Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session Q13: Focus Session: Fe-based Superconductors-STM and Neutrons |
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Sponsoring Units: DMP Chair: Andreas Kreyssig, Iowa State University Room: 207 |
Wednesday, March 5, 2014 2:30PM - 3:06PM |
Q13.00001: The electronic phase diagram of NaFe$_{\mathrm{1-x}}$Co$_{\mathrm{x}}$As studied by scanning tunneling microscopy Invited Speaker: Yayu Wang Similar to the high $T_{\mathrm{C}}$ cuprates, the iron pnictide superconductors also lie in close proximity to a magnetically ordered phase. The interplay between magnetism and superconductivity (SC) is a central issue concerning the pairing mechanism. A key step for resolving this issue is to acquire a comprehensive picture regarding the nature of various phases and interactions in the iron-based compounds. In this talk we present doping, temperature, and spatial evolutions of the electronic structure of NaFe$_{\mathrm{1-x}}$Co$_{\mathrm{x}}$As studied by scanning tunneling microscopy. The spin density wave (SDW) gap in the parent state is directly observed, which shows a strongly asymmetric lineshape that is incompatible with conventional Fermi surface nesting. In the underdoped regime the SDW and SC phases are shown to microscopically coexist and compete with each other. The optimally doped sample exhibits a single SC gap, but in the overdoped regime another asymmetric gap-like feature emerges near the Fermi level. In contrast to the rich variations of the low energy electronic states, the high energy spectra of the NaFe$_{\mathrm{1-x}}$Co$_{\mathrm{x}}$As system remain nearly unchanged until the system enters the strongly overdoped non-SC regime. The implications of the local electronic structures on the pairing mechanism of the iron pnictides will be discussed. [Preview Abstract] |
Wednesday, March 5, 2014 3:06PM - 3:18PM |
Q13.00002: The upper critical field of NaFe$_{1-x}$Co$_x$As superconductors Saman Ghannadzadeh, Jack D. Wright, Francesca R. Foronda, Stephen J. Blundell, Simon J. Clarke, Paul A. Goddard Measurement of the upper critical field $H_{\rm c2}$ is a crucial part of the experimental effort to understand unconventional superconductivity. Studying the temperature dependence of $H_{\rm c2}$ and the interplay between the various pair-breaking mechanisms may hint towards the underlying pair-forming interaction. $H_{\rm c2}$ and its anisotropy are also sensitive to the cooper-pair symmetry, as well as the underlying dimensionality and electronic structure of the system. Here, we present the results of upper critical field measurements on the 111 system NaFe$_{1-x}$Co$_x$As in fields up to 45 T, across the phase diagram from the parent to the over-doped compound. We show that a multi-band model is required to describe $H_{\rm c2}$. We find the in-plane critical field to be strongly dominated by paramagnetic pair-breaking. In the parent compound the paramagnetic limit is shown to be equal to the BCS value; however addition of Co leads to a significant enhancement of the paramagnetic limit above the BCS value, most likely due to a combination of spin-orbit and strong-coupling effects. [Preview Abstract] |
Wednesday, March 5, 2014 3:18PM - 3:30PM |
Q13.00003: Directly Visualizing Bogoliubov Quasiparticle Interference of LiFeAs: a Way Toward Understanding Superconductivity in Iron Pnictides Shun Chi, S. Johnston, G. Levy, S. Grothe, R. Szedlak, B. Ludbrook, Ruixing Liang, P. Dosanjh, S.A. Burke, A. Damascelli, D.A. Bonn, W.N. Hardy, Y. Pennec Imaging quasiparticle interference (QPI) is a way to probe the electronic states of a wide range of materials. In superconductors, QPI of Bogoliubov excitations is directly sensitive to the sign changes of the superconducting order parameter. In this talk, I present our investigation of QPI in superconducting LiFeAs by means of scanning tunneling microscopy/spectroscopy, angle resolved photoemission spectroscopy, and multi-orbital tight binding calculations. Using this combination we identify intra- and interband scattering vectors between the hole ($h$) and electron ($e$) bands in the QPI maps. Bogoliubov QPI, with a clear antisymmetric phase at positive and negative bias voltages near the superconducting gap, is revealed in the spatial modulations of the local density of states. The observation of both h-h and e-h scattering intensity variations is exploited using scattering selection rules for Bogoliubov quasiparticles. From this we infer an $s+-$ gap structure, where a sign change occurs in the superconducting order parameter between the $e$ and $h$ bands. [Preview Abstract] |
