### Session NB: Superconductivity

Chair: Roy Goodrich, Louisiana State University
Room: Nicholson Hall 109

 Saturday, October 23, 2010 10:45AM - 10:57AM NB.00001: Synthesis and Physical Properties of BaCo2As2 Single Crystals Amar Karki , Yimin Xiong , Rongying Jin We report synthesis and physical properties of BaCo2As2 single crystals grown by the self -flux method. While it does not show any magnetic and structural transition, the results reveal that BaCo2As2 is a usual paramagnetic metal with Wilson ratio well exceeding unity. We will discuss the implication by comparing its electrical, magnetic and thermodynamic properties with that of BaFe2As2. Saturday, October 23, 2010 10:57AM - 11:09AM NB.00002: BaFe2As2 Surface Domains and Domain Walls: Mirroring the Bulk Spin Structure Guorong Li , Xiaobo He , Ang Li , Shuheng Pan , Jiandi Zhang , Rongying Jin , Athena S. Sefat , Michael A. McGuire , David G. Mandrus , Brain C. Sales , Ward Plummer We have used scanning tunneling microscopy/spectroscopy (STM/STS) to investigate the geometric and electronic structure at the (001) surface of the parent compound BaFe$_{2}$As$_{2}$. While high-resolution STM measurements reveals a (1x1) As-terminated unit cell on the (001) surface, there are significant differences of the surface unit cell compared to the bulk: only one of the two As atoms in the unit cell is imaged and domain walls between different (1x1) regions display a C2 symmetry at the surface. It should have been C2v if the STM image reflected the geometric structure of the surface or the orthorhombic bulk. The inequivalent As atoms and the bias dependence of the domain walls indicate that the origin of the STM image is primarily electronic not geometric. We argue that the surface electronic topography mirrors the bulk spin structure of BaFe2As2, via strong orbital-spin coupling. Saturday, October 23, 2010 11:09AM - 11:21AM NB.00003: High-Temperature Electrical and Magnetic Properties of Undoped Iron Pnicties Jianneng Li , Y. Xiong , Y. Yang , R. Jin , T. Qi , G. Cao , J.R. Thompson , H. Wang , B.C. Sales , A.S. Sefat , M.A. McGuire , V. Keppens , D. Mandrus We have investigated the electrical and magnetic properties of several parent compounds of Fe-based superconductors in a wide temperature range. In addition to the well-known transitions (one structural transition at T$_{s}$ and one spin-density-wave (SDW) transition at T$_{M})$, all investigated parent compounds (BaFe$_{2}$As$_{2}$,$_{ }$SrFe$_{2}$As$_{2}$, LaFeAsO, FeTe) show unusual features in both magnetic susceptibility and electrical resistivity at high temperatures (T). Above T$_{s}$, their magnetic susceptibility increases more or less linearly with increasing temperature up to at least 700 K. On the other hand, the electrical resistivity exhibits non-monotonic temperature dependence, revealing non-metallic character at high temperatures. The implication of these results will be discussed. Saturday, October 23, 2010 11:21AM - 11:33AM NB.00004: Unusual Temperature and Field Dependence of Transport Properties Ba(Fe1-xCox)2As2 Yimin Xiong , Jianneng Li , Rongying Jin The in-plane transport properties of Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ single crystals with x = 0.02 $\sim$ 0.28 was measured as a function of temperature (2 $\sim$ 300 K) and magnetic field (up to 14 Tesla). A Cobalt doping composition-temperature (x-T) phase diagram was plotted and shows a non-Fermi liquid (NFL) transport behavior around the optimal doing level. The Hall effect and magneto-resistance results also show an anomaly and a change of field dependence around the optimal doing. The underlying physics of such unusual temperature and field dependence of transport properties $ab$ plane will be discussed. Saturday, October 23, 2010 11:33AM - 11:45AM NB.00005: Possible free flux flow phase in single crystals of optimally doped Ba(Fe$_{1-x}$Co$_{x})_{2}$As$_{2}$ A.A. Gapud , O. Gafarov , D.K. Christen , J.R. Thompson The possibility of a new magnetic component to the superconductivity in the recently discovered iron-containing superconductors -- something previously deemed impossible -- has attracted a wide breadth of studies. One area of interest is in magnetic phase transitions in the mixed-state flux medium'' comprised