Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session V41: Fe-based Superconductivity. SpectroscopiesFocus
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Sponsoring Units: DMP DCMP Chair: Makariy Tanatar, Ames Laboratory Room: 388 |
Thursday, March 16, 2017 2:30PM - 3:06PM |
V41.00001: Superconducting energy gap structure in KFe$_2$As$_2$ and BaFe$_2$(As$_{1-x}$P$_x$)$_2$ Invited Speaker: Antony Carrington The $k$-space structure of the superconducting energy gap $\Delta$ in the iron-based materials is perhaps our best handle on the structure of the pairing interacting and hence the mechanism of superconductivity. Previous measurements have indicated that, unlike the cuprates, there is considerable variety in this structure between the different materials. In some $\Delta$ is almost isotropic whereas in other is it highly anisotropic and in a few case maybe nodal. However, there is still considerable controversy with different measurement techniques indicating different structures. Here I will discuss our measurement of magnetic penetration depth $\lambda$ and specific heat $C$ of the putative nodal materials KFe$_2$As$_2$ and BaFe$_2$(As$_{1-x}$P$_x$)$_2$. Measurements of the field dependence of $\lambda$ are shown to be very useful for distinguishing the different pairing states, and we conclude that the best samples of KFe$_2$As$_2$ have a small but finite residual gap which may be impurity induced. For BaFe$_2$(As$_{1-x}$P$_x$)$_2$ we discuss some unusual features which appear when this material is tuned close the quantum critical point. I will discuss the significance of these for other superconductors close to a quantum critical point. [Preview Abstract] |
Thursday, March 16, 2017 3:06PM - 3:18PM |
V41.00002: Localization via exchange splitting in NaFe$_{1-x}$Cu$_x$As Aliaksei Charnukha, Zhiping Yin, Yu Song, Chongde Cao, Pengcheng Dai, Dimitri Basov Iron-based high-temperature superconductors have emerged as a distinct material family believed to bridge the wide gap in understanding between conventional low-temperature and unconventional high-temperature copper-based superconductors. And yet, compounds that bear close resemblance to strongly correlated superconducting cuprates have been hard to come by. Recently, copper substitution in a quintessential iron pnictide, NaFeAs, has been demonstrated to result in a semiconducting transport behavior, suggesting the possibility of a strongly correlated Mott insulating electronic state. Here we use optical spectroscopy and dynamical mean-field theory to demonstrate explicitly that the excitation spectrum of NaFe0.5Cu0.5As possesses a sizable gap below the Neel temperature and remains unchanged up to room temperature due to the persistence of short-range antiferromagnetic correlations. We show that all of the observed experimental properties can be explained remarkably well as a result of exchange splitting in the predominantly Fe-$d$--derived electronic band structure induced by local antiferromagnetic order. On-site repulsion, on the contrary, is insufficient to drive localization. Our results paint a fuller picture of the intermediate character of correlations in iron-pnictides. [Preview Abstract] |
Thursday, March 16, 2017 3:18PM - 3:30PM |
V41.00003: Doping dependence of competing pairing channels in ${\rm Ba_{1-x}K_xFe_2As_2}$ Thomas Boehm, Florian Kretzschmar, Andreas Baum, Michael Rehm, Daniel Jost, Ramez Hosseinian Ahangharnejhad, Ronny Thomale, Christian Platt, Thomas Maier, Werner Hanke, Brian Moritz, Thomas Devereaux, Douglas Scalapino, Saurabh Maiti, Peter Hirschfeld, Peter Adelmann, Thomas Wolf, Hai-Hu Wen, Rudi Hackl We study the doping dependence of competing pairing channels in the superconducting state of ${\rm Ba_{1-x}K_xFe_2As_2}$ for $0.22\le x \le 0.70$. Around optimal doping symmetry-resolved Raman spectra reveal the existence of two collective exciton-like Bardasis-Schrieffer modes in $B_{1g}$-symmetry below the gap edge. These modes correspond to two sub-leading pairing interactions of $d_{x^2-y^2}$ symmetry, which compete with the dominant $s$-wave interaction. The experimental doping dependence of the corresponding pairing strengths is compared with a functional renormalization group study and a spin-fluctuation based approach via the random-phase approximation. Both techniques yield good agreement with each other and the experiment. This provides evidence for repulsive spin-fluctuations to dominate the present system, not only generating the $s\pm$-wave ground state but also the BS modes. [Preview Abstract] |
