Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session T13: Focus Session: Fe-Based Superconductors-STEM,11's,Various |
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Sponsoring Units: DMP Chair: Amlan Biswas, University of Florida Room: 207 |
Thursday, March 6, 2014 11:15AM - 11:51AM |
T13.00001: Nanoscale probe of magnetism, orbital occupation, and structural distortions in iron-based superconductors Invited Speaker: Claudia Cantoni Local probes of atomic and electronic structures with sub-nanometer spatial resolution can provide additional insights into the physics of iron-based superconductors (FBS) by resolving the influence of inhomogeneities that are typically averaged over by bulk-sensitive techniques. Here we apply aberration-corrected scanning transmission electron microscopy coupled with electron energy loss spectroscopy to a wide class of iron-based superconductors and parent compounds to decipher the interplay between crystal distortions, local magnetic moment, orbital occupancy, and charge doping in these complex materials. In addition to revealing universal trends for hole concentration and local magnetic moment across families of FBS, we directly observe the effects of magneto-elastic coupling in 122 arsenides at room temperature, well above the structural and antiferromagnetic transition. The presence of atomic displacements indicates that the C$_{4}$ tetragonal symmetry is already broken at room temperature in unstrained crystals, lowering the symmetry to orthorhombic (\textit{I2mm}), and that all of the crystals are twinned with domains the size of a few nanometers. By tracking these local atomic displacements as a function of doping level $x$, in Ba(Fe$_{\mathrm{1-x}}$Co$_{x})_{2}$As$_{2}$, we find that the domain size correlates with the magnitude of the dynamic Fe moment, and both are enhanced near optimal doping where the ordered moment is suppressed. The non-monotonic behavior of the local Fe magnetic moment is linked to the strong coupling between lattice, spin, and orbital degrees of freedom. [Preview Abstract] |
Thursday, March 6, 2014 11:51AM - 12:03PM |
T13.00002: Bulk Superconductivity in Fe(Te,Se) Single Crystals Induced by Post Annealing Tsuyoshi Tamegai, Yue Sun, Yuji Tsuchiya, Tatsuhiro Yamada, Toshihiro Taen, Sunseng Pyon, Zhixiang Shi Fe(Te,Se) has the simplest structure among all iron-based superconductors. However, as-grown crystals of Fe(Te,Se) do not show superconductivity, and post treatment is necessary to induce superconductivity. We found that the annealing in controlled O$_{\mathrm{2}}$ atmosphere or other chalcogen (Te, Se, S) atmosphere at relatively low temperatures is very effective to induce superconductivity. During the annealing process, some iron oxides or iron chalcogenides are formed on the surface of the crystal, that effectively extract excess iron from the crystal. Physical properties such as resistivity, Hall coefficient, magnetization, specific heat, and the critical current density are measured before and after the post annealing to discussed the intrinsic properties of Fe(Te,Se) superconductors. [Preview Abstract] |
Thursday, March 6, 2014 12:03PM - 12:15PM |
T13.00003: Cu substition of FeTe Kaya Kobayashi, Shinya Kawamoto, Kazumasa Horigane, Jun Akimitsu One of the typical Fe-based superconductors, Fe(Te,Se) has the simplest structure. One end of the material, FeTe does not show superconducting transition but the structural transition around 60 K which is a sharp contrast to another end, FeSe. The structural transition is suppressed when Se is substituted for Te for certain amount, accompanied by superconductivity. We have synthesized various ratio of Cu-substituted FeTe to suppress the structural transition. The suppression and the superconductivity are independent against our expectation and new intermediate magnetic transitions were observed. [Preview Abstract] |
Thursday, March 6, 2014 12:15PM - 12:27PM |
T13.00004: ABSTRACT WITHDRAWN |
Thursday, March 6, 2014 12:27PM - 12:39PM |
T13.00005: Thickness dependence of superconductivity in FeSe$_{0.5}$Te$_{0.5}$ nanodevices Chunlei Yue, Jin Hu, Xue Liu, Zhiqiang Mao, Jiang Wei We investigated the thickness dependence of superconductivity on thin film single-crystal FeSe$_{0.5}$Te$_{0.5}$ nanodevices. We designed two independent approaches of exfoliation and ion milling to reduce the crystal thickness. On both methods, we discovered that once the thickness of crystal is reduced below 20nm, the superconductivity disappears. When the thickness is approaching to the critical thickness of 20nm, the normal state becomes more insulating, and transition temperature (14K) shifts toward lower temperature. In addition, ion milling method reveals that there is always about 6nm of non- stoichiometric FeSe$_x$Te$_{1-x}$ developed on the surface of FeSe$_{0.5}$Te$_{0.5}$ single crystal in ambient environment. [Preview Abstract] |
Thursday, March 6, 2014 12:39PM - 12:51PM |
