Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session M5: Focus Session: Intercalated FeSe and Other Fe-chalcogenide Superconductors |
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Sponsoring Units: DMP DCOMP Chair: Stephen Wilson, University of California, Santa Barbara Room: Juan Gorman Room 005 |
Wednesday, March 4, 2015 11:15AM - 11:51AM |
M5.00001: High transition temperatures in molecular intercalates of FeSe Invited Speaker: Stephen Blundell Molecular groups can now be intercalated into iron-based superconductors with dramatic consequences on the superconducting properties. These species act as charge reservoirs, sources of electrical polarization, and also make subtle structural modifications to superconducting layers, all of which can make novel adjustments to the band structure that in turn can control superconducting properties. By synthesizing the compound Lix(NH2)y(NH3)1$-$yFe2Se2 (x $\sim$ 0.6; y $\sim$ 0.2), in which lithium ions, lithium amide and ammonia (NH3) act as the spacer layer between FeSe layers, we have turned a 9 K superconductor into a 43 K superconductor [1]. Further chemical modification allow us to produce a range of new superconducting materials which we have studied using a variety of techniques including muon-spin rotation. Recently, we have used hydrothermal reactions to produce layered lithium iron selenide hydroxides with chemical formula Li1--xFex(OH)Fe1--ySe and thereby producing compounds whose transition temperature can be tuned from zero up to about 40 K [2]. Minimizing the concentration of iron vacancies in the iron selenide layer and simultaneously increasing the electron count on iron in the selenide layers enhance the superconducting properties in this family. Future prospects for new superconducting materials using these novel synthetic routes will be discussed, as will also our current understanding of the superconductivity in these materials. (Work performed in collaboration with S. J. Clarke and coworkers at Oxford, RAL and Durham, UK.) \\[4pt] [1] M. Burrard-Lucas et al., Nature Materials \textbf{12}, 15 (2013).\\[0pt] [2] H. Sun et al. arXiv:1408.4350 [Preview Abstract] |
Wednesday, March 4, 2015 11:51AM - 12:03PM |
M5.00002: Unified picture of the doping dependence of superconducting transition temperatures in alkali metal/ammonia intercalated FeSe Daniel Guterding, Harald Jeschke, Peter Hirschfeld, Roser Valenti We present a theoretical investigation of alkali metal/ammonia intercalated iron selenide [1]. Using ab-initio density functional theory we unravel how charge doping and dimensionality of the electronic structure can be controlled through the chemical composition of the intercalated molecules. Within random phase approximation spin fluctuation theory we analyze the impact of intercalation on the superconducting pairing strength. We find that high $T_c$ is to be expected away from perfect nesting. While experimental studies have focused on the intercalation of larger molecules in the spacer layer so far, we argue that no higher $T_c$ can be achieved this way. This work was supported by Deutsche Forschungsgemeinschaft under Grant No. SPP 1458, the National Science Foundation under Grant No. PHY11-25915 and the Department of Energy under Grant No. DE-FG02-05ER46236. \\[4pt] [1] D. Guterding et al., arXiv:1410.7565 [Preview Abstract] |
Wednesday, March 4, 2015 12:03PM - 12:15PM |
M5.00003: Neutron scattering studies on semiconducting Rb$_{0.8}$Fe$_{1.5}$S$_2$ Meng Wang, Patrick Valdivia, Robert Birgeneau, Wei Tian, Songxue Chi, Pengcheng Dai, Edith Bourret-Courchesne We report neutron scattering and transport measurements on semiconducting Rb$_{0.8}$Fe$_{1.5}$S$_2$, a compound isostructural and isoelectronic to the well-studied $A_{0.8}$Fe$_{y}$Se$_2 (A=$ K, Rb, Cs, Tl/K) superconducting systems. Both resistivity and dc susceptibility measurements reveal a magnetic phase transition at $T=275$ K. Neutron diffraction studies show that the 275 K transition originates from a phase with rhombic iron vacancy order which exhibits an in-plane stripe antiferromagnetic ordering below 275 K. Based on the close similarities of the in-plane antiferromagnetic structures, moments sizes, and ordering temperatures in semiconducting Rb$_{0.8}$Fe$_{1.5}$S$_2$ and K$_{0.81}$Fe$_{1.58}$Se$_2$, we argue that the in-plane antiferromagnetic order arises from strong coupling between local moments. The spin waves of the stripe AF order will also be presented. [Preview Abstract] |
Wednesday, March 4, 2015 12:15PM - 12:27PM |
