Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session L3: Gap Structure of the Ba-122 Iron Based Superconductors |
Hide Abstracts |
Sponsoring Units: DCMP Chair: Peter Hirschfeld, University of Florida Room: Ballroom A3 |
Tuesday, March 22, 2011 2:30PM - 3:06PM |
L3.00001: Electronic structure studies of Ba/EuFe$_2$As$_2$ based superconductors by angle and time-resolved photoemission spectroscopy Invited Speaker: We report high-resolution ARPES studies on the evolution of the electronic structure of Ba/EuFe$_2$As$_2$ compounds upon n-type doping by replacing Fe by Co and applying chemical pressure by substituting As by P. In particular, we have investigated the nesting conditions between the hole pockets in the centre and the electron pocket at the corner of the Brillouin zone (BZ) for various wave vectors perpendicular to the FeAs layers. In the case of chemically doped systems we observe a shift of the Fermi level in an almost rigid band system. These changes of the electronic structure upon doping cause a reduction of the nesting conditions, possibly yielding a microscopic explanation of the phase diagrams in which antiferromagnetic (AF) order is destroyed, followed by the appearance and disappearance of superconductivity at higher doping concentration. On the basis of the almost equivalent phase diagram obtained upon chemically pressurizing the compound, one expects a similar change of the electronic structure. However, in this case, with increasing P concentration, we observe a non-rigid-band-like change of the electronic structure in the centre of the BZ. In spite of this difference, also here the nesting conditions decrease with increasing P substitution, possibly providing a microscopic explanation for the phase diagram. Finally, we have performed femtosecond time-resolved ARPES studies on undoped and doped Ba/EuFe$_2$As$_2$ after optical pumping. Regarding the relaxation processes we obtain information on the complex dynamics of the excited electronic state in these semi metallic systems. Furthermore, we derive a small electron-phonon coupling constant making electron-phonon coupling an unlikely candidate for the mechanism of high-$T_c$ superconductivity in these compounds. This work is performed in collaboration with S. Thirupathaiah, E. Rienks, H. A. D\"urr, S. de Jong, E. van Heumen, E. Slooten, Y. Huang, R. Huisman, M. S. Golden, L. Rettig, R. Cortes, U. Bovensiepen, M. Wolf, A. Erb, T. Wolf, H.S. Jeevan, P. Gegenwart. [Preview Abstract] |
Tuesday, March 22, 2011 3:06PM - 3:42PM |
L3.00002: Symmetry of spin excitation spectra in 122-ferropnictides Invited Speaker: We have studied the symmetry of spin excitation spectra in 122-ferropnictide superconductors by comparing the results of first-principles calculations with inelastic neutron scattering (INS) measurements on Ni- and Co-doped BaFe$_2$As$_2$ samples close to the optimal doping level, which exhibit neither static magnetic phases nor structural phase transitions. In both the normal and superconducting (SC) states, the spectrum does not follow the $I4/mmm$ space group of the crystal, but instead inherits its symmetry from the unfolded Brillouin zone of the Fe- sublattice. This is manifest both in the in-plane anisotropy of the normal- and SC-state spin dynamics and in the out-of-plane dispersion of the spin-resonance mode and the SC spin gap. The in-plane anisotropy is temperature-independent and can be qualitatively reproduced in normal-state density-functional theory calculations without invoking a symmetry-broken (``nematic'') ground state that was previously proposed as an explanation for this effect. Below the SC transition, the energy of the magnetic resonant mode, as well as its intensity and the SC spin gap, inherit the normal-state intensity modulation along the out-of-plane direction. Apparently, it can be traced back to the three-dimensional band structure and the superconducting gap, both of which were reported to disperse along the out-of- plane direction. [Preview Abstract] |
Tuesday, March 22, 2011 3:42PM - 4:18PM |
L3.00003: London penetration depth as a sensitive tool for determining the superconducting gap structure in iron-pnictide superconductors Invited Speaker: In the high$-T_c$ cuprates, experiments and theories have relied on a single-band picture that is essentially two- dimensional with a single superconducting gap, which provided a simple way to understand the angular dependence of the superconducting order parameter. In iron-based superconductors, the experimental mapping of the superconducting gap structure is complicated by the doping- dependent, multi-band electronic structure with three- dimensional character and the existence of at least two distinct superconducting gaps. Focusing on precision measurements of the London penetration depth, $\lambda(T)$, in ``122'' Ba(Fe$_{1-x}$T$_{x}$)$_{2}$As$_{2}$ (T=Co,Ni,Ru,Pt,Pd,Co+Cu) single crystals, I will discuss the systematics of the ubiquitous power law temperature variation of the