconductors. [Preview Abstract] |
Wednesday, March 17, 2010 11:51AM - 12:03PM |
Q40.00002: ARPES studies of the iron pnictide superconductors Ming Yi, Donghui Lu, James Analytis, Jiun-Haw Chu, Sung-Kwan Mo, Ruihua He, Makoto Hashimoto, Rob Moore, Igor Mazin, David Singh, Ian Fisher, Zhi-Xun Shen A lot of progress has been made in understanding the family of iron-based pnictide superconductors since its discovery. However, many issues still remain. Here we use angle-resolved photoemission to study the electronic structures of various pnictide materials. Specifically, through a systematic high-resolution study of the parent compounds (Ba,Sr)Fe$_{2}$As$_{2}$, we show that the electronic structures of these materials are significantly reconstructed across the spin density wave transition, which cannot be described by a simple folding scenario of conventional density wave ordering. Moreover, we find that LDA calculations with an incorporated suppressed magnetic moment of 0.5$\mu _{B}$ can match well the details in the reconstructed electronic structure, suggesting that the nature of magnetism in the pnictides is more itinerant than local, while the origin of suppressed magnetic moment remains an important issue for future investigations. [Preview Abstract] |
Wednesday, March 17, 2010 12:03PM - 12:15PM |
Q40.00003: Effect of multiple bands on point contact spectra in iron chalcogenide superconductors Cassandra R. Hunt, H.Z. Arham, W.K. Park, L.H. Greene, Z.J. Xu, J.S. Wen, Z.W. Lin, Q. Li, G. Gu We present point-contact spectroscopy (PCS) measurements on single crystal Fe$_{1+y}$Te$_{1-x}$Se$_{x}$ using a nanoscale Au tip contact. Analysis of PCS measurements using BTK theory[1] is well-established, and recent work[2] that extends BTK to two-band superconductors with a relative band phase may shed new light on the nature of the SC order parameter (OP) of these iron-based materials. Recent experiments suggests that they have $s_\pm$ symmetry, but $d$-wave and anisotropic $s$-wave OPs cannot yet be ruled out[3]. The two-band model predicts that $s_\pm$-wave leads to interference effects between bands that result in conductance profiles distinct from a $d$-wave OP. Preliminary fitting is discussed for the chalcogenides as well as the conventional multi-band $s_{++}$-wave superconductor, MgB$_2$. [1] G E Blonder, M Tinkham, T M Klapwijk, PRB \textbf{25}, 4515 (1982); [2] A A Golubov, \emph{et al.} PRL \textbf{103}, 077003 (2009); [3] M V Sadovskii, UFN \textbf{178} 1243 (2008) [Preview Abstract] |
Wednesday, March 17, 2010 12:15PM - 12:27PM |
Q40.00004: X-ray magnetic circular dichroism spectroscopy investigation of ferromagnetic ordering in NdCoAsO Vemuru Krishnamurthy, Dave Keavney, Micheal McGuire, Brian Sales, Athena Sefat, David Mandrus Recently, the electronic properties of RMOAs (R= rare earth, M = transition metal such as Fe or Co) type 3d-transition metal based layered oxypnictides have attracted considerable interest, in view of the discovery of superconductivity in the FeAs compounds. Here, we would like to present the results of ferromagnetic ordering, element specific magnetic moments of Nd and Co in NdCoAsO, obtained by x-ray magnetic circular dichroism spectroscopy at Nd and Co sites. The nature of magnetic coupling between Co and Nd and its temperature dependence in relation to the bulk magnetization will be discussed. [Preview Abstract] |
Wednesday, March 17, 2010 12:27PM - 12:39PM |
Q40.00005: Spin-induced Symmetry Breaking of the Electronic Structure of CaFe2As2 Qiang Wang, Zhe Sun, Eli Rotenberg, Filip Ronning, Eric Bauer, Hsin Lin, Robert Markiewicz, Matti Lindroos, Bernardo Barbiellini, Arun Bansil, Daniel Dassau Neutron scattering experiments have shown that the ground state of undoped Fe-As compounds exhibits collinear magnetic structure, namely, FM coupling and AFM coupling exist simultaneously along the orthogonal Fe-Fe bonds. However, the corresponding electronic structure is still a mystery. Using ARPES, we measured the low-temperature antiferromagnetic (AFM) phase of high quality crystals of CaFe2As2. We found that, consistent with collinear magnetic structure, the electronic structure exhibits symmetry breaking along the orthogonal Fe-Fe bonds. This is also consistent with the LDA calculation. We also found that FM coupling and AFM coupling results in exotic band dispersions perpendicular to the Fe-As layers. These properties shed light on the true band structure of undoped Fe-As compounds and are indispensable for the study of superconductivity and paring mechanism in Fe-As superconductors. [Preview Abstract] |