M5.00004: Block magnetic excitations in the orbitally-selective Mott insulator BaFe$_2$Se$_3$ M. Mourigal, Shan Wu, M.B. Stone, J.R. Neilson, J.M. Caron, T.M. McQueen, C.L. Broholm We investigate the spectrum of magnetic excitations in the Fe-based two-leg ladder material BaFe$_2$Se$_3$ by means of broad-band inelastic neutron scattering. BaFe$_2$Se$_3$ garnered recent attention due to its quasi-1D structure and as hosting an exotic block magnetic ground-state where 4 Fe spins co-align to form Fe$_4$ plaquettes. Our neutron results provide a detailed understanding of magnetic excitations originating from the Fe$_4$ block ground-state. Consisting of a 50 meV wide band of quasi-1D acoustic spin-waves and three high-energy modes around 100 meV and 200 meV, the spin fluctuations and the static moment carry a total squared magnetic moment of 16 $\mu_{\rm B}^2$ per Fe, indicative of orbital selectiveness for localized spins. We develop an effective Heisenberg model that accounts for the observed spectrum and provides a set of exchange interactions to understand how exotic magnetism stems from strong lattice, orbital and electronic correlations in iron chalcogenides. [Preview Abstract] |
Wednesday, March 4, 2015 12:27PM - 12:39PM |
M5.00005: Neutron investigation of possible (ferro)magnetic order in the new superconductor Li$_{1-x}$Fe$_x$OD)(Fe$_{1-y}$Se Jeffrey Lynn, Xiuquan Zhou, Christopher K. H. Borg, Efrain Rodriguez The system Li$_{1-x}$Fe$_{x}$)OH(Fe$_{1-y}$Li$_{y}$)Se has recently been reported to become superconducting at T$_{C}\approx 43\ $K and then developing ferromagnetism in terms of a spontaneous vortex lattice below T$_{M}\approx 10\ $K [1]. We have prepared the tetragonal (Li$_{1-x}$Fe$_{x}$ OD)(Fe$_{1-y}$Se) phase by hydrothermal synthesis in D$_{2}$O to reduce the neutron incoherent scattering cross section. We also prepared the samples using isotropically purified $^{7}$LiOD as a starting material to reduce the neutron adsorption cross section. The lattice parameters according to neutron and X-ray diffraction varied with 3.79 \AA\ $<$ a $<$ 3.82 \AA\ and 9.16 \AA\ $<$ c $<$ 9.20 \AA. We have carried out bulk magnetization, high resolution powder diffraction measurements to determine the crystal structure, high intensity diffraction measurements for the magnetic structure, and small angle scattering measurements to elucidate the interaction between the magnetism and superconductivity. The results of the neutron and bulk measurements will be discussed in detail. \\[4pt] [1] U. Pachmayr, et al., arXiv:1411.3304. [Preview Abstract] |
Wednesday, March 4, 2015 12:39PM - 12:51PM |
M5.00006: Multiband transport and nonmetallic low-temperature state of K$_{0.50}$Na$_{0.24}$Fe$_{1.52}$Se$_{2}$ Hyejin Ryu, F. Wolff-Fabris, J.B. Warren, M. Uhlarz, J. Wosnitza, C. Petrovic We report evidence for multiband transport and an insulating low-temperature normal state in superconducting K$_{0.50}$Na$_{0.24}$Fe$_{1.52}$Se$_{2}$ with T$_{\mathrm{c}} \approx $ 20 K. The temperature-dependent upper critical field H$_{\mathrm{c2}}$ is well described by a two-band model. After the superconductivity is suppressed by applying pulsed magnetic field at low temperature, the normal-state resistance is found to increase logarithmically as T $\to $ 0. This is similar as for high-T$_{\mathrm{c}}$ copper oxides and granular superconductors, suggesting that the superconductor-insulator transition is related to intrinsic nanoscale phase separation.\\[4pt] Work at Brookhaven is supported by the U.S. DOE under Contract No. DE-AC02-98CH10886 and in part by the Center for Emergent Superconductivity, an Energy Frontier Research Center funded by the U.S. DOE, Office for Basic Energy Science (C.P.). We acknowledge the support of the HLD at HZDR, member of the European Magnet Field Laboratory (EMFL). C.P. acknowledges support by the Alexander von Humboldt Foundation. [Preview Abstract] |
Wednesday, March 4, 2015 12:51PM - 1:03PM |
M5.00007: First-principles theory of electron-spin fluctuation coupling and superconducting instabilities in iron selenide Johannes Lischner, Timur Bazhirov, Allan H. MacDonald, Marvin L. Cohen, Steven G. Louie We present first-principles calculations of the coupling of quasiparticles to spin fluctuations in iron selenide and discuss which types of superconducting instabilities this coupling gives rise to. We find that strong antiferromagnetic stripe-phase spin fluctuations lead to large coupling constants for superconducting gaps with s$+$/- -symmetry, but these coupling constants are significantly reduced by other spin fluctuations with small wave vectors. An accurate description of this competition and an inclusion of band structure and Stoner parameter renormalization effects lead to a value of the coupling constant for an s$+$/- symmetric gap which can produce a superconducting transition temperature consistent with experimental measurements. [Preview Abstract] |