Thursday, March 16, 2017 3:30PM - 3:42PM |
V41.00004: Multiple Bardasis-Schrieffer modes and role of vertex corrections in Raman Spectroscopy Saurabh Maiti, Thomas Maier, Thomas Boehm, Rudi Hackl, Peter Hirschfeld We formulate a multiband theory of Raman Response by accounting for vertex corrections and using the same microscopic interactions that lead to pairing. Doing so, in the B$_{1g}$ channel, we find that each attractive B$_{1g}$ solution within a microscopic pairing theory gives a Bardasis-Schrieffer(BS) mode: thus there can be multiple BS modes. We argue that the entire Raman spectrum is actually dominated by contributions from vertex corrections. We also argue that the same holds in the A$_{1g}$ channel and that the previously used screening diagrams are not relevant for Raman spectroscopy. We discuss the results in the context of recent measurements on heavily hole doped Ba$_{1-x}$K$_x$Fe$_2$As$_2$, where the theory makes it possible to deduce more information about the pairing interaction than previously anticipated. [Preview Abstract] |
Thursday, March 16, 2017 3:42PM - 3:54PM |
V41.00005: Pseudogaplike feature of slightly underdoped (BaK)Fe$_{2}$As$_{2}$ studied by ultrafast spectroscopy Kung-Hsuan Lin, Kuan-Jen Wang, Chung-Chieh Chang, Yu-Chieh Wen, Bing Lv, Paul Ching-Wu Chu, Maw-Kuen Wu We have utilized ultrafast optical spectroscopy to study carrier dynamics in slightly underdoped (BaK)Fe$_{2}$As$_{2}$ crystals without magnetic transition. The photoelastic signals due to coherent acoustic phonons have been quantitatively investigated. According to our temperature-dependent results, we found that the relaxation component of superconducting quasiparticles persisted from the superconducting state up to at least 70 K in the normal state. Our findings suggest that the pseudogaplike feature in the normal state is possibly the precursor of superconductivity. We also highlight that the pseudogap feature of K-doped BaFe$_{2}$As$_{2}$ is different from that of other iron-based superconductors, including Co-doped or P-doped BaFe$_{2}$As$_{2}$. [Preview Abstract] |
Thursday, March 16, 2017 3:54PM - 4:06PM |
V41.00006: Local magnetic moment in hydrogen-doped 1111-type iron pnictides probed by Electron Energy Loss Spectroscopy Takashi Sasaki, Soshi Iimura, Masato Sasase, Hideo Hosono Since superconductivity in Fe pnictides emerges adjacent to the antiferromagnetic phase, the magnetic properties have been intensively investigated by neutron scattering, NMR and muSR. Since their magnetic moments are strongly screened by itinerant electrons [1], it is needed to be probed in much faster process such as electron dynamics, for measuring the “bare” moment. Recently, it was proved that Electron Energy Loss Spectroscopy (EELS) is able to capture the bare moment in Ba(Fe$_{\mathrm{1-}}_{x}$Co$_{x})_{\mathrm{2}}$As$_{\mathrm{2}}$ [2]. In this study, we performed the EELS measurement on LaFeAsO$_{\mathrm{1-}}_{x}$H$_{x}$ with a double dome structure, and found that the Neel temperature and the critical temperature ($T_{\mathrm{N}}$, $T_{\mathrm{c}})$, especially in high-doped region, are related to the bare magnetic moment. In this talk, we also present the results in SmFeAsO$_{\mathrm{1-}}_{x}$H$_{x}$ and LaFePO$_{\mathrm{1-}}_{x}$H$_{x}$, and discuss the effects of the electron-doping and the local structure around Fe on the moment in the hydrogen-doped 1111-type iron pnictides [1] P. Hansmann et al., PRL 104, 197002 (2010). [2] C. Cantoni et al., Adv. Mater. 26, 6193 (2014). [Preview Abstract] |
Thursday, March 16, 2017 4:06PM - 4:18PM |