Wednesday, March 5, 2014 3:30PM - 3:42PM |
Q13.00004: Doping dependence of nematicity in NaFe$_{\mathrm{1-x}}$Co$_{\mathrm{x}}$As imaged with scanning tunneling spectroscopy Ethan Rosenthal, Erick Andrade, Carlos Arguello, Rafael Fernandes, Li-Ying Xing, Xian-Cheng Wang, Chang-Qing Jin, Andrew Millis, Abhay Pasupathy Multiple experiments have found evidence for broken C$_{\mathrm{4}}$ rotational symmetry (nematicity) in the electronic structure of iron-based superconductors above the bulk magnetic and structural transition temperatures and across the doping phase diagram. Deducing the relationship between this broken symmetry state and the proximal superconducting state is essential to understanding the nature of the unconventional superconductivity. Many aspects of the nematic state still remain unknown given the bulk-probing nature of many measurements and their inability to probe beneath the superconducting dome. We use atomic-resolution, scanning tunneling spectroscopy (STS) to examine electronic nematicity across the phase diagram in NaFe$_{\mathrm{1-x}}$Co$_{\mathrm{x}}$As. We find that electronic anisotropy persists both above and below the superconducting dome from the parent compound to the overdoped regime. The strength of the nematicity decreases with increased doping and finally disappears in heavily doped, non-superconducting samples. With the spectral resolution of STS, we will discuss the energy dependence of the nematicity, as well as its interplay with magnetic and superconducting order. [Preview Abstract] |
Wednesday, March 5, 2014 3:42PM - 3:54PM |
Q13.00005: Multiple superconducting gaps in hole-doped Ba$_{0.85}$K$_{0.15}$Fe$_{2}$As$_{2}$ observed by nano-scale Andreev reflection spectroscopy Guoxiong Su Recently, intensive attention has been paid to iron-based superconductors owing to their high transition temperature and intriguing physical properties, especially the mechanism for the superconductivity. Here we investigate the gap structure of Ba$_{0.85}$K$_{0.15}$Fe$_{2}$As$_{2}$ with Tc $\sim$ 25 K. By employing a novel experimental approach to point-contact Andreev reflection spectroscopy, experimental support has been found for the multiple superconducting gaps in hole-doped Ba$_{0.85}$K$_{0.15}$Fe$_{2}$As$_{2}$. The effects of temperature and magnetic field will be discussed as well. We also investigate the behavior of Andreev reflection spectrum at high biased voltage range. This work opens up new opportunity to understand the pairing mechanism and study gap structures in iron-based superconductors. [Preview Abstract] |
Wednesday, March 5, 2014 3:54PM - 4:06PM |
Q13.00006: The low temperature magnetic structure of superconducting FeTeO$_{\mathrm{x}}$ films L.K. Narangammana, J.I. Budnick, W.A. Hines, C. Niedermayer, E.E. Alp, W. Bi, D.G. Hinks, D.E. Brown, B.O. Wells We compare the temperature dependent magnetic structure of superconducting FeTeO$_{\mathrm{x}}$ and non-superconducting FeTe films using neutron diffraction. Both show an antiferromagnetic transition below 70 K. The major difference between the two is that the superconducting FeTeO$_{\mathrm{x}}$ film shows a distinct reduction in magnetic order around the superconducting transition temperature (13 K) while the non-superconducting FeTe film shows a smoothly developing magnetic order as a function of temperature. Preliminary M\"{o}ssbauer spectroscopy studies done on the superconducting FeTeO$_{\mathrm{x}}$ indicate that film undergoes an antiferromagnetic transition below 50 K and magnetic order still exists in superconducting state. [Preview Abstract] |
Wednesday, March 5, 2014 4:06PM - 4:18PM |
