Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session B34: Focus Session: Iron Pnictides and Other Novel Superconductors II: Quantum Oscillations, Electronic Structure and Magnetism |
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Sponsoring Units: DMP Chair: Vladimir Antropov, Ames Laboratory Room: 404 |
Monday, March 16, 2009 11:15AM - 11:27AM |
B34.00001: Fermi surface of superconducting LaFePO determined from quantum oscillations Amalia Coldea, J. Fletcher, A. Carrington, J. Analytis, C. Andrew, A. Bangura, J.-H. Chu, A. Erickson, I. Fisher, N. Hussey, R. McDonald We report extensive measurements of quantum oscillations in the normal state of LaFePO, using low temperature torque magnetometry and transport in high static magnetic fields (45 T). LaFePO is a bulk Fe-based superconductor (\textit{Tc$\sim $ }6 K) which can be grown in high quality single crystalline form being isostructural to LaFeAsO but without being affected by magnetic or structural transitions at low temperatures. We find that the Fermi surface is that of a compensated metal in broad agreement with the band-structure calculations with frequencies varying between 2.8{\%} to 9{\%} of the basal plane area of the Brillouin zone. The effective masses vary between 1.7-2.1 me and the electronic correlations in LaFePO are moderate corresponding to a mass enhancement of about \textit{$\sim $}2. The observed variation in the electronic scattering between different bands may be related to their different orbital character. The quasi-two dimensional Fermi surface consists of nearly-nested electron and hole pockets, suggesting proximity to a spin/charge density wave instability. The upper critical field anisotropy is a factor $\sim $10 (at 0.325 K) which may linked to its increased two-dimensionality (PRL \textbf{101}, 216402 (2008)). [Preview Abstract] |
Monday, March 16, 2009 11:27AM - 11:39AM |
B34.00002: Quantum Oscillations in the parent pnictide BaFe2As2: Fermi surface reconstruction of the magnetic ground state James Analytis, Jiun-Haw Chu, Ian Firsher, Ross McDonald, Igor Mazin We have measured quantum oscillations in the magnetically ordered ground state of BaFe2As2, a parent compound of the superconducting ternary pnictides. Measurements were performed in 65T pulsed field at the Los Alamos National High Magentic Field Laboratory, using an atomic force microscope torque cantilever. We also perform detailed band-structure calculations for the spin-density wave ground state and find agreement with our observations of small quasi-two dimensional pockets. These results place significant constraints on our understanding of the magnetism associated with the Fe-As layers and demonstrates that coherent quasiparticles persist in the magnetic ground state, providing an important clue about the nature of superconductivity which emerges when this compound is doped. [Preview Abstract] |
Monday, March 16, 2009 11:39AM - 11:51AM |
B34.00003: Quantum oscillations in the parent magnetic phase of an iron arsenide high temperature superconductor Neil Harrison, Suchitra Sebastian, J. Gillet, P. Lau, David Singh, Charles Mielke, Gilbert Lonzarich We report quantum oscillation measurements in SrFe$_2$As$_2$, which is known to become superconducting under doping and pressure. The magnetic field and temperature dependences of the oscillations between 20 and 55~T suggest that the electronic excitations are those of a Fermi liquid. We show that the observed Fermi surface comprising small pockets is consistent with the formation of a spin-density wave. Our measurements thus demonstrate that high $T_{\rm c}$ superconductivity can occur on doping or pressurizing a conventional metallic spin-density wave state. [Preview Abstract] |
Monday, March 16, 2009 11:51AM - 12:03PM |
B34.00004: Fermi surface changes in LaFeAsO$_{1-x}$F$_x$ using supercells, rigid band shifts, and the virtual crystal approximation Paul Larson, Sashi Satpathy There is currently great interest in the properties of the superconducting material LaFeAsO. While numerous calculations have been performed for this material, questions arise to which approximations describe the changes in the Fermi surface with doping. We have performed {\it ab initio} density functional studies of F-doping in the non-magnetic state using supercell calculations and compared these results to those obtained using rigid band shifts and the virtual crystal approximation (VCA). The Fermi surface consists mainly of Fe d and As p states with La and O states lying far from the Fermi level. Significant differences are found by comparing the supercell results with those of the rigid band shifts, but remarkable agreement is found for the Fermi surface using VCA calculations where the nuclear and electron charge are changed continuously to mimic the addition of electrons. P.Larson and S. Satpathy, condmat:arXiv 0810.4605v1. [Preview Abstract] |
