Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session D13: Focus Session: Fe-Based Superconductors-STM, FeSe, Ca10(Pt3As8)(Fe2As2)5 |
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Sponsoring Units: DMP Chair: Kee Hoon Kim, Seoul National University Room: 207 |
Monday, March 3, 2014 2:30PM - 2:42PM |
D13.00001: STM/S study of the interplay between local and average effects of single atomic impurities in Fe(Te,Se) Zhiyang Ye, Jiaxin Yin, Zheng Wu, Jihui Wang, Ang Li, Xuejin Liang, Jian Li, Chin-Sen Ting, Pei-Herng Hor, Hong Ding, Shuheng H Pan In materials with electron correlations, simple defects can affect both the local and the macroscopic properties of the system. We use low temperature scanning tunneling microscopy/spectroscopy (STM/S) to systematically investigate the local and average effects due to single atomic impurities in Fe(Te,Se). We have found that these single atomic impurities cause strong scattering and destroy superconductivity locally. On the other hand, the average quasi particle density of states at Fermi level exhibits a near square-root dependence on impurity concentration, suggesting that the impurity scattering is in the unitary limit. More interestingly, the superconducting gap magnitude is not affected by the impurities, despite a huge reduction of the super-fluid density and the superconducting transition temperature. We have also observed the local and the average doping effects of these single atomic impurities. Based on these observations, we will highlight the interplay between the local and the average impurity scattering effects in this iron-based superconducting system. [Preview Abstract] |
Monday, March 3, 2014 2:42PM - 2:54PM |
D13.00002: STM/S study of the mid-gap impurity bound states in Fe(Te,Se) Jiaxin Yin, Zheng Wu, Jihui Wang, Zhiyang Ye, Jing Gong, Xingyuan Hou, Lei Shan, Ang Li, Xuejin Liang, Xianxin Wu, Jian Li, Chin-Sen Ting, Jiangping Hu, Pei-Herng Hor, Hong Ding, Shuheng H Pan In superconductors, the microscopic phenomenon of quasi-particles scattering off an impurity is sensitive to the symmetry of the order parameter. Therefore, it has been used to probe the pairing symmetry of the superconducting state. We use low temperature scanning tunneling microscopy/spectroscopy (STM/S) to study the scattering effects of single atomic impurities in Fe(Te,Se). The tunneling spectrum on the pristine sample exhibits two fully opened superconducting energy gaps, which are consistent with our photoemission results. On samples with impurities, we have found that an isotropic sharp peak of mid-gap states emerges in the tunneling spectrum at the impurity site within a short length scale of $\sim$ 10 {\AA}. The results of the temperature dependence study indicate that this peak of impurity bound-states is intimately related to the state of superconductivity. We have also studied the response of this impurity induced spectrum feature to a magnetic field (up to 8 Tesla) and to impurity-impurity interactions. We will discuss the implication of our observations in the context of pairing symmetry. [Preview Abstract] |
Monday, March 3, 2014 2:54PM - 3:06PM |
D13.00003: Quasi-particle interference and possible orbital order in FeSe T. Hanaguri, T. Watashige, Y. Kohsaka, K. Iwaya, Y. Fu, S. Kasahara, D. Watanabe, Y. Mizukami, T. Mikami, Y. Kawamoto, S. Kurata, T. Shibauchi, Y. Matsuda, A. B{\"o}hmer, T. Wolf, C. Meingast, H. v. L{\"o}hneysen Spontaneous formation of unidirectional electronic states or nematicity in iron-based superconductors has attracted much attention but spectroscopic understanding of nematicity is still elusive. Using scanning tunneling microscopy/spectroscopy, we have investigated FeSe which becomes superconducting in an orthorhombic phase. Spectroscopic imaging over wide energy range ($\pm$50 meV) revealed clear quasi-particle interference (QPI) patterns. In the Fourier-transformed spectroscopic images, we identified unidirectional electron- and hole-like QPI branches, which disperse in orthogonal directions. We argue this behavior in connection to the orbital ordering in the orthorhombic phase. [Preview Abstract] |
Monday, March 3, 2014 3:06PM - 3:42PM |