Wednesday, March 4, 2015 1:03PM - 1:15PM |
M5.00008: ``Forbidden'' phonon in the iron chalcogenide series David M. Fobes, Igor A. Zaliznyak, Zhijun Xu, Genda Gu, John M. Tranquada Recently, we uncovered evidence for the formation of a bond-order wave (BOW) leading to ferro-orbital order at low temperature, acting to stabilize the bicollinear AFM order, in the iron-rich parent compound, Fe$_{1+y}$Te [D. Fobes \textit{et al}., Phys. Rev. Lett. 112, 187202 (2014)]. Investigating the inelastic spectra centered near (100) in Fe$_{1+y}$Te, a signature peak for the BOW formation in the monoclinic phase, we observed an acoustic phonon dispersion in both tetragonal and monoclinic phases. While a structural Bragg peak accompanies the mode in the monoclinic phase, in the tetragonal phase Bragg scattering at this \textbf{Q} is forbidden by symmetry, and we observed no elastic peak. This phonon mode was also observed in superconducting FeTe$_{0.6}$Se$_{0.4}$, where structural and magnetic transitions are suppressed. LDA frozen phonon calculations suggested that this mode could result from a spin imbalance between neighboring Fe atoms, but polarized neutron measurements revealed no additional magnetic scattering. We propose that this ``forbidden'' phonon mode may originate from dynamically broken symmetry, perhaps related to the strong dynamic spin correlations in these materials. [Preview Abstract] |
Wednesday, March 4, 2015 1:15PM - 1:27PM |
M5.00009: Synthesis and superconductivity in spark plasma sintered pristine and graphene-doped FeSe$_{0.5}$Te$_{0.5}$ Pooja Puneet, Ramakrishna Podila, Jian He, Apparao Rao, Austin Howard, Nicholas Cornell, Anvar A. Zakhidov Replace this text with your abstract body. Here, we present a new ball-milling and spark plasma sintering based technique for the facile synthesis FeSe$_{0.5}$Te$_{0.5}$ superconductors (SC) without the need for pre-alloying. This method is advantageous since it is quick and flexible for incorporating other dopants such as graphene for vortex pinning. We observed that FeSe$_{0.5}$Te$_{0.5}$ exhibits a coexistence of ferromagnetic (FM) and SC signature plausibly arising from a FM core-SC shell structure. More importantly, the Hc2 values observed from resistivity data are higher than 7 T indicating that SPS process synthesized FeSe$_{0.5}$Te$_{0.5}$ samples could lead to next{\-}generation superconducting wires and cables. [Preview Abstract] |
Wednesday, March 4, 2015 1:27PM - 1:39PM |
M5.00010: An ab-initio Quantum Monte Carlo analysis of pressure and magnetism in the unconventional superconductor, FeSe Brian Busemeyer, Lucas K. Wagner We report the results of many-body ab-initio fixed-node diffusion Monte Carlo calculations performed on the unconventional superconductor, FeSe. FeSe shows a strong pressure-dependent critical temperature, and because magnetism is generally expected to play an important role in understanding unconventional superconductivity, for pressures ranging from ambient to 11 GPa, we investigate the single and many-body properties of three ordered magnetic configurations, including the two found in similar iron-based superconductors. Our calculations find that at least two magnetic orders (collinear and bicollinear) are nearly degenerate in energy, becoming closer in energy as pressure increases. We also analyze how correlations between the electrons change as a function of pressure, and discuss what this could mean for superconductivity. [Preview Abstract] |
Wednesday, March 4, 2015 1:39PM - 1:51PM |
M5.00011: Recent studies on the isovalent phosphorous-substituted 122-type iron pnictides Yuanyuan Zhao, Anrris Tai, C.S. Ting Recent experiments demonstrate that isovalent doping system gives the similar phase diagram as the heterovalent doped cases: with the P-doping, the magnetic order is suppressed and the superconductivity emerges. With the help of tight-binding model calculation and self-consistent lattice Bogoliubov-de Gennes (BdG) equation calculation, we choose a minimal two-orbital model and obtain the phase diagram of BaFe2(As1-xPx)2 against the P content x, which could be qualitatively comparable with the experimental results. Besides, we will compare these results with the recent experiments shown QCP in this system. [Preview Abstract] |
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