T13.00006: ``Proximity fingerprint'' as a measure of order parameter symmetry with planar tunnel junctions grown on Fe chalcogenides Han Zhao, Can Zhang, Juan Atkinson, Hamood Arham, Wan Kyu Park, Genda Gu, Dale Van Harlingen, James Eckstein, Laura Greene A fundamental question regarding the iron-based superconductors is: what is their order parameter symmetry? To answer this question, we are performing the experiment proposed by Koshelev and Stanev [1] called the ``proximity fingerprint.'' In this experiment, a thin film of a conventional s-wave superconductor is directly deposited onto the surface of an iron-based superconductor. Planar tunneling spectra on this layered structure are then analyzed to differentiate between the s$++$ and s$+$- symmetries. Our experiments have been performed using both Nb and Al as the s-wave proximity layer on MBE grown Fe chalcogenide thin films, and Nb on bulk single crystals. Preliminary tunneling spectra will be presented. This work is supported by the Center for Emergent Superconductivity, an Energy Frontier Research Center funded by the US DOE, Office of Science, Award No. DE-AC0298CH1088.\\[4pt] [1]. A. E. Koshelev and V. Stanev, EPL 96, 27014 (2011). [Preview Abstract] |
Thursday, March 6, 2014 12:51PM - 1:03PM |
T13.00007: Pressure dependent resistivity and magnetic measurements on superconducting KFe$_2$As$_2$ Udhara Kaluarachchi, Valentin Taufour, Makariy A. Tanatar, Stella K. Kim, Yong Liu, Thomas A. Lograsso, Sergey L. Bud'ko, Paul C. Canfield, Neda Foroozani, Jinhyuk Lim, James S. Schilling Ba$_{1-x}$K$_x$Fe$_2$As$_2$ shows superconductivity at $T_c\approx 38$~K at the optimal doping ($x\approx 0.4$). However, superconductivity is still observed up to the extreme hole doping ($x=1$) in KFe$_2$As$_2$ with a reduced $T_c\approx 3.4$~K. At this extreme limit, there is no observed electron pocket in this compound. The superconducting state is believed to be of a different symmetry than in the other 122 iron based superconductors. By means of resistivity, magnetization and AC susceptibility under pressure, we investigate the properties of this material. The pressure dependence of $T_c$ has a change of slope around $2$~GPa possibly consistent with a transition to a superconducting state of a different symmetry [F. F. Tafti, et al., Nature Physics 9, 349 (2013)]. We will compare measurements performed in different pressure media and discuss the evolution of the electronic correlations with applied pressure. Work at Ames Laboratory supported by AFOSR-MURI grant FA9550-09-1-0603 and by US DOE under the Contract No. DE-AC02-07CH11358. Work at Washington University supported by NSF Grant No. DMR-1104742 and by the Carnegie/DOE Alliance Center through NNSA/DOE Grant No. DE-FC52-08NA28554. [Preview Abstract] |
Thursday, March 6, 2014 1:03PM - 1:15PM |
T13.00008: Neutron scattering and gap structure in KFe$_2$Se$_2$ Thomas Maier, Andreas Kreisel, Yan Wang, Peter Hirschfeld, Douglas Scalapino The structure of the superconducting gap in the alkali metal iron selenide KFe$_2$Se$_2$ remains controversial. Due to the absence of Fermi surface hole-pockets, the usual sign-changing $s^\pm$ state is unlikely and node-less $d$-wave as well as bonding-anti-bonding $s$-wave gap structures have been suggested. Here we use an RPA BCS approximation for a realistic 3D 10-orbital tight-binding model to calculate the neutron scattering response for different gap structures. We show that both $d$-wave and $s$-wave states are consistent with a neutron resonance in the superconducting state, and discuss possible ways to distinguish between the different gap structures. [Preview Abstract] |
Thursday, March 6, 2014 1:15PM - 1:27PM |
T13.00009: Infrared Hall effect measurements in iron pnictide superconductors Alok Mukherjee, M. Murat Arik, Hui Xing, Payam Taheri, Hao Zeng, John Cerne, Hikaru Sato, Hidenori Hiramatsu, Hideo Hosono Recent longitudinal conductivity $\sigma_{xx}(\omega )_{\thinspace }$measurements on Ba122 superconductors have found many rich features including infrared pseudogap phase, related to spin density waves. In addition, iron superconductors exhibit unusual DC Hall conductivity. We expand the range of study by measuring the frequency-dependent Hall conductivity $\sigma_{xy}(\omega )$. We measure the polarization sensitive complex Faraday angle $\theta_{F,}$ which is proportional to $\sigma _{xy}(\omega )$. The complex $\theta_{\mathrm{F}}$ in Ba122 superconducting films and reference iron films are measured as a function of energy (0.1- 3 eV), temperature (10-300 K) and magnetic field ($B=$ 0-7T). Surprisingly, the infrared (0.1-0.4 eV) $\theta_{F}$ in Ba122 films is consistent with a soft ferromagnet having a step like feature near $B=$0, followed by a linear dependence at higher $B$. The step near $B=$0 ($\Delta \theta_{F})$ is due to the magnetization-dependent anomalous Hall effect of iron impurities, while the linear behavior ($\theta_{F,slope})$ at higher $B$ (after magnetization in Fe gets saturated) reflects the ordinary Hall response of Ba122. The Fe reference films show little temperature dependence in both $\Delta \theta_{F}$ and $\theta_{F,slope}$ at any energy, which is consistent with its high Curie temperature of $\sim$ 1000 K. On the other hand $\theta_{F,slope}$ for the Ba122 films shows strong, non-monotonic temperature dependence with a peak near 50 K [Preview Abstract] |