Monday, March 16, 2009 12:03PM - 12:15PM |
B34.00005: Momentum density, Fermi surface and directional Compton profile in the Iron-based superconductor LaOFeAs Y. J. Wang, Hsin Lin, B. Barbiellini , P.E. Mijnarends, S. Kaprzyk, W. Al-sawai, R.S. Markiewicz, A. Bansil We have carried out first principles all-electron calculations of the (001) projected 2D electron momentum density (2D-EMD) and directional Compton profiles (CPs) along the [100], [001], and [110] directions in iron-based superconductor LaO$_{1-x}$F$_{x}$FeAs for various doping concentrations $x$ within the framework of the local density approximation (LDA). We have identified Fermi surface features both in the 2D-EMD and in the CPs. Bonding effects related to the character of wave functions near the Fermi level are revealed by the autocorrelation function B(r) defined as the Fourier transform of the momentum density. Work supported in part by the US Department of Energy. [Preview Abstract] |
Monday, March 16, 2009 12:15PM - 12:27PM |
B34.00006: First Principles studies of 122 Ferropnictide Surface Alexander Kemper, P.J. Hirschfeld, H-P. Cheng We present DFT-GGA calculations for the Ba and Sr-122 ferropnictide materials on the effect of surfaces on the electronic structure. It has been established that there is a strong modulation of the electronic structure by the Fe-As distance, which decreases near a free surface due to surface reconstruction. Indeed, we see significant changes of both Fermi velocities and Fermi surfaces due to these effects for both the paramagnetic and collinear spin density wave states. These changes of the electronic structure, which we exhibit here, will be crucial for the interpretation of surface probes like ARPES and STM. [Preview Abstract] |
Monday, March 16, 2009 12:27PM - 12:39PM |
B34.00007: Structural, spin, and orbital phase transitions in LaOFeAs: I. Total energy calculations Wei Ku, Chi-Cheng Lee, Wei-Guo Yin Recent experimental studies on iron pnictides showed the existence of local Fe magnetic moments even in the superconducting phase, indicating strong fluctuations in a short time scale. We report a investigation of the local electron-lattice and electron-spin couplings via total energy calculations within first-principles density functional theory. Strong coupling in both channels were found to be closely tied to a ferro-orbital order, which drives the structure transition and the stripy magnetic (SDW) transition at high temperature in the undoped system. We suggest that this orbital degree of freedom leads to stronger coupling upon doping and thus possible enhancement of superconductivity. [Preview Abstract] |
Monday, March 16, 2009 12:39PM - 12:51PM |
B34.00008: Structural, spin, and orbital phase transitions in LaOFeAs: II. Wannier function analysis Wei-Guo Yin, Chi-Cheng Lee, Wei Ku A realistic low-energy effective interacting Hamiltonian of doped LaOFeAs is derived quantitatively from a novel first-principles Wannier function analysis. Strong local orbital, spin, and lattice coupling to the doped charge is found associated with the orbital degree of freedom, which suggests a possible explanation of the high transition temperature. The fluctuations in the orbital sector fundamentally distinguish the iron pnictides from the copper oxide superconductors, and ensure the essential role of phonons in the mechanism of superconductivity in this new class of materials, in addition to electronic origin. [Preview Abstract] |
Monday, March 16, 2009 12:51PM - 1:03PM |