V41.00007: Tuning across the BCS-BEC crossover in superconducting $\mbox{Fe}_{1+y}\mbox{Se}_{x}\mbox{Te}_{1-x}$: An angle-resolved photoemission study Shahar Rinott, Amit Ribak, Khanan Chashka, Mohit Randeria, Amit Kanigel The crossover from Bardeen-Cooper-Schrieffer (BCS) superconductivity to Bose-Einstein condensation (BEC) was never realized in quantum materials. It is difficult to realize because, unlike in ultra cold atoms, one cannot tune the pairing interaction. We realize the BCS-BEC crossover in a nearly compensated semimetal $\mbox{Fe}_{1+y}\mbox{Se}_{x}\mbox{Te}_{1-x}$ by tuning the Fermi energy $\epsilon_{F}$ via chemical doping, which permits us to systematically change $\Delta/\epsilon_{F}$ from $0.16$ to $0.50$, where $\Delta$ is the superconducting (SC) gap. We use angle-resolved photoemission spectroscopy to measure the Fermi energy, the SC gap and characteristic changes in the SC state electronic dispersion as the system evolves from a BCS to a BEC regime. Our results raise important questions about the crossover in multi-band superconductors which go beyond those addressed in the context of cold atoms. [Preview Abstract] |
Thursday, March 16, 2017 4:18PM - 4:30PM |
V41.00008: Angle Resolved Photoemission Spectroscopy Study of CaKFe$_4$As$_4$ Superconductor Daixiang Mou, Tai Kong, William Meier, Felix Lochner, Lin-Lin Wang, Qisheng Lin, Yun Wu, Sergey Bud'ko, Ilya Eremin, David Johnson, Paul Canfield, Adam Kaminski CaKFe$_4$As$_4$ is a new superconductor with high Tc of 35K at stoichiometric composition. This presents unique opportunity to study properties of iron arsenic superconductors in absence of disorder caused by doping or substitution. We use high resolution angle resolved photoemission spectroscopy and density functional theory to study the electronic properties of this new material. We find that the Fermi surface consists of three hole pockets at $\Gamma$ and two electron pockets at the M point. The values of the superconducting gap are nearly isotropic, but significantly different for each of the FS sheets. The overall momentum dependence of the gap magnitudes displays a deviation from the simple $cos(k_x)cos(k_y)$ functional form. Instead, the maximum value of the gap is observed for FS sheets that are closest to the ideal nesting condition. Our results provide strong support for the multiband character of superconductivity in CaKFe$_4$As$_4$, in which Cooper pairing forms on the electron and the hole bands interacting via dominant interband repulsive interaction, enhanced by FS nesting. [Preview Abstract] |
Thursday, March 16, 2017 4:30PM - 4:42PM |
V41.00009: Electronic Interactions in Iron-Based Superconductors Daniil Evtushinsky Iron superconductors, offering the possibility to study different and yet related materials, provided an important insight into the problem of unconventional superconductivity---in particular it is becoming clear that the strong electron interaction with other electrons and with the magnetic excitation spectrum is the feature that unifies all relevant compounds from an empirical point of view. At the same time even isostructural materials, exhibiting low levels of the electronic interactions, never show high critical temperatures. I will review the electronic self energy, determined for the entire $3d$ band from the angle-resolved photoemission spectroscopy (ARPES) measurements of iron pnictides and chalcogenides of 11, 111, 122, and 1111 families in the bulk and thin film form, and will compare it to the situation in the compounds with iron substituted by other $d$ metals. Matching ARPES data with the results of calculations for correlated systems shows detailed agreement as for the large-scale structure of the spectral function, implying that major deviations from the single-electron model are to be attributed to the electronic correlations of moderate strength. The low-energy spectral anomalies correspond well to the shape of the spin-fluctuation spectrum. [Preview Abstract] |
Thursday, March 16, 2017 4:42PM - 4:54PM |
V41.00010: The magnetic and nematic phase diagram of Ba$_{\mathrm{1-x}}$Sr$_{\mathrm{x}}$Fe$_{\mathrm{2-y}}$Ni$_{\mathrm{y}}$As$_{\mathrm{2}}$ Dongliang Gong, Shiliang Li The correlation between magnetic and nematic orders has been widely studied in iron-based superconductors. The magnetic and nematic phase transitions may be both first order as in SrFe$_{\mathrm{2}}$As$_{\mathrm{2}}$, or both second order as in BaFe$_{\mathrm{2-x}}$Ni$_{\mathrm{x}}$As$_{\mathrm{2}}$. Within spin-nematic scenario, it is possible for a system to establish the nematic phase as second order while keeping the magnetic transition first-ordered. Experimentally, it is rather hard to distinguish a second-order transition from a weakly first-order transition. Here we have systematically studied the nematic susceptibility and magnetic susceptibility in the iron-based superconductor Ba$_{\mathrm{1-x}}$Sr$_{\mathrm{x}}$Fe$_{\mathrm{2-y}}$Ni$_{\mathrm{y}}$As$_{\mathrm{2}}$ by elastoresistivity and magnetic susceptibility measurements, respectively. The evolutions of the nematic and magnetic transitions from first order to second order can be continuously tuned by the substitution of Sr by Ba. Our results give a phase diagram that is consistent with the spin-nematic theory. [Preview Abstract] |
Thursday, March 16, 2017 4:54PM - 5:06PM |
V41.00011: ARPES observation of non-Fermi liquid behavior in hole-doped LiFe$_{1-x}$V$_x$As and electron-doped LiFe$_{1-x}$Co$_x$As superconductors Pierre Richard, L. Y. Xing, X. Shi, X. C. Wang, Q. Q. Liu, B. Q. Lv, J.-Z. Ma, B. B. Fu, L.-Y. Kong, T. Qian, H. Ding, C. Q. Jin, H. Miao, T. K. Kim, M. Hoesch As with other Fe-based superconductors, the Fermi surface of LiFeAs is composed of multiple hole and electron Fermi surface pockets mainly derived from $3d_{xy}$, $3d_{yz}$ and $3d_{xz}$ orbitals. With its direct momentum resolution, ARPES is a powerful technique able to track the evolution of the Fermi surface upon doing. Here we reveal a non-Fermi liquid behavior in both Co-doped (electron) and V-doped (hole) LiFeAs characterized by a sub-square temperature dependence of the electrical resistivity. We show that for both types of carrier doping, the less Fermi liquid like behavior coincides with good Fermi nesting conditions between a hole and an electron Fermi surface pockets with different orbital characters that are separated by the ``antiferromagnetic" wave vector $\Gamma$-M. Our results suggest that the non-Fermi liquid behavior is driven by low-energy inter-orbital antiferromagnetic fluctuations, but that this observation is not correlated with superconductivity. [Preview Abstract] |
Thursday, March 16, 2017 5:06PM - 5:18PM |
V41.00012: ARPES Studies of Monolayer FeSe on MBE Controlled Titanate Heterostructures Slavko Rebec, Tao Jia, Hafiz Sohail, Robert Moore, Zhi-Xun Shen For 2D films, interface interactions can play a critical role in determining the prevailing physics of the system. In the case of 1ML FeSe on SrTiO3, interfacial electron-phonon (e-ph) coupling greatly increases its superconducting Tc over its bulk counterpart. Here we study this coupling through MBE control of the titanate heterostructures, with the hope of generalizing this enhancement to other systems. Here we present recent in-situ angle-resolved photo emission studies of the interfacial e-ph coupling on MBE grown ML FeSe samples. [Preview Abstract] |
Thursday, March 16, 2017 5:18PM - 5:30PM |
V41.00013: Infrared probe of the superconducting gap across the phase diagram of Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$ R.P.S.M. Lobo We measured the optical conductivity of superconducting single crystals of Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$ with $x$ ranging from 0.40 (optimal doping, $T_c = 39$~K) down to 0.20 (underdoped, $T_c = 16$~K) where a magnetic order coexists with superconductivity. In the superconducting state, the formation of the superconducting condensate leads to a low-frequency suppression of the optical conductivity spectral weight. In the coexistence region, the superfluid density is significantly suppressed, and the weight of unpaired carriers rapidly increases. Whereas the gap is isotropic near the optimally doped region, a significant residual conductivity appears in the underdoped regime. We attribute these results to anisotropic changes in the superconducting gap due to the strong interplay between magnetism and superconductivity in underdoped Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$. [Preview Abstract] |
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