Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session X23: Focus Session: Iron Based Superconductors -- Gap Symmetry |
Hide Abstracts |
Sponsoring Units: DMP DCOMP Chair: Laura Greene, University of Illinois Room: D165 |
Thursday, March 24, 2011 2:30PM - 3:06PM |
X23.00001: Nodal and nodeless Superconductivity in Iron-Based Superconductors Invited Speaker: The superconducting phase in Iron-based superconductors (pnictides) exhibits a variety of different properties depending on the doping regime and specific parameters such as band structure and interaction which describe the different compounds. The question of the symmetry of the superconducting order parameter combined with the role of interaction-induced anisotropies plays a decisive role to distinguish material- dependent effects from universal mechanisms in this family of compounds. In our talk we attempt to provide an overview of the superconducting phases currently discussed for different classes of pnictides. Specifically, we report on our work on functional renormalization group (FRG) calculations for the pnictides and how it can contribute to our understanding of the different compounds. We discuss from first principles why LaOFeAs shows nodeless while LaOFeP shows nodal anisotropic extended $s$-wave superconductivity, which we find to be dictated by the existence / non-existence of an additional hole pocket at $\rm{M}=(\pi,\pi)$ in the unfolded Brillouin zone. We also elaborate on the nodal phase in KFe${}_2$As${}_2$ as being of $d$-wave symmetry type and explain its microscopic origin related to the absence of electron pockets which are gapped out at large hole doping. In particular, we will draw a direct line from band structure and interaction parameter calculations to FRG which accomplishes to study Fermi surface instabilities from a truly ab initio starting point, and illustrate this approach for the LiFeAs compound. [Preview Abstract] |
Thursday, March 24, 2011 3:06PM - 3:18PM |
X23.00002: Do theoretical calculations really predict nodes in Fe-based superconductors? Igor Mazin It is well established that calculations based on the LDA band structure and the Hubbard model, with the parameters $U\sim 1.3-1.6$ eV, and $J\sim 0.2-0.3 J$ (a ``UJ'' model), yield strongly anisotropic, and sometimes nodal gaps. The physical origin of this effect is well understood: the two leading terms in the model are $\sum{Un_{i\uparrow}n_{i\downarrow}}$ and $\sum ' {U n_{i}n_{j}}$. The former ensures that the coupling to spin fluctuations proceeds only through the like orbitals, and the latter, not being renormalized by the standard Tolmachev-Morel-Anderson logarithm, tends to equalize the positive and the negative order parameters. Both these features are suspect on a general physics basis: the leading magnetic interaction in itinerant systems is the Hund-rule coupling, which couples every orbital with all the others, and the pnictides, with the order parameter less than 20 meV, should have nearly as strong renormalization of the Coulomb pseudopotential as the conventional superconductors. I will argue that, instead of the UJ model, in pnictides one should use the ``I'' model, derived from the density functional theory (which is supposed to describe the static susceptibility on the mean field level very accurately). The ``I'' here is simply the Stoner factor, the second variation of the LSDA magnetic energy. Unfortunately, this approach is very unlikely to produce gap nodes as easily as the UJ model, indicating that one has to look elsewhere for the nodes origin. [Preview Abstract] |
Thursday, March 24, 2011 3:18PM - 3:30PM |
X23.00003: Competing Pairing Symmetries in a Generalized Two-Orbital Model for the Pnictides A. Nicholson, W. Ge, X. Zhang, J. Riera, M. Daghofer, A. Oles, G. Martins, A. Moreo, E. Dagotto An extended ``$t$-$U$-$J$'' two-orbital model [1] for the pnictides will be introduced that includes Heisenberg terms deduced from the strong coupling expansion of the Hubbard model. This extension allows us to enhance the strength of the $(\pi,0)$-$(0,\pi)$ spin order and favors the presence of tightly bound pairing states even in the small clusters that are exactly diagonalized. The $A_{\rm 1g}$ and $B_{\rm 2g}$ pairing symmetries are found to compete in the realistic spin-ordered and metallic regime. The dynamical pairing susceptibility additionally unveils low-lying $B_{\rm 1g}$ states, suggesting that small changes in parameters may render any of the three channels stable. These results contribute to understanding the puzzling results in pnictides where both nodeless and nodal states have been reported.\\[4pt] [1] A. Moreo {\em et. al.}, Phys. Rev. {\bf B} 79, 134502 (2009) \\[0pt] [2] A. Nicholson {\em et. al.}, preprint [Preview Abstract] |