Q13.00007: Determination of a Magnetic Component to the Superconducting Condensation Energy for Fe$_{\mathrm{1+\delta }}$Se$_{\mathrm{x}}$Te$_{\mathrm{1-x}}$ Jonathan Leiner, Vivek Thampy, Mark Lumsden, Andrew Christianson, Douglas Abernathy, Brian Sales, Athena Sefat, Zhiqiang Mao, Jin Hu, Wei Bao, Collin Broholm A quantitative method to extract a magnetic component of the superconducting condensation energy from inelastic neutron scattering data is described and applied to Fe$_{\mathrm{1+\delta }}$Se$_{\mathrm{0.4}}$Te$_{\mathrm{0.6}}$. Based on the first moment sum-rule for the dynamic correlation function, the method is sensitive to changes in the inter-site magnetic correlation energy, $\Delta E_{\mathrm{ij}}$, associated with superconductivity. We find the length scale over which $\Delta E_{\mathrm{ij\thinspace }}$is appreciable coincides with the superconducting coherence length as determined by Scanning Tunneling Microscopy. The overall change in inter-site magnetic correlation energy is compared to the superconducting condensation energy determined through specific heat measurements. [Preview Abstract] |
Wednesday, March 5, 2014 4:18PM - 4:30PM |
Q13.00008: Neutron Scattering Study of Low Energy Magnetic Excitation in FeTeSe System Zhijun Xu, Jinsheng Wen, John Schneeloch, Masaaki Matsuda, A.D. Christianson, Genda Gu, I.A. Zaliznyak, Guangyong Xu, J.M. Tranquada, R.J. Birgeneau We have performed neutron scattering and magnetization/transport measurements on a series of FeTe1-xSex system single crystals to study the interplay between magnetism and superconductivity. Comparing to pure FeTe1-xSex compounds, extra Fe and Ni/Cu doping on Fe-site can change physics properties of these samples, including resistivity, magnetization and superconducting properties. Our neutron scattering studies also show the Fe-site doping change low energy magnetic spectrum, including the magnetic excitations intensity, position and magnetic correlation length in these samples. On the other hand, the temperature dependence of the low energy magnetic fluctuations are also found to be different depending on the composition. This work is supported by the Office of Basic Energy Sciences, DOE. [Preview Abstract] |
Wednesday, March 5, 2014 4:30PM - 4:42PM |
Q13.00009: Observation of acoustic-phonon-like mode driven by magnetic imbalance between neighboring Fe atoms in Fe$_{1+y}$Te ($y < 0.12$) David Fobes, Igor Zaliznyak, Zhijun Xu, Genda Gu, John M. Tranquada, Xu-Gang He, Wei Ku, Ovidiu Garlea We have studied the evolution with temperature of the low-energy inelastic spectra of Fe$_{1+y}$Te ($y < 0.12$), a parent compound of the iron-chalcogenide superconductor family, revealing an acoustic mode at an unexpected position. Recently, we found evidence for the formation of a bond-order wave leading to ferro-orbital order in the monoclinic phase, in part due to the observation of an elastic structural peak at (100) in the low-temperature monoclinic phase [D. Fobes, \textit{et al.}, arXiv:1307.7162]. In the inelastic spectra we observe a sharp acoustic-phonon-like mode dispersing out of the (100) position in the monoclinic phase. Surprisingly, the mode survives in the tetragonal phase, despite the absence of a Bragg peak at (100); such a peak is forbidden by symmetry. LDA calculations suggest this mode could involve significant magnetic scattering. By assuming in-phase virtual displacement of the Fe atoms from their equilibrium position in a frozen phonon calculation, we have found a small but significant imbalance in the magnetic moments between the two Fe atoms within the unit cell, suggesting magnetic contribution to the mode. [Preview Abstract] |
Wednesday, March 5, 2014 4:42PM - 4:54PM |
Q13.00010: The non-magnetic collapsed tetragonal phase of CaFe$_{2}$As$_{2}$ and superconductivity in the iron pnictides J.H. Soh, G.S. Tucker, D.K. Pratt, D.L. Abernathy, M.B. Stone, S. Ran, S.L. Bud'ko, P.C. Canfield, A. Kreyssig, R.J. McQueeney, A.I. Goldman The relationship between antiferromagnetic spin fluctuations and superconductivity has become a central topic of research in studies of superconductivity in the iron pnictides. We present unambiguous evidence of the absence of magnetic fluctuations in the non-superconducting collapsed tetragonal phase of CaFe$_{2}$As$_{2}$ via inelastic neutron scattering time-of-flight data, which is consistent with the view that spin fluctuations are a necessary ingredient for unconventional superconductivity in the iron pnictides. We demonstrate that the collapsed tetragonal phase of CaFe$_{2}$As$_{2}$ is non-magnetic, and discuss this result in light of recent reports of high-temperature superconductivity in the collapsed tetragonal phase of closely related compounds. [Preview Abstract] |