Thursday, March 6, 2014 1:27PM - 1:39PM |
T13.00010: Phase diagram of Fe-based superconductor Sr2FeAs(Mg,Ti)O3 Hiraku Ogino, Shiv Jee Singh, Akiyasu Yamamoto, Kohji Kishio, Jun-ichi Shimoyama In iron-based superconductors, many compounds having perovskite-type blocking layers such as Sr2FeAs(Mg,Ti)O3 and Ca4Fe2As2(Mg,Ti)3O8 were discovered[1]. There compounds have chemical and structural varieties, and have much thicker blocking layers compared to other phases. Generally superconducting transitions appear without intentional carrier doping, and Tc reaches as high as 47 K. On the other hand, electronic state and electronic phase diagram of these compounds are much less studied compared to other phases, and there are no clear observation of antiferromagnetic ordering in these compounds. In this study, we have systematically investigated phase diagram of Sr2FeAs(Mg,Ti)O3 phase by controlling carriers through oxygen composition and post-annealing. Relationship between crystal structure, chemical compositions and physical properties will be discussed. [1] S. Sato et al., Supercond. Sci. Technol. 23 (2010) 045001 [Preview Abstract] |
Thursday, March 6, 2014 1:39PM - 1:51PM |
T13.00011: Anisotropy and effects of oxygen deficiencies in single crystals of superconducting Sr$_{2}$VFeAsO$_{3}$ Takao Sasagawa, Takao Katagiri, Hiromi Kashiwaya, Satoshi Kashiwaya With a series of different oxygen deficiencies, single crystals of Sr$_{2}$VFeAsO$_{3}$ were successfully grown by a self-flux technique in an evacuated double quartz tube. Highly anisotropic properties were observed in the mixed state. From the angular dependence of the resistivity at various temperatures and fields, the anisotropy parameter ($=H_{\mathrm{c2}}^{\mathrm{//ab}}$/$H_{\mathrm{c2}}^{\mathrm{//c}}$) was quantitatively evaluated by using the anisotropic Ginzburg-Landau theory. The obtained value for the optimally doped crystal (with almost no oxygen deficiencies) amounts to 25, which is several times higher than other typical iron-based superconductors and comparable to the cuprate superconductor (La,Sr)$_{2}$CuO$_{4}$. In crystals with sufficient oxygen deficiencies, a ferromagnetic transition was found to appear above the superconducting transition. Upon increasing the oxygen deficiencies, a monotonic increase of the Curie temperature together with counter suppression of superconductivity was observed. In oxygen deficient crystals, it is highly likely that a natural superlattice with the periodic stack of superconducting FeAs and ferromagnetic Sr$_{2}$VO$_{3}$ layers, corresponding to SFS Josephson junctions, is realized. [Preview Abstract] |
Thursday, March 6, 2014 1:51PM - 2:03PM |
T13.00012: CaFe$_2$As$_2$ Under In-Plane Uniaxial Pressure Miles Frampton, Rena Zieve, Adam Dioguardi Many unconventional superconductors have a planar crystal structure, with a resulting two-dimensional character that favors superconductivity. They tend to have anisotropic behavior and can be very sensitive to uniaxial pressure. Since these materials often grow preferentially as platelets perpendicular to the crystalline $c$ axis, applying in-plane pressure is challenging. We present a new setup for studying thin samples under uniaxial pressure and our results on CaFe$_2$As$_2$. CaFe$_2$As$_2$ undergoes a magnetic transition simultaneously with a tetragonal-to-orthorhombic structural transition. In-plane uniaxial pressure detwins the orthorhombic phase and accentuates the difference between the axes. We find a significant change in $T_s$ as well as anisotropy of the in-plane resistivity that increases with pressure. [Preview Abstract] |
Thursday, March 6, 2014 2:03PM - 2:15PM |
T13.00013: Superconductivity and magnetism in naturally occurring minerals Renxiong Wang, S.R. Saha, Xiangfeng Wang, R.L. Greene, J. Paglione, C. Santelli, J. Post In a new and unique venture in collaboration with the Smithsonian Museum of Natural History's Department of Mineral Sciences, we present preliminary results from a project focusing on the search for superconductivity in mineral specimens provided by Geologists/Curators of the Smithsonian Institution. Including magnetization and transport studies of Wittichenite, Pyrrhotite, Nagyagite, Pyrargyrite and other related compounds, we report preliminary findings of the physical properties of mineral specimens at low temperatures, including several unreported magnetic phases and unconvetional behaviors. [Preview Abstract] |
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