Thursday, March 24, 2011 3:30PM - 3:42PM |
X23.00004: What the magnitude of $2\Delta/T_c$ on the hole pockets can tell us about the structure of the gaps on electron pockets in Fe-based superconductors? Saurabh Maiti, Andrey Chubukov There is evidence from transport and penetration depth measurements that some Fe-based superconductors (pnictides) are nodal and some nodeless. Most notable example of nodal behavior is $BaFe_2(As_{1-x}P_x)_2$. But as of this date, there has been no direct probes of the gap structure in this material. ARPES is a direct probe to measure the gap evolution along the Fermi surfaces (FS), but in $BaFe_2(As_{1-x}P_x)_2$ accurate laser ARPES data are only available for hole FSs at $\Gamma$ point, along which the gaps are nearly identical and are nearly angle-independent. We addressed the issue whether one can use ARPES data for $2\Delta_h/T_c$ on the hole FSs to predict the gap structure and magnitude along the two electron FSs. For this, we considered the non-linear gap equations in realistic 2D multi-pocket models. We found that, in the 4-pocket model, at least in certain limits, the electronic gaps have accidental nodes if $2\Delta_h/T_c$ is below a certain value close to the BCS result, and have no nodes if $2\Delta_h/T_c$ exceeds this value. This, combined with the experimental input on the $2\Delta_h/T_c$, allows us to predict the forms of the electronic gaps based on the ARPES data for the gaps on the hole pockets The verification of these results by the ARPES measurements along the electron FSs will be a crucial test for 2D itinerant multi-pocket models for Fe-pnictides. [Preview Abstract] |
Thursday, March 24, 2011 3:42PM - 3:54PM |
X23.00005: Observation of a ubiquitous three-dimensional superconducting gap function in optimally-doped Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$ Y.-M. Xu, Y.-B. Huang, X.-Y. Cui, E. Razzoli, M. Radovic, M. Shi, G.-F. Chen, P. Zheng, N.-L. Wang, C.-L. Zhang, P.-C. Dai, J.-P. Hu, Z. Wang, H. Ding The knowledge of the quasi-three-dimensional (3D) superconducting (SC) gap is essential for understanding the superconducting mechanism of the iron-pnictides highlighted by their multiband and quasi-3D electronic structure. By using the $k_z$-capability of angle-resolved photoemission, we completely determined the SC gap on all five FSs in 3D on Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$ samples. We found a significant $k_z$ dispersion of the SC gap which can only derive from interlayer pairing. Remarkably, the SC energy gaps can be described by a single 3D gap function with two energy scales characterizing the strengths of intra-layer ($\Delta_1$) and interlayer ($\Delta_2$) pairing. The anisotropy ratio $\Delta_2/\Delta_1$, determined from the gap function, is close to the $c$-axis anisotropy ratio of the magnetic exchange coupling $J_c/J_{ab}$ in the parent compound. The ubiquitous gap function for all the 3D FSs reveals that pairing is short-ranged and strongly constrain the possible pairing force in the pnictides. [Preview Abstract] |
Thursday, March 24, 2011 3:54PM - 4:06PM |
X23.00006: Field dependence of the zero energy density of states of an anisotropic $s_{\pm}$ superconductor Yan Wang, Peter Hirschfeld, Siegfried Graser The pairing symmetry in iron-based superconductors (SC) is generally believed to be an $s_{\pm}$-wave state. Although ARPES suggests a mainly isotropic gap on all Fermi surface sheets, different thermodynamic and transport measurements are still inconclusive about the existence and orientation of gap nodes. Specific heat measurements in a magnetic field showing a square root like dependence of the Sommerfeld coefficient $\gamma(B)$ have been reported, contradicting the linear behavior expected for a fully gapped system. For a $d$-wave SC, $\gamma(B)\propto\sqrt{B}$ is well-known as Volovik effect. For a fully gapped $s_{\pm}$-wave SC with $\Delta_+\ne\Delta_-$, a similar concave field dependence is expected. To distinguish these two effects we apply a two-band model using the Riccati parametrization of the Eilenberger equation to study the density of states around a single vortex and compare it with self-consistent calculations in the vortex lattice. Different models for the momentum dependence of the gap on each band relevant to the iron-based SC, ranging from isotropic to strongly anisotropic and nodal gaps are investigated. Partial support was provided by DOE DE-FG02-05ER46236 (PJH). [Preview Abstract] |
Thursday, March 24, 2011 4:06PM - 4:18PM |