Wednesday, March 5, 2014 4:54PM - 5:06PM |
Q13.00011: The structural and magnetic properties of Cs$_{\mathrm{x}}$Fe$_{\mathrm{2-y}}$Se$_{2}$ as determined by x ray and neutron scattering of powder and single crystal samples Keith Taddei, Omar Chmaissem, Mihai Sturza, Sevda Avci, Helmut Claus, Mercouri Kanatzidis, Stephan Rosenkranz, Ray Osborn The A$_{\mathrm{x}}$Fe$_{\mathrm{2-y}}$Se$_{2}$ family of iron selenides (A $=$ K, Rb and Cs) has proven an intricate system for the study of unconventional superconductivity, exhibiting high temperature superconductivity ($\sim$ 30 K) and a complex structural phase transition into a biphasic state coupled with a high temperature magnetic transition ($\sim$ 500 K). While isostructural to the 122 arsenides, significant structural differences are identified. In the selenides, iron vacancies in the tetrahedral FeSe layers become ordered below a high temperature structural transition defining a main phase $\surd 5 \times \surd 5$ superstructure. Coexistent with the main phase, a secondary phase of a previously contested structure is observed and it is in this biphasic state that superconductivity arises at $\sim$ 30 K. Both powder and single crystal samples show similar phase separation and coexistence. In this talk, I will discuss structural results and lattice parameter evolution obtained from neutron powder diffraction as well as single crystal x-ray diffraction with an emphasis on a novel magnetic structural model, the identification of the secondary phase, and the nature of coincidence of the magnetic, structural and secondary phase transitions. [Preview Abstract] |
Wednesday, March 5, 2014 5:06PM - 5:18PM |
Q13.00012: Magnetic Excitations in Cr-Ru Superconducting Alloy Mehmet Ramazanoglu, Robert J. McQueeney, Thomas A. Lograsso, Deborah L. Schagel, Andreas Kreyssig, Alan I. Goldman, Daniel Pratt, Jeffrey W. Lynn, Garreth Granroth We have studied the spin fluctuations in the normal state of a cubic superconductor Cr$_{(1-x)}$Ru$_{x}$ for x$=$0.2. The electron doping created by Ru ions in Cr$_{(1-x)}$Ru$_{x}$ monotonically decreases the antiferromagnetic (AFM) spin density wave (SDW) transition temperature, T$_{N}$. As the Ru fraction increases through x$=$0.17 [1], the long-range SDW order is completely suppressed and the alloy becomes a superconductor. We have conducted a series of inelastic neutron scattering (INS) experiments with Ru concentration of x$=$0.2 with a superconducting transition temperature of Tc $\sim$1.6 K. The SDW fluctuations are found to be commensurate with the magnetic propagation vector of Q$_{AFM}=$(100). At high energy transfers, the strong spin fluctuations appearing near Q$_{AFM}$ reach beyond dE$=$120 meV, not unlike metallic Cr, where spin excitations are very energetic and can reach up to several hundreds of meV [2]. These excitations are also found to be down to energies of dE$=$2 meV. We discuss the possibility that superconductivity found in the Cr-Ru alloy system is unconventional. \\[4pt] [1] K. Chatani and Y. Endoh, J. Phys. Soc. Of Jpn, 72 , 17 , (2003)\\[0pt] [2] E. Fawcett, et. al. Rev. Mod. Phys. 66, 25 (1994) [Preview Abstract] |
Wednesday, March 5, 2014 5:18PM - 5:30PM |
Q13.00013: Physical properties of Rh substituted CaFe$_{2}$As $_{2}$ tuned by annealing/quenching Sheng Ran, Sergey Bud'ko, Paul Canfield Our previous work on CaFe$_{2}$As$_{2}$ single crystal grown out of FeAs flux has shown that a process of annealing and quenching can be used as an additional control parameter which can tune the ground state of CaFe$_{2}$As$_{2}$ systematically. We have also shown that CaFe$_{2}$As$_{2}$ is very pressure sensitive. Therefore, unlike the BaFe$_{2}$As$_{2}$ system, the effect of 4d transition metal substitution on CaFe$_{2}$As$_{2}$ is expected to be largely different from that of 3d transition metal substitution (e.g. cobalt or nickel substitution). In this talk we will present results of measurements on a Rh substituted CaFe$_{2}$As$_{2}$ system with different annealing/quenching temperatures. Phase diagrams with substitution level and annealing/quenching temperature as independent parameters are constructed and compared with that of other transition metal substitutions. [Preview Abstract] |
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