of interacting magnetic flux quanta (or vortices) which are found in Type II superconductors. Not surprisingly, the flux dynamics in these materials already show novelties not yet completely understood. Recent work from various groups do agree on a vortex liquid'' phase at highest fields and temperatures, with a melting'' transition line from a phase in which interactions between vortices become significant -- all while still affected by pinning mechanisms. To test the proposed phases, the present study explores the possibility of achieving the highly ordered free flux flow (FFF) phase in optimally doped Ba(Fe$_{1-0.92}$Co$_{0.08})_{2}$As$_{2}$. This may well be the first time such a measurement is reported for iron arsenides, which are also known for strong pinning. Results and preliminary analyses are discussed. Saturday, October 23, 2010 11:45AM - 11:57AM NB.00006: Free flux flow in two single crystals of V$_{3}$Si with slightly different pinning strengths O. Gafarov , A.A. Gapud , S. Moraes , J.R. Thompson , D.K. Christen , A.P. Reyes Results of recent measurements on two very clean, single-crystal samples of the A15 superconductor V$_{3}$Si are presented. Magnetization and transport data already confirmed the clean'' quality of both samples, as manifested by: (i) high residual resistivity ratio, (ii) very low critical current densities, and (iii) a peak'' effect in the field dependence of critical current. The (H,T) phase line for this peak effect is shifted in the slightly dirtier'' sample, which consequently also has higher critical current density J$_{c}$(H). High-current Lorentz forces are applied on mixed-state vortices in order to induce the highly ordered free flux flow (FFF) phase, using the same methods as in previous work. A traditional model by Bardeen and Stephen (BS) predicts a simple field dependence of flux flow resistivity $\rho _{f}$(H), presuming a field-independent flux core size. A model by Kogan and Zelezhina (KZ) takes core size into account, and predict a clear deviation from BS. In this study, $\rho _{f}$(H) is confirmed to be consistent with predictions of KZ, as will be discussed. Saturday, October 23, 2010 11:57AM - 12:09PM NB.00007: Deviation in magnetoresistive Kohler's rule due to Martensitic transformation in V$_{3}$Si S. Moraes , O. Gafarov , A.A. Gapud , J.R. Thompson , D.K. Christen , A.P. Reyes Preliminary results are presented on a comparison between two very clean, single-crystal samples of the A15 superconductor V$_{3}$Si. Three independent measurements on the same samples -- namely: (i) resistivity versus temperature, (ii) magnetic susceptibility, and (iii) nuclear magnetic resonance -- confirm that (a) both samples have a high residual resistivity ratio and (b) the Martensitic transformation is manifest in one sample, but suppressed in the other. This provides the opportunity to study how the Martensitic transformation causes the magnetoresistivity of V$_{3}$Si to deviate from Kohler's Rule, adding more detailed information to results obtained previously. Results and analysis are discussed. Saturday, October 23, 2010 12:09PM - 12:21PM NB.00008: Inductive Critical Currents in Mo/Nb layered structures Phillip Broussard , David Myers , James Veldhorst We have carried out measurements of inductive critical currents in Mo/Nb layered films. The films were grown by magnetron sputtering onto silicon substrates from separate sources. We grew films with the structure (N/S)$^{m}$, where the Mo/Nb bilayer is repeated $m$ times. Here the base bilayer unit is composed of a Mo layer 36.9 nm thick and a Nb layer 43.2 nm thick, while $m$ varies from 1 to 4. The films grow with (110) orientation, as expected for bcc materials. Inductive critical currents were measured using a third harmonic technique, while superconducting transition temperatures ($T_{c}$) were measured both resistively and inductively. The films were cooled by a cryocooler down to temperatures of approximately 6 K. We find the $T_{c}$ and the critical current density ($J_{c}$) are nearly independent of $m$. $J_{c}$ varies as $(1-t)^{3/2}$ as expected from Ginzburg-Landau theory (here $t$ is the reduced temperature, $T/T_{c}$). Measurements of $J_{c}$ have also been made in low magnetic fields, and will be discussed.