B34.00009: Emergence of complex magnetism in three dimensional, yet quasi-layered, iron pnictides: CaFe$_{4}$As$_{3}$ Arthur J. Freeman, Giancarlo Trimarchi, Mercouri Kanatzidis, Iliya Todorov, Duck-Young Chung The class of iron pnictides has been the focus of much attention for the discovery of superconductivity in the layered compounds LaOFeAs, CaFe$_{2}$As$_{2}$, and related ones; the phase diagrams of these pnictides remain still largely unexplored. Here, we report on the electronic and magnetic structure of the recently synthesized CaFe$_{4}$As$_{3}$ compound. This material, as opposed to the layered CaFe$_{2}$As$_{2}$, shows FeAs slabs parallel to the $b$-direction and approximately perpendicular to each other, defining tunnels filled by the Ca atoms. No sign of superconductivity was found in this compound. Instead, the system shows a complex ferromagnetic state at low temperature. DFT calculations performed on the refined crystal structure using the highly precise FLAPW method\footnote{Wimmer, Krakauer, Weinert, and Freeman, PRB, {\bf 24}, 864 (1981)} show a pronounced stabilization for the ferromagnetic state which is characterized by four distinct Fe sites with magnetic moments of between 1 $\mu_{B}$ and 2 $\mu_{B}$. The influence of the local topology of the crystal structure on the the electronic and magnetic state is analyzed. [Preview Abstract] |
Monday, March 16, 2009 1:03PM - 1:15PM |
B34.00010: `Unscreening' Effect on Fe-Pnictide Superconductor Masao Ogata, Yuki Fuseya, Toshikaze Kariyado We study a microscopic mechanism of Fe-pnictide superconductor, considering the screening effects of Coulomb interaction in addition to the conventional spin-fluctuation mechanism. It is shown that, by electron doping, the transition temperature of superconductivity increases due to the `unscreening' effect even though the density of states decrease, while that of spin-density wave rapidly decreases due to breaking of nesting conditions. Our results give a clear interpretation to the mystery of interrelation between $T$c and the density of states observed in the Fe-pnictide superconductors. [Preview Abstract] |
Monday, March 16, 2009 1:15PM - 1:27PM |
B34.00011: Quasiparticle scattering interference in superconducting Oxypnictides Yanyang Zhang, Kangjun Seo, Xiaoting Zhou, B. Andrei Bernevig, Jiangping Hu Based on a two-band model of the superconducting iron oxypnictides, we study the effects of single-impurity scattering on the local density of states by exact T-matrix calculations. We compare the quasi-particle interference patterns in different pairing states, as well as different kinds of impurities. The results of this calculation can be used to test and distinguish different sorts of pairing symmetries and impurities in the experiments. [Preview Abstract] |
Monday, March 16, 2009 1:27PM - 1:39PM |
B34.00012: Quasi-particle properties in iron-based superconductors Xiang Hu, Chin-Sen Ting, Jian-Xin Zhu The pairing symmetry is one of the major issues in the study of iron-based superconductors. We adopted a minimum two band model, to introduce the two-band Bogoliubov de Gennes equations. By solving those equations numerically, we checked the possibilities of different pairing symmetry, and made some predictions which can be tested by future experiments in spectrum tunnelling microscopy. [Preview Abstract] |
Monday, March 16, 2009 1:39PM - 1:51PM |
B34.00013: Nature of the SDW state in FeAs-based Compounds Xi Dai, Zhong Fang, Guangtao Wang, Gang Xu We show that the significant underestimation (about 10\%) of Fe-As bond length in FeAs-based compounds by LDA is due to the strong correlation effect. By properly taking into acount the on-site correlation, we are able to reproduce experimental values (to about 1\%) using self-consistent LDA+Gutzwiller method. Also we will show that the strong on-site orbital fluctuation will dramatically reduce the anti-ferromagnetic long range order in the parent compound. All these results are in good agreement with experiments. [Preview Abstract] |
Monday, March 16, 2009 1:51PM - 2:03PM |
B34.00014: High-temperature Superconductivity: Status John D. Dow, Dale R. Harshman, Anthony R. Fiory A theory of high-temperature superconductivity is presented which (i) explains the cuprates, with cuprate-planes; (ii) describes the superconducting ruthenates without cuprate-planes, such as Ba$_2$YRuO$_6$; (iii) treats the rutheno-cuprates, such as GdSr$_2$Cu$_2$RuO$_8$, whose cuprate-planes do not superconduct; (iv) treats the molecule $\kappa$-[BEDT-TTF]$_2$Cu [NCS]$_2$ which superconducts via S, and (v)explains the pnictides. In YBa$_2$Cu$_3$O$_7$, the theory is consistent with the observation that no Cu-containing plane superconducts and the observed superconductivity is $s$-wave, not $d$-wave, once fluxon-de-pinning has been properly accounted for. The superconducting layers are BaO layers, are $p$-type, and are adjacent to the $n$-type cuprate-planes. The theory is consistent with many data which were previously beyond explanation. [Preview Abstract] |
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