X23.00007: Measurement of a sign-changing two-gap superconducting phase in Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$ single crystals using scanning tunneling spectroscopy (STS) M.L. Teague, G.K. Drayna, G.P. Lockhart, T.P. Wu, N.-C. Yeh, P. Chen, B. Shen, H.-H. Wen We present STS studies of the iron pnictide superconductors Ba(Fe$_{1-x}$Co$_{x})_{2}$As$_{2}$ (x=0.06, 0.08, 0.12). Studies on single crystals, cleaved at room temperature in a pure argon atmosphere, reveal direct spectroscopic evidence for predominantly two-gap superconductivity. These gaps decrease with increasing temperature and vanish immediately above the superconducting transition, T$_{C}$. Fourier transformation of the tunneling conductance demonstrates slight doping- and energy-dependent quasiparticle scattering interferences (QPI) near the nesting wave-vectors ($\pm \pi $,0)/(0,$\pm \pi )$ and also near ($\pm $2$\pi $,0)/(0,$\pm $2$\pi )$. The dominant QPI near ($\pm \pi $,0)/(0,$\pm \pi )$ and the two-gap nature are consistent with sign-changing s-wave superconductivity. The excess zero-bias conductance and the large gap-to-T$_{C}$ ratios suggest significant unitary impurity scattering. Further studies of the magnetic field dependence will be discussed. This work was supported by NSF Grant DMR-0907251. [Preview Abstract] |
Thursday, March 24, 2011 4:18PM - 4:30PM |
X23.00008: Local measurement of the superfluid density in the pnictide superconductors $Ba(Fe_{1-x} Co_x )_2 As_2 $ across the superconducting dome Lan Luan, T.M. Lippman, C.W. Hicks, O.M. Auslaender, J.A. Bert, Jiun-Haw Chu, J.G. Analytis, I.R. Fisher, K.A. Moler We locally measure the superfluid density $\rho _s (T)$ in $Ba(Fe_{1-x} Co_x)_2 As_2 $ single crystals with magnetic force microscopy and scanning SQUID susceptometry. These high-precision, local-probe-based techniques enable us to measure both the zero temperature value of the superfluid density $\rho _s (0)$ and the temperature variation, to distinguish homogeneous from spatially varying responses, and to report systematic behavior as a function of Co doping across the superconducting dome. We find that $\rho _s (T)$ increases sharply with decreasing temperature below the superconducting transition temperature $T_c $ of both optimally doped and underdoped compounds, and that $\rho _s (0)$ falls more quickly with $T_c $ on the underdoped side of the dome than on the overdoped. These observations, as well as the increasing temperature induced change of $\rho _s (T)$ at low temperatures upon underdoping, are consistent with magnetic fluctuation mediated pairing and the coexistence of magnetism and superconductivity. [Preview Abstract] |
Thursday, March 24, 2011 4:30PM - 4:42PM |
X23.00009: A sign of change: pinning down the pairing symmetry of the iron-based superconductors Erez Berg, Netanel Lindner, Tamar Pereg-Barnea Understanding the structure of the order parameter of the iron-based superconductors is the key to unveil their pairing mechanism. Although there has been much theoretical and experimental indications that the order parameter changes its sign in momentum space, direct evidence is still lacking. The difficulty stems from the fact that the order parameter is likely to be of s-wave symmetry, and therefore designing a phase sensitive experiment that would clearly reveal the sign change is non-trivial. Here, we examine a contact between a sign-changing superconductor and an ordinary, uniform-sign superconductor. If the the barrier between the two superconductors is not too high, the frustration of the Josephson coupling between different portions of the Fermi surface across the contact can lead to surprising consequences, such as time-reversal symmetry breaking at the interface and unusual energy-phase relations with multiple local minima. We propose this mechanism as a possible explanation for the half-integer flux quantum transitions in niobium-iron pnictide loops, which were discovered in a recent experiment (C-T. Chen et. al., Nature Physics 6, 260 (2010)). [Preview Abstract] |
Thursday, March 24, 2011 4:42PM - 4:54PM |
X23.00010: Probing the Superconducting Order Parameter of Ba(Fe$_{1-x}$Co$_{x})_{2}$As$_{2}$ by Josephson Interferometry J.M. Atkinson, D.J. Van Harlingen, P. Canfield, N. Ni, J.D. Strand Since the discovery of the first Fe-based superconductors in 2006, extensive effort has been directed toward characterizing and modeling the novel properties of these exotic materials, in particular, the symmetry of their superconducting order parameter. We probe the order parameter in Co-doped BaFe$_{2}$As$_{2}$ single crystals by fabricating Josephson junctions on polished faces orthogonal to the c-axis. It has been proposed that the Fe-pnictides form electron and hole pockets in the Fermi surface that have s-wave Cooper pair symmetry but opposite phases, the so-called s$\pm $ model. The modulation of the critical current I$_{C}$ as a function of magnetic flux applied along the c-axis is different for junctions fabricated on a corner (between [100] and [110] faces) or on an edge (either [100] or [110]). In the same way, the product I$_{C}$R should be different for each type of junction. The combination of these effects may help us map the phase anisotropy and test for this pairing symmetry. We will present preliminary results of these studies and attempts to match them with existing theoretical models. [Preview Abstract] |