in-plane penetration depth, $\lambda_{ab}(T)=\lambda_{ab} (0)+\beta T^n$, and of the absolute value, $\lambda_{ab}(0)$, with the doping level, $x$. To understand the role of disorder and pairbreaking scattering, the effect of heavy ion irradiation has been systematically studied and the results are compared with other systems, most notably stoichiometric LiFeAs. Together with the doping dependence of the out-of- plane London penetration depth, $\lambda_c(T)$, and comparisons to thermal conductivity and specific heat data, these results strongly suggest the development of a significant in-plane anisotropy of the superconducting gap(s) and are also consistent with the appearance of accidental c-axis nodes (not imposed by symmetry) for concentrations moving away from optimal doping. By taking pairbreaking scattering into account, the data for the optimally doped compounds are well described by weak-coupling superconductivity with two nodeless superconducting gaps having amplitudes that differ by about a factor of two. I conclude by emphasizing the significant role of three-dimensionality and scattering in determining the electrodynamics of iron-based superconductors. [Preview Abstract] |
Tuesday, March 22, 2011 4:18PM - 4:54PM |
L3.00004: Phase diagram and superconducting gap structure of the iron-pnictide superconductor (Ba,K)Fe$_{2}$As$_{2}$ Invited Speaker: Measurements of the Nernst and Seebeck coefficients were used to delineate the T-x phase diagram of the iron-pnictide superconductor Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$. The sensitivity of these two coefficients to the reconstruction of the Fermi surface caused by the onset of antiferromagnetic order below a temperature T$_{N}$ allowed us to track T$_{N}$ precisely as a function of concentration x, even when the electrical resistivity, for example, shows no anomaly at the magnetic transition. In the region of concentrations where superconductivity appears out of an antiferromagnetic normal state (T$_{c} \quad <$ T$_{N})$, we investigate the evolution of the superconducting gap structure of Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$ by measuring the thermal conductivity in the T=0 limit. This is a sensitive and directional probe of nodal quasiparticles. As the concentration x is reduced, we find a sudden change in the gap structure from a full gap without nodes to a gap with nodes. We ascribe this change to the onset of antiferromagnetism below a critical doping x$_{N}$ inside the superconducting phase, whose effect is most likely to alter both the Fermi surface and the angular dependence of the gap. We compare these results with our earlier study on Ba(Fe$_{1-x}$Co$_{x})_{2}$As$_{2}$ [1,2]. \\[4pt] [1] M. Tanatar {\it et al.}, Physical Review Letters {\bf 104}, 067002 (2010).\\[0pt] [2] J.-Ph. Reid {\it et al.}, Physical Review B {\bf 82}, 064501 (2010). [Preview Abstract] |
Tuesday, March 22, 2011 4:54PM - 5:30PM |
L3.00005: Superconductivity in a 3D tight-binding model for Ba-122 Invited Speaker: Theoretical investigations of the superconducting state in the iron pnictides have shown that weak-coupling approaches based on a tight-binding parametrization of the LDA band structure can be successfully applied to describe both magnetism and superconductivity in these materials. FLEX, RPA, and fRG studies find in most cases a superconducting state with $s$-wave symmetry that exhibits a $\pi$-phase shift between the gap on the electron and the hole Fermi surfaces, often called a sign-changing $s$-wave state. Besides this general agreement about the symmetry of the superconducting state these studies have also revealed that the momentum dependence of the gap, including the possibility of gap nodes, is highly sensitive to details of the electronic structure, in particular to the orbital composition of the Fermi surface. Since superconductivity in these materials is restricted to the FeAs layers that, at least for the 1111 compounds, are well separated and only weakly coupled most tight-binding models used to study the superconducting state have been limited to two dimensions. On the other hand the 122 compounds as well as the binary compounds show a very pronounced 3D electronic structure with changing orbital weights on the Fermi surfaces along the $k_z$ direction. An RPA based calculation of the spin susceptibility for the Ba-122 material demonstrates that the necessary averaging over the full three dimensional Brillouin zone leads to a broader and more commensurate spin response compared to a corresponding two dimensional calculation in agreement with experimental observations. In addition changes of the orbital character of the Fermi surface lead to a complicated three dimensional gap structure exhibiting V-shaped or near horizontal nodes on the hole sheets near the zone boundary that can in part explain the puzzling transport measurements. [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. |
© 2025 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