D13.00004: Controlled impurity study and observation of a bosonic mode in iron based superconductors by STM measurements: implications for the pairing symmetry and mechanism Invited Speaker: Hai-Hu Wen The pairing mechanism in the iron pnictides remains unresolved yet. The pairing model based on the magnetic origin predicts a sign reversal gap on the electron and hole Fermi pockets, leading to the S$^{\pm }$ pairing, however, a more conventional S$^{++}$ pairing gap was suggested based on the orbital fluctuation mediated pairing. Here we show the clear evidence of the in-gap quasi-particle states induced by the non- or very weak magnetic Cu impurities in Na(Fe$_{0.97-x}$Co$_{0.03}$Cu$_{x})$As by measuring the scanning tunneling spectroscopy, giving strong evidence of the S$^{\pm }$ pairing. Furthermore, we show the presence of the bosonic mode with the energy identical to that of the neutron resonance with a simple linear relation $\Omega $/k$_{B}$T$_{c\, }\approx $ 4.3 in several systems. This mode can also be explained very well as the consequence of the S$^{\pm}$ pairing. These observations strongly suggest that the antiferromagnetic spin fluctuation is the key factor for superconductivity. In collaboration with: Huan Yang, Zhenyu Wang, Delong Fang, Lei Shan, Qiangua Wang, Chenglin Zhang, and Pengcheng Dai, et al. \\[4pt] [1] Zhenyu Wang, et al., Nature Physics \textbf{9}, 42-48(2013). \\[0pt] [2] Lei Shan, et al. Phys. Rev. Lett. \textbf{108}, 227002 (2012). \\[0pt] [3] Huan Yang et al., Nature Communications \textbf{4}, 2947 (2013). [Preview Abstract] |
Monday, March 3, 2014 3:42PM - 3:54PM |
D13.00005: Influence of twin boundaries on superconducting gap nodes in FeSe single crystal studied by STM/STS T. Watashige, T. Hanaguri, Y. Kohsaka, K. Iwaya, Y. Fu, S. Kasahara, D. Watanabe, Y. Mizukami, T. Mikami, Y. Kawamoto, S. Kurata, T. Shibauchi, Y. Matsuda, A.E. B{\"o}hmer, T. Wolf, C. Meingast, H. v. L{\"o}hneysen We performed scanning tunneling microscopy (STM) and spectroscopy (STS) measurements on high-quality FeSe single crystals grown by vapor transport technique [1] to examine the superconducting-gap structure. In MBE-grown FeSe thin films, based on the V-shaped tunneling spectra, nodal superconductivity is suggested [2]. It is interesting to investigate how the nodes are affected by various kinds of defects. We found that twin boundaries bring about drastic effects on the gap nodes. With approaching to the twin boundary, V-shaped spectra gradually change to U-shaped ones. Interestingly, in the area between the twin boundaries separated by about 30 nm, the gap node is completely lifted and there appears a finite gap over $\pm$0.4 meV. This unusual twin-boundary effect will give us a hint to elucidate the superconducting-gap structure.\\[4pt] [1] A. E. B{\"o}hmer {\it et al.,} Phys. Rev. B {\bf 87,} 180505(R) (2013).\\[0pt] [2] C. -L. Song {\it et al.,} Science {\bf 332,} 1410 (2011). [Preview Abstract] |
Monday, March 3, 2014 3:54PM - 4:06PM |
D13.00006: Imaging the Electron-Boson Coupling in Superconducting FeSe Can-Li Song, Yi-Lin Wang, Ye-Ping Jiang, Zhi Li, Lili Wang, Ke He, Xi Chen, Jennifer E. Hoffman, Xu-Cun Ma, Qi-Kun Xue Scanning tunneling spectroscopy has been used to reveal signatures of a bosonic mode in the local quasiparticle density of states of superconducting FeSe films. The mode appears below $T_c$ as a `dip-hump' feature at energy $\Omega\sim 4.7 k_B T_c$ beyond the superconducting gap $\Delta$. Spectra on strained regions of the FeSe films reveal simultaneous decreases in $\Delta$ and $\Omega$. This contrasts with all previous reports on other high-$T_c$ superconductors, where $\Delta$ locally anti-correlates with $\Omega$. A local strong coupling model is found to reconcile the discrepancy well, and to provide a unified picture of the electron-boson coupling in unconventional superconductors. [Preview Abstract] |
Monday, March 3, 2014 4:06PM - 4:18PM |