Thursday, March 24, 2011 4:54PM - 5:06PM |
X23.00011: Robust nodal gap structure in BaFe$_2$(As$_{1-x}$P$_x$)$_2$ with P doping revealed by magnetic penetration depth measurements Kenichiro Hashimoto, Ryo Katsumata, Sho Tonegawa, Shigeru Kasahara, Takahito Terashima, Takasada Shibauchi, Yuji Matsuda, Alessandro Serafin, Antony Carrington A number of experimental studies show that the non-universal superconducting gap structures with and without nodes is realized in iron pnictides, depending on the doping materials and its doping level. It has been suggested that in a framework of s++ wave symmetry, vertical nodal gap structure occurs during the crossover from s++ to s+- state due to the competition between the orbital and magnetic fluctuations as well as the impurity scattering effect. On the other hand, within the spin-fluctuation mediated pairing mechanism, three dimensional nodal structures is discussed. Therefore, it is important to uncover the doping dependence of the superconducting gap structure and its impurity effect. Here we report the magnetic penetration depth results measured down to 100 mK in the P-doped Ba122 system indicate robust nodal gap structure. We especially focus on doping evolution of the superfluid density with P doping. We also discuss the impurity effect introduced by Pb heavy ion beam respective to the in-plane. [Preview Abstract] |
Thursday, March 24, 2011 5:06PM - 5:18PM |
X23.00012: Two-Gap Paring of the Optimal Doped (M,K)Fe$_{2}$As$_{2}$ with M = Ba, Sr Fengyan Wei, Bing Lv, Feng Chen, Yuyi Xue, Chingwu Chu The gap structure revealed by the specific heat of iron pnictides remains unsettled. Not only do the reported characters vary for similar Ba$_{0.6}$K$_{0.4}$Fe$_{2}$As$_{2}$ and Sr$_{0.55}$K$_{0.45}$Fe$_{2}$As$_{2}$, single gap and two-gap pairings have also been suggested in the crystals with the same nominal composition of Ba$_{0.6}$K$_{0.4}$Fe$_{2}$As$_{2}$. It seems that either the gap structure is unusually sensitive to the sample details or some analysis procedures need to be refined Here we explored both the (Sr,K)Fe$_{2}$As$_{2}$ and (Ba,K)Fe$_{2}$As$_{2}$ systems, and different procedures were used to extract the phonon background. In the case of (Sr,K)Fe$_{2}$As$_{2}$, the phonon background seems to be insensitive to both the procedures and the potassium doping. For (Ba,K)Fe$_{2}$As$_{2}$, however, the data suggest a significant doping dependency of the soft phonons. The observations cast doubts on the previous procedures of using either BaFe$_{2}$As$_{2}$ or Ba(Fe$_{0.9}$Co$_{0.1})_{2}$As$_{2}$ to estimate the phonon background of Ba$_{0.6}$K$_{0.4}$Fe$_{2}$As$_{2}$. A new procedure, therefore, is developed. The result will be presented and discussed. [Preview Abstract] |
Thursday, March 24, 2011 5:18PM - 5:30PM |
X23.00013: Superconducting gap measurements on Co-doped SrFe2As2 single crystals by point contact spectroscopy Cassandra R. Hunt, H.Z. Arham, W.K. Park, L.H. Greene, J. Gillett, S. Sebastian We present point contact spectroscopy results on single crystal Co-doped SrFe$_2$As$_2$. Two sets of Andreev-like enhancements in conductance are seen with nominally $c$-axis contacts. For temperatures up to $T_c$ = 14.5 K, the conductance is fit to a Blonder-Tinkham-Klapwijk (BTK) model extended to two independent bands with lifetime broadening [1]. We also consider recently proposed $s_\pm$-wave extensions to BTK [2,3]. Many recent reports claim multiple gaps in the 122 compounds, however care must be taken to distinguish the presence of Andreev peaks from other excitation modes. We find robust evidence of an SC gap at 6 meV and evidence of another conductance enhancement at 12 mV that tracks the inner gap. The origin of this feature, and of multi-gap features as measured by PCS, are discussed. [1] G. E. Blonder, M. Tinkham, and T. M. Klapwijk, PRB \textbf{25}, 45154532 (1982). [2] A. A. Golubov, \emph{et al}. PRL \textbf{103}, 077003 (2009). [3] I. B. Sperstad, J. Linder, A. Sudbo, PRB \textbf{80}, 144507 (2009). [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700