Wednesday, March 17, 2010 12:39PM - 12:51PM |
Q40.00006: Symmetry breaking and Orbital fluctuation in iron pnictides Zhiping Yin, Warren Pickett To perform a local orbital analysis of electronic and magnetic interactions, we construct Wannier functions of the Fe $3d$ orbitals in several parent compounds of the newly discovered iron pnictide superconductors in both nonmagnetic and antiferromagnetic phases, and use a tight binding representation to fit the DFT-LSDA Fe-derived bands. The calculated hopping parameters indicate that electrons in the Fe $3d_{xz}$ ( $3d_{yz}$) orbital have a larger amplitude to hop in the $y$ ($x$) direction rather than the $x$ ($y$) direction. Weak stripe antiferromagnetism causes the spin-majority electron in Fe $3d_{xz}$ orbital hop in both $x$ and $y$ directions, but not so for the $3d_{yz}$ (possible because tetragonal symmetry is lost). This difference reinforces anisotropy which is accompanied by a large broken symmetry in the $3d_{xy}$ orbital. To take advantage of a kinetic energy gain from this additional hopping process, orbital fluctuation is favored, which reduces the ordered Fe magnetic moment in the stripe antiferromagnetic phase, consistent with experimental observations. It will also be shown how the pnictide atom is influential in forming the stripe antiferromagnetism. Interlayer hopping of Fe $3d$ electrons in the $z$ direction may inhibit fluctuations and thereby help to stabilize the ordered magnetic moment of Fe in the stripe antiferromagnetic phase. [Preview Abstract] |
Wednesday, March 17, 2010 12:51PM - 1:03PM |
Q40.00007: Evidence for weak electronic correlations in Fe pnictides Adam Sorini, Wanli Yang, Cheng-Chien Chen, Brian Moritz, Wei-Sheng Lee, Jiun-Haw Chu, Jim Analytis, Ian Fisher, B. Delley, Francois Vernay, P. Olalde-Velasco, J. D. Denlinger, Zahid Hussain, J. Yang, W. Lu, Z. X. Zhao, Z. A. Ren, Jeroen van den Brink, Z.-X. Shen, Tom Devereaux Using a combination of theoretical techniques we study the XAS and RIXS spectra of three different Fe-pnictide superconductors and parent compounds. Experimental data show that the XAS of the Fe-pnictides is similar to that of Fe metal. Motivated by this fact we analyze the size of electronic correlations, typified by Hubbard's ``U'' and Hund's ``J'', in the Fe-pnictides. Our exact diagonalization cluster calculations include explicit correlations and put an upper limit on U and J. Our relativistic Dirac atomic calculations show the relative unimportance of core-hole multiplet effects and spin-orbit coupling on the spectra. Finally, we present DFT based calculations of the XAS and XES spectra for three Fe-pnictides and Fe metal using the relativistic FEFF8 code and the WIEN2k code. [Preview Abstract] |
Wednesday, March 17, 2010 1:03PM - 1:15PM |
Q40.00008: The Electronic Structure of the Nickel Pnictide Superconductor BaNi$_{2}$As$_{2}$ R. G. Moore, D. H. Lu, F. Ronning, E. D. Bauer, S. -K. Mo, M. Hashimoto, F. Schmitt, J. Schwede, Z. -X. Shen The electronic structure of the nickel pnictide superconductor BaNi$_{2}$As$_{2}$ is investigated with angle-resolved photoemission spectroscopy. While the iron pnictide superconductors display unconventional superconducting properties, superconductivity in the non-magnetic nickel compounds is believed to be more conventional. Here we present the electronic structure of BaNi$_{2}$As$_{2}$ and compare it to the iron pnictide superconductors. While differences in the electronic structures exist, similarities raise questions about the nature of the nickel pnictide superconductivity. Low energy electron diffraction also reveals distinct differences in the surface structure of nickel and iron compounds. Temperature dependence across the T$_{c}$ = 140K structural phase transition is also investigated. [Preview Abstract] |
Wednesday, March 17, 2010 1:15PM - 1:27PM |
Q40.00009: Surface Superconductivity in the `Partial' Superconductor BaFe$_{2}$As$_{2}$ T. D. Blasius, J. S. Kim, E. G. Kim, G. R. Stewart, T. Vojta The partial superconductivity (resistive transition to $\rho $=0, but no bulk anomalies) in undoped BaFe$_{2}$As$_{2}$, as well as in other MFe$_{2}$As$_{2}$ compounds, has been a puzzle. Sample dependence plays a large role in the superconductivity, and hints have been found for a sensitivity of the surface to exposure to air. Based on a fairly reliable ability to produce such superconducting samples (with a yield of $\sim $ 50{\%}), and the clue from a previous work [1] that the critical current of the superconductivity is abnormally low, we show here that the superconductivity is a 2-dimensional affect -- presumably on the surface. [1] J. S. Kim, et al., J. Phys.: Condens. Matter \textbf{21}, 342201 (2009). [Preview Abstract] |
Wednesday, March 17, 2010 1:27PM - 1:39PM |
Q40.00010: Superconductivity in Fe$_{1.08}$Te:O$_x$ Epitaxial Thin Films Weidong Si, Qing Jie, Lijun Wu, Juan Zhou, Genda Gu, Peter Johnson, Qiang Li Superconducting thin films of Fe$_{1.08}$Te:O$_x$ have been epitaxially grown on SrTiO$_3$ substrates by pulsed-laser deposition in controlled oxygen atmosphere. Although the bulk Fe$_{1.08}$Te is not superconducting, thin films with oxygen are superconducting with an onset and a zero resistance transition temperature around 12\,K and 8\,K respectively. Oxygen was found to be crucial to the superconducting properties of these films, suggesting the oxygen can induce superconductivity possibly through substitution of Te. A metal-insulator transition is found at about 60\,K, lower than that of bulk ($\sim$\,70\,K). From magnetoresistive measurements, we obtained the irreversibility line and the upper critical field. [Preview Abstract] |
Wednesday, March 17, 2010 1:39PM - 1:51PM |
Q40.00011: Induced Superconductivity in Iron Telluride Films Yuefeng Nie, Donald Telesca, Joseph Budnick, Boris Sinkovic, Barrett Wells The 11-type iron chalcogenide superconductors have the simplest crystal structure of the iron-based supercontuctors, consisting only of buckled Fe-anion planes. We grew FeTe films epitaxially on MgO substrates by pulsed laser deposition. X-ray diffraction shows our films to be tetragonal with c axis aligned with the substrate normal. By adding oxygen to the system, superconductivity was induced in the FeTe films. The onset temperature is near 10 K. X-ray absorption measurements show that in the FeTe films which become superconducting, the nominal Fe valence state increases from largely 2+ to 3+. This suggests that the path towards achieving superconductivity in O doped FeTe is quite different from the more standard procedure of substitutionally exchanging isoelectronic Se onto Te sites. [Preview Abstract] |
Wednesday, March 17, 2010 1:51PM - 2:03PM |
Q40.00012: Electronic structure of the iron arsenic parent compound LaFeAsO studied by angle resolved photoemission spectroscopy Chang Liu, Yongbin Lee, Ari Palczewski, Jiaqiang Yan, Takeshi Kondo, Bruce Harmon, Adam Kaminski, Tom Lograsso The current availability of millimeter-sized single crystals of LaFeAsO opened the door to the detailed investigation on the physical properties of this iron pnictide parent compound. In this talk we report our latest results on the electronic structure of this material measured by angle resolved photoemission spectroscopy (ARPES) with special focus on the observation of a surface-driven electronic state. By comparison with theoretical calculations we show that at least the extra large $\Gamma$-pocket seen by ARPES is generated by the surface atomic layer of the crystal. Detailed Fermi surface topology and other related topics will also be discussed. [Preview Abstract] |
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