D13.00007: STM/STS study of superconducting properties in Ca$_{10}$(Pt$_{4}$As$_{8}$)(Fe$_{2}$As$_{2}$)$_{5}$ Jisun Kim, Hyoungdo Nam, Guorong Li, Amar Karki, Chih-Kang Shih, Jiandi Zhang, Rongying Jin, E.W. Plummer Newly discovered iron-based superconductor, Ca$_{10}$(Pt$_{4}$As$_{8}$)(Fe$_{2}$As$_{2}$)$_{5}$ (T$_{c}$ = 34 K) is studied using scanning tunneling microscopy/spectroscopy (STM/S). Given the symmetry of the crystal structure, several surface terminations are expected with roughly same probability: 1) Ca or partial Ca layer on top Fe$_{2}$As$_{2}$; 2) Ca or partial Ca layer on top Pt$_{4}$As$_{8}$ layer; 3) A Fe$_{2}$As$_{2}$ layer, and; 4) A Pt$_{4}$As$_{8}$ layer. Surprisingly, Fe$_{2}$As$_{2}$ related layers (1 \& 3) are rarely observed (less than 1\%). Instead, we observe Pt$_{4}$As$_{8}$ layers separated by unit-cell-high ($\sim$ 1 nm) steps accompanied with Ca or partial Ca layer on top Pt$_{4}$As$_{8}$ layer (1 - 2 {\AA} step height). Scanning tunneling spectroscopy reveals different spectra for each surface, with superconducting coherence peaks seen only on Ca layers. We argue that intermediary layers are proximity-coupled to superconducting Fe$_{2}$As$_{2}$ layers. The results from Ca$_{10}$(Pt$_{4}$As$_{8}$)(Fe$_{2}$As$_{2}$)$_{5}$ are discussed with the properties observed in other iron-based superconductors. [Preview Abstract] |
Monday, March 3, 2014 4:18PM - 4:30PM |
D13.00008: Two band superconductivity in optimal doped Ca$_{10}$(Pt$_{3}$As$_{8})$(Fe$_{2}$As$_{2})_{5}$ superconductors revealed by anisotropic H$_{c2}$ measurement up to 65T Ni Ni, Eundeok Mun, Shan Jiang, Vivien Zapf, Robert J. Cava, Eric D. Bauer, Filip Ronning Anisotropic upper critical field, H$_{c2}$(T), has been determined using radio-frequency contactless penetration depth method in pulsed magnetic fields up to 65T for optimal Pt doped and La-doped Ca$_{10}$(Pt$_{3}$As$_{8})$(Fe$_{2}$As$_{2})_{5}$ single crystals with magnetic field applied parallel and perpendicular to \textit{ab} planes. For both compounds, with decreasing temperature, H$_{c2}^{//ab}$(T) shows a downward curvature while H$_{c2}^{\bot ab}$(T) shows an upward curvature, indicating their multiband superconductivity nature. [Preview Abstract] |
Monday, March 3, 2014 4:30PM - 4:42PM |
D13.00009: Field-induced nematic-like magnetic transition in an iron pnictide superconductor, Ca$_{10}$(Pt$_{3}$As$_{8}$)((Fe$_{1-x}$Pt$_{x}$)$_{2}$As$_{2}$)$_{5}$ Matthew Watson, Amalia Coldea, Sam Blake, Alix McCollam, David Vignolles, Loic Drigo, Igor Mazin, Daniel Guterding, Harald Jeschke, Roser Valenti, Ni Ni, Robert Cava We report a high magnetic field study up to 55~T of the nearly optimally doped iron-pnictide superconductor Ca$_{10}$(Pt$_{3}$As$_{8}$)((Fe$_{1-x}$Pt$_{x}$)$_{2}$As$_{2}$)$_{5}$ with a $T_{c} \approx$ 10 K using magnetic torque, tunnel diode oscillator technique and transport measurements. We determine the superconducting phase diagram, revealing an anisotropy of the irreversibility field up to a factor of 10 near $T_{c}$ and signatures of multiband superconductivity. Unexpectedly, we find a spin-flop like anomaly in magnetic torque at 22~T, when the magnetic field is applied perpendicular to the {\it ab} planes, which becomes significantly more pronounced as the temperature is lowered 0.33~K. As our superconducting sample lies well outside the antiferromagnetic region of the phase diagram, the observed field-induced transition in torque indicates a spin-flop transition \textit{not of long-range ordered moments}, but of nematic antiferromagnetic fluctuations. This work was supported by the EPSRC (EP/I004475/1) and EuroMagNET II. [Preview Abstract] |
Monday, March 3, 2014 4:42PM - 4:54PM |
D13.00010: Lattice Distortion and Magnetic Order in Ca$_{10}$(Fe$_{2}$As$_{2}$)$_{5}$(Pt$_{3}$As$_{8}$) Aashish Sapkota, Gregory S. Tucker, Mehmet Ramazanoglu, Wei Tian, Ni Ni, Alan I. Goldman, Robert J. McQueeney, Andreas Kreyssig The Ca$_{\mathrm{10}}$(Fe$_{\mathrm{2}}$As$_{\mathrm{2}})_{\mathrm{5}}$(Pt$_{\mathrm{3}}$As$_{\mathrm{8}})$ compound is a member of the Fe-based high-temperature superconductor family. Recent work showed that this compound has a complex chemical structure with a skutterudite-type Pt-As intermediary layer sandwiched between tetrahedral Fe-As layers. We have studied the structural and magnetic properties of Ca$_{\mathrm{10}}$(Fe$_{\mathrm{2}}$As$_{\mathrm{2}})_{\mathrm{5}}$(Pt$_{\mathrm{3}}$As$_{\mathrm{8}})$ by single-crystal x-ray and neutron diffraction. Similar to other compounds in the iron arsenide family, we observed a lattice distortion from tetragonal to orthorhombic symmetry below approximately 110 K and stripe-like antiferromagnetic order below approximately 100 K. Both phase transitions are 2$^{\mathrm{nd}}$ order in nature. The magnetic order and lattice distortion in the iron arsenides appear to be robust against deviations from simple structure motifs. [Preview Abstract] |
Monday, March 3, 2014 4:54PM - 5:06PM |
D13.00011: Intrinsic Josephson Junctions in the iron-based multi-band superconductor (V$_2$Sr$_4$O$_6$)Fe$_2$As$_2$ Philip Moll, Xiyu Zhu, Peng Cheng, Hai-Hu Wen, Batlogg Bertram We have observed clear experimental evidence for intrinsic Josephson junction (iJJ) behavior in the iron-based superconductor (V$_2$Sr$_4$O$_6$)Fe$_2$As$_2$ (T$_c \approx 20K$). The iJJs are identified by periodic oscillations of the flux flow voltage for out-of-plane (c-axis) currents upon increasing a well aligned in-plane magnetic field. Their periodicity is well explained by commensurability effects between the Josephson vortex lattice and the crystal structure, which is a hallmark signature of Josephson vortices confined into iJJ stacks. Essential for reliable c-axis transport measurements on the available microcrystals are Focused Ion Beam microstructuring and contacting techniques. The insulating temperature behavior of $\rho_c$ indicates S-I-S type junctions. This finding adds (V$_2$Sr$_4$O$_6$)Fe$_2$As$_2$ as the first iron-based, multi-band superconductor to the copper-based iJJ materials of interest for Josephson junction applications, and in particular novel devices based on multi-band Josephson coupling may be realized. [Preview Abstract] |
Monday, March 3, 2014 5:06PM - 5:18PM |
D13.00012: Magnetic and Electronic Phase Diagram of Sr$_{2}$VFeAsO$_{\mathrm{3-d}}$ Yujiro Tojo, Taizo Shibuya, Tetsuro Nakamura, Koichiro Shoji, Masanori Matoba, Shintaro Yasui, Mitsuru Itoh, Shinji Kitao, Makoto Seto, Yoichi Kamihara Ae$_{2}$MFePnO$_{3}$ (so-called 21113 systems) with perovskite-type layers such as Ae$_{2}$MO$_{3}$, where Ae denotes alkaline earth metals, M does Sc, Ti, Cr, V and other transition metal atoms and Pn does As and P shows superconductivity at T \textless 46 K. Sr$_{2}$VFeAsO$_{\mathrm{3-d}}$ is a representative compound in 21113 systems. [Zhu et al, Phys. Rev. B (2009); Ogino et al, Supercond. Sci. Technol. (2009); Ogino et al, Supercond. Sci. Technol. (2009)] Although the oxygen deficiency (d) as a function of $T_{\mathrm{c}}$ is still controversial in Sr$_{2}$VFeAsO$_{\mathrm{3-d}}$, many samples have been reported as superconductors with $T_{\mathrm{c}} =$ 24-37 K. In this study, a polycrystalline Sr$_{2}$VFeAsO$_{\mathrm{3-d}}$ (d $=$ $\sim$ 0.1- 0.6) were prepared by a solid state reaction using an alumina tube in a sealed silica tube. DC electrical resistivity was measured by a four-probe technique. Magnetization measurements were performed on a superconducting quantum interference device. $^{57}$Fe Mossbauer spectra were obtained using conventional equipment. The electronic and magnetic phase diagram of Sr$_{2}$VFeAsO$_{\mathrm{3-d}}$ is elucidated. [Preview Abstract] |
Monday, March 3, 2014 5:18PM - 5:30PM |
D13.00013: Weak coupling BCS-like superconductivity in the pnictide oxide Ba$_{1-x}$Na$_{x}$Ti$_{2}$Sb$_{2}$O B. Lorenz, M. Gooch, P. Doan, Z. Tang, A.M. Guloy, C.W. Chu We report the results of low-temperature heat capacity measurements of the pnictide oxide superconductor BaTi$_{2}$Sb$_{2}$O and the optimally Na-doped compound Na$_{0.15}$Ba$_{0.85}$Ti$_{2}$Sb$_{2}$O. Temperature- and field-dependent heat capacity data are well described by a single-gap BCS theory. The estimated values for the normal-state Sommerfeld constant, the heat capacity jump at $T_{c}$, and the electron-phonon coupling constant are in favor of a conventional weak coupling superconductivity, possibly mediated by electron-phonon interaction. The results are discussed with regard to and compared with recent first-principles calculations. [Preview